Wireless Networks With RF Energy Harvesting: A Contemporary Survey
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paper_title: Space solar power programs and microwave wireless power transmission technology

paper_content:

Future large-scale space solar power (SSP) will form a very complex integrated system of systems requiring numerous significant advances in current technology and capabilities. Ongoing technology developments have narrowed many of the gaps, but major technical, regulatory, and conceptual hurdles remain. Continuing systems concept studies and analyses will be critical to success, as will following a clear strategic R&T road map. This road map must assure both an incremental and evolutionary approach to developing needed technologies and systems is followed, with significant and broadly applicable advances with each increment. In particular, the technologies and systems needed for SPS must support highly leveraged applicability to needs in space science, robotic and human exploration, and the development of space. Considerable progress has been made in the critical area of microwave power transmission. At 5.8 GHz, DC-RF converters with efficiencies over 80% are achievable today. Rectennas developed at 5.8 GHz have also been measured with efficiencies greater than 80%. With optimized components in both the transmitter and rectenna, an SPS system has the potential of a DC-to-DC efficiency of 45%.
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paper_title: Transporting information and energy simultaneously

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The fundamental tradeoff between the rates at which energy and reliable information can be transmitted over a single noisy line is studied. Engineering inspiration for this problem is provided by powerline communication, RFID systems, and covert packet timing systems as well as communication systems that scavenge received energy. A capacity-energy function is defined and a coding theorem is given. The capacity-energy function is a non-increasing concave cap function. Capacity-energy functions for several channels are computed.
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paper_title: Enabling Wireless Power Transfer in Cellular Networks: Architecture, Modeling and Deployment

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Microwave power transfer (MPT) delivers energy wirelessly from stations called power beacons (PBs) to mobile devices by microwave radiation. This provides mobiles practically infinite battery lives and eliminates the need of power cords and chargers. To enable MPT for mobile charging, this paper proposes a new network architecture that overlays an uplink cellular network with randomly deployed PBs for powering mobiles, called a hybrid network. The deployment of the hybrid network under an outage constraint on data links is investigated based on a stochastic-geometry model where single-antenna base stations (BSs) and PBs form independent homogeneous Poisson point processes (PPPs) and single-antenna mobiles are uniformly distributed in Voronoi cells generated by BSs. In this model, mobiles and PBs fix their transmission power at p and q, respectively; a PB either radiates isotropically, called isotropic MPT, or directs energy towards target mobiles by beamforming, called directed MPT. The model is applied to derive the tradeoffs between the network parameters including p, q, and the BS/PB densities under the outage constraint. First, consider the deployment of the cellular network. It is proved that the outage constraint is satisfied so long as the product the BS density decreases with increasing p following a power law where the exponent is proportional to the path-loss exponent. Next, consider the deployment of the hybrid network assuming infinite energy storage at mobiles. It is shown that for isotropic MPT, the product between q, the PB density, and the BS density raised to a power proportional to the path-loss exponent has to be above a given threshold so that PBs are sufficiently dense; for directed MPT, a similar result is obtained with the aforementioned product increased by the array gain. Last, similar results are derived for the case of mobiles having small energy storage.
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paper_title: Resource Allocation in Wireless Networks with RF Energy Harvesting and Transfer

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RF energy harvesting and transfer techniques have recently become alternative methods to power the next generation of wireless networks. As this emerging technology enables proactive replenishment of wireless devices, it is advantageous in supporting applications with QoS requirements. This article focuses on the resource allocation issues in wireless networks with RF energy harvesting capability, referred to as RF-energy harvesting networks (RF-EHNs). First we present an overview of the RF-EHNs, followed by a review of a variety of issues regarding resource allocation. Then we present a case study of designing in the receiver operation policy, which is of paramount importance in RF-EHNs. We focus on QoS support and service differentiation, which have not been addressed in previous literature. We also outline open research directions.
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paper_title: RF Energy Harvesting and Transport for Wireless Sensor Network Applications: Principles and Requirements

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This paper presents an overview of principles and requirements for powering wireless sensors by radio-frequency (RF) energy harvesting or transport. The feasibility of harvesting is discussed, leading to the conclusion that RF energy transport is preferred for powering small sized sensors. These sensors are foreseen in future Smart Buildings. Transmitting in the ISM frequency bands, respecting the transmit power limits ensures that the International Commission on Non-Ionizing Radiation Protection (ICNIRP) exposure limits are not exceeded. With the transmit side limitations being explored, the propagation channel is next discussed, leading to the observation that a better than free-space attenuation may be achieved in indoors line-of-sight environments. Then, the components of the rectifying antenna (rectenna) are being discussed: rectifier, dc-dc boost converter, and antenna. The power efficiencies of all these rectenna subcomponents are being analyzed and finally some examples are shown. To make RF energy transport a feasible powering technology for low-power sensors, a number of precautions need to be taken. The propagation channel characteristics need to be taken into account by creating an appropriate transmit antenna radiation pattern. All subcomponents of the rectenna need to be impedance matched, and the power transfer efficiencies of the rectifier and the boost converter need to be optimized.
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paper_title: Shannon meets Tesla: Wireless information and power transfer

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The problem considered here is that of wireless information and power transfer across a noisy coupled-inductor circuit, which is a frequency-selective channel with additive white Gaussian noise. The optimal tradeoff between the achievable rate and the power transferred is characterized given the total power available. The practical utility of such systems is also discussed.
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paper_title: Wireless power transfer and applications to sensor networks

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Energy constraints are widely regarded as a fundamental limitation of wireless and mobile devices. For sensor networks, a limited lifetime due to battery constraint poses a performance bottleneck and barrier for large scale deployment. Recently, wireless power transfer has emerged as a promising technology to address energy and lifetime bottlenecks in a sensor network. In this article, we give a review of the history of wireless power transfer and describe its recent developments. We show how such technologies can be applied to sensor networks and address their energy constraints.
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paper_title: Determination of far-field antenna patterns from near-field measurements

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In many cases, it is impractical or impossible to make antenna pattern measurements on a conventional far-field range; the distance to the radiating far field may be too long, it may be impractical to move the antenna from its operating environment to an antenna range, or the desired amount of pattern data may require too much time on a far-field range. For these and other reasons, it is often desirable or necessary to determine far-field antenna patterns from measurements made in the radiating near-field region; three basic techniques for accomplishing this have proven to be successful. In the first technique, the aperture phase and amplitude distributions are sampled by a scanning field probe, and then the measured distributions are transformed to the far field. In the second technique, a plane wave that is approximately uniform in amplitude is created by a feed and large reflector in the immediate vicinity of the test antenna. And in the third technique, the test antenna is focused within the radiating near-field region, patterns are measured at the reduced range, and then the antenna is refocused to infinity. Each of these techniques is discussed, and the various advantages and limitations of each technique are presented.
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paper_title: Experiments Involving a Microwave Beam to Power and Position a Helicopter

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This paper describes two different experiments involving a microwavebeam and a helicopter. The first experiment utilized a CW microwavebeam to supply a small helicopter with all of the power that it neededfor its propulsion. The second experiment utilized an unmodulated CW microwave beam for supplying a position reference to the helicopterer with respect to roll, pitch, yaw, and horizontal translation. Thesecond experiment also involved the construction of a fully articulatedhelicopter and a complete control system carried on board theli-helicopter. The paper also attempts to relate the small scale experimentsand the present state of component technology to practical, full scalemicrowave-powered helicopter systems that can operate at altitudesof up to 50 000 feet.
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paper_title: Wireless charger networking for mobile devices: fundamentals, standards, and applications

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Wireless charging is a technique of transmitting power through an air gap to an electrical device for the purpose of energy replenishment. Recently, wireless charging technology has significantly advanced in terms of efficiency and functionality. This article first presents an overview and fundamentals of wireless charging. We then provide the review of standards, that is, Qi and the Alliance for Wireless Power, and highlight their communication protocols. Next, we propose a novel concept of wireless charger networking that allows chargers to be connected to facilitate information collection and control. We demonstrate the application of the wireless charger network in user-charger assignment, which clearly shows the benefit in terms of reduced costs for users to identify the best chargers to replenish energy for their mobile devices.
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paper_title: Energy-Efficient Optimization for Wireless Information and Power Transfer in Large-Scale MIMO Systems Employing Energy Beamforming

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In this letter, we consider a large-scale multiple-input multiple-output (MIMO) system where the receiver should harvest energy from the transmitter by wireless power transfer to support its wireless information transmission. The energy beamforming in the large-scale MIMO system is utilized to address the challenging problem of long-distance wireless power transfer. Furthermore, considering the limitation of the power in such a system, this letter focuses on the maximization of the energy efficiency of information transmission (bit per Joule) while satisfying the quality-of-service (QoS) requirement, i.e. delay constraint, by jointly optimizing transfer duration and transmit power. By solving the optimization problem, we derive an energy-efficient resource allocation scheme. Numerical results validate the effectiveness of the proposed scheme.
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paper_title: Wireless power transfer via strongly coupled magnetic reso- nances

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Using self-resonant coils in a strongly coupled regime, we experimentally demonstrated efficient nonradiative power transfer over distances up to 8 times the radius of the coils. We were able to transfer 60 watts with ∼40% efficiency over distances in excess of 2 meters. We present a quantitative model describing the power transfer, which matches the experimental results to within 5%. We discuss the practical applicability of this system and suggest directions for further study.
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paper_title: Prototype implementation of ambient RF energy harvesting wireless sensor networks

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Energy harvesting is a key technique that can be used to overcome the barriers that prevent the real world deployment of wireless sensor networks (WSNs). In particular, solar energy harvesting has been commonly used to overcome this barrier. However, it should be noted that WSNs operating on solar power suffer form energy shortage during nighttimes. Therefore, to solve this problem, we exploit the use of TV broadcasts airwaves as energy sources to power wireless sensor nodes. We measured the output of a rectenna continuously for 7 days; from the results of this measurement, we showed that Radio Frequency (RF) energy can always be harvested. We developed an RF energy harvesting WSN prototype to show the effectiveness of RF energy harvesting for the usage of a WSN. We also proposed a duty cycle determination method for our system, and verified the validity of this method by implementing our system. This RF energy harvesting method is effective in a long period measurement application that do not require high power consumption.
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paper_title: The wireless power transmission: inductive coupling, radio wave, and resonance coupling

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This paper describes various wireless power transmission (WPT) technologies including inductive coupling, radio waves, and resonance coupling. Theoretically, these three WPT technologies are similar, in that all of them depend on Maxwell's equations. However, there are pros and cons for each application of the WPT. Therefore, a suitable WPT technology must be selected for each application. Herein, the theory, technologies, and applications of WPT are discussed. WPT is a useful and convenient technology that can be employed to charge the batteries in mobile phones, notebook PCs, electric vehicles, light-emitting diodes, integrated circuits, and other equipment without the need for a wire connection. For systems that use very low power, the battery can be removed altogether, and the systems can be run on energy harvested from ambient radio frequency and microwave radiation. Therefore, the number of batteries can be reduced when wireless power is available from various locations, and because batteries can be charged wirelessly, concerns about the shortage of batteries can also be reduced. WPT via microwaves, for example, can be applied in the future to stable and CO2-free space-based solar power satellites. Overall, WPT will support both future energy production and the environment. ::: ::: This article is categorized under: ::: ::: Concentrating Solar Power > Systems and Infrastructure ::: Energy Infrastructure > Systems and Infrastructure ::: Energy Research & Innovation > Systems and Infrastructure
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paper_title: Multi-Antenna Wireless Powered Communication With Energy Beamforming

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The newly emerging wireless powered communication networks (WPCNs) have recently drawn significant attention, where radio signals are used to power wireless terminals for information transmission. In this paper, we study a WPCN where one multi-antenna access point (AP) coordinates energy transfer and information transfer to/from a set of single-antenna users. A harvest-then-transmit protocol is assumed where the AP first broadcasts wireless power to all users via energy beamforming in the downlink (DL), and then, the users send their independent information to the AP simultaneously in the uplink (UL) using their harvested energy. To optimize the users' throughput and yet guarantee their rate fairness, we maximize the minimum throughput among all users by a joint design of the DL-UL time allocation, the DL energy beamforming, and the UL transmit power allocation, as well as receive beamforming. We solve this nonconvex problem optimally by two steps. First, we fix the DL-UL time allocation and obtain the optimal DL energy beamforming, UL power allocation, and receive beamforming to maximize the minimum signal-to-interference-plus-noise ratio of all users. This problem is shown to be still nonconvex; however, we convert it equivalently to a spectral radius minimization problem, which can be solved efficiently by applying the alternating optimization based on the nonnegative matrix theory. Then, the optimal time allocation is found by a one-dimensional search to maximize the minimum rate of all users. Furthermore, two suboptimal designs of lower complexity are also proposed, and their throughput performance is compared against that of the optimal solution.
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paper_title: MIMO Broadcasting for Simultaneous Wireless Information and Power Transfer

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Wireless power transfer (WPT) is a promising new solution to provide convenient and perpetual energy supplies to wireless networks. In practice, WPT is implementable by various technologies such as inductive coupling, magnetic resonate coupling, and electromagnetic (EM) radiation, for short-/mid-/long-range applications, respectively. In this paper, we consider the EM or radio signal enabled WPT in particular. Since radio signals can carry energy as well as information at the same time, a unified study on simultaneous wireless information and power transfer (SWIPT) is pursued. Specifically, this paper studies a multiple-input multiple-output (MIMO) wireless broadcast system consisting of three nodes, where one receiver harvests energy and another receiver decodes information separately from the signals sent by a common transmitter, and all the transmitter and receivers may be equipped with multiple antennas. Two scenarios are examined, in which the information receiver and energy receiver are separated and see different MIMO channels from the transmitter, or co-located and see the identical MIMO channel from the transmitter. For the case of separated receivers, we derive the optimal transmission strategy to achieve different tradeoffs for maximal information rate versus energy transfer, which are characterized by the boundary of a so-called rate-energy (R-E) region. For the case of co-located receivers, we show an outer bound for the achievable R-E region due to the potential limitation that practical energy harvesting receivers are not yet able to decode information directly. Under this constraint, we investigate two practical designs for the co-located receiver case, namely time switching and power splitting, and characterize their achievable R-E regions in comparison to the outer bound.
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paper_title: Wireless power transfer using weakly coupled magnetostatic resonators

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Wireless power transfer can create the illusion of portable devices with infinite power supplies and enable applications that are currently unimaginable because of power constraints. Magnetic induction has been extensively used for wireless power transfer, but its efficiency depends on magnetic coupling that decays as the inverse cube of distance. At long enough distances, the magnetic coupling is weak enough that the effect of the receiver coil on the sender coil can be neglected. In this weakly coupled limit, series resonance in both the sender and the receiver increases the power transfer. Compared to magnetic induction, the power transfer increases by the sum of the quality factors of the sender and the receiver times the quality factor of the sender. Similarly, the efficiency increases by half of the product of the quality factors of the sender and the receiver. However, the overall efficiency of the power transfer is less than 50% for all weakly coupled series resonators. Resonators with a Q of 1,000 should be able to send power over a distance 9 times the radius of the devices with an efficiency of 10%.
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paper_title: MIMO Broadcasting for Simultaneous Wireless Information and Power Transfer

paper_content:

Wireless power transfer (WPT) is a promising new solution to provide convenient and perpetual energy supplies to wireless networks. In practice, WPT is implementable by various technologies such as inductive coupling, magnetic resonate coupling, and electromagnetic (EM) radiation, for short-/mid-/long-range applications, respectively. In this paper, we consider the EM or radio signal enabled WPT in particular. Since radio signals can carry energy as well as information at the same time, a unified study on simultaneous wireless information and power transfer (SWIPT) is pursued. Specifically, this paper studies a multiple-input multiple-output (MIMO) wireless broadcast system consisting of three nodes, where one receiver harvests energy and another receiver decodes information separately from the signals sent by a common transmitter, and all the transmitter and receivers may be equipped with multiple antennas. Two scenarios are examined, in which the information receiver and energy receiver are separated and see different MIMO channels from the transmitter, or co-located and see the identical MIMO channel from the transmitter. For the case of separated receivers, we derive the optimal transmission strategy to achieve different tradeoffs for maximal information rate versus energy transfer, which are characterized by the boundary of a so-called rate-energy (R-E) region. For the case of co-located receivers, we show an outer bound for the achievable R-E region due to the potential limitation that practical energy harvesting receivers are not yet able to decode information directly. Under this constraint, we investigate two practical designs for the co-located receiver case, namely time switching and power splitting, and characterize their achievable R-E regions in comparison to the outer bound.
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paper_title: Antenna Theory: Analysis and Design

paper_content:

The most-up-to-date resource available on antenna theory and design. Expanded coverage of design procedures and equations makes meeting ABET design requirements easy and prepares readers for authentic situations in industry. New coverage of microstrip antennas exposes readers to information vital to a wide variety of practical applications.Computer programs at end of each chapter and the accompanying disk assist in problem solving, design projects and data plotting.-- Includes updated material on moment methods, radar cross section, mutual impedances, aperture and horn antennas, and antenna measurements.-- Outstanding 3-dimensional illustrations help readers visualize the entire antenna radiation pattern.
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paper_title: DRIFT: Differentiated RF Power Transmission for Wireless Sensor Network deployment in the smart grid

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Smart grid calls for low-cost, fine-grained and long-lasting monitoring solutions, to be able to provide reliable service to customers, enhance situational awareness capabilities and refine the operation of the grid and the microgrids, in addition to enabling prompt utility reaction to emergencies. Wireless Sensor Networks (WSNs) are promising candidates for monitoring the smart grid, given their capability to cover a large geographic region at low-cost. However, they do not provide long-lasting operation capability due to the limited battery lifetime of the sensors. Particularly, when sensors are deployed in hard-to-reach or hazardous environments, replacing the batteries of the sensors increase the cost of monitoring significantly. In the literature, energy-efficient protocols and ambient energy harvesting have been proposed to extend the lifetime of the sensors while neither of those offer a concrete solution for the smart grid. In this context, recent advances in Radio Frequency (RF) energy harvesting offers a unique solution to make WSNs operationally ready for smart grid monitoring tasks. Studies on RF energy harvesting have focused on uniform power delivery to all sensors, however it becomes essential to differentiate between critical zones and less critical zones in smart grid monitoring tasks. In this paper, we propose the Differentiated RF Power Transmission (DRIFT) scheme which is based on an Integer Linear Programming (ILP) model that maximizes the power received by the high priority sensor nodes. We compare the performance of DRIFT with a path length minimizing approach, namely Sustainable wireless Rechargeable Sensor network (SuReSense). We show that DRIFT is able to provide more power to high priority nodes than SuReSense. We also show that there is a tradeoff between power maximization and path length minimization.
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paper_title: Performance Analysis of Ambient RF Energy Harvesting: A Stochastic Geometry Approach

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Ambient RF (Radio Frequency) energy harvesting techniques have recently been proposed as a potential solution to provide proactive energy replenishment for wireless devices. This paper aims to analyze the performance of a battery-free wireless sensor powered by ambient RF energy harvesting using a stochastic-geometry approach. Specifically, we consider a random network model in which ambient RF sources are distributed as a Ginibre α-determinantal point process which recovers the Poisson point process when α approaches zero. We characterize the expected RF energy harvesting rate. We also perform a worst-case study which derives the upper bounds of both power outage and transmission outage probabilities. Numerical results show that our upper bounds are accurate and that better performance is achieved when the distribution of ambient sources exhibits stronger repulsion.
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paper_title: Opportunistic Wireless Energy Harvesting in Cognitive Radio Networks

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Wireless networks can be self-sustaining by harvesting energy from ambient radio-frequency (RF) signals. Recently, researchers have made progress on designing efficient circuits and devices for RF energy harvesting suitable for low-power wireless applications. Motivated by this and building upon the classic cognitive radio (CR) network model, this paper proposes a novel method for wireless networks coexisting where low-power mobiles in a secondary network, called secondary transmitters (STs), harvest ambient RF energy from transmissions by nearby active transmitters in a primary network, called primary transmitters (PTs), while opportunistically accessing the spectrum licensed to the primary network. We consider a stochastic-geometry model in which PTs and STs are distributed as independent homogeneous Poisson point processes (HPPPs) and communicate with their intended receivers at fixed distances. Each PT is associated with a guard zone to protect its intended receiver from ST's interference, and at the same time delivers RF energy to STs located in its harvesting zone. Based on the proposed model, we analyze the transmission probability of STs and the resulting spatial throughput of the secondary network. The optimal transmission power and density of STs are derived for maximizing the secondary network throughput under the given outage-probability constraints in the two coexisting networks, which reveal key insights to the optimal network design. Finally, we show that our analytical result can be generally applied to a non-CR setup, where distributed wireless power chargers are deployed to power coexisting wireless transmitters in a sensor network.
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paper_title: RF energy harvesting system and circuits for charging of mobile devices

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RF energy harvesting holds a promise able future for generating a small amount of electrical power to drive partial circuits in wirelessly communicating electronics devices. This paper presents the overview and progress achieved in RF energy harvesting field. A modified form of existing CMOS based voltage doubler circuit is presented to achieve 160% increase in output power over traditional circuits at 0 dBm input power. A schottky diode based RF energy harvesting circuit performance is also studied with practical and simulations results.
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paper_title: Design of an efficient ambient WiFi energy harvesting system

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This study is focused on equipping wireless devices (including sensors) with novel, high-efficiency circuitry to harvest and convert ambient radio frequency (RF) power to direct current (dc). Key components of this technology are (a) miniaturised antenna and (b) high-efficiency rectifying circuit. The first is responsible for capturing the RF waves, and the latter converts the RF energy to dc. A major challenge is the design of novel circuitry to generate a battery-like voltage from very low incoming RF energy. Under this study, the authors designed a novel RF power harvesting front-end whose conversion efficiency is significantly improved at low RF power levels (<;-20 dBm) as compared to existing technologies. Thus, the new circuitry can harvest ambient and widely available RF energy, making a game changing technology for powering mobile devices. In this study, the authors demonstrate this technology by using it to power a commercially available temperature and humidity meter with an LCD display. The latter is powered using nothing more than ambient WiFi signals in an office environment.
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paper_title: RF energy harvesting system from cell towers in 900MHz band

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An experimental RF energy harvesting system to harvest energy from cell towers is presented in this paper. An electromagnetically-coupled square microstrip antenna is designed and fabricated for deployment in the presented system. Antenna gain of 9.1dB and bandwidth from 877 MHz to 998 MHz is achieved. A Schottky diode-based single stage voltage doubler and six-stage voltage doubler has also been designed and fabricated for DC voltage generation. Measured results show that a voltage of 2.78V is obtained at a distance of 10m from the cell tower and a voltage of 0.87V is obtained at a distance of 50m.
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paper_title: Optimized CMOS RF-DC converters for remote wireless powering of RFID applications

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In this paper, we present for the first time a novel optimization procedure which allows to maximize the efficiency of RF-DC energy harvester converters, taking into account the contributions of the matching network. Thanks to this procedure, we have designed and realized a CMOS RF-DC converter operating in a very wide range of input power −14÷+1dBm with a peak efficiency of 45% The RF-DC converter provides a constant output voltage ∼2V in the whole input power range thanks to a smart voltage regulator integrated with the converter.
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paper_title: Low power analog circuit design for RFID sensing circuits

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This paper presents low power circuit design for voltage regulator and resistor to digital converter that are used in a RFID-like sensing circuit for measuring the resistance of a sol-gel sensor. The regulator circuit has simple circuit structure and consumes zero DC current (except the current drained by its load). The resistor to digital converter consists of a cascoded current mirror, a reference resistor and a charge-redistribution (CR) ADC circuit. The proposed circuit techniques reduce the size of the ADC capacitor array by half compared to traditional CR ADCs with the same resolution. The proposed techniques also significantly reduce the circuit power consumption with enhanced measurement accuracy. The proposed circuits and a RFID energy harvesting circuit for powering the proposed circuits are implemented using a 0.13µ CMOS technology. Circuit simulations are presented to examine the performance of the proposed circuits.
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paper_title: Efficient ambient WiFi energy harvesting technology and its applications

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This paper is focused on powering wireless devices (including sensors) with high efficiency circuitry that can harvest ambient WiFi/Bluetooth energy and convert into direct current (dc). Herewith, we introduce a novel circuitry to generate a battery-like voltage from very low incoming RF energy (<−20 dBm). The new RF energy harvesting front-end is demonstrated to have much better conversion efficiency than state-of-the-art commercial RF harvesting modules, notably for low power levels. Realization of this rectification circuitry is a game-changing technology in powering mobile devices. To verify our energy harvesting design, we carry out an experiment where our RF harvesting module provides enough power to drive a commercial temperature/humidity meter with an LCD display using nothing more than ambient WiFi signals in a typical office environment.
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paper_title: Power harvester design for semi-passive UHF RFID Tag using a tunable impedance transformation

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This paper proposes a design of a power harvester for a semi-passive RFID Tag. A tunable impedance transformation circuit is inserted between the antenna and the rectifier unit to maximize the power delivered to the rectifier of a tag and minimize reflections from the antenna input port. A power harvester for the UHF band of 950 MHz has been designed based on a 0.35 µm CMOS technology. This impedance matching is designed to maximize efficiency by characterizing both the tunable impedance transformation circuit and the conventional rectifier circuit. The antenna is modeled as an RF source with series impedance Z S of 50Ω while the rectifier is modeled an impedance Z L , which is approximated from the process and design parameters of the NMOS transistors used in the conventional NMOS rectifier. These two impedances are used to find optimal matching parameters of the chosen lowpass LL matching circuit. The simulation results show that the required voltage for a 1.2V secondary battery can be achieved with the tunable impedance matching circuit and conventional 2-stage rectifier. The DC output of the rectifier yields the maximum DC voltage of 1.77V, the maximum average current of 4.72µA, and the power conversion efficiency of 15.85% for the input RF power of −14.47dBm at the battery voltage of 1.2V.
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paper_title: Survivable RFID Systems: Issues, Challenges, and Techniques

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Radio frequency identification (RFID) technique has been applied in high-security and high-integrity settings such as national defense, healthcare, and citizen identification. A tiny RFID tag is attached to a mobile object, which can be scanned and recognized by a reader. RFID offers opportunities for real-time item identification and inventory tracking. For applications using resource-restricted RFID tags and mobile hand-held readers, however, various risks could threaten their abilities to provide essential services to users. High mobility of the RFID system components and the open nature make an RFID system vulnerable to various attacks. Currently, although some techniques exist that might help improve survivability; there is still no complete proposal on survivability of an RFID system despite its growing popularity and importance in many applications. In this paper, we study survivability issues related to RFID systems and survey existing techniques whose usability and adaptability toward survivability would be beneficial. Survivability is defined as the ability of a system to continuously provide essential services to support the system's mission even in the presence of malicious attacks or system failures. We discuss the issues and challenges in developing survivable RFID systems and propose research directions.
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paper_title: Wi-Pie: Energy Harvesting in Mobile Electronic Devices

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A recent study puts the number of mobile phones in use around the world at an astonishing 5 billion and these numbers are growing rapidly. This places the increase in energy consumption of mobile phones in the world at an alarming level. Wi-Pie or Wireless-Piezoelectric technique combines RF energy scavenging and piezoelectric energy harvesting technologies to provide an alternative method of powering mobile electronic devices. By placing piezoelectric crystals beneath the keypad of a mobile phone, we intend to generate a small voltage with every key press which could be stored in a secondary storage device like a capacitor. The charge accumulated in the device can then be transferred to the primary storage or the battery so that the use of external chargers can be minimized. Additional power management circuitry is used to intercept the known and the unknown sources of ambient RF radiations. The received RF power is converted to DC and conditioned before it's delivered to the load. Although the RF energy levels are low, a combination of the harvesting and scavenging techniques will provide a significant source of power.
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paper_title: Antennas and circuits for ambient RF energy harvesting in wireless body area networks

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In this paper, we identify the spectrum opportunities for radio frequency (RF) energy harvesting through power density measurements from 350 MHz to 3 GHz. The field trials have been performed in Covilha by using the NAKDA-SMR spectrum analyser with a measuring antenna. Based on the identification of the most promising opportunities, a dual-band band printed antenna operating at GSM bands (900/1800) is proposed, with gains of the order 1.8-2.06 dBi and efficiency 77.6-84%. Guidelines for the design of RF energy harvesting circuits and choice of textile materials for a wearable antenna are also discussed. Besides, we address the guidelines for designing circuits to harvest energy in a scenario where a wireless body area network (WBAN) is being sustained by a TX91501 Powercasf® RF dedicated transmitter and a five-stage Dickson voltage multiplier responsible for harvesting the RF energy. The IRIS motes, considered for our WBAN scenario, can perpetually operate if the RF received power attains at least -10 dBm.
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paper_title: Design and analysis of very low voltage charge pumps for RFID tags

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The design and analysis of very low-voltage driven charge pumps powered by RF telemetry is proposed. The use of thick oxide zero threshold voltage transistors along with appropriately sized boosting capacitors and matching techniques allows for charge pumps capable of achieving high voltage DC outputs with very low input voltages. Two test chips have been fabricated, an 11 stage pump and a 12 stage pump in 1.2 V 0.13-mum standard CMOS process. The pumps are capable of generating an output voltage above 1.2 volts with input voltages below 100 mV making them ideal for generating DC supplies from low RF scavenged sources.
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paper_title: A dual-band wireless energy transfer protocol for heterogeneous sensor networks powered by RF energy harvesting

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Radio frequency (RF) energy harvesting promises to realize battery-less sensor networks by converting energy contained in electromagnetic waves into useful electrical energy. We consider a network architecture that allows heterogeneous frequency harvesting. One class of sensors harvests RF energy on the DTV band (614 MHz) while another uses the 915 MHz ISM band. We study the effective energy transfer that is achieved under these circumstances, and then design a link layer protocol called RF-HSN that optimizes the energy delivery to energy-hungry sensors with the optimal duty cycle. To the best of our knowledge, this is the first wireless energy transfer protocol for heterogeneous frequency RF energy harvesting, and through a combination of experimentation and simulation studies, we demonstrate over 59% higher duty cycle and 66% average network throughput improvement over the classical CSMA MAC protocol.
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paper_title: A Battery-Free Tag for Wireless Monitoring of Heart Sounds

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We have developed a wearable, battery-free tag that monitors heart sounds. The tag powers up by harvesting ambient RF energy, and contains a low-power integrated circuit, an antenna and up to four microphones. The chip, which consumes only 1.0uW of power, generates digital events when the outputs of any of the microphones exceeds a programmable threshold voltage, combines such events together by using a programmable logic array, and transmits them to a base station by using backscatter modulation. The chip can also be programmed to trade-off microphone sensitivity for power consumption. In this paper, we demonstrate that the tag, when attached to the chest, can reliably measure heart rate at distances up to 7m from an FCC-compliant RF power source. We also suggest how delays between signals measured by microphones at the wrist and neck can be used to provide information about relative blood-pressure variations.
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paper_title: 0.56 V, –20 dBm RF-Powered, Multi-Node Wireless Body Area Network System-on-a-Chip With Harvesting-Efficiency Tracking Loop

paper_content:

A battery-less, multi-node wireless body area network (WBAN) system-on-a-chip (SoC) is demonstrated. An efficiency tracking loop is proposed that adjusts the rectifier's threshold voltage to maximize the wireless harvesting operation, resulting in a minimum RF sensitivity better than –20 dBm at 904.5 MHz. Each SoC node is injection-locked and time-synchronized with the broadcasted RF basestation power (up to a sensitivity of –33 dBm) using an injection-locked frequency divider (ILFD). Hence, every sensor node is phase-locked with the basestation and all nodes can wirelessly transmit TDMA sensor data concurrently. Designed in a 65 nm-CMOS process, the fabricated sensor SoC contains the energy harvesting rectifier and bandgap, duty-cycled ADC, digital logic, as well as the multi-node wireless clock synchronization and MICS-band transmitter. For a broadcasted basestation power of 20 dBm (30 dBm), experimental measurements verify correct powering, sensor reading, and wireless data transfer for a distance of 3 m (9 m). The entire biomedical system application is verified by reception of room and abdominal temperature monitoring.
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paper_title: Low-Power Far-Field Wireless Powering for Wireless Sensors

paper_content:

This paper discusses far-field wireless powering for low-power wireless sensors, with applications to sensing in environments where it is difficult or impossible to change batteries and where the exact position of the sensors might not be known. With expected radio-frequency (RF) power densities in the 20-200- μW/cm2 range, and desired small sensor overall size, low-power nondirective wireless powering is appropriate for sensors that transmit data at low duty cycles. The sensor platform is powered through an antenna which receives incident electromagnetic waves in the gigahertz frequency range, couples the energy to a rectifier circuit which charges a storage device (e.g., thin-film battery) through an efficient power management circuit, and the entire platform, including sensors and a low-power wireless transmitter, and is controlled through a low-power microcontroller. For low incident power density levels, codesign of the RF powering and the power management circuits is required for optimal performance. Results for hybrid and monolithic implementations of the power management circuitry are presented with integrated antenna rectifiers operating in the 1.96-GHz cellular and in 2.4-GHz industrial-scientific-medical (ISM) bands.
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paper_title: RF Energy Transmission for a Low-Power Wireless Impedance Sensor Node

paper_content:

The proper management of energy resources is essential for any wireless sensing system. With applications that span industrial, civil, and aerospace infrastructure, it is necessary for sensors and sensor nodes to be physically robust and power efficient. In many applications, a sensor network must operate in locations that are difficult to access, and often these systems have a desired operational lifespan which exceeds that of conventional battery technologies. In the present study, the use of microwave energy is examined as an alternate method for powering compact, deployable wireless sensor nodes. A prototype microstrip patch antenna has been designed to operate in the 2.4 GHz ISM band and is used to collect directed radio frequency (RF) energy to power a wireless impedance device that provides active sensing capabilities for structural health monitoring applications. The system has been demonstrated in the laboratory, and was deployed in field experiments on the Alamosa Canyon Bridge in New Mexico in August 2007. The transmitted power was limited to 1 W in field tests, and was able to charge the sensor node to 3.6 V in 27 s. This power level was sufficient to measure two piezoelectric sensors and transmit data back to a base station on the bridge.
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paper_title: Low-profile planar rectenna for batteryless RFID sensors

paper_content:

Radio Frequency Identification (RFID) technology is highly desirable for sensing applications, where form-factor, unit price and lifetime are most important. However, current passive RFID technology is mostly limited to only identifying and inventorying items placed near the reader.
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paper_title: A batteryless 19μW MICS/ISM-band energy harvesting body area sensor node SoC

paper_content:

Recent advances in ultra-low power chip design techniques, many originally targeting wireless sensor networks, will enable a new generation of body-worn devices for health monitoring. We utilize the state-of-the-art in low power RF transmitters, low voltage boost circuits, subthreshold processing, biosignal front-ends, dynamic power management, and energy harvesting to realize an integrated reconfigurable wireless body-area-sensor node (BASN) SoC capable of autonomous power management for battery-free operation.
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paper_title: Rectenna architecture based energy harvester for low power RFID application

paper_content:

Large scale implementation of active RFID tag technology has been restricted by the need for battery replacement. Prolonging battery lifespan may potentially promote active RFID tags which offer obvious advantages over passive RFID systems. This paper explores some opportunities to simulate and develop a prototype RF energy harvester for 2.4 GHz band specifically designed for low power active RFID tag application. This system employs a rectenna architecture which is a receiving antenna attached to a rectifying circuit that efficiently converts RF energy to DC current. Initial ADS simulation results show that 2 V output voltage can be achieved using a 7 stage Cockroft-Walton rectifying circuitry with −4.881 dBm (0.325 mW) output power under −4 dBm (0.398 mW) input RF signal. These results lend support to the idea that RF energy harvesting is indeed promising.
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paper_title: An efficiency enhancement technique for CMOS rectifiers with low start-up voltage for UHF RFID tags

paper_content:

A high-efficiency CMOS rectifier with low start-up voltage for ultra-high-frequency (UHF) radio-frequency identification (RFID) applications is presented. To enhance the power conversion efficiency (PCE) of the conventional rectifier when the input voltage (power) is low, appropriate gate-drive voltages for each stage of the rectifier are generated using a chain of auxiliary floating rectifier cells. Floating rectifier cells are optimized to generate shifted versions of the intermediate voltage of each stage to boost the drive voltage of NMOS and PMOS switching transistors and accordingly improve the PCE. The proposed rectifier architecture is designed in a standard 0.13μm CMOS technology. For a 950 MHz RF input and 50 kΩ output load, simulation results show that the rectifier achieves a PCE of 54% for a small input signal with an amplitude of 200 mV (-19 dBm) which is well below the nominal standard threshold voltage of MOS transistors in the technology used.
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paper_title: Prototype implementation of ambient RF energy harvesting wireless sensor networks

paper_content:

Energy harvesting is a key technique that can be used to overcome the barriers that prevent the real world deployment of wireless sensor networks (WSNs). In particular, solar energy harvesting has been commonly used to overcome this barrier. However, it should be noted that WSNs operating on solar power suffer form energy shortage during nighttimes. Therefore, to solve this problem, we exploit the use of TV broadcasts airwaves as energy sources to power wireless sensor nodes. We measured the output of a rectenna continuously for 7 days; from the results of this measurement, we showed that Radio Frequency (RF) energy can always be harvested. We developed an RF energy harvesting WSN prototype to show the effectiveness of RF energy harvesting for the usage of a WSN. We also proposed a duty cycle determination method for our system, and verified the validity of this method by implementing our system. This RF energy harvesting method is effective in a long period measurement application that do not require high power consumption.
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paper_title: Integrated UHF a-IGZO energy harvester for passive RFID tags

paper_content:

We present an energy harvester composed of a dipole antenna, matching network and a rectifier based on thin-film metal-oxide (amorphous Indium-Gallium-Zinc Oxide, IGZO) semiconductor Schottky diodes, operating at 868MHz. S-parameters measurements show that the integrated single IGZO Schottky diodes have a cutoff frequency over 1.8GHz at 0V bias. Large signal analysis of the integrated double-half wave rectifier shows a cut-off frequency of 1.1GHz. Our harvester generates 1.3V at 15 cm from an emitter emitting 15dBm at 868MHz.
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paper_title: Data delivery scheme for Wireless Sensor Network powered by RF energy harvesting

paper_content:

Wireless Sensor Networks (WSNs) have traditionally been powered by portable energy sources, viz. batteries, which limit their operational lifetime. To ensure WSN sustainability, researchers have turned to alternative energy sources for power. Harvesting ambient energy to power WSNs is a promising approach but energy harvesting devices of the same footprint as wireless sensors are unable to provide sufficient energy for sustained operation; availability of energy is also highly unpredictable. Harvesting energy from radio waves can provide energy-on-demand using a radio transmitter to broadcast over the sensor network when the sensors need to be powered up to operate. However, only minute amounts of energy can be extracted from radio waves, presenting a monumental constraint in terms of the duty cycle and accomplishable tasks. We present our design of a data delivery scheme which aims to optimize the usage of the minute amounts of harvested energy through a sink synchronized protocol. The design has been implemented and experimentally validated utilizing commercially available RF energy harvesting devices.
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paper_title: Energy extraction from RF/ Microwave signal

paper_content:

The objective of this work is to use the concept of energy harvesting i.e. to receive Radio Frequency (RF) and Microwave signals present in the atmosphere and converting these signals into DC, in order to store it or power some circuit like charging a mobile. To implement this concept, an antenna is required to capture RF and microwave frequencies of GSM and WiFi signals in the surrounding. Designing an antenna is the focal point in realization of wireless cellular charging device. RF signals cannot be used to drive a load such as battery of a mobile phone, therefore rectification and a boost up is required done in the third phase which is Voltage Boosting and Rectification. A complete implemented system is being proposed in this paper with successful demonstration of using GSM-900 and GSM-1800 signals for charging a cellular phone. The research paper is a contribution towards the concept of wireless energy transmission at the same time it proves the idea of collecting and converting GSM and WiFi signals, to drive our low power consuming devices, from Radio towers and other wireless transceivers that is of no use otherwise.
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paper_title: A 90-nm CMOS 5-Mbps Crystal-Less RF-Powered Transceiver for Wireless Sensor Network Nodes

paper_content:

This paper presents a 90-nm CMOS batteryless transceiver for RF-powered wireless sensor networks. The circuit is made up of a RF energy harvesting module that is implemented through a multi-stage rectifier, a power management unit, and a PLL-based RF front-end that enables TX carrier synthesis by exploiting the RF input signal as a reference frequency. This avoids the use of a local crystal oscillator, thus implementing a highly integrated low-cost wireless transceiver. An active narrowband transmission scheme is adopted with the aim of overcoming the reader self-jamming that limits the operating range of backscattering-based RF-powered devices. The circuit supports a 915-MHz FSK downlink and a 2.45-GHz OOK uplink, while achieving a data rate up to 5 Mbps. It operates with an RF input power as low as -17.1 dBm.
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paper_title: Energy harvesting from ambient RF sources

paper_content:

This paper presents a feasibility study of energy harvesting from ambient radio frequency (RF) sources. A preliminary assessment of the ambient RF energy was carried out by calculation of power spectrum density and measurement of received power in a common suburban area. The results showed that the median value of received power obtained −25dBm from 800 MHz band mobile telephone base station (BS) was averagely 13dB stronger than that from Digital TV broadcasting. A prototype multi-stage RF-DC conversion circuit was designed and tested. Experimental results showed that a 1.0 F electric double layer capacitor can be charged up to 320 mV in 3900 minutes at common suburban area. We confirm that the possibility of energy harvesting from ambient RF sources in suburban area, and clarify the problems in devices for the future energy harvesting systems.
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paper_title: A UHF micro-power CMOS rectifier using a novel diode connected CMOS transistor for micro-sensor and RFID applications

paper_content:

The design strategy and efficiency optimization of UHF micro-power rectifiers using a novel diode connected MOS transistor is presented. The proposed diode connected MOS transistor uses a new bulk connection which leads to reduce the threshold voltage and leakage current in compare to conventional diode connected transistors. Using the proposed diode in typical rectifiers makes a significant improvement in output voltage and current therefore the efficiency is increased comparing to the same rectifier architectures using conventional diodes. Also a design procedure for efficiency optimization is presented and a superposition method is used to optimize the performance of multiple output rectifiers. Finally a five stage double output rectifier is designed based on the proposed optimization method. Simulation results verified the improvement in efficiency and area. All circuits are designed in a standard O.18um CMOS Technology.
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paper_title: Design Optimization and Implementation for RF Energy Harvesting Circuits

paper_content:

A new design for an energy harvesting device is proposed in this paper, which enables scavenging energy from radiofrequency (RF) electromagnetic waves. Compared to common alternative energy sources like solar and wind, RF harvesting has the least energy density. The existing state-of-the-art solutions are effective only over narrow frequency ranges, are limited in efficiency response, and require higher levels of input power. This paper has a twofold contribution. First, we propose a dual-stage energy harvesting circuit composed of a seven-stage and ten-stage design, the former being more receptive in the low input power regions, while the latter is more suitable for higher power range. Each stage here is a modified voltage multiplier, arranged in series and our design provides guidelines on component choice and precise selection of the crossover operational point for these two stages between the high (20 dBm) and low power (-20 dBm) extremities. Second, we fabricate our design on a printed circuit board to demonstrate how such a circuit can run a commercial Mica2 sensor mote, with accompanying simulations on both ideal and non-ideal conditions for identifying the upper bound on achievable efficiency. With a simple yet optimal dual-stage design, experiments and characterization plots reveal approximately 100% improvement over other existing designs in the power range of -20 to 7 dBm.
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paper_title: High-Efficiency Harmonically Terminated Diode and Transistor Rectifiers

paper_content:

This paper presents a theoretical analysis of harmonically terminated high-efficiency power rectifiers and experimental validation on a class-C single Schottky-diode rectifier and a class- F-1 GaN transistor rectifier. The theory is based on a Fourier analysis of current and voltage waveforms, which arise across the rectifying element when different harmonic terminations are presented at its terminals. An analogy to harmonically terminated power amplifier (PA) theory is discussed. From the analysis, one can obtain an optimal value for the dc load given the RF circuit design. An upper limit on rectifier efficiency is derived for each case as a function of the device on-resistance. Measured results from fundamental frequency source-pull measurement of a Schottky diode rectifier with short-circuit terminations at the second and third harmonics are presented. A maximal device rectification efficiency of 72.8% at 2.45 GHz matches the theoretical prediction. A 2.14-GHz GaN HEMT rectifier is designed based on a class-F-1 PA. The gate of the transistor is terminated in an optimal impedance for self-synchronous rectification. Measurements of conversion efficiency and output dc voltage for varying gate RF impedance, dc load, and gate bias are shown with varying input RF power at the drain. The rectifier demonstrates an efficiency of 85% for a 10-W input RF power at the transistor drain with a dc voltage of 30 V across a 98-Ω resistor.
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paper_title: Dual-band monopole antenna for energy harvesting system

paper_content:

A planar dual-band monopole antenna is presented for Global System for Mobile Communications (GSM) band applications, which also have the potential to be used for energy harvesting system. The proposed antenna comprises of a ground plane at the back of the FR4 substrate and three microstrip lines which are physically connected with each other at the top surface of the substrate. The monopole antenna achieves good return loss at resonance frequencies of 915 MHz and 1800 MHz with a bandwidth value of 124.2 MHz and 196.9 MHz respectively. The antenna gains of 1.97 dB and 3.05 dB are achieved at resonance frequencies of 900 MHz and 1800 MHz. Experimental results show good agreement with simulated performance. The output from the receiving antenna is also observed in order to analyze the relationship of the power level and the distance between transmitting and receiving antenna. This study is an early investigation in designing the RF energy harvesting system to support green technology and sustainable development particularly for Wireless Sensor Network (WSN) applications.
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paper_title: RF energy harvesting system from cell towers in 900MHz band

paper_content:

An experimental RF energy harvesting system to harvest energy from cell towers is presented in this paper. An electromagnetically-coupled square microstrip antenna is designed and fabricated for deployment in the presented system. Antenna gain of 9.1dB and bandwidth from 877 MHz to 998 MHz is achieved. A Schottky diode-based single stage voltage doubler and six-stage voltage doubler has also been designed and fabricated for DC voltage generation. Measured results show that a voltage of 2.78V is obtained at a distance of 10m from the cell tower and a voltage of 0.87V is obtained at a distance of 50m.
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paper_title: Triple bands antenna and high efficiency rectifier design for RF energy harvesting at 900, 1900 and 2400 MHz

paper_content:

We present the design and development of a novel antenna that effectively radiates at three frequency bands to harvest RF energy from cellular network frequency bands (900 MHz and 1900 MHz) and Wi-Fi sources (2.4 GHz) available in ambience. The antenna is designed using the combination of three different design techniques including composite right/left hand transmission line (CRLH). In addition, a high efficiency, sensitive triple-band rectifier is also proposed in this work. The rectifier provides a maximum conversion efficiency of 80%, 46% and 42% at 940MHz, 1.95GHz and 2.44 GHz, respectively. The experimental results demonstrate that the proposed triple-band RF energy harvesting system can collect 6.6 times more power than the single 900MHz band one and 3.4 times more power than three individual bands combined.
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paper_title: Broadband differential antenna for full-wave RF energy scavenging system

paper_content:

This research work demonstrated in this paper is part of a larger project that aims to design high efficiency miniaturized RF energy scavenging system. In this paper, a broadband differential antenna is designed for a full-wave RF energy scavenging system. Simulation results demonstrate that the proposed antenna shows a wide bandwidth of 3.48 GHz based on S parameters <; -10 dB, covering most of the wireless communication bands which include GPS (L1: 1575.42 MHz, L2: 1227.60 MHz, L3: 1381.05 MHz, L4: 1379.913 MHz, L5: 1176.45 MHz), DCS (1710-1880 MHz), PCS (1850-1990 MHz), UMTS (1920-2170 MHz), WiFi/WLAN/Hiper LAN/IEEE 802.11 2.4 GHz (2412-2484 MHz), 3.6 GHz (3657.5-3690.0 MHz), Bluetooth (2400-2484 MHz), WiMAX 2.3 GHz (2.3-2.5 GHz), 2.5 GHz (2500-2690 MHz). 3.3 GHz and 3.5 GHz (3400-3600 MHz) & LTE applications.
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paper_title: Current Developments of RF Energy Harvesting System for Wireless Sensor Networks

paper_content:

Energy harvesting or energy scavenging is basically a conversion process of the ambient energy into the electrical energy. The ambient energy exists around us in many different forms including thermal, chemical, electrical and radio frequency (RF). This technique significantly reduces the costs of replacing batteries periodically. Hence, energy harvesting offers various environmental friendly alternative energy sources, which include the vibration, electromagnetic wave, wind energy and solar power. This study will focus on RF energy harvesting that involves the generating of a small amount of the electrical power to drive circuits in wireless communication electronics devices. Recently, wireless ::: sensor network (WSN) has been a crucial part of our daily life. The importance of WSN can be described by the use of sensors in many devices for home security including light sensors, room thermostat and alarm systems. This paper presents an overview and the progress achieved in RF ::: energy harvesting, which involves the integration of antenna with rectifying circuit. Different combinations of antenna and rectifier topologies yield diverse results. Therefore, this study is expected to give an indication on the appropriate techniques to develop an efficient RF energy harvesting system.
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paper_title: Multi-band simultaneous radio frequency energy harvesting

paper_content:

This paper presents a multi-band simultaneous RF energy harvesting system. The input impedance and the output voltage of a voltage multiplier are investigated. The maximum number of cascaded rectifying stages are identified for three different operating frequencies, 900 MHz, 1800 MHz and 2.45 GHz. The harvesting system combines the DC output voltage of three single-frequency RF harvesters. The combined system allows an efficiency increase by 15 % compared to the efficiency of each sub-system. At an input power level of -15 dBm, the system efficiency reaches 45 % at 900 MHz, 46 % at 1800 MHz and 25 % at 2.45 GHz. Compared to the state-of-the-art, the suggested system has the ability to harvest RF power at different operating frequencies.
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paper_title: Efficient RF energy harvesting by using multiband microstrip antenna arrays with multistage rectifiers

paper_content:

This paper covers the design, implementation and usage of a matched multiband microstrip antenna intended to collect RF energy efficiently in commonly used portions of the selected cellular frequency spectrum including the ISM bands. Multistage Schottky rectifier antenna (rectenna) array circuit is proposed for high energy conversion efficiency to improve battery life and portability. Design details for the multiband microstrip antenna prototype is presented.
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paper_title: Broadband printed antenna for radiofrequency energy harvesting

paper_content:

In this work a broadband UHF antenna with high inductive input impedance for radiofrequency energy harvesting is presented. It consists of a small feeding loop and a biconical radiating dipole. A prototype has been fabricated on a FR4 substrate and tested. Experimental results show a −3dB power transmission bandwidth of about 135MHz (840MHz–975MHz).
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paper_title: Radiofrequency Energy Harvesting

paper_content:

Chapter 6 is dedicated to remote RF powering of autonomous sensors and in particular to RF energy harvesters. The RF energy transducer (antenna), matching networks, and ensuing rectifiers are presented. Based on circuits proposed in the literature, extensive simulations for several incoming power levels at the antenna (from −10 dBm to 10 dBm) were performed. We determine that circuit efficiency slightly depends on the number of stages used for the voltage rectifier multiplier, but varies widely with the received power (ranging, when using a shunt-inductor matching network, from 10% at −10 dBm to 80% at 10 dBm). Additionally, as the power level increases, so does the output voltage corresponding to maximum efficiency. For low power levels, LC matching networks provided higher efficiencies than shunt-inductor networks, at the cost of greater sensitivity to output voltage variations and to the value of the inductor. Experimental tests were performed with a folded dipole antenna (about 300 Ω), shunt inductor matching, a three-stage rectifier, and a storage unit composed of two series connected NiMH batteries.
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paper_title: An Efficient RF Energy Harvester with Tuned Matching Circuit

paper_content:

Microstrip line with π matching circuits are very attractive because of high output power and good impedance matching which makes it an alternative over earlier matching circuit. This paper presents an RF energy harvester with microstrip line in series with tuned π-matching circuit that enables efficient power conversion at different RF input power under different load conditions. Matching circuit parameters were optimized for better efficiency. We have focused for specific input power range -15 to 10dBm for 3-stage, 5-stage and 7-stage of energy harvesting circuit. Optimum efficiency of approximately 80% is achieved at input power 0 to 10dBm for higher stages. Effect of load variation also shows that better efficiency is achieved for input power -10 to 10dBm for 3-stage, 5-stage and 7-stage of the harvesting circuit.
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paper_title: Modeling of RF energy scavenging for batteryless wireless sensors with low input power

paper_content:

RF energy scavenging enables batteryless operation of wireless sensors. In particular, a system with a central controller that transfers wireless energy to and exchanges information with RF energy scavenging sensors is very suitable for a wide range of applications. State-of-the-art analysis of RF energy scavenging is mostly based on RF-DC rectifier models operating with relatively high input power to achieve high rectification efficiency. However, to enable larger distance between the central controller and sensors and/or to increase the operating frequencies, which can lead to small and low-cost smart dust like sensors, a good model describing the RF-DC rectification with low input power is needed to aid system design and optimization. In this paper, we develop such a model. Using the model, we derive closed-form solutions for the equilibrium voltage and the input resistance of the rectifier. We further propose a quasi-static model to describe the dynamic charging of the capacitor in the rectifier. A comparison with circuit simulations using Cadence Virtuoso Spectre circuit simulator shows good match between our model and the circuit simulation.
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paper_title: Recycling ambient microwave energy with broad-band rectenna arrays

paper_content:

This paper presents a study of reception and rectification of broad-band statistically time-varying low-power-density microwave radiation. The applications are in wireless powering of industrial sensors and recycling of ambient RF energy. A 64-element dual-circularly-polarized spiral rectenna array is designed and characterized over a frequency range of 2-18 GHz with single-tone and multitone incident waves. The integrated design of the antenna and rectifier, using a combination of full-wave electromagnetic field analysis and harmonic balance nonlinear circuit analysis, eliminates matching and filtering circuits, allowing for a compact element design. The rectified dc power and efficiency is characterized as a function of dc load and dc circuit topology, RF frequency, polarization, and incidence angle for power densities between 10/sup -5/-10/sup -1/ mW/cm/sup 2/. In addition, the increase in rectenna efficiency for multitone input waves is presented.
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paper_title: Radiofrequency Energy Harvesting

paper_content:

Chapter 6 is dedicated to remote RF powering of autonomous sensors and in particular to RF energy harvesters. The RF energy transducer (antenna), matching networks, and ensuing rectifiers are presented. Based on circuits proposed in the literature, extensive simulations for several incoming power levels at the antenna (from −10 dBm to 10 dBm) were performed. We determine that circuit efficiency slightly depends on the number of stages used for the voltage rectifier multiplier, but varies widely with the received power (ranging, when using a shunt-inductor matching network, from 10% at −10 dBm to 80% at 10 dBm). Additionally, as the power level increases, so does the output voltage corresponding to maximum efficiency. For low power levels, LC matching networks provided higher efficiencies than shunt-inductor networks, at the cost of greater sensitivity to output voltage variations and to the value of the inductor. Experimental tests were performed with a folded dipole antenna (about 300 Ω), shunt inductor matching, a three-stage rectifier, and a storage unit composed of two series connected NiMH batteries.
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paper_title: Design and Optimization of Passive UHF RFID Systems

paper_content:

Radio Frequency IDentification (RFID) stores and retrieves data using devices called RFID tags: objects attached to or incorporated into a product, animal or person which communicate with an RFID reader or interrogator. This book proposes a linear two-port model for an N-stage modified-Greinacher full wave rectifier, predicting the overall conversion efficiency at low power levels where the diodes are operating near their threshold voltage. Included is an experimental procedure to measure how impedance modulation in the tag affects the signal at the reader, and a useful tool for choosing the most appropriate impedances.
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paper_title: Analytical models for low-power rectenna design

paper_content:

The design of a low-cost rectenna for low-power applications is presented. The rectenna is designed with the use of analytical models and closed-form analytical expressions. This allows for a fast design of the rectenna system. To acquire a small-area rectenna, a layered design is proposed. Measurements indicate the validity range of the analytical models.
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paper_title: Fully integrated wideband high-current rectifiers for inductively powered devices

paper_content:

This paper describes the design and implementation of fully integrated rectifiers in BiCMOS and standard CMOS technologies for rectifying an externally generated RF carrier signal in inductively powered wireless devices, such as biomedical implants, radio-frequency identification (RFID) tags, and smartcards to generate an on-chip dc supply. Various full-wave rectifier topologies and low-power circuit design techniques are employed to decrease substrate leakage current and parasitic components, reduce the possibility of latch-up, and improve power transmission efficiency and high-frequency performance of the rectifier block. These circuits are used in wireless neural stimulating microsystems, fabricated in two processes: the University of Michigan's 3-/spl mu/m 1M/2P N-epi BiCMOS, and the AMI 1.5-/spl mu/m 2M/2P N-well standard CMOS. The rectifier areas are 0.12-0.48 mm/sup 2/ in the above processes and they are capable of delivering >25mW from a receiver coil to the implant circuitry. The performance of these integrated rectifiers has been tested and compared, using carrier signals in 0.1-10-MHz range.
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paper_title: On the Transfer of Information and Energy in Multi-User Systems

paper_content:

The problem of joint transfer of information and energy for wireless links has been recently investigated in light of emerging applications such as RFID and body area networks. Specifically, recent work has shown that the additional requirements of providing sufficient energy to the receiver significantly affects the design of the optimal communication strategy. In contrast to most previous works, this letter focuses on baseline multi-user systems, namely multiple access and multi-hop channels, and demonstrates that energy transfer constraints call for additional coordination among distributed nodes of a wireless network. The analysis is carried out using information-theoretic tools, and specific examples are worked out to illustrate the main conclusions.
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paper_title: Transporting information and energy simultaneously

paper_content:

The fundamental tradeoff between the rates at which energy and reliable information can be transmitted over a single noisy line is studied. Engineering inspiration for this problem is provided by powerline communication, RFID systems, and covert packet timing systems as well as communication systems that scavenge received energy. A capacity-energy function is defined and a coding theorem is given. The capacity-energy function is a non-increasing concave cap function. Capacity-energy functions for several channels are computed.
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paper_title: Wireless information and power transfer: Architecture design and rate-energy tradeoff

paper_content:

Simultaneous information and power transfer over the wireless channels potentially offers great convenience to mobile users. Yet practical receiver designs impose technical constraints on its hardware realization, as practical circuits for harvesting energy from radio signals are not yet able to decode the carried information directly. To make theoretical progress, we propose a general receiver operation, namely, dynamic power splitting (DPS), which splits the received signal with adjustable power ratio for energy harvesting and information decoding, separately. Three special cases of DPS, namely, time switching (TS), static power splitting (SPS) and on-off power splitting (OPS) are investigated. The TS and SPS schemes can be treated as special cases of OPS. Moreover, we propose two types of practical receiver architectures, namely, separated versus integrated information and energy receivers. The integrated receiver integrates the front-end components of the separated receiver, thus achieving a smaller form factor. The rate-energy tradeoff for the two architectures are characterized by a so-called rate-energy (R-E) region. The optimal transmission strategy is derived to achieve different rate-energy tradeoffs. With receiver circuit power consumption taken into account, it is shown that the OPS scheme is optimal for both receivers. For the ideal case when the receiver circuit does not consume power, the SPS scheme is optimal for both receivers. In addition, we study the performance for the two types of receivers under a realistic system setup that employs practical modulation. Our results provide useful insights to the optimal practical receiver design for simultaneous wireless information and power transfer (SWIPT).
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paper_title: Shannon meets Tesla: Wireless information and power transfer

paper_content:

The problem considered here is that of wireless information and power transfer across a noisy coupled-inductor circuit, which is a frequency-selective channel with additive white Gaussian noise. The optimal tradeoff between the achievable rate and the power transferred is characterized given the total power available. The practical utility of such systems is also discussed.
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paper_title: Simultaneous Information and Power Transfer for Broadband Wireless Systems

paper_content:

Far-field microwave power transfer (MPT) will free wireless sensors and other mobile devices from the constraints imposed by finite battery capacities. Integrating MPT with wireless communications to support simultaneous wireless information and power transfer (SWIPT) allows the same spectrum to be used for dual purposes without compromising the quality of service. A novel approach is presented in this paper for realizing SWIPT in a broadband system where orthogonal frequency division multiplexing and transmit beamforming are deployed to create a set of parallel sub-channels for SWIPT, which simplifies resource allocation. Based on a proposed reconfigurable mobile architecture, different system configurations are considered by combining single-user/multi-user systems, downlink/uplink information transfer, and variable/fixed coding rates. Optimizing the power control for these configurations results in a new class of multi-user power-control problems featuring the circuit-power constraints, specifying that the transferred power must be sufficiently large to support the operation of the receiver circuitry. Solving these problems gives a set of power-control algorithms that exploit channel diversity in frequency for simultaneously enhancing the throughput and the MPT efficiency. For the system configurations with variable coding rates, the algorithms are variants of water-filling that account for the circuit-power constraints. The optimal algorithms for those configurations with fixed coding rates are shown to sequentially allocate mobiles their required power for decoding in ascending order until the entire budgeted power is spent. The required power for a mobile is derived as simple functions of the minimum signal-to-noise ratio for correct decoding, the circuit power and sub-channel gains.
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paper_title: MIMO Broadcasting for Simultaneous Wireless Information and Power Transfer

paper_content:

Wireless power transfer (WPT) is a promising new solution to provide convenient and perpetual energy supplies to wireless networks. In practice, WPT is implementable by various technologies such as inductive coupling, magnetic resonate coupling, and electromagnetic (EM) radiation, for short-/mid-/long-range applications, respectively. In this paper, we consider the EM or radio signal enabled WPT in particular. Since radio signals can carry energy as well as information at the same time, a unified study on simultaneous wireless information and power transfer (SWIPT) is pursued. Specifically, this paper studies a multiple-input multiple-output (MIMO) wireless broadcast system consisting of three nodes, where one receiver harvests energy and another receiver decodes information separately from the signals sent by a common transmitter, and all the transmitter and receivers may be equipped with multiple antennas. Two scenarios are examined, in which the information receiver and energy receiver are separated and see different MIMO channels from the transmitter, or co-located and see the identical MIMO channel from the transmitter. For the case of separated receivers, we derive the optimal transmission strategy to achieve different tradeoffs for maximal information rate versus energy transfer, which are characterized by the boundary of a so-called rate-energy (R-E) region. For the case of co-located receivers, we show an outer bound for the achievable R-E region due to the potential limitation that practical energy harvesting receivers are not yet able to decode information directly. Under this constraint, we investigate two practical designs for the co-located receiver case, namely time switching and power splitting, and characterize their achievable R-E regions in comparison to the outer bound.
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paper_title: Interactive Joint Transfer of Energy and Information

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In some communication networks, such as passive RFID systems, the energy used to transfer information between a sender and a recipient can be reused for successive communication tasks. In fact, from known results in physics, any system that exchanges information via the transfer of given physical resources, such as radio waves, particles and qubits, can conceivably reuse, at least part, of the received resources. This paper aims at illustrating some of the new challenges that arise in the design of communication networks in which the signals exchanged by the nodes carry both information and energy. To this end, a baseline two-way communication system is considered in which two nodes communicate in an interactive fashion. In the system, a node can either send an "onquotedblright symbol (or "1quotedblright), which costs one unit of energy, or an "offquotedblright signal (or "0quotedblright), which does not require any energy expenditure. Upon reception of a "1quotedblright signal, the recipient node "harvestsquotedblright, with some probability, the energy contained in the signal and stores it for future communication tasks. Inner and outer bounds on the achievable rates are derived. Numerical results demonstrate the effectiveness of the proposed strategies and illustrate some key design insights.
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paper_title: Multi-user scheduling schemes for simultaneous wireless information and power transfer

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In this paper, the downlink multi-user scheduling problem is studied for a time-slotted system with simultaneous wireless information and power transfer. In particular, in each time slot, a single user is scheduled to receive information, while the remaining users opportunistically harvest the ambient radio frequency (RF) energy. We devise novel scheduling schemes in which the tradeoff between the users' ergodic capacities and their average amount of harvested energy can be controlled. To this end, two fair scheduling schemes used in information-only transfer systems are modified. First, proportionally fair maximum normalized signal-to-noise ratio (N-SNR) scheduling is modified by scheduling the user having the j ::: th ::: ascendingly ordered (rather than the maximum) N-SNR. We refer to this scheme as order-based N-SNR scheduling. Second, conventional equal-throughput (ET) fair scheduling is modified by scheduling the user having the minimum moving average throughput among the set of users whose N-SNR orders fall into a certain set of allowed orders S ::: a ::: (rather than the set of all users). We refer to this scheme as order-based ET scheduling. The feasibility conditions required for the users to achieve ET with this scheme are also derived. It is shown that the smaller the selection order j for the order-based N-SNR scheme, and the lower the orders in S ::: a ::: for the order-based ET scheme, the higher the average amount of energy harvested by the users at the expense of a reduction in their ergodic capacities. The performance of the considered scheduling schemes is analyzed for independent and non-identically distributed (i.n.d.) Ricean fading channels, and closed-form results for the special case of i.n.d. Rayleigh fading are provided.
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paper_title: Performance Analysis of Multiuser Selection Diversity

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In this paper, the authors study the performance of scheduling algorithms exploiting the multiuser selection diversity. Schedulers with affordable-rate transmission and adaptive transmission based on the absolute signal-to-noise ratio (SNR) and the normalized SNR are considered. In contrast to previous studies on the multiuser-diversity systems, the channel dynamics is taken into consideration in this paper by a novel formulation based on the level crossing analysis of stochastic processes. Then, the authors make the connection between the Doppler frequency shift, which indicates the channel temporal correlation, and the average (channel) access time, the average waiting time (AWT) between accesses, and the average access rate (AAR) of active users. These properties are important for the scheduler design, especially for applications where delay is a concern. In addition, analytical expressions for the system throughput and the degree of fairness (DOF) when users have nonidentical average channel conditions are presented. These expressions quantify the effect of disparateness in users' average channel conditions on the system performance
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paper_title: Competitive wireless energy transfer bidding: A game theoretic approach

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Wireless or RF energy transfer will be capable of supplying energy to mobile nodes for packet transmission without physical battery changing or replacement. In this paper, we consider the network in which the access point can transfer wireless energy to the nodes and the nodes use that energy to transmit packets. The nodes can send requests (i.e., bids) to the access point for transferring wireless energy. As the access point adopts auction mechanism for wireless energy transfer, we formulate a noncooperative game to analyze the competitive wireless energy transfer bidding of the nodes. The Nash equilibrium is considered to be a solution of the game. To reach the Nash equilibrium, the nodes as players can adapt their strategies based on stochastic response dynamic. The Markov chain is used to analyze the properties of the game (e.g., convergence). The numerical results clearly show the convergence of the bidding strategy adaptation of the nodes toward the Nash equilibrium.
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paper_title: Throughput Optimization for Massive MIMO Systems Powered by Wireless Energy Transfer

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This paper studies a wireless-energy-transfer (WET) enabled massive multiple-input-multiple-output (MIMO) system (MM) consisting of a hybrid data-and-energy access point (H-AP) and multiple single-antenna users. In the WET-MM system, the H-AP is equipped with a large number $M$ of antennas and functions like a conventional AP in receiving data from users, but additionally supplies wireless power to the users. We consider frame-based transmissions. Each frame is divided into three phases: the uplink channel estimation (CE) phase, the downlink WET phase, as well as the uplink wireless information transmission (WIT) phase. Firstly, users use a fraction of the previously harvested energy to send pilots, while the H-AP estimates the uplink channels and obtains the downlink channels by exploiting channel reciprocity. Next, the H-AP utilizes the channel estimates just obtained to transfer wireless energy to all users in the downlink via energy beamforming. Finally, the users use a portion of the harvested energy to send data to the H-AP simultaneously in the uplink (reserving some harvested energy for sending pilots in the next frame) . To optimize the throughput and ensure rate fairness, we consider the problem of maximizing the minimum rate among all users. In the large- $M$ regime, we obtain the asymptotically optimal solutions and some interesting insights for the optimal design of WET-MM system.
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paper_title: Optimal decentralized control policy for wireless communication systems with wireless energy transfer capability

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In this paper, we consider a decentralized wireless communication system with wireless energy transfer capability. We aim to minimize the total number of packets waiting at wireless nodes for the whole system. We first formulated the optimization problem as a decentralized partially observable Markov decision process (DEC-POMDP). To solve an optimization problem with constraints, we applied the Lagrangian multiplier and the policy gradient method. In addition, to reduce the complexity of DEC-POMDP, we proposed a decentralized online learning algorithm with minimum communication among the wireless nodes. Under appropriate conditions, we showed that the proposed algorithm converges to a local optimal solution. The simulation results clearly showed the convergence as well as the efficiency of the proposed algorithm.
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paper_title: Optimal Resource Allocation in Full-Duplex Wireless-Powered Communication Network

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This paper studies optimal resource allocation in the wireless-powered communication network (WPCN), where one hybrid access point (H-AP) operating in full duplex (FD) broadcasts wireless energy to a set of distributed users in the downlink (DL) and, at the same time, receives independent information from the users via time-division multiple access in the uplink (UL). We design an efficient protocol to support simultaneous wireless energy transfer (WET) in the DL and wireless information transmission (WIT) in the UL for the proposed FD-WPCN. We jointly optimize the time allocations to the H-AP for DL WET and different users for UL WIT and the transmit power allocations over time at the H-AP to maximize the users' weighted sum rate of UL information transmission with harvested energy. We consider both the cases with perfect and imperfect self-interference cancellation (SIC) at the H-AP, for which we obtain optimal and suboptimal time and power allocation solutions, respectively. Furthermore, we consider the half-duplex (HD) WPCN as a baseline scheme and derive its optimal resource allocation solution. Simulation results show that the FD-WPCN outperforms the HD-WPCN when effective SIC can be implemented and more stringent peak power constraint is applied at the H-AP.
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paper_title: Zero-forcing methods for downlink spatial multiplexing in multiuser MIMO channels

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The use of space-division multiple access (SDMA) in the downlink of a multiuser multiple-input, multiple-output (MIMO) wireless communications network can provide a substantial gain in system throughput. The challenge in such multiuser systems is designing transmit vectors while considering the co-channel interference of other users. Typical optimization problems of interest include the capacity problem - maximizing the sum information rate subject to a power constraint-or the power control problem-minimizing transmitted power such that a certain quality-of-service metric for each user is met. Neither of these problems possess closed-form solutions for the general multiuser MIMO channel, but the imposition of certain constraints can lead to closed-form solutions. This paper presents two such constrained solutions. The first, referred to as "block-diagonalization," is a generalization of channel inversion when there are multiple antennas at each receiver. It is easily adapted to optimize for either maximum transmission rate or minimum power and approaches the optimal solution at high SNR. The second, known as "successive optimization," is an alternative method for solving the power minimization problem one user at a time, and it yields superior results in some (e.g., low SNR) situations. Both of these algorithms are limited to cases where the transmitter has more antennas than all receive antennas combined. In order to accommodate more general scenarios, we also propose a framework for coordinated transmitter-receiver processing that generalizes the two algorithms to cases involving more receive than transmit antennas. While the proposed algorithms are suboptimal, they lead to simpler transmitter and receiver structures and allow for a reasonable tradeoff between performance and complexity.
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paper_title: Throughput Maximization in Wireless Powered Communication Networks

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This paper studies the newly emerging wireless powered communication network in which one hybrid access point (H-AP) with constant power supply coordinates the wireless energy/information transmissions to/from a set of distributed users that do not have other energy sources. A "harvest-then-transmit" protocol is proposed where all users first harvest the wireless energy broadcast by the H-AP in the downlink (DL) and then send their independent information to the H-AP in the uplink (UL) by time-division-multiple-access (TDMA). First, we study the sum-throughput maximization of all users by jointly optimizing the time allocation for the DL wireless power transfer versus the users' UL information transmissions given a total time constraint based on the users' DL and UL channels as well as their average harvested energy values. By applying convex optimization techniques, we obtain the closed-form expressions for the optimal time allocations to maximize the sum-throughput. Our solution reveals an interesting "doubly near-far" phenomenon due to both the DL and UL distance-dependent signal attenuation, where a far user from the H-AP, which receives less wireless energy than a nearer user in the DL, has to transmit with more power in the UL for reliable information transmission. As a result, the maximum sum-throughput is shown to be achieved by allocating substantially more time to the near users than the far users, thus resulting in unfair rate allocation among different users. To overcome this problem, we furthermore propose a new performance metric so-called common-throughput with the additional constraint that all users should be allocated with an equal rate regardless of their distances to the H-AP. We present an efficient algorithm to solve the common-throughput maximization problem. Simulation results demonstrate the effectiveness of the common-throughput approach for solving the new doubly near-far problem in wireless powered communication networks.
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paper_title: Simultaneous Information and Power Transfer Scheme for Energy Efficient MIMO Systems

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This letter studies the energy efficiency (EE) optimization problem for the simultaneous wireless information and power transfer (SWIPT) MIMO systems with statistical channel state information (CSI) feedback. Considering the further transmission of the energy harvesting node, the potential capacity produced by the transferred energy is counted in system capacity. Both the optimal algorithm and the low complexity sub-optimal algorithm are proposed. Simulation results show the superiority of the proposed schemes on EE.
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paper_title: Cooperative bidding of data transmission and wireless energy transfer

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This paper proposes a model and a mechanism for cooperative bidding of wireless data transmission and energy transfer in a decentralized wireless communication system. We aim to minimize the total number of packets waiting at wireless nodes and the total bid prices from the nodes. We first formulated the optimization problem as a decentralized partially observable Markov decision process (DEC-POMDP) with constraints for wireless nodes and then proposed a decentralized learning algorithm to obtain an optimal policy for the DEC-POMDP. Through simulations, we showed the convergence as well as the significantly better performance of the proposed algorithm compared with other algorithms.
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paper_title: Simultaneous Information and Power Transfer for Broadband Wireless Systems

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Far-field microwave power transfer (MPT) will free wireless sensors and other mobile devices from the constraints imposed by finite battery capacities. Integrating MPT with wireless communications to support simultaneous wireless information and power transfer (SWIPT) allows the same spectrum to be used for dual purposes without compromising the quality of service. A novel approach is presented in this paper for realizing SWIPT in a broadband system where orthogonal frequency division multiplexing and transmit beamforming are deployed to create a set of parallel sub-channels for SWIPT, which simplifies resource allocation. Based on a proposed reconfigurable mobile architecture, different system configurations are considered by combining single-user/multi-user systems, downlink/uplink information transfer, and variable/fixed coding rates. Optimizing the power control for these configurations results in a new class of multi-user power-control problems featuring the circuit-power constraints, specifying that the transferred power must be sufficiently large to support the operation of the receiver circuitry. Solving these problems gives a set of power-control algorithms that exploit channel diversity in frequency for simultaneously enhancing the throughput and the MPT efficiency. For the system configurations with variable coding rates, the algorithms are variants of water-filling that account for the circuit-power constraints. The optimal algorithms for those configurations with fixed coding rates are shown to sequentially allocate mobiles their required power for decoding in ascending order until the entire budgeted power is spent. The required power for a mobile is derived as simple functions of the minimum signal-to-noise ratio for correct decoding, the circuit power and sub-channel gains.
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paper_title: Full-Duplex Wireless-Powered Communication Network With Energy Causality

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In this paper, we consider a wireless communication network with a full-duplex hybrid access point (HAP) and a set of wireless users with energy harvesting capabilities. The HAP implements the full-duplex through two antennas: one for broadcasting wireless energy to users in the downlink and one for receiving independent information from users via time-division-multiple-access (TDMA) in the uplink at the same time. All users can continuously harvest wireless power from the HAP until its transmission slot, i.e., the energy causality constraint is modeled by assuming that energy harvested in the future cannot be used for tranmission. Hence, latter users' energy harvesting time is coupled with the transmission time of previous users. Under this setup, we investigate the sum-throughput maximization (STM) problem and the total-time minimization (TTM) problem for the proposed multi-user full-duplex wireless-powered network. The STM problem is proved to be a convex optimization problem. The optimal solution strategy is then obtained in closed-form expression, which can be computed with linear complexity. It is also shown that the sum throughput is non-decreasing with increasing of the number of users. For the TTM problem, by exploiting the properties of the coupling constraints, we propose a two-step algorithm to obtain an optimal solution. Then, for each problem, two suboptimal solutions are proposed and investigated. Finally, the effect of user scheduling on STM and TTM are investigated through simulations. It is also shown that different user scheduling strategies should be used for STM and TTM.
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paper_title: User cooperation in wireless powered communication networks

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This paper studies user cooperation in the emerging wireless powered communication network (WPCN) for throughput optimization. For the purpose of exposition, we consider a two-user WPCN, in which one hybrid access point (H-AP) broadcasts wireless energy to two distributed users in the downlink (DL) and the users transmit their independent information using their individually harvested energy to the H-AP in the uplink (UL) through time-division-multiple-access (TDMA). We propose user cooperation in the WPCN where the user that is nearer to the H-AP and in general has a better channel for DL energy harvesting as well as UL information transmission uses part of its allocated UL time and DL harvested energy to help relay the far user's information to the H-AP, in order to achieve more balanced throughput. We maximize the weighted sum-rate (WSR) of the two users by jointly optimizing the time and power allocations in the network for both wireless energy transfer in the DL and wireless information transmission and relaying in the UL. Simulation results show that the proposed user cooperation scheme can effectively improve the achievable throughput in the WPCN with desired user fairness.
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paper_title: A Low-Complexity Algorithm for Throughput Maximization in Wireless Powered Communication Networks

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This paper investigates a wireless powered communication network (WPCN) under the protocol of harvest-then-transmit,where a hybrid access point with constant power supply replenishes the passive user nodes by wireless power transfer in the downlink,then each user node transmit independent information to the hybrid AP in a time division multiple access (TDMA) scheme in the uplink.The sum-throughput maximization and min-throughput maximization problems are considered in this paper.The optimal time allocation for the sum-throughput maximization is proposed based on the Jensen's inequality,which provides more insight into the design of WPCNs.A low-complexity fixed-point iteration algorithm for the min-throughput maximization problem,which promises a much better computation complexity than the state-of-the-art algorithm.Simulation results confirm the effectiveness of the proposed algorithm.
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paper_title: Admission control policy for wireless networks with RF energy transfer

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With RF (radio frequency) energy transfer capability, an access point not only communicates with wireless nodes, but also supplies them with energy for data transmission. In this paper, we consider the wireless network with RF energy transfer. To support quality of service (QoS) in the network, we introduce an admission control policy, which decides whether incoming nodes can be admitted into the network or not. Also, the policy determines the RF energy transfer strategy to maximize the reward of the network, while the performance requirement in terms of node throughput is maintained at the target level. We present optimization and queueing models to obtain an optimal admission control policy and performance measures of the node in the network, respectively. The performance evaluation shows that the admission control policy can successfully achieve the network objective and meet the node constraint on QoS requirement.
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paper_title: Cooperation in Delay-Tolerant Networks With Wireless Energy Transfer: Performance Analysis and Optimization

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We consider a delay-tolerant network (DTN) whose mobile nodes are assigned to collect packets from data sources and deliver them to a sink (i.e., a gateway). Each mobile node operates by using energy transferred wirelessly from the gateway. For such a network, two main issues are studied. First, when a mobile node is at the data source, this node must decide on whether to accept the packet received from the data source or not. In contrast, whenever a mobile node is at the gateway, it has to decide on whether to transmit the packets collected from the data sources or to request wireless energy transfer. Second, multiple mobile nodes can cooperate and form coalitions to help one another in the delivery of packets from their associated data sources. However, this cooperation may not be always beneficial due to the limited buffer and energy resources. Moreover, some mobile nodes may secretly decide to deviate from a given coalition, thus taking advantage of the other innocent mobile nodes. To address these two issues, this paper introduces a performance analysis and optimization framework, which is based on a joint optimization and game-theoretic framework. The optimization model is used to obtain the packet delivery policy of each individual mobile node. Then, a novel game-theoretic model, namely a repeated coalition formation game , is developed to analyze the cooperation strategies of multiple mobile nodes.
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paper_title: Multi-Antenna Wireless Powered Communication With Energy Beamforming

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The newly emerging wireless powered communication networks (WPCNs) have recently drawn significant attention, where radio signals are used to power wireless terminals for information transmission. In this paper, we study a WPCN where one multi-antenna access point (AP) coordinates energy transfer and information transfer to/from a set of single-antenna users. A harvest-then-transmit protocol is assumed where the AP first broadcasts wireless power to all users via energy beamforming in the downlink (DL), and then, the users send their independent information to the AP simultaneously in the uplink (UL) using their harvested energy. To optimize the users' throughput and yet guarantee their rate fairness, we maximize the minimum throughput among all users by a joint design of the DL-UL time allocation, the DL energy beamforming, and the UL transmit power allocation, as well as receive beamforming. We solve this nonconvex problem optimally by two steps. First, we fix the DL-UL time allocation and obtain the optimal DL energy beamforming, UL power allocation, and receive beamforming to maximize the minimum signal-to-interference-plus-noise ratio of all users. This problem is shown to be still nonconvex; however, we convert it equivalently to a spectral radius minimization problem, which can be solved efficiently by applying the alternating optimization based on the nonnegative matrix theory. Then, the optimal time allocation is found by a one-dimensional search to maximize the minimum rate of all users. Furthermore, two suboptimal designs of lower complexity are also proposed, and their throughput performance is compared against that of the optimal solution.
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paper_title: Power Allocation in Multiuser MIMO Systems for Simultaneous Wireless Information and Power Transfer

paper_content:

Far field wireless recharging based on microwave power transfer (MPT) will release mobile terminals from interruption due to finite battery storage. Wireless energy transfer (WET), which integrating MPT with wireless communications, is a promising technology to provide perpetual energy supplies to wireless networks. In this paper, we propose a novel power allocation algorithm for Multiuser MIMO (MU MIMO) systems with simultaneous wireless information and power transfer (SIPT). For the purpose of universal relevance, a K +L+1 node network is investigated, in which K receivers decode information and the other L receivers harvest energy from the signals broadcasted by the base station. The proposed integrated scheme, based on the well-known Block Diagonalization (BD) preceding method, can completely suppress the multiuser interference and perform an outer-bound which is characterized by the boundary of the rate-energy (R-E) region as well.
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paper_title: Energy-Efficient Resource Allocation in Multiuser OFDM Systems with Wireless Information and Power Transfer

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In this paper, we study the resource allocation algorithm design for multiuser orthogonal frequency division multiplexing (OFDM) downlink systems with simultaneous wireless information and power transfer. The algorithm design is formulated as a non-convex optimization problem for maximizing the energy efficiency of data transmission (bit/Joule delivered to the users). In particular, the problem formulation takes into account the minimum required system data rate, heterogeneous minimum required power transfers to the users, and the circuit power consumption. Subsequently, by exploiting the method of timesharing and the properties of nonlinear fractional programming, the considered non-convex optimization problem is solved using an efficient iterative resource allocation algorithm. For each iteration, the optimal power allocation and user selection solution are derived based on Lagrange dual decomposition. Simulation results illustrate that the proposed iterative resource allocation algorithm achieves the maximum energy efficiency of the system and reveal how energy efficiency, system capacity, and wireless power transfer benefit from the presence of multiple users in the system.
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paper_title: Joint Transmit Beamforming and Receive Power Splitting for MISO SWIPT Systems

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This paper studies a multi-user multiple-input single-output (MISO) downlink system for simultaneous wireless information and power transfer (SWIPT), in which a set of single-antenna mobile stations (MSs) receive information and energy simultaneously via power splitting (PS) from the signal sent by a multi-antenna base station (BS). We aim to minimize the total transmission power at BS by jointly designing transmit beamforming vectors and receive PS ratios for all MSs under their given signal-to-interference-plus-noise ratio (SINR) constraints for information decoding and harvested power constraints for energy harvesting. First, we derive the sufficient and necessary condition for the feasibility of our formulated problem. Next, we solve this non-convex problem by applying the technique of semidefinite relaxation (SDR). We prove that SDR is indeed tight for our problem and thus achieves its global optimum. Finally, we propose two suboptimal solutions of lower complexity than the optimal solution based on the principle of separating the optimization of transmit beamforming and receive PS, where the zero-forcing (ZF) and the SINR-optimal based transmit beamforming schemes are applied, respectively.
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paper_title: Power efficient and secure multiuser communication systems with wireless information and power transfer

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In this paper, we study resource allocation algorithm design for power efficient secure communication with simultaneous wireless information and power transfer (WIPT) in multiuser communication systems. In particular, we focus on power splitting receivers which are able to harvest energy and decode information from the received signals. The considered problem is modeled as an optimization problem which takes into account a minimum required signal-to-interference-plus-noise ratio (SINR) at multiple desired receivers, a maximum tolerable data rate at multiple multi-antenna potential eavesdroppers, and a minimum required power delivered to the receivers. The proposed problem formulation facilitates the dual use of artificial noise in providing efficient energy transfer and guaranteeing secure communication. We aim at minimizing the total transmit power by jointly optimizing transmit beamforming vectors, power splitting ratios at the desired receivers, and the covariance of the artificial noise. The resulting non-convex optimization problem is transformed into a semidefinite programming (SDP) and solved by SDP relaxation. We show that the adopted SDP relaxation is tight and achieves the global optimum of the original problem. Simulation results illustrate the significant power saving obtained by the proposed optimal algorithm compared to suboptimal baseline schemes.
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paper_title: Energy-Efficient Power Allocation in OFDM Systems with Wireless Information and Power Transfer

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This paper considers an orthogonal frequency division multiplexing (OFDM) downlink point-to-point system with simultaneous wireless information and power transfer. It is assumed that the receiver is able to harvest energy from noise, interference, and the desired signals. We study the design of power allocation algorithms maximizing the energy efficiency of data transmission (bit/Joule delivered to the receiver). In particular, the algorithm design is formulated as a high-dimensional non-convex optimization problem which takes into account the circuit power consumption, the minimum required data rate, and a constraint on the minimum power delivered to the receiver. Subsequently, by exploiting the properties of nonlinear fractional programming, the considered non-convex optimization problem, whose objective function is in fractional form, is transformed into an equivalent optimization problem having an objective function in subtractive form, which enables the derivation of an efficient iterative power allocation algorithm. In each iteration, the optimal power allocation solution is derived based on dual decomposition and a one-dimensional search. Simulation results illustrate that the proposed iterative power allocation algorithm converges to the optimal solution, and unveil the trade-off between energy efficiency, system capacity, and wireless power transfer: (1) In the low transmit power regime, maximizing the system capacity may maximize the energy efficiency. (2) Wireless power transfer can enhance the energy efficiency, especially in the interference limited regime.
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paper_title: Interference Alignment and Wireless Energy Transfer via Antenna Selection

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This paper investigates simultaneous transfer of information and energy over wireless link in the multiuser MIMO systems in which interference is aligned at the reduced signal subspace at each receiver. We propose to utilize interference that falls into the reduced subspace as an energy source, which was formerly discarded in the conventional interference alignment system. This makes the receive antenna selection achieve better rate and energy tradeoff. Therefore, the receive antenna selection is beneficial in order to partition the receive signal space into two orthogonal spaces for information and energy transfer.
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paper_title: Antenna Selection for Simultaneous Wireless Information and Power Transfer in MIMO Systems

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For simultaneous wireless information and power transfer in multiple-input multiple-output broadcast systems, we propose to investigate the antenna selection (AS) design problem. The problem is formulated as joint AS and transmit covariance matrix design optimization problem which maximizes the achievable rate from the transmitter to the information-decoding receiver subject to the energy-harvesting constraint and the transmit power constraint. To solve the problem, we relax the binary constraints on the AS matrices and restrict the transmit covariance matrix to be diagonal. The AS matrices and the transmit covariance matrix are optimized iteratively by our proposed iterative AS algorithm. We also propose a low-complexity non-iterative norm-based algorithm which optimizes the AS matrices and the transmit covariance matrix sequentially. It is shown from simulation results that the achievable rates of proposed algorithms approach that of the AS scheme which is optimized by exhaustive search.
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paper_title: Spectral efficient optimization in OFDM systems with wireless information and power transfer

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This paper considers an orthogonal frequency division multiplexing (OFDM) point-to-point wireless communication system with simultaneous wireless information and power transfer. We study a receiver which is able to harvest energy from the desired signal, noise, and interference. In particular, we consider a power splitting receiver which dynamically splits the received power into two power streams for information decoding and energy harvesting. We design power allocation algorithms maximizing the spectral efficiency (bit/s/Hz) of data transmission. In particular, the algorithm design is formulated as a nonconvex optimization problem which takes into account the constraint on the minimum power delivered to the receiver. The problem is solved by using convex optimization techniques and a one-dimensional search. The optimal power allocation algorithm serves as a system benchmark scheme due to its high complexity. To strike a balance between system performance and computational complexity, we also propose two suboptimal algorithms which require a low computational complexity. Simulation results demonstrate the excellent performance of the proposed suboptimal algorithms.
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paper_title: Wireless information and power transfer: Architecture design and rate-energy tradeoff

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Simultaneous information and power transfer over the wireless channels potentially offers great convenience to mobile users. Yet practical receiver designs impose technical constraints on its hardware realization, as practical circuits for harvesting energy from radio signals are not yet able to decode the carried information directly. To make theoretical progress, we propose a general receiver operation, namely, dynamic power splitting (DPS), which splits the received signal with adjustable power ratio for energy harvesting and information decoding, separately. Three special cases of DPS, namely, time switching (TS), static power splitting (SPS) and on-off power splitting (OPS) are investigated. The TS and SPS schemes can be treated as special cases of OPS. Moreover, we propose two types of practical receiver architectures, namely, separated versus integrated information and energy receivers. The integrated receiver integrates the front-end components of the separated receiver, thus achieving a smaller form factor. The rate-energy tradeoff for the two architectures are characterized by a so-called rate-energy (R-E) region. The optimal transmission strategy is derived to achieve different rate-energy tradeoffs. With receiver circuit power consumption taken into account, it is shown that the OPS scheme is optimal for both receivers. For the ideal case when the receiver circuit does not consume power, the SPS scheme is optimal for both receivers. In addition, we study the performance for the two types of receivers under a realistic system setup that employs practical modulation. Our results provide useful insights to the optimal practical receiver design for simultaneous wireless information and power transfer (SWIPT).
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paper_title: Wireless Information and Power Transfer: Energy Efficiency Optimization in OFDMA Systems

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This paper considers orthogonal frequency division multiple access (OFDMA) systems with simultaneous wireless information and power transfer. We study the resource allocation algorithm design for maximization of the energy efficiency of data transmission (bits/Joule delivered to the receivers). In particular, we focus on power splitting hybrid receivers which are able to split the received signals into two power streams for concurrent information decoding and energy harvesting. Two scenarios are investigated considering different power splitting abilities of the receivers. In the first scenario, we assume receivers which can split the received power into a continuous set of power streams with arbitrary power splitting ratios. In the second scenario, we examine receivers which can split the received power only into a discrete set of power streams with fixed power splitting ratios. For both scenarios, we formulate the corresponding algorithm design as a non-convex optimization problem which takes into account the circuit power consumption, the minimum data rate requirements of delay constrained services, the minimum required system data rate, and the minimum amount of power that has to be delivered to the receivers. By exploiting fractional programming and dual decomposition, suboptimal iterative resource allocation algorithms are developed to solve the non-convex problems. Simulation results illustrate that the proposed iterative resource allocation algorithms approach the optimal solution within a small number of iterations and unveil the trade-off between energy efficiency, system capacity, and wireless power transfer: (1) wireless power transfer enhances the system energy efficiency by harvesting energy in the radio frequency, especially in the interference limited regime; (2) the presence of multiple receivers is beneficial for the system capacity, but not necessarily for the system energy efficiency.
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paper_title: Wireless information and power transfer in multiuser OFDM systems

paper_content:

In this paper, we study the optimal design for simultaneous wireless information and power transfer (SWIPT) in downlink multiuser orthogonal frequency division multiplexing (OFDM) systems. For information transmission, we consider two types of multiple access schemes, namely, time division multiple access (TDMA) and orthogonal frequency division multiple access (OFDMA). At the receiver side, due to the practical limitation that circuits for harvesting energy from radio signals are not yet able to decode the carried information directly, each user applies either time switching (TS) or power splitting (PS) to coordinate the energy harvesting (EH) and information decoding (ID) processes. For the TDMA-based information transmission, we employ TS at the receivers; for the OFDMA-based information transmission, we employ PS at the receivers. Under the above two scenarios, we address the problem of maximizing the weighted sum-rate over all users by varying the time/frequency power allocation and either TS or PS ratio, subject to a minimum harvested energy constraint on each user as well as a peak and/or total transmission power constraint. For the TS scheme, by an appropriate variable transformation the problem is reformulated as a convex problem, for which the optimal power allocation and TS ratio are obtained by the Lagrange duality method. For the PS scheme, we propose an iterative algorithm to optimize the power allocation, subcarrier (SC) allocation and the PS ratio for each user. The performances of the two schemes are compared numerically as well as analytically for the special case of single-user setup. It is revealed that the peak power constraint imposed on each OFDM SC as well as the number of users in the system play a key role in the rate-energy performance comparison by the two proposed schemes.
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paper_title: Training-Based SWIPT: Optimal Power Splitting at the Receiver

paper_content:

We consider a point-to-point system with simultaneous wireless information and power transfer (SWIPT) over a block-fading channel. Each transmission block consists of a training phase and a data transmission phase. Pilot symbols are transmitted during the training phase for channel estimation at the receiver. To enable SWIPT, the receiver adopts a power-splitting design, such that a portion of the received signal is used for channel estimation or data detection, while the rest is used for energy harvesting. We optimally design the power-splitting ratios for both training and data phases to achieve the best ergodic capacity performance while maintaining a required energy harvesting rate. Our result shows how a power-splitting receiver can make the best use of the received pilot and data signals to obtain optimal SWIPT performance.
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paper_title: A Novel Mode Switching Scheme Utilizing Random Beamforming for Opportunistic Energy Harvesting

paper_content:

Since radio signals carry both energy and information at the same time, a unified study on simultaneous wireless information and power transfer (SWIPT) has recently drawn a significant attention for achieving wireless powered communication networks. In this paper, we study a multiple-input single-output (MISO) multicast SWIPT network with one multi-antenna transmitter sending common information to multiple single-antenna receivers simultaneously along with opportunistic wireless energy harvesting at each receiver. From the practical consideration, we assume that the channel state information (CSI) is only known at each respective receiver but is unavailable at the transmitter. We propose a novel receiver mode switching scheme for SWIPT based on a new application of the conventional random beamforming technique at the multi-antenna transmitter, which generates artificial channel fading to enable more efficient energy harvesting at each receiver when the received power exceeds a certain threshold. For the proposed scheme, we investigate the achievable information rate, harvested average power and power outage probability, as well as their various trade-offs in quasi-static fading channels. Compared to a reference scheme of periodic receiver mode switching without random transmit beamforming, the proposed scheme is shown to be able to achieve better rate-energy trade-offs when the harvested power target is sufficiently large. Particularly, it is revealed that employing one single random beam for the proposed scheme is asymptotically optimal as the transmit power increases to infinity, and also performs the best with finite transmit power for the high harvested power regime of most practical interests, thus leading to an appealing low-complexity implementation. Finally, we compare the rate-energy performances of the proposed scheme with different random beam designs.
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paper_title: Wireless information transfer with opportunistic energy harvesting

paper_content:

Energy harvesting is a promising solution to prolong the operation of energy-constrained wireless networks. In particular, scavenging energy from ambient radio signals, namely wireless energy harvesting (WEH), has recently drawn significant attention. In this paper, we consider a point-to-point wireless link over the narrowband flat-fading channel subject to time-varying co-channel interference. It is assumed that the receiver has no fixed power supplies and thus needs to replenish energy opportunistically via WEH from the unintended interference and/or the intended signal sent by the transmitter. We further assume a single-antenna receiver that can only decode information or harvest energy at any time due to the practical circuit limitation. Therefore, it is important to investigate when the receiver should switch between the two modes of information decoding (ID) and energy harvesting (EH), based on the instantaneous channel and interference condition. In this paper, we derive the optimal mode switching rule at the receiver to achieve various trade-offs between wireless information transfer and energy harvesting. Specifically, we determine the minimum transmission outage probability for delay-limited information transfer and the maximum ergodic capacity for no-delay-limited information transfer versus the maximum average energy harvested at the receiver, which are characterized by the boundary of so-called"outage-energy"region and"rate-energy"region, respectively. Moreover, for the case when the channel state information (CSI) is known at the transmitter, we investigate the joint optimization of transmit power control, information and energy transfer scheduling, and the receiver's mode switching. Our results provide useful guidelines for the efficient design of emerging wireless communication systems powered by opportunistic WEH.
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paper_title: MIMO Broadcasting for Simultaneous Wireless Information and Power Transfer

paper_content:

Wireless power transfer (WPT) is a promising new solution to provide convenient and perpetual energy supplies to wireless networks. In practice, WPT is implementable by various technologies such as inductive coupling, magnetic resonate coupling, and electromagnetic (EM) radiation, for short-/mid-/long-range applications, respectively. In this paper, we consider the EM or radio signal enabled WPT in particular. Since radio signals can carry energy as well as information at the same time, a unified study on simultaneous wireless information and power transfer (SWIPT) is pursued. Specifically, this paper studies a multiple-input multiple-output (MIMO) wireless broadcast system consisting of three nodes, where one receiver harvests energy and another receiver decodes information separately from the signals sent by a common transmitter, and all the transmitter and receivers may be equipped with multiple antennas. Two scenarios are examined, in which the information receiver and energy receiver are separated and see different MIMO channels from the transmitter, or co-located and see the identical MIMO channel from the transmitter. For the case of separated receivers, we derive the optimal transmission strategy to achieve different tradeoffs for maximal information rate versus energy transfer, which are characterized by the boundary of a so-called rate-energy (R-E) region. For the case of co-located receivers, we show an outer bound for the achievable R-E region due to the potential limitation that practical energy harvesting receivers are not yet able to decode information directly. Under this constraint, we investigate two practical designs for the co-located receiver case, namely time switching and power splitting, and characterize their achievable R-E regions in comparison to the outer bound.
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paper_title: Wireless Information and Energy Transfer in Multi-Antenna Interference Channel

paper_content:

This paper considers the transmitter design for wireless information and energy transfer (WIET) in a multiple-input single-output (MISO) interference channel (IFC). The design problem is to maximize the system throughput (i.e., the weighted sum rate) subject to individual energy harvesting constraints and power constraints. Different from the conventional IFCs without energy harvesting, the cross-link signals in the considered scenario play two opposite roles in information detection (ID) and energy harvesting (EH). It is observed that the ideal scheme, where the receivers can simultaneously perform ID and EH from the received signal, may not always achieve the best tradeoff between information transfer and energy harvesting, but simple practical schemes based on time splitting may perform better. We therefore propose two practical time splitting schemes, namely time division mode switching (TDMS) and time division multiple access (TDMA), in addition to a power splitting (PS) scheme which separates the received signal into two parts for ID and EH, respectively. In the two-user scenario, we show that beamforming is optimal to all the schemes. Moreover, the design problems associated with the TDMS and TDMA schemes admit semi-analytical solutions. In the general K-user scenario, a successive convex approximation method is proposed to handle the WIET problems associated with the ideal scheme and the PS scheme, which are known to be NP-hard in general. The K-user TDMS and TDMA schemes are shown efficiently solvable as convex problems. Simulation results show that stronger cross-link channel powers actually improve the information sum rate under energy harvesting constraints. Moreover, none of the schemes under consideration can dominate another in terms of the sum rate performance.
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paper_title: Wireless Information and Power Transfer: A Dynamic Power Splitting Approach

paper_content:

Energy harvesting is a promising solution to prolong the operation time of energy-constrained wireless networks. In particular, scavenging energy from ambient radio signals, namely wireless energy harvesting (WEH), has recently drawn significant attention. In this paper, we consider a point-to-point wireless link over the flat-fading channel, where the receiver has no fixed power supplies and thus needs to replenish energy via WEH from the signals sent by the transmitter. We first consider a SISO (single-input single-output) system where the single-antenna receiver cannot decode information and harvest energy independently from the same signal received. Under this practical constraint, we propose a dynamic power splitting (DPS) scheme, where the received signal is split into two streams with adjustable power levels for information decoding and energy harvesting separately based on the instantaneous channel condition that is assumed to be known at the receiver. We derive the optimal power splitting rule at the receiver to achieve various trade-offs between the maximum ergodic capacity for information transfer and the maximum average harvested energy for power transfer, which are characterized by the boundary of a so-called "rate-energy (R-E)" region. Moreover, for the case when the channel state information is also known at the transmitter, we investigate the joint optimization of transmitter power control and receiver power splitting. The achievable R-E region by the proposed DPS scheme is also compared against that by the existing time switching scheme as well as a performance upper bound by ignoring the practical receiver constraint. Finally, we extend the result for optimal DPS to the SIMO (single-input multiple-output) system where the receiver is equipped with multiple antennas. In particular, we investigate a low-complexity power splitting scheme, namely antenna switching, which achieves the near-optimal rate-energy trade-offs as compared to the optimal DPS.
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paper_title: Space solar power programs and microwave wireless power transmission technology

paper_content:

Future large-scale space solar power (SSP) will form a very complex integrated system of systems requiring numerous significant advances in current technology and capabilities. Ongoing technology developments have narrowed many of the gaps, but major technical, regulatory, and conceptual hurdles remain. Continuing systems concept studies and analyses will be critical to success, as will following a clear strategic R&T road map. This road map must assure both an incremental and evolutionary approach to developing needed technologies and systems is followed, with significant and broadly applicable advances with each increment. In particular, the technologies and systems needed for SPS must support highly leveraged applicability to needs in space science, robotic and human exploration, and the development of space. Considerable progress has been made in the critical area of microwave power transmission. At 5.8 GHz, DC-RF converters with efficiencies over 80% are achievable today. Rectennas developed at 5.8 GHz have also been measured with efficiencies greater than 80%. With optimized components in both the transmitter and rectenna, an SPS system has the potential of a DC-to-DC efficiency of 45%.
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paper_title: Simultaneous Information and Power Transfer for Broadband Wireless Systems

paper_content:

Far-field microwave power transfer (MPT) will free wireless sensors and other mobile devices from the constraints imposed by finite battery capacities. Integrating MPT with wireless communications to support simultaneous wireless information and power transfer (SWIPT) allows the same spectrum to be used for dual purposes without compromising the quality of service. A novel approach is presented in this paper for realizing SWIPT in a broadband system where orthogonal frequency division multiplexing and transmit beamforming are deployed to create a set of parallel sub-channels for SWIPT, which simplifies resource allocation. Based on a proposed reconfigurable mobile architecture, different system configurations are considered by combining single-user/multi-user systems, downlink/uplink information transfer, and variable/fixed coding rates. Optimizing the power control for these configurations results in a new class of multi-user power-control problems featuring the circuit-power constraints, specifying that the transferred power must be sufficiently large to support the operation of the receiver circuitry. Solving these problems gives a set of power-control algorithms that exploit channel diversity in frequency for simultaneously enhancing the throughput and the MPT efficiency. For the system configurations with variable coding rates, the algorithms are variants of water-filling that account for the circuit-power constraints. The optimal algorithms for those configurations with fixed coding rates are shown to sequentially allocate mobiles their required power for decoding in ascending order until the entire budgeted power is spent. The required power for a mobile is derived as simple functions of the minimum signal-to-noise ratio for correct decoding, the circuit power and sub-channel gains.
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paper_title: Energy-Efficient Optimization for Wireless Information and Power Transfer in Large-Scale MIMO Systems Employing Energy Beamforming

paper_content:

In this letter, we consider a large-scale multiple-input multiple-output (MIMO) system where the receiver should harvest energy from the transmitter by wireless power transfer to support its wireless information transmission. The energy beamforming in the large-scale MIMO system is utilized to address the challenging problem of long-distance wireless power transfer. Furthermore, considering the limitation of the power in such a system, this letter focuses on the maximization of the energy efficiency of information transmission (bit per Joule) while satisfying the quality-of-service (QoS) requirement, i.e. delay constraint, by jointly optimizing transfer duration and transmit power. By solving the optimization problem, we derive an energy-efficient resource allocation scheme. Numerical results validate the effectiveness of the proposed scheme.
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paper_title: MIMO Broadcasting for Simultaneous Wireless Information and Power Transfer

paper_content:

Wireless power transfer (WPT) is a promising new solution to provide convenient and perpetual energy supplies to wireless networks. In practice, WPT is implementable by various technologies such as inductive coupling, magnetic resonate coupling, and electromagnetic (EM) radiation, for short-/mid-/long-range applications, respectively. In this paper, we consider the EM or radio signal enabled WPT in particular. Since radio signals can carry energy as well as information at the same time, a unified study on simultaneous wireless information and power transfer (SWIPT) is pursued. Specifically, this paper studies a multiple-input multiple-output (MIMO) wireless broadcast system consisting of three nodes, where one receiver harvests energy and another receiver decodes information separately from the signals sent by a common transmitter, and all the transmitter and receivers may be equipped with multiple antennas. Two scenarios are examined, in which the information receiver and energy receiver are separated and see different MIMO channels from the transmitter, or co-located and see the identical MIMO channel from the transmitter. For the case of separated receivers, we derive the optimal transmission strategy to achieve different tradeoffs for maximal information rate versus energy transfer, which are characterized by the boundary of a so-called rate-energy (R-E) region. For the case of co-located receivers, we show an outer bound for the achievable R-E region due to the potential limitation that practical energy harvesting receivers are not yet able to decode information directly. Under this constraint, we investigate two practical designs for the co-located receiver case, namely time switching and power splitting, and characterize their achievable R-E regions in comparison to the outer bound.
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paper_title: Joint Wireless Information and Energy Transfer in a $K$-User MIMO Interference Channel

paper_content:

Recently, joint wireless information and energy transfer (JWIET) methods have been proposed to relieve the battery limitation of wireless devices. However, the JWIET in a general K-user MIMO interference channel (IFC) has been unexplored so far. In this paper, we investigate for the first time the JWIET in K-user MIMO IFC, in which receivers either decode the incoming information data (information decoding, ID) or harvest the RF energy (energy harvesting, EH). In the K-user IFC, we consider three different scenarios according to the receiver mode -- i) multiple EH receivers and a single ID receiver, ii) multiple IDs and a single EH, and iii) multiple IDs and multiple EHs. For all scenarios, we have found a common necessary condition of the optimal transmission strategy and, accordingly, developed the transmission strategy that satisfies the common necessary condition, in which all the transmitters transferring energy exploit a rank-one energy beamforming. Furthermore, we have also proposed an iterative algorithm to optimize the covariance matrices of the transmitters that transfer information and the powers of the energy beamforming transmitters simultaneously, and identified the corresponding achievable rate-energy tradeoff region. Finally, we have shown that by selecting EH receivers according to their signal-to-leakage-and-harvested energy-ratio (SLER), we can improve the achievable rate-energy region further.
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paper_title: Resource allocation for coordinated multipoint networks with wireless information and power transfer

paper_content:

This paper studies the resource allocation algorithm design for multiuser coordinated multipoint (CoMP) networks with simultaneous wireless information and power transfer (SWIPT). In particular, remote radio heads (RRHs) are connected to a central processor (CP) via capacity-limited backhaul links to facilitate CoMP joint transmission. Besides, the CP transfers energy to the RRHs for more efficient network operation. The considered resource allocation algorithm design is formulated as a non-convex optimization problem with a minimum required signal-to-interference-plus-noise ratio (SINR) constraint at multiple information receivers and a minimum required power transfer constraint at the energy harvesting receivers. By optimizing the transmit beamforming vectors at the CP and energy sharing between the CP and the RRHs, we aim at jointly minimizing the total network transmit power and the maximum capacity consumption per backhaul link. The resulting non-convex optimization problem is NP-hard. In light of the intractability of the problem, we reformulate it by replacing the non-convex objective function with its convex hull, which enables the derivation of an efficient iterative resource allocation algorithm. In each iteration, a non-convex optimization problem is solved by semi-definite programming (SDP) relaxation and the proposed iterative algorithm converges to a local optimal solution of the original problem. Simulation results illustrate that our proposed algorithm achieves a close-to-optimal performance and provides a significant reduction in backhaul capacity consumption compared to full cooperation. Besides, the considered CoMP network is shown to provide superior system performance as far as power consumption is concerned compared to a traditional system with multiple antennas co-located.
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paper_title: Robust Beamforming for Wireless Information and Power Transmission

paper_content:

In this letter, we study the robust beamforming problem for the multi-antenna wireless broadcasting system with simultaneous information and power transmission, under the assumption of imperfect channel state information (CSI) at the transmitter. Following the worst-case deterministic model, our objective is to maximize the worst-case harvested energy for the energy receiver while guaranteeing that the rate for the information receiver is above a threshold for all possible channel realizations. Such problem is nonconvex with infinite number of constraints. Using certain transformation techniques, we convert this problem into a relaxed semidefinite programming problem (SDP) which can be solved efficiently. We further show that the solution of the relaxed SDP problem is always rank-one. This indicates that the relaxation is tight and we can get the optimal solution for the original problem. Simulation results are presented to validate the effectiveness of the proposed algorithm.
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paper_title: Two-way relay beamforming for sum-rate maximization and energy harvesting

paper_content:

Weighted sum-rate maximization with transmit power limit and energy harvesting constraints is considered in the context of two-way collaborative relay systems. We discuss an ideal scheme where the source nodes can decode information signal and harvest energy, simultaneously. An iterative algorithm based on semidefinite relaxation (SDR) and successive convex approximation (SCA) is developed to obtain (at least) a local optimal solution. In view of the limitation of the current circuits, we propose a practical ping-pong scheme wherein the source node can only perform one operation at a time. Furthermore, the iterative algorithm and its convergence are extended to such a case. The efficacy of the proposed algorithms is validated by numerical experiments.
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paper_title: Sum rate maximization and energy harvesting for two-way af relay systems with imperfect CSI

paper_content:

This paper studies the beamforming design of two-way relay systems for simultaneous information and power transfer with consideration of imperfect channel state information (CSI). To this end, we seek the robust beamforming design to maximize the weighted sum rate under the power constraint at relay nodes and the energy harvesting constraints at the two source nodes. Due to the infinitely many nonconvex constraints, the robust beamforming design problem is challenging. Therefore, we firstly propose an approximate to the signal to interference plus noise ratio (SINR). Then an iterative algorithm is developed based on the semidefinite relaxation (SDR), S-procedure and successive convex approximation (SCA) techniques. Furthermore, the effectiveness of the proposed algorithm is validated by numerical experiments.
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paper_title: Multiuser MISO Beamforming for Simultaneous Wireless Information and Power Transfer

paper_content:

This paper studies a multiuser multiple-input single-output (MISO) broadcast system for simultaneous wireless information and power transfer (SWIPT), where a multi-antenna access point (AP) sends information and energy simultaneously via beamforming to multiple single-antenna receivers. We maximize the weighted sum-power transferred to energy harvesting (EH) receivers subject to a set of minimum signal-to-interference-and-noise ratio (SINR) constraints at information decoding (ID) receivers. In particular, we consider two types of ID receivers, namely Type I and Type II receivers, without and with the capability of cancelling the interference from energy signals, respectively. For each type of ID receivers, we formulate the joint information and energy transmit beamforming problem as a non-convex quadratically constrained quadratic program (QC-QP), for which the globally optimal solution is obtained by applying the technique of semidefinite relaxation (SDR). It is shown that for Type I ID receivers, dedicated energy beamforming is not needed to achieve the optimal solution, while for Type II ID receivers, employing no more than one energy beam is optimal.
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paper_title: Joint Wireless Information and Energy Transfer in a Two-User MIMO Interference Channel

paper_content:

This paper investigates joint wireless information and energy transfer in a two-user MIMO interference channel, in which each receiver either decodes the incoming information data (information decoding, ID) or harvests the RF energy (energy harvesting, EH) to operate with a potentially perpetual energy supply. In the two-user interference channel, we have four different scenarios according to the receiver mode -- ($ID_1$, $ID_2$), ($EH_1$, $EH_2$), ($EH_1$, $ID_2$), and ($ID_1$, $EH_2$). While the maximum information bit rate is unknown and finding the optimal transmission strategy is still open for ($ID_1$, $ID_2$), we have derived the optimal transmission strategy achieving the maximum harvested energy for ($EH_1$, $EH_2$). For ($EH_1$, $ID_2$), and ($ID_1$, $EH_2$), we find a necessary condition of the optimal transmission strategy and, accordingly, identify the achievable rate-energy (R-E) tradeoff region for two transmission strategies that satisfy the necessary condition - maximum energy beamforming (MEB) and minimum leakage beamforming (MLB). Furthermore, a new transmission strategy satisfying the necessary condition - signal-to-leakage-and-energy ratio (SLER) maximization beamforming - is proposed and shown to exhibit a better R-E region than the MEB and the MLB strategies. Finally, we propose a mode scheduling method to switch between ($EH_1$, $ID_2$) and ($ID_1$, $EH_2$) based on the SLER.
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paper_title: Energy harvesting balancing technique for robust beamforming in multiuser MISO SWIPT system

paper_content:

In this paper, a novel energy harvesting (EH) balancing technique for robust beamformers design in multiuser multiple-input single-output (MISO) broadcast system with simultaneous wireless information and power transfer (SWIPT) is proposed. Our design objective is to maximize the worst energy receiver harvested power under both the signal-to-interference-and-noise ratio (SINR) constraints at information receivers and total transmission power constraints at the transmitter. Considering imperfect channel state information (CSI) at the transmitter, the aforementioned max-min problem for EH fair robust beamformers design is non-convex in general. In order to solve this non-convex optimization problem, in this paper, we first derive equivalent conditions for the EH constraints and SINR constraints, and then convert original non-convex optimization problem into a relaxed semi-definite programming (SDP) problem, which can be solved efficiently by standard interior-point methods. Moreover, we propose an iterative algorithm based on bisection method to obtain the robust beamforming solutions. Finally, simulation results are provided to demonstrate the effectiveness and robustness of our algorithm.
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paper_title: Joint Wireless Information and Energy Transfer With Reduced Feedback in MIMO Interference Channels

paper_content:

To determine the transmission strategy for the joint wireless information and energy transfer (JWIET) in the MIMO interference channel (IFC), the information access point (IAP) and energy access point (EAP) require the channel state information (CSI) of their associated links to both the information-decoding (ID) mobile stations (MSs) and energy-harvesting (EH) MSs (so-called local CSI). In this paper, to reduce the feedback overhead of MSs for the JWIET in two-user MIMO IFC, we propose a Geodesic energy beamforming scheme that requires partial CSI at the EAP. Furthermore, in the two-user MIMO IFC, it is proved that the Geodesic energy beamforming is the optimal non-cooperative strategy under local CSIT assumption. By adding a rank-one constraint on the transmit signal covariance of IAP, we can further reduce the feedback overhead to IAP by exploiting Geodesic information beamforming. Under the rank-one constraint of IAP's transmit signal, we prove that Geodesic information/energy beamforming approach is the optimal non-cooperative strategy for JWIET in the two-user MIMO IFC. We also discuss the extension of the proposed rank-one Geodesic information/energy beamforming strategies to general K-user MIMO IFC. Finally, by analyzing the achievable rate-energy performance statistically under imperfect partial CSIT, we propose an adaptive bit allocation strategy for both EH MS and ID MS.
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paper_title: Beamforming for MISO Interference Channels with QoS and RF Energy Transfer

paper_content:

We consider a multiuser multiple-input single-output interference channel where the receivers are characterized by both quality-of-service (QoS) and radio-frequency (RF) energy harvesting (EH) constraints. We consider the power splitting RF-EH technique where each receiver divides the received signal into two parts a) for information decoding and b) for battery charging. The minimum required power that supports both the QoS and the RF-EH constraints is formulated as an optimization problem that incorporates the transmitted power and the beamforming design at each transmitter as well as the power splitting ratio at each receiver. We consider both the cases of fixed beamforming and when the beamforming design is incorporated into the optimization problem. For fixed beamforming we study three standard beamforming schemes, the zero-forcing (ZF), the regularized zero-forcing (RZF) and the maximum ratio transmission (MRT); a hybrid scheme, MRT-ZF, comprised of a linear combination of MRT and ZF beamforming is also examined. The optimal solution for ZF beamforming is derived in closed-form, while optimization algorithms based on second-order cone programming are developed for MRT, RZF and MRT-ZF beamforming to solve the problem. In addition, the joint-optimization of beamforming and power allocation is studied using semidefinite programming (SDP) with the aid of rank relaxation.
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paper_title: MIMO Broadcasting for Simultaneous Wireless Information and Power Transfer

paper_content:

Wireless power transfer (WPT) is a promising new solution to provide convenient and perpetual energy supplies to wireless networks. In practice, WPT is implementable by various technologies such as inductive coupling, magnetic resonate coupling, and electromagnetic (EM) radiation, for short-/mid-/long-range applications, respectively. In this paper, we consider the EM or radio signal enabled WPT in particular. Since radio signals can carry energy as well as information at the same time, a unified study on simultaneous wireless information and power transfer (SWIPT) is pursued. Specifically, this paper studies a multiple-input multiple-output (MIMO) wireless broadcast system consisting of three nodes, where one receiver harvests energy and another receiver decodes information separately from the signals sent by a common transmitter, and all the transmitter and receivers may be equipped with multiple antennas. Two scenarios are examined, in which the information receiver and energy receiver are separated and see different MIMO channels from the transmitter, or co-located and see the identical MIMO channel from the transmitter. For the case of separated receivers, we derive the optimal transmission strategy to achieve different tradeoffs for maximal information rate versus energy transfer, which are characterized by the boundary of a so-called rate-energy (R-E) region. For the case of co-located receivers, we show an outer bound for the achievable R-E region due to the potential limitation that practical energy harvesting receivers are not yet able to decode information directly. Under this constraint, we investigate two practical designs for the co-located receiver case, namely time switching and power splitting, and characterize their achievable R-E regions in comparison to the outer bound.
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paper_title: SWIPT in MISO Multicasting Systems

paper_content:

This letter considers simultaneous wireless information and power transfer (SWIPT) in multiple-input single-output (MISO) multicasting systems where each receiver is equipped with a power splitting device and can receive both information and energy from the base station (BS) continuously at the same time. We investigate the joint multicast transmit beamforming and receive power splitting problem for minimizing the transmit power of the BS subject to signal-to-noise ratio (SNR) and energy harvesting constraints at each receiver. Both scenarios of perfect and imperfect channel state information (CSI) at the BS are studied. Due to non convexity of the problems, we use semidefinite relaxation (SDR) technique to solve the problems. Interestingly, we show that the SDR is in fact tight in certain scenarios.
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paper_title: Multi-Objective Resource Allocation for Secure Communication in Cognitive Radio Networks with Wireless Information and Power Transfer

paper_content:

In this paper, we study resource allocation for multiuser multiple-input single-output secondary communication systems with multiple system design objectives. We consider cognitive radio networks where the secondary receivers are able to harvest energy from the radio frequency when they are idle. The secondary system provides simultaneous wireless power and secure information transfer to the secondary receivers. We propose a multi-objective optimization framework for the design of a Pareto optimal resource allocation algorithm based on the weighted Tchebycheff approach. In particular, the algorithm design incorporates three important system objectives: total transmit power minimization, energy harvesting efficiency maximization, and interference power leakage-to-transmit power ratio minimization. The proposed framework takes into account a quality of service requirement regarding communication secrecy in the secondary system and the imperfection of the channel state information of potential eavesdroppers (idle secondary receivers and primary receivers) at the secondary transmitter. The adopted multi-objective optimization problem is non-convex and is recast as a convex optimization problem via semidefinite programming (SDP) relaxation. It is shown that the global optimal solution of the original problem can be constructed by exploiting both the primal and the dual optimal solutions of the SDP relaxed problem. Besides, two suboptimal resource allocation schemes for the case when the solution of the dual problem is unavailable for constructing the optimal solution are proposed. Numerical results not only demonstrate the close-to-optimal performance of the proposed suboptimal schemes, but also unveil an interesting trade-off between the considered conflicting system design objectives.
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paper_title: Rank-two transmit beamformed Alamouti space-time coding for physical-layer multicasting

paper_content:

In physical-layer multicasting over a multiuser MISO downlink channel, transmit beamforming using semidefinite relaxation (SDR) has been a popular approach. In this paper, we propose a rank-2 transmit beamformed Alamouti space-time code scheme, which may be seen as a generalization of the previous SDR-based beamforming framework. The beamforming problem arising from the proposed scheme is a rank-2 constrained semidefinite program (SDP).We deal with it using the SDR technique, but this time using rank-2 approximation rather than rank-1 approximation in the previous transmit beamforming. An analysis on the worst-case approximation accuracy of the rank-2 SDR approximation is provided, which reveals that the approximation accuracy degrades at a rate of √M, where M is the number of users served. This improves upon the case of transmit beamforming, where the worst-case approximation accuracy degrades at the higher rate of M. Simulation results further show that the proposed scheme performs better than the transmit beamforming scheme.
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paper_title: Rank-Two Beamformed Secure Multicasting for Wireless Information and Power Transfer

paper_content:

In this letter, we investigate secure multicasting for simultaneous wireless information and power transfer (SWIPT) in the presence of multiple energy receivers who have potential to eavesdrop on the messages of information receivers. Under both, signal-interference-noise ratio (SINR) and harvested energy constraints, we propose a novel secure multicast design to minimize the total power by using the techniques of rank-two beamformed Alamouti coding and semidefinite relaxation (SDR). The sufficient conditions are derived under which the proposed secure multicast design is optimal. We also present the corresponding rank-two Gaussian randomization procedure to obtain a suboptimal solution when SDR is not tight. Simulation results are then provided to demonstrate the efficacy of the proposed design in power saving.
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paper_title: Robust Beamforming for Secure Communication in Systems with Wireless Information and Power Transfer

paper_content:

This paper considers a multiuser multiple-input single-output (MISO) downlink system with simultaneous wireless information and power transfer. In particular, we focus on secure communication in the presence of passive eavesdroppers and potential eavesdroppers (idle legitimate receivers). We study the design of a resource allocation algorithm minimizing the total transmit power for the case when the legitimate receivers are able to harvest energy from radio frequency signals. Our design advocates the dual use of both artificial noise and energy signals in providing secure communication and facilitating efficient wireless energy transfer. The algorithm design is formulated as a non-convex optimization problem. The problem formulation takes into account artificial noise and energy signal generation for protecting the transmitted information against both considered types of eavesdroppers when imperfect channel state information (CSI) of the potential eavesdroppers and no CSI of the passive eavesdroppers are available at the transmitter. In light of the intractability of the problem, we reformulate the considered problem by replacing a non-convex probabilistic constraint with a convex deterministic constraint. Then, a semi-definite programming (SDP) relaxation approach is adopted to obtain the optimal solution for the reformulated problem. Furthermore, we propose a suboptimal resource allocation scheme with low computational complexity for providing communication secrecy and facilitating efficient energy transfer. Simulation results demonstrate a close-to-optimal performance achieved by the proposed schemes and significant transmit power savings by optimization of the artificial noise and energy signal generation.
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paper_title: Secure Layered Transmission in Multicast Systems with Wireless Information and Power Transfer

paper_content:

This paper considers downlink multicast transmit beamforming for secure layered transmission systems with wireless simultaneous information and power transfer. We study the power allocation algorithm design for minimizing the total transmit power in the presence of passive eavesdroppers and energy harvesting receivers. The algorithm design is formulated as a non-convex optimization problem. Our problem formulation promotes the dual use of energy signals in providing secure communication and facilitating efficient energy transfer. Besides, we take into account a minimum required power for energy harvesting at the idle receivers and heterogeneous quality of service (QoS) requirements for the multicast video receivers. In light of the intractability of the problem, we reformulate the considered problem by replacing a non-convex probabilistic constraint with a convex deterministic constraint which leads to a smaller feasible solution set. Then, a semidefinite programming relaxation (SDR) approach is adopted to obtain an upper bound solution for the reformulated problem. Subsequently, sufficient conditions for the global optimal solution of the reformulated problem are revealed. Furthermore, we propose two suboptimal power allocation schemes based on the upper bound solution. Simulation results demonstrate the excellent performance and significant transmit power savings achieved by the proposed schemes compared to isotropic energy signal generation.
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paper_title: Secrecy wireless information and power transfer with MISO beamforming

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The dual use of radio signals for simultaneous wireless information and power transfer (SWIPT) has recently drawn significant attention. To meet the practical requirement that energy receivers (ERs) operate with much higher received power than information receivers (IRs), ERs need to be deployed closer to the transmitter than IRs. However, due to the broadcast nature of wireless channels, one critical issue is that the messages sent to IRs cannot be eavesdropped by ERs, which possess better channels from the transmitter. In this paper, we address this new secrecy communication problem in a multiuser multiple-input single-output (MISO) SWIPT system where a multi-antenna transmitter sends information and energy simultaneously to one IR and multiple ERs, each with a single antenna. By optimizing transmit beamforming vectors and their power allocation, we maximize the weighted sum-energy transferred to ERs subject to a secrecy rate constraint for the information sent to the IR. We solve this non-convex problem optimally by reformulating it into a two-stage problem. First, we fix the signal-to-interference-plus-noise ratio (SINR) at the IR and obtain the optimal beamforming solution by applying the technique of semidefinite relaxation (SDR). Then the original problem is solved by a one-dimension search over the optimal SINR value for the IR. Furthermore, two suboptimal low-complexity beamforming schemes are proposed, and their achievable (secrecy) rate-energy (R-E) regions are compared against that by the optimal scheme.
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paper_title: Secure Relay Beamforming for Simultaneous Wireless Information and Power Transfer in Nonregenerative Relay Networks

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For simultaneous wireless information and power transfer (SWIPT), secure communication is an important issue. In this correspondence, we study the secure relay beamforming (SRB) scheme for SWIPT in a nonregenerative multiantenna relay network. We propose a constrained concave convex procedure (CCCP)-based iterative algorithm that is able to achieve a local optimum, where the secrecy rate is maximized, and the relay transmit power and energy harvesting constraints are satisfied. Simulation results have shown that our proposed CCCP-based iterative algorithm achieves a larger secrecy rate and lower computational complexity than the convectional SRB schemes. Since the CCCP-based iterative algorithm is still complex, we propose a semidefinite programming (SDP)-based noniterative suboptimal algorithm and a closed-form suboptimal algorithm. It is shown that when the maximum transmit power of the relay to noise power ratio is high, the SDP-based noniterative suboptimal algorithm performs close to the CCCP-based iterative algorithm.
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paper_title: Secure Beamforming for MIMO Broadcasting With Wireless Information and Power Transfer

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This paper considers a basic MIMO information-energy (I-E) broadcast system, where a multi-antenna transmitter transmits information and energy simultaneously to a multi-antenna information receiver and a dual-functional multi-antenna energy receiver which is also capable of decoding information. Due to the open nature of wireless medium and the dual purpose of information and energy transmission, secure information transmission while ensuring efficient energy harvesting is a critical issue for such a broadcast system. Assuming that physical layer security techniques are applied to the system to ensure secure transmission from the transmitter to the information receiver, we study beamforming design to maximize the achievable secrecy rate subject to a total power constraint and an energy harvesting constraint. First, based on semidefinite relaxation, we propose global optimal solutions to the secrecy rate maximization (SRM) problem in the single-stream case and a specific full-stream case where the difference of Gram matrices of the channel matrices is positive semidefinite. Then, we propose a simple iterative algorithm named inexact block coordinate descent (IBCD) algorithm to tackle the SRM problem of general case with arbitrary number of streams. We proves that the IBCD algorithm can monotonically converge to a Karush-Kuhn-Tucker (KKT) solution to the SRM problem. Furthermore, we extend the IBCD algorithm to the joint beamforming and artificial noise design problem. Finally, simulations are performed to validate the performance of the proposed beamforming algorithms.
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paper_title: Collaborative Wireless Energy and Information Transfer in Interference Channel

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This paper studies the simultaneous wireless information and power transfer (SWIPT) in a multiuser wireless system, in which distributed transmitters send independent messages to their respective receivers, and at the same time cooperatively transmit wireless power to the receivers via energy beamforming. Accordingly, from the wireless information transmission (WIT) perspective, the system of interest can be modeled as the classic interference channel, while it also can be regarded as a distributed multiple-input multiple-output (MIMO) system for collaborative wireless energy transmission (WET). To enable both information decoding (ID) and energy harvesting (EH) in SWIPT, we adopt the low-complexity time switching operation at each receiver to switch between the ID and EH modes over scheduled time. For the hybrid system, we aim to characterize the achievable rate-energy (R–E) trade-offs by various transmitter-side collaboration schemes. Specifically, to facilitate the collaborative energy beamforming, we propose a new signal splitting scheme at the transmitters, where each transmit signal is generally split into an information signal and an energy signal for WIT and WET, respectively. With this new scheme, first, we study the two-user SWIPT system over the fading channel and derive the optimal mode switching rule at the receivers as well as the corresponding transmit signal optimization to achieve various R-E trade-offs. We also compare the R-E performance of our proposed scheme with transmit energy beamforming and signal splitting against two existing schemes with partial or no cooperation of the transmitters. Next, the general case of SWIPT systems with more than two users is studied, for which we propose a practical transmit collaboration scheme by extending the result for the two-user case: we group users into different pairs and apply the cooperation schemes obtained in the two-user case to each paired group. Furthermore, we present a benchmarking scheme based on joint cooperation of all the transmitters inspired by the principle of interference alignment , against which the performance of the proposed scheme is compared.
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paper_title: On Nonlinear Fractional Programming

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The main purpose of this paper is to delineate an algorithm for fractional programming with nonlinear as well as linear terms in the numerator and denominator. The algorithm presented is based on a theorem by Jagannathan Jagannathan, R. 1966. On some properties of programming problems in parametric form pertaining to fractional programming. Management Sci.12 609--615. concerning the relationship between fractional and parametric programming. This theorem is restated and proved in a somewhat simpler way. Finally, it is shown how the given algorithm can be related to the method of Isbell and Marlow Isbell, J. R., W. H. Marlow. 1956. Attrition games. Naval Res. Logist. Quart.3 71--93. for linear fractional programming and to the quadratic parametric approach by Ritter Ritter, K. 1962. Ein Verfahren zur Losung parameterabhangiger, nichtlinearer Maximum-Probleme. Unternehmensforschung, Band 6, S. 149--166.. The Appendix contains a numerical example.
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paper_title: Energy-Efficient Optimization for Wireless Information and Power Transfer in Large-Scale MIMO Systems Employing Energy Beamforming

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In this letter, we consider a large-scale multiple-input multiple-output (MIMO) system where the receiver should harvest energy from the transmitter by wireless power transfer to support its wireless information transmission. The energy beamforming in the large-scale MIMO system is utilized to address the challenging problem of long-distance wireless power transfer. Furthermore, considering the limitation of the power in such a system, this letter focuses on the maximization of the energy efficiency of information transmission (bit per Joule) while satisfying the quality-of-service (QoS) requirement, i.e. delay constraint, by jointly optimizing transfer duration and transmit power. By solving the optimization problem, we derive an energy-efficient resource allocation scheme. Numerical results validate the effectiveness of the proposed scheme.
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paper_title: Dynamic Resource Allocation for Multiple-Antenna Wireless Power Transfer

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We consider a point-to-point multiple-input-single-output (MISO) system where a receiver harvests energy from a transmitter. To achieve high-efficiency wireless power transfer (WPT), the transmitter performs energy beamforming by using an instantaneous channel state information (CSI). The CSI is estimated at the receiver by training via a preamble and fed back to the transmitter. In this paper, we address the key challenge of balancing the time resource used for channel estimation and WPT to maximize the harvested energy and also investigate the allocation of energy resource used for WPT. First, we consider the general scenario where the preamble length is allowed to vary dynamically depending on channel conditions. The optimal preamble length is obtained online by solving a dynamic programming (DP) problem. The DP problem is proved to reduce to an optimal stopping problem. The optimal policy is then shown to depend only on the channel estimate power. Next, we consider the scenario in which the preamble length is fixed by an offline optimization. Furthermore, we derive the optimal power allocation schemes. For the dynamic-length-preamble scenario, the power is allocated according to both the optimal preamble length and the channel estimate power, while for the fixed-length-preamble scenario, the power is allocated according to only the channel estimate power. By numerical simulations, our results show that with optimal power allocation, the energy harvested by using the optimized fixed-length preamble is close to that harvested by using a dynamic-length preamble, hence allowing a low-complexity yet close-to-optimal WPT system to be implemented in practice.
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paper_title: Joint Beamforming Design and Time Allocation for Wireless Powered Communication Networks

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This paper investigates a multi-input single-output (MISO) wireless powered communication network (WPCN) under the protocol of harvest-then-transmit. The power station (PS) with reliable power supply can replenish the passive user nodes by wireless power transfer (WPT) in the downlink (DL), then each user node transmits independent information to the sink by a time division multiple access (TDMA) scheme in the uplink (UL). We consider the joint time allocation and beamforming design to maximize the system sum-throughput. The semidefinite relaxation (SDR) technique is applied to solve the nonconvex design problem. The tightness of SDR approximation, thus the global optimality, is proved. This implies that only one single energy beamformer is required at the PS. Then a fast semiclosed form solution is proposed by exploiting the inherent structure. Simulation results demonstrate the efficiency of the proposed algorithms from the perspectives of time complexity and information throughput.
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paper_title: Ergodic Interference Alignment

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Consider a K-user interference channel with timevarying fading. At any particular time, each receiver will see a signal from most transmitters. The standard approach to such a scenario results in each transmitter-receiver pair achieving a rate proportional to 1/K the single user rate. However, given two well chosen time indices, the channel coefficients from interfering users can be made to exactly cancel. By adding up these two signals, the receiver can see an interference-free version of the desired transmission. We show that this technique allows each user to achieve at least half its interference-free ergodic capacity at any SNR. Prior work was only able to show that half the interference-free rate was achievable as the SNR tended to infinity. We examine a finite field channel model and a Gaussian channel model. In both cases, the achievable rate region has a simple description and, in the finite field case, we prove it is the ergodic capacity region.
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paper_title: A simple gradient sign algorithm for transmit antenna weight adaptation with feedback

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In this paper, a simple algorithm for adaptation of the complex baseband weights of a transmit antenna array using feedback from the receiver is proposed and analyzed. The system utilizes stochastic gradient adaptation to maximize the power delivered to the receiver for a constrained transmission power, which provides both fading diversity and beam steering gain. Dual perturbed transmission weight vectors are time multiplexed onto the pilot signal, and the receiver generates feedback selecting the perturbed weight vector which delivers greater power. This feedback is used to provide weight adaptation at the transmitter, and this adaptation is shown to be an update by a coarse estimate of the gradient of the delivered power. The performance of the algorithm is analyzed in terms of convergence and tracking of an AR1 fading channel, with simulations confirming the analysis. Bit error rate (BER) simulations in a dynamic fading channel show that the algorithm outperforms previously proposed vector selection feedback, and in slower fading, the algorithm substantially outperforms diversity space time coding.
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paper_title: The One-Bit Null Space Learning Algorithm and its Convergence

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This paper proposes a new algorithm for MIMO cognitive radio Secondary Users (SU) to learn the null space of the interference channel to the Primary User (PU) without burdening the PU with any knowledge or explicit cooperation with the SU. The knowledge of this null space enables the SU to transmit in the same band simultaneously with the PU by utilizing separate spatial dimensions than the PU. Specifically, the SU transmits in the null space of the interference channel to the PU. We present a new algorithm, called the One-Bit Null Space Learning Algorithm (OBNSLA), in which the SU learns the PU's null space by observing a binary function that indicates whether the interference it inflicts on the PU has increased or decreased in comparison to the SU's previous transmitted signal. This function is obtained by listening to the PU transmitted signal or control channel and extracting information from it about whether the PU's Signal to Interference plus Noise power Ratio (SINR) has increased or decreased. In addition to introducing the OBNSLA, this paper provides a thorough convergence analysis of this algorithm. The OBNSLA is shown to have a linear convergence rate and an asymptotic quadratic convergence rate. Finally, we derive bounds on the interference that the SU inflicts on the PU as a function of a parameter determined by the SU. This lets the SU control the maximum level of interference, which enables it to protect the PU completely blindly with minimum complexity. The asymptotic analysis and the derived bounds also apply to the recently proposed Blind Null Space Learning Algorithm.
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paper_title: Wireless Energy and Information Transfer Tradeoff for Limited-Feedback Multiantenna Systems With Energy Beamforming

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In this paper, we consider a multiantenna system where the receiver should harvest energy from the transmitter by wireless energy transfer to support its wireless information transmission. To maximize the harvesting energy, we propose the performance of adaptive energy beamforming according to the instantaneous channel state information (CSI). To help the transmitter obtain the CSI for energy beamforming, we further propose a win-win CSI quantization feedback strategy to improve the efficiencies of both power and information transmission. The focus of this paper is on the tradeoff of wireless energy and information transfer by adjusting the transfer duration with a total duration constraint. By revealing the relationship between transmit power, transfer duration, and feedback amount, we derive two wireless energy and information transfer tradeoff schemes by maximizing an upper bound and an approximate lower bound of the average information transmission rate, respectively. Moreover, the impact of imperfect CSI at the receiver is investigated, and the corresponding wireless energy and information transfer tradeoff scheme is also given. Finally, numerical results validate the effectiveness of the proposed schemes.
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paper_title: Energy Beamforming with One-Bit Feedback

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Multi-antenna or multiple-input multiple-output (MIMO) techniques are appealing to enhance the transmission efficiency and range for radio frequency (RF) signal enabled wireless energy transfer (WET). In order to reap the energy beamforming gain in MIMO WET, acquiring the channel state information (CSI) at the energy transmitter (ET) is an essential task. This task is particularly challenging, since existing channel training and feedback methods used for communication receivers may not be implementable at the energy receiver (ER) due to its hardware limitation. To tackle this problem, we consider in this paper a multiuser MIMO WET system, and propose a new channel learning method that requires only one feedback bit from each ER to the ET per feedback interval. Specifically, each feedback bit indicates the increase or decrease of the harvested energy by each ER in the present as compared to the previous intervals, which can be measured without changing the existing structure of the ER. Based on such feedback information, the ET adjusts transmit beamforming in subsequent training intervals and at the same time obtains improved estimates of the MIMO channels to different ERs by applying an optimization technique called analytic center cutting plane method (ACCPM). For the proposed ACCPM based channel learning algorithm, we analyze its worst-case convergence, from which it is revealed that the algorithm is able to estimate multiuser MIMO channels simultaneously without reducing the analytic convergence speed. Also, we provide extensive simulations to show its performances in terms of both convergence speed and energy transfer efficiency.
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paper_title: Optimal training for wireless energy transfer

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Wireless energy transfer (WET) is potentially a promising solution to provide convenient and reliable energy supplies for energy-constrained networks, and has drawn growing interests recently. To overcome the significant propagation loss over distance, employing multi-antennas at the energy transmitter (ET) to more efficiently direct wireless energy to desired energy receivers (ERs), termed energy beamforming, is an essential technique for WET. However, the achievable gain of energy beamforming crucially depends on the available channel state information (CSI) at the ET, which needs to be acquired practically. In this paper, we study the optimal design of one efficient channel-acquisition method for a point-to-point multiple-input multiple-output (MIMO) WET system, by exploiting the channel reciprocity based on which the ET estimates the CSI via dedicated reverse-link training from the ER. Considering the limited energy availability at the ER, the training strategy should be carefully designed so that the channel can be estimated with sufficient accuracy, and yet without consuming excessive energy at the ER. To this end, we propose to maximize the net energy at the ER, which is the total energy harvested offset by that used for channel training. The optimal training design, including the number of receive antennas to be trained, as well as the training time and power allocated, is derived. Our result shows that training helps only when the following conditions are satisfied: (i) the channel coherence time is sufficiently large; (ii) the number of antennas at the ET is large enough; and (iii) the effective signal-to-noise ratio (ESNR) is sufficiently high; otherwise, no training should be applied and isotropic energy transmission is optimal.
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paper_title: Relaying Protocols for Wireless Energy Harvesting and Information Processing

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An emerging solution for prolonging the lifetime of energy constrained relay nodes in wireless networks is to avail the ambient radio-frequency (RF) signal and to simultaneously harvest energy and process information. In this paper, an amplify-and-forward (AF) relaying network is considered, where an energy constrained relay node harvests energy from the received RF signal and uses that harvested energy to forward the source information to the destination. Based on the time switching and power splitting receiver architectures, two relaying protocols, namely, i) time switching-based relaying (TSR) protocol and ii) power splitting-based relaying (PSR) protocol are proposed to enable energy harvesting and information processing at the relay. In order to determine the throughput, analytical expressions for the outage probability and the ergodic capacity are derived for delay-limited and delay-tolerant transmission modes, respectively. The numerical analysis provides practical insights into the effect of various system parameters, such as energy harvesting time, power splitting ratio, source transmission rate, source to relay distance, noise power, and energy harvesting efficiency, on the performance of wireless energy harvesting and information processing using AF relay nodes. In particular, the TSR protocol outperforms the PSR protocol in terms of throughput at relatively low signal-to-noise-ratios and high transmission rate.
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paper_title: A Low Complexity Antenna Switching for Joint Wireless Information and Energy Transfer in MIMO Relay Channels

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In this paper, we investigate a low-complexity technique for simultaneous wireless information and energy transfer in multiple-input multiple-output relay channels. The proposed technique exploits the array configuration at the relay node and uses the antenna elements either for conventional decoding or for rectifying (rectennas). In order to keep the complexity low, a dynamic antenna switching between decoding/rectifying is proposed based on the principles of the generalized selection combiner (GSC); the L strongest paths are allocated for decoding while the remaining channel paths for rectifying (and vice versa). The optimal L as well as the allocation strategy that minimizes the outage probability are investigated via theoretical and numerical results. In addition, two performance bounds that provide the optimal performance without the limitation of GSC are proposed by solving a linear programming and a binary knapsack problem, respectively. The proposed technique is extended to scenarios with multi-user interference, where a zero-forcing receiver is used at the relay node; closed-forms expressions for the outage probability are also derived.
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paper_title: RF Energy Transfer for Cooperative Networks: Data Relaying or Energy Harvesting?

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This letter deals with a three-node cooperative network where the relay node harvests energy from radio frequency (RF) radiation. The source node is the only available RF generator and introduces a fundamental switching between energy harvesting and data relaying. A greedy switching (GS) policy where the relay node transmits when its residual energy ensures decoding at the destination is investigated. The GS policy is modeled as a Markov chain for a discretized battery; the stationary distribution and the outage probability of the system are derived in closed form expressions. In addition, an optimal switching policy that incorporates a-priori knowledge of the channel coefficients is proposed and solved by a mixed-integer linear programming formulation.
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paper_title: Wireless Energy Harvesting and Information Relaying: Adaptive Time-Switching Protocols and Throughput Analysis.

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This paper considers amplify-and-forward (AF) and decode-and-forward (DF) relaying networks where an energy constrained relay node first harvests energy through the received radio-frequency signal from the source and then uses the harvested energy to forward the source information to the destination node. We propose adaptive time-switching protocols for wireless energy harvesting (EH) and information transmission by adapting the timeduration of EH at the relay node based on the available harvested energy and the source-to-relay channel quality. Specifically, we consider both continuous and discrete time-switching protocols and account for energy accumulation at the relay. For continuous time-switching protocols, the EH time can be set to any percentage of the total transmission block time. For discrete time-switching protocols, the whole transmission block is either used for EH or information transmission. We derive analytical expressions of the achievable throughput for both the continuous and the discrete time-switching protocols. The derived expressions provide practical insights into the effect of various parameters, such as relay transmission power, noise power, and detection threshold signal-to-noise ratio, on the system performance. Interestingly, we find that the discrete time-switching protocols, despite being simpler to implement, yield more throughput than the continuous time-switching protocols for a wide range of scenarios.
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paper_title: Analysis of Instantaneous Power Distributions for Non-Regenerative and Regenerative Relaying Signals

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In this paper, we analyze the instantaneous power distributions of the transmitting signals at the relay in the non-regenerative and regenerative cooperation scenarios with linear modulation. We study the following four relay function scenarios: fixed gain amplify and forward (AF) relay, variable gain AF relay, estimate and forward (EF) relay, and decode and forward (DF) relay. The analytical results show that the transmitting signals from non-regenerative relaying tend to suffer from high peak power, which is even prohibitive when the power amplifier (PA) efficiency at the relay is of primary concern.
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paper_title: Throughput Maximization in Wireless Powered Communication Networks

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This paper studies the newly emerging wireless powered communication network in which one hybrid access point (H-AP) with constant power supply coordinates the wireless energy/information transmissions to/from a set of distributed users that do not have other energy sources. A "harvest-then-transmit" protocol is proposed where all users first harvest the wireless energy broadcast by the H-AP in the downlink (DL) and then send their independent information to the H-AP in the uplink (UL) by time-division-multiple-access (TDMA). First, we study the sum-throughput maximization of all users by jointly optimizing the time allocation for the DL wireless power transfer versus the users' UL information transmissions given a total time constraint based on the users' DL and UL channels as well as their average harvested energy values. By applying convex optimization techniques, we obtain the closed-form expressions for the optimal time allocations to maximize the sum-throughput. Our solution reveals an interesting "doubly near-far" phenomenon due to both the DL and UL distance-dependent signal attenuation, where a far user from the H-AP, which receives less wireless energy than a nearer user in the DL, has to transmit with more power in the UL for reliable information transmission. As a result, the maximum sum-throughput is shown to be achieved by allocating substantially more time to the near users than the far users, thus resulting in unfair rate allocation among different users. To overcome this problem, we furthermore propose a new performance metric so-called common-throughput with the additional constraint that all users should be allocated with an equal rate regardless of their distances to the H-AP. We present an efficient algorithm to solve the common-throughput maximization problem. Simulation results demonstrate the effectiveness of the common-throughput approach for solving the new doubly near-far problem in wireless powered communication networks.
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paper_title: Relay Selection for Simultaneous Information Transmission and Wireless Energy Transfer: A Tradeoff Perspective

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In certain applications, relay terminals can be employed to simultaneously deliver information and energy to a designated receiver and a radio frequency (RF) energy harvester, respectively. In such scenarios, the relay that is preferable for information transmission does not necessarily coincide with the relay with the strongest channel to the energy harvester, since the corresponding channels fade independently. Relay selection thus entails a tradeoff between the efficiency of the information transfer to the receiver and the amount of energy transferred to the energy harvester. The study of this tradeoff is the subject on which this work mainly focuses. Specifically, we investigate the behavior of the ergodic capacity and the outage probability of the information transmission to the receiver, for a given amount of energy transferred to the RF energy harvester. We propose two relay selection methods that apply to any number of available relays. Furthermore, for the case of two relays, we develop the optimal relay selection method in a maximum capacity / minimum outage probability sense, for a given energy transfer constraint. A close-to-optimal selection method that is easier to analyze than the optimal one is also examined. Closed-form expressions for the capacity-energy and the outage-energy tradeoffs of the developed schemes are provided and corroborated by simulations. Interesting insights on the aforementioned tradeoffs are obtained.
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paper_title: Harvest-Then-Cooperate: Wireless-Powered Cooperative Communications

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In this paper, we consider a wireless-powered cooperative communication network consisting of one hybrid access-point (AP), one source, and one relay. In contrast to conventional cooperative networks, the source and relay in the considered network have no embedded energy supply. They need to rely on the energy harvested from the signals broadcasted by the AP for their cooperative information transmission. Based on this three-node reference model, we propose a harvest-then-cooperate (HTC) protocol, in which the source and relay harvest energy from the AP in the downlink and work cooperatively in the uplink for the source's information transmission. Considering a delay-limited transmission mode, the approximate closed-form expression for the average throughput of the proposed protocol is derived over Rayleigh fading channels. Subsequently, this analysis is extended to the multi-relay scenario, where the approximate throughput of the HTC protocol with two popular relay selection schemes is derived. The asymptotic analyses for the throughput performance of the considered schemes at high signal-to-noise radio are also provided. All theoretical results are validated by numerical simulations. The impacts of the system parameters, such as time allocation, relay number, and relay position, on the throughput performance are extensively investigated.
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paper_title: Energy Harvesting Cooperative Networks: Is the Max-Min Criterion Still Diversity-Optimal?

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This paper considers a general energy harvesting cooperative network with M source-destination (SD) pairs and one relay, where the relay schedules only m user pairs for transmissions. For the special case of m = 1, the addressed scheduling problem is equivalent to relay selection for the scenario with one SD pair and M relays. In conventional cooperative networks, the max-min selection criterion has been recognized as a diversity-optimal strategy for relay selection and user scheduling. The main contribution of this paper is to show that the use of the max-min criterion will result in loss of diversity gains in energy harvesting cooperative networks. Particularly when only a single user is scheduled, analytical results are developed to demonstrate that the diversity gain achieved by the max-min criterion is only (M+1)/2, much less than the maximal diversity gain M. The max-min criterion suffers this diversity loss because it does not reflect the fact that the source-relay channels are more important than the relay-destination channels in energy harvesting networks. Motivated by this fact, a few user scheduling approaches tailored to energy harvesting networks are developed and their performance is analyzed. Simulation results are provided to demonstrate the accuracy of the developed analytical results and facilitate the performance comparison.
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paper_title: Simultaneous Information and Energy Transfer in Large-Scale Networks with/without Relaying

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Energy harvesting (EH) from ambient radio-frequency (RF) electromagnetic waves is an efficient solution for fully autonomous and sustainable communication networks. Most of the related works presented in the literature are based on specific (and small-scale) network structures, which although give useful insights on the potential benefits of the RF-EH technology, cannot characterize the performance of general networks. In this paper, we adopt a large-scale approach of the RF-EH technology and we characterize the performance of a network with random number of transmitter-receiver pairs by using stochastic-geometry tools. Specifically, we analyze the outage probability performance and the average harvested energy, when receivers employ power splitting (PS) technique for "simultaneous" information and energy transfer. A non-cooperative scheme, where information/energy are conveyed only via direct links, is firstly considered and the outage performance of the system as well as the average harvested energy are derived in closed form in function of the power splitting. For this protocol, an interesting optimization problem which minimizes the transmitted power under outage probability and harvesting constraints, is formulated and solved in closed form. In addition, we study a cooperative protocol where sources' transmissions are supported by a random number of potential relays that are randomly distributed into the network. In this case, information/energy can be received at each destination via two independent and orthogonal paths (in case of relaying). We characterize both performance metrics, when a selection combining scheme is applied at the receivers and a single relay is randomly selected for cooperative diversity.
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paper_title: Cooperative energy harvesting communications with relaying and energy sharing

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This paper considers two-hop communication networks where the transmitters harvest their energy in an intermittent fashion. In this network, communication is carried out by signal cooperation, i.e., relaying. Additionally, the transmitters have the option of transferring energy to one another, i.e., energy cooperation. Energy is partially lost during transfer, exposing a trade-off between energy cooperation and use of harvested energy for transmission. A multi-access relay model is considered and transmit power allocation and energy transfer policies that jointly maximize the sum-rate are found. It is shown that a class of power policies achieves the optimal sum-rate, allowing a separation of optimal energy transfer and optimal power allocation problems. The optimal energy transfer policy is shown to be an ordered node selection, where nodes with better energy transfer efficiency and worse channels transfer all their energy to the relay or other source nodes via the relay. For the special case of single source, the optimal policy requires the direction of energy transfer to remain unchanged unless either node depletes all of its energy. Overall, the findings provide the insight that cooperation of the source nodes by sharing energy with the relay node leads to them indirectly cooperating with each other, and that such cooperation can be carried out in a last-minute fashion.
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paper_title: Two-way and multiple-access energy harvesting systems with energy cooperation

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We study the capacity regions of two-way and multiple-access energy harvesting communication systems with one-way wireless energy transfer. In these systems, energy required for data transmission is harvested by the users from nature throughout the communication duration, and there is a separate unit that enables energy transfer from the first user to the second user with an efficiency of α. Energy harvests are known by the transmitters a priori. We first investigate the capacity region of the energy harvesting Gaussian two-way channel (TWC) with one-way energy transfer. We show that the boundary of the capacity region is achieved by a generalized two-dimensional directional water-filling algorithm. Then, we study the capacity region of the energy harvesting Gaussian multiple access channel (MAC) with one-way energy transfer. We show that if the priority of the first user is higher, then energy transfer is not needed. In addition, if the priority of the second user is sufficiently high, then the first user must transfer all of its energy to the second user.
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paper_title: Power Allocation Strategies in Energy Harvesting Wireless Cooperative Networks

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In this paper, a wireless cooperative network is considered, in which multiple source-destination pairs communicate with each other via an energy harvesting relay. The focus of this paper is on the relay's strategies to distribute the harvested energy among the multiple users and their impact on the system performance. Specifically, a non-cooperative strategy that uses the energy harvested from the i-th source as the relay transmission power to the i-th destination is considered first, and asymptotic results show that its outage performance decays as log SNR/SNR. A faster decay rate, 1/SNR, can be achieved by two centralized strategies proposed next, of which a water filling based one can achieve optimal performance with respect to several criteria, at the price of high complexity. An auction based power allocation scheme is also proposed to achieve a better tradeoff between system performance and complexity. Simulation results are provided to confirm the accuracy of the developed analytical results.
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paper_title: Time-Switching Uplink Network-Coded Cooperative Communication with Downlink Energy Transfer

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In this work, we consider a multiuser cooperative wireless network where the energy-constrained sources have independent information to transmit to a common destination, which is assumed to be externally powered and responsible for transferring energy wirelessly to the sources. The source nodes may cooperate, under either decode-and-forward or network coding-based protocols. Taking into account the fact that the energy harvested by the source nodes is a function of the fading realization of inter-user channels and user-destination channels, we obtain a closed-form approximation for the system outage probability, as well as an approximation for the optimal energy transfer period that minimizes such outage probability. It is also shown that, even though the achievable diversity order is reduced due to wireless energy transfer process, it is very close to t he one achieved for a network without energy constraints. Numerical results are also presented to validate the theoretical resu lts.
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paper_title: Optimum Performance Boundaries of OSTBC Based AF-MIMO Relay System With Energy Harvesting Receiver

paper_content:

This paper studies the optimum performance boundaries of a two-hop multi-antenna amplify-and-forward (AF) relay system with a multi-antenna energy harvesting (EH) receiver. The source and relay nodes employ orthogonal space-time block codes for data transmission. When instantaneous channel state information (CSI) is available, we design joint optimal source and relay precoders to achieve different tradeoffs between the energy and information transfers, which are characterized by the boundary of the rate-energy (R-E) region. For this purpose, the optimization problem is formulated as a relaxed convex problem but its optimality is confirmed with a proof that rank-one optimal precoders can always be obtained. As a consequence, it is shown that the full-rate OSTBC, like the Alamouti code, can be employed for an arbitrary number of antennas at the transmit nodes (source and relay) and support up to seven simultaneously existing EH receivers. When only second order statistics of the CSI is available, the tradeoff between outage probability (OP) and energy is characterized by the boundary of the OP-energy (OP-E) region. In this case, the precoder design problem is formulated using a convex upper bound approximation to the OP, since the exact OP expression is difficult for tractable optimization. Numerical results show that the OP-E region obtained with the upper bound of the OP is better than that with the approach based on maximization of the long-term average signal-to-noise ratio (SNR). The role of the different parameters such as average SNR, numbers of antennas, and spatial correlation on the boundaries of the R-E and OP-E regions is demonstrated via simulations.
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paper_title: Opportunistic Wireless Energy Harvesting in Cognitive Radio Networks

paper_content:

Wireless networks can be self-sustaining by harvesting energy from ambient radio-frequency (RF) signals. Recently, researchers have made progress on designing efficient circuits and devices for RF energy harvesting suitable for low-power wireless applications. Motivated by this and building upon the classic cognitive radio (CR) network model, this paper proposes a novel method for wireless networks coexisting where low-power mobiles in a secondary network, called secondary transmitters (STs), harvest ambient RF energy from transmissions by nearby active transmitters in a primary network, called primary transmitters (PTs), while opportunistically accessing the spectrum licensed to the primary network. We consider a stochastic-geometry model in which PTs and STs are distributed as independent homogeneous Poisson point processes (HPPPs) and communicate with their intended receivers at fixed distances. Each PT is associated with a guard zone to protect its intended receiver from ST's interference, and at the same time delivers RF energy to STs located in its harvesting zone. Based on the proposed model, we analyze the transmission probability of STs and the resulting spatial throughput of the secondary network. The optimal transmission power and density of STs are derived for maximizing the secondary network throughput under the given outage-probability constraints in the two coexisting networks, which reveal key insights to the optimal network design. Finally, we show that our analytical result can be generally applied to a non-CR setup, where distributed wireless power chargers are deployed to power coexisting wireless transmitters in a sensor network.
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paper_title: Dynamic Spectrum Access in Cognitive Radio Networks with RF Energy Harvesting

paper_content:

Spectrum efficiency and energy efficiency are two critical issues in designing wireless networks. Through dynamic spectrum access, cognitive radios can improve the spectrum efficiency and capacity of wireless networks. On the other hand radio frequency (RF) energy harvesting has emerged as a promising technique to supply energy to wireless networks and thereby increase their energy efficiency. Therefore, to achieve both spectrum and energy efficiencies, the secondary users in a cognitive radio network (CRN) can be equipped with the RF energy harvesting capability, and such a network can be referred to as an RF-powered cognitive radio network. In this article we provide an overview of the RF-powered CRNs and discuss the challenges that arise for dynamic spectrum access in these networks. Focusing on the trade-off among spectrum sensing, data transmission, and RF energy harvesting, we then discuss the dynamic channel selection problem in a multi-channel RF-powered CRN. In the RF-powered CRN a secondary user can adaptively select a channel to transmit data when the channel is not occupied by any primary user. Alternatively, the secondary user can harvest RF energy for data transmission if the channel is occupied. The optimal channel selection policy of the secondary user can be obtained by formulating a Markov decision process (MDP) problem. We present some numerical results obtained by solving this MDP problem.
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paper_title: Dynamic Spectrum Access in Cognitive Radio Networks with RF Energy Harvesting

paper_content:

Spectrum efficiency and energy efficiency are two critical issues in designing wireless networks. Through dynamic spectrum access, cognitive radios can improve the spectrum efficiency and capacity of wireless networks. On the other hand radio frequency (RF) energy harvesting has emerged as a promising technique to supply energy to wireless networks and thereby increase their energy efficiency. Therefore, to achieve both spectrum and energy efficiencies, the secondary users in a cognitive radio network (CRN) can be equipped with the RF energy harvesting capability, and such a network can be referred to as an RF-powered cognitive radio network. In this article we provide an overview of the RF-powered CRNs and discuss the challenges that arise for dynamic spectrum access in these networks. Focusing on the trade-off among spectrum sensing, data transmission, and RF energy harvesting, we then discuss the dynamic channel selection problem in a multi-channel RF-powered CRN. In the RF-powered CRN a secondary user can adaptively select a channel to transmit data when the channel is not occupied by any primary user. Alternatively, the secondary user can harvest RF energy for data transmission if the channel is occupied. The optimal channel selection policy of the secondary user can be obtained by formulating a Markov decision process (MDP) problem. We present some numerical results obtained by solving this MDP problem.
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paper_title: Optimal mode selection for cognitive radio sensor networks with RF energy harvesting

paper_content:

This paper investigates an optimal mode selection policy for cognitive radio sensor networks powered by RF energy harvesting. The RF energy harvesting enables the sensor node to operate with a potentially perpetual lifetime. We assume that the sensor node harvests RF energy received from the primary network and it cannot carry out RF energy harvesting and opportunistic spectrum access at the same time. Therefore, the sensor node should decide whether to access the spectrum or to harvest RF energy in each time slot to maximize an expected total throughput. We develop the optimal mode selection policy by casting this decision making problem in the framework of partially observable Markov decision process (POMDP). Numerical results show that the developed optimal policy finds a balance between obtaining the immediate throughput and harvesting the RF energy for future use.
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paper_title: Cognitive and Energy Harvesting-Based D2D Communication in Cellular Networks: Stochastic Geometry Modeling and Analysis

paper_content:

While cognitive radio enables spectrum-efficient wireless communication, radio frequency (RF) energy harvesting from ambient interference is an enabler for energy-efficient wireless communication. In this paper, we model and analyze cognitive and energy harvesting-based device-to-device (D2D) communication in cellular networks. The cognitive D2D transmitters harvest energy from ambient interference and use one of the channels allocated to cellular users (in uplink or downlink), which is referred to as the D2D channel, to communicate with the corresponding receivers. We investigate two spectrum access policies for cellular communication in the uplink or downlink, namely, random spectrum access (RSA) policy and prioritized spectrum access (PSA) policy. In RSA, any of the available channels including the channel used by the D2D transmitters can be selected randomly for cellular communication, while in PSA the D2D channel is used only when all of the other channels are occupied. A D2D transmitter can communicate successfully with its receiver only when it harvests enough energy to perform channel inversion toward the receiver, the D2D channel is free, and the signal-to-interference-plus-noise ratio $({\ssr SINR} ) $ at the receiver is above the required threshold; otherwise, an outage occurs for the D2D communication. We use tools from stochastic geometry to evaluate the performance of the proposed communication system model with general path-loss exponent in terms of outage probability for D2D and cellular users. We show that energy harvesting can be a reliable alternative to power cognitive D2D transmitters while achieving acceptable performance. Under the same ${\ssr SINR} $ outage requirements as for the non-cognitive case, cognitive channel access improves the outage probability for D2D users for both the spectrum access policies. When compared with the RSA policy, the PSA policy provides a better performance to the D2D users. Also, using an uplink channel provides improved performance to the D2D users in dense networks when compared to a downlink channel. For cellular users, the PSA policy provides almost the same outage performance as the RSA policy.
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paper_title: Opportunistic Channel Access and RF Energy Harvesting in Cognitive Radio Networks

paper_content:

Radio frequency (RF) energy harvesting is a promising technique to sustain operations of wireless networks. In a cognitive radio network, a secondary user can be equipped with RF energy harvesting capability. In this paper, we consider such a network where the secondary user can perform channel access to transmit a packet or to harvest RF energy when the selected channel is idle or occupied by the primary user, respectively. We present an optimization formulation to obtain the channel access policy for the secondary user to maximize its throughput. Both the case that the secondary user knows the current state of the channels and the case that the secondary knows the idle channel probabilities of channels in advance are considered. However, the optimization requires model parameters (e.g., the probability of successful packet transmission, the probability of successful RF energy harvesting, and the probability of channel to be idle) to obtain the policy. To obviate such a requirement, we apply an online learning algorithm that can observe the environment and adapt the channel access action accordingly without any a prior knowledge about the model parameters. We evaluate both the efficiency and convergence of the learning algorithm. The numerical results show that the policy obtained from the learning algorithm can achieve the performance in terms of throughput close to that obtained from the optimization.
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paper_title: Opportunistic Wireless Energy Harvesting in Cognitive Radio Networks

paper_content:

Wireless networks can be self-sustaining by harvesting energy from ambient radio-frequency (RF) signals. Recently, researchers have made progress on designing efficient circuits and devices for RF energy harvesting suitable for low-power wireless applications. Motivated by this and building upon the classic cognitive radio (CR) network model, this paper proposes a novel method for wireless networks coexisting where low-power mobiles in a secondary network, called secondary transmitters (STs), harvest ambient RF energy from transmissions by nearby active transmitters in a primary network, called primary transmitters (PTs), while opportunistically accessing the spectrum licensed to the primary network. We consider a stochastic-geometry model in which PTs and STs are distributed as independent homogeneous Poisson point processes (HPPPs) and communicate with their intended receivers at fixed distances. Each PT is associated with a guard zone to protect its intended receiver from ST's interference, and at the same time delivers RF energy to STs located in its harvesting zone. Based on the proposed model, we analyze the transmission probability of STs and the resulting spatial throughput of the secondary network. The optimal transmission power and density of STs are derived for maximizing the secondary network throughput under the given outage-probability constraints in the two coexisting networks, which reveal key insights to the optimal network design. Finally, we show that our analytical result can be generally applied to a non-CR setup, where distributed wireless power chargers are deployed to power coexisting wireless transmitters in a sensor network.
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paper_title: Information and Energy Cooperation in Cognitive Radio Networks

paper_content:

Cooperation between the primary and secondary systems can improve the spectrum efficiency in cognitive radio networks. The key idea is that the secondary system helps to boost the primary system's performance by relaying, and, in return, the primary system provides more opportunities for the secondary system to access the spectrum. In contrast to most of existing works that only consider information cooperation, this paper studies joint information and energy cooperation between the two systems, i.e., the primary transmitter sends information for relaying and feeds the secondary system with energy as well. This is particularly useful when the secondary transmitter has good channel quality to the primary receiver but is energy constrained. We propose and study three schemes that enable this cooperation. First, we assume there exists an ideal backhaul between the two systems for information and energy transfer. We then consider two wireless information and energy transfer schemes from the primary transmitter to the secondary transmitter using power splitting and time splitting energy harvesting techniques, respectively. For each scheme, the optimal and zero-forcing solutions are derived. Simulation results demonstrate promising performance gain for both systems due to the additional energy cooperation. It is also revealed that the power splitting scheme can achieve larger rate region than the time splitting scheme when the efficiency of the energy transfer is sufficiently large.
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paper_title: Dynamic Spectrum Access in Cognitive Radio Networks with RF Energy Harvesting

paper_content:

Spectrum efficiency and energy efficiency are two critical issues in designing wireless networks. Through dynamic spectrum access, cognitive radios can improve the spectrum efficiency and capacity of wireless networks. On the other hand radio frequency (RF) energy harvesting has emerged as a promising technique to supply energy to wireless networks and thereby increase their energy efficiency. Therefore, to achieve both spectrum and energy efficiencies, the secondary users in a cognitive radio network (CRN) can be equipped with the RF energy harvesting capability, and such a network can be referred to as an RF-powered cognitive radio network. In this article we provide an overview of the RF-powered CRNs and discuss the challenges that arise for dynamic spectrum access in these networks. Focusing on the trade-off among spectrum sensing, data transmission, and RF energy harvesting, we then discuss the dynamic channel selection problem in a multi-channel RF-powered CRN. In the RF-powered CRN a secondary user can adaptively select a channel to transmit data when the channel is not occupied by any primary user. Alternatively, the secondary user can harvest RF energy for data transmission if the channel is occupied. The optimal channel selection policy of the secondary user can be obtained by formulating a Markov decision process (MDP) problem. We present some numerical results obtained by solving this MDP problem.
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paper_title: Energy adaptive MAC protocol for wireless sensor networks with RF energy transfer

paper_content:

Recently, various energy harvesting techniques from ambient environments were proposed as alternative methods for powering sensor nodes, which convert the ambient energy from environments into electricity to power sensor nodes. However, those techniques are not applicable to the wireless sensor networks (WSNs) in the environment with no ambient energy source. To overcome this problem, an RF energy transfer method was proposed to power wireless sensor nodes. However, the RF energy transfer method also has a problem of unfairness among sensor nodes due to the significant difference between their energy harvesting rates according to their positions. In this paper, we propose a medium access control (MAC) protocol for WSNs based on RF energy transfer. The proposed MAC protocol adaptively manages the duty cycle of sensor nodes according to their the amount of harvested energy as well as the contention time of sensor nodes considering fairness among them. Through simulations, we show that our protocol can achieve a high degree of fairness, while maintaining duty cycle of sensor nodes appropriately according to the amount of their harvested energy.
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paper_title: Medium access control protocol design for sensors powered by wireless energy transfer

paper_content:

Wireless transfer of energy will help realize perennially operating sensors, where dedicated transmitters replenish the residual node battery level through directed radio frequency (RF) waves. However, as this radiative transfer is in-band, it directly impacts data communication in the network, requiring a fresh perspective on medium access control (MAC) protocol design for appropriately sharing the channel for these two critical functions. Through an experimental study, we first demonstrate how the placement, the chosen frequency, and number of the RF energy transmitters affect the sensor charging time. These studies are then used to design a MAC protocol called RFMAC that optimizes energy delivery to desirous sensor nodes on request. To the best of our knowledge, this is the first distributed MAC protocol for RF energy harvesting sensors, and through a combination of experimentation and simulation studies, we demonstrate 112% average network throughput improvement over the modified unslotted CSMA MAC protocol.
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paper_title: Prolonging Sensor Network Lifetime Through Wireless Charging

paper_content:

The emerging wireless charging technology is a promising alternative to address the power constraint problem in sensor networks. Comparing to existing approaches, this technology can replenish energy in a more controllable manner and does not require accurate location of or physical alignment to sensor nodes. However, little work has been reported on designing and implementing a wireless charging system for sensor networks. In this paper, we design such a system, build a proof-of-concept prototype, conduct experiments on the prototype to evaluate its feasibility and performance in small-scale networks, and conduct extensive simulations to study its performance in large-scale networks. Experimental and simulation results demonstrate that the proposed system can utilize the wireless charging technology effectively to prolong the network lifetime through delivering energy by a robot to where it is needed. The effects of various configuration and design parameters have also been studied, which may serve as useful guidelines in actual deployment of the proposed system in practice.
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paper_title: Routing and Link Layer Protocol Design for Sensor Networks with Wireless Energy Transfer

paper_content:

Wireless sensor networks are equipped with batteries with limited charge, and are often deployed in conditions that make their retrieval and replacement infeasible. Thus, energy conservation has been a primary consideration for protocol design for such networks. Recent advancements in the transfer of energy wirelessly over large distances, such as through radio frequency electromagnetic (EM) waves and magnetic coupling, may give rise to a new class of networks that allow the sensors to be charged on the field, thereby prolonging the network lifetime. Moreover, wireless charging though EM waves may be undertaken in the same unlicensed band as that used for communication, leading to several unique protocol design challenges for such a network. The contribution of this paper is threefold: First, a set of experiments is undertaken to investigate the effect of distance and location on the energy transfer through EM waves. Second, a new routing metric based on the charging ability of the sensor nodes is proposed. Finally, an optimization framework is developed to determine the optimal charging and transmission cycle for the sensor network, resulting in enhanced lifetime of the network under user-specified end-to-end constraints of throughput and latency.
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paper_title: How Wireless Power Charging Technology Affects Sensor Network Deployment and Routing

paper_content:

As wireless power charging technology emerges, some basic principles in sensor network design are changed accordingly. Existing sensor node deployment and data routing strategies cannot exploit wireless charging technology to minimize overall energy consumption. Hence, in this paper, we (a) investigate the impact of wireless charging technology on sensor network deployment and routing arrangement, (b) formalize the deployment and routing problem, (c) prove it as NP-complete, (d) develop heuristic algorithms to solve the problem, and (e) evaluate the performance of the solutions through extensive simulations. To the best of our knowledge, this is the first effort on adapting sensor network design to leverage wireless charging technology.
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paper_title: J-RoC: A Joint Routing and Charging scheme to prolong sensor network lifetime

paper_content:

The emerging wireless charging technology creates a controllable and perpetual energy source to provide wireless power over distance. Schemes have been proposed to make use of wireless charging to prolong the sensor network lifetime. Unfortunately, existing schemes only passively replenish sensors that are deficient in energy supply, and cannot fully leverage the strengths of this technology. To address the limitation, we propose J-RoC — a practical and efficient Joint Routing and Charging scheme. Through proactively guiding the routing activities in the network and delivering energy to where it is needed, J-RoC not only replenishes energy into the network but also effectively improves the network energy utilization, thus prolonging the network lifetime. To evaluate the performance of the J-RoC scheme, we conduct experiments in a small-scale testbed and simulations in large-scale networks. Evaluation results demonstrate that J-RoC significantly elongates the network lifetime compared to existing wireless charging based schemes.
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paper_title: Impact of mobile transmitter sources on radio frequency wireless energy harvesting

paper_content:

Wireless energy harvesting sensor networks constitute a new paradigm, where the motes deployed in the field are no longer constrained by the limited battery resource, but are able to re-charge themselves through directed electromagnetic energy transfer. The energy sources, whom we call actors, are mobile and move along pre-decided patterns while radiating an appropriate level of energy, sufficient enough to charge the sensors at an acceptable rate. This is the first work that investigates the impact of energy transfer, especially concerning the energy gain in the sensors, the energy spent by the actors, and the overall lifetime in the resulting mobile sensor-actor networks. We propose two event-specific mobility models, where the events occur at the centers of a Voronoi tessellation, and the actors move along either (i) the edges of the Voronoi cells, or (ii) directly from one event center to another. We undertake a comprehensive simulation based study using traces obtained from our experimental energy harvesting circuits powering Mica2 motes. Our results reveal several non-intuitive outcomes, and provide guidelines on which mobility model may be adopted based on the distribution of the events and actors.
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paper_title: Network Information Flow

paper_content:

We introduce a new class of problems called network information flow which is inspired by computer network applications. Consider a point-to-point communication network on which a number of information sources are to be multicast to certain sets of destinations. We assume that the information sources are mutually independent. The problem is to characterize the admissible coding rate region. This model subsumes all previously studied models along the same line. We study the problem with one information source, and we have obtained a simple characterization of the admissible coding rate region. Our result can be regarded as the max-flow min-cut theorem for network information flow. Contrary to one's intuition, our work reveals that it is in general not optimal to regard the information to be multicast as a "fluid" which can simply be routed or replicated. Rather, by employing coding at the nodes, which we refer to as network coding, bandwidth can in general be saved. This finding may have significant impact on future design of switching systems.
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paper_title: Wireless Energy Harvesting in Two-Way Network Coded Cooperative Communications: A Stochastic Approach for Large Scale Networks

paper_content:

In this letter, we study the performance of network coding (NC)-aided cooperative communications in large scale networks, where the relays are able to harvest energy emitted by wireless transmissions. In particular, we derive theoretical expressions for key network performance metrics, i.e., the probability of successful data exchange and the network lifetime gain. The proposed analytical expressions are verified via extensive Monte Carlo simulations, demonstrating the potential benefits of the energy harvested by the wireless transmissions.
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paper_title: How dangerous are mobile phones, transmission masts, and electricity pylons?

paper_content:

Electrical power and mobile communications deliver enormous benefit to society, but there are concerns whether the electric and magnetic field (EMF) emissions associated with the delivery of this benefit are linked to cancer or other health hazards. This article reviews the strength of the available epidemiological and laboratory evidence and notes that this falls short of what is normally required to establish a causal link. However, because of scientific uncertainty a cautious approach is often advocated, but here, too, there may be a tendency to judge these risks more harshly than those in other areas with similar strength of evidence.
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paper_title: Efficient Far-Field Radio Frequency Energy Harvesting for Passively Powered Sensor Networks

paper_content:

An RF-DC power conversion system is designed to efficiently convert far-field RF energy to DC voltages at very low received power and voltages. Passive rectifier circuits are designed in a 0.25 mum CMOS technology using floating gate transistors as rectifying diodes. The 36-stage rectifier can rectify input voltages as low as 50 mV with a voltage gain of 6.4 and operates with received power as low as 5.5 muW(22.6 dBm). Optimized for far field, the circuit operates at a distance of 44 m from a 4 W EIRP source. The high voltage range achieved at low load current make it ideal for use in passively powered sensor networks.
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Title: Wireless Networks With RF Energy Harvesting: A Contemporary Survey

Section 1: INTRODUCTION
Description 1: Introduce the rise in research interest in RF energy harvesting and its significance in powering energy-constrained wireless networks. Discuss various wireless energy transfer techniques, historical developments, and the motivation for this survey.

Section 2: OVERVIEW OF RF ENERGY HARVESTING NETWORKS
Description 2: Provide a comprehensive overview of the architecture, propagation models, energy harvesting techniques, and existing applications of RF-EHNs.

Section 3: CIRCUIT DESIGN
Description 3: Introduce the hardware circuit designs related to RF energy harvesting devices, including antenna design, matching network, rectifier, and receiver architecture design.

Section 4: MULTI-USER SCHEDULING
Description 4: Discuss multi-user scheduling in RF-EHNs to achieve the best utilization of resources under energy harvesting constraints, categorized into throughput fairness scheduling, throughput maximization scheduling, and utility optimization scheduling.

Section 5: RECEIVER OPERATION POLICY
Description 5: Review the receiver operation policies that handle various tradeoffs in the physical and MAC layers, with a focus on both time-switching and power-splitting architectures.

Section 6: MULTI-ANTENNA RF-EHNS
Description 6: Examine the use of multi-antenna and beamforming techniques to improve the efficiency of RF energy transfer and SWIPT, including beamforming for secure communication and energy beamforming designs.

Section 7: MULTI-HOP RF-EHNS
Description 7: Review cooperation strategies and relay operation policies in multi-hop RF-EHNs, including aspects such as relay selection and power allocation.

Section 8: RF-POWERED COGNITIVE RADIO NETWORKS
Description 8: Discuss the integration of RF energy harvesting in cognitive radio networks, focusing on dynamic spectrum access, spectrum sensing, spectrum management, and spectrum handoff.

Section 9: COMMUNICATION PROTOCOLS
Description 9: Highlight the design metrics for MAC and routing protocols in RF-EHNs, and review existing protocols to ensure QoS support and fairness for information transmissions.

Section 10: FUTURE DIRECTIONS AND PRACTICAL CHALLENGES
Description 10: Discuss open research issues and practical challenges in the field of RF energy harvesting, including distributed energy beamforming, interference management, energy trading, mobility, network coding, health impact, and practical limitations.

Section 11: CONCLUSION
Description 11: Summarize the survey's findings and emphasize the new opportunities as well as challenges brought by RF energy harvesting in wireless networks.