A Survey of Adaptive Carrier Sensing Mechanisms for IEEE 802.11 Wireless Networks
---
paper_title: Hidden Problems with the Hidden Node Problem

paper_content:

We discuss a few problems introduced by the RTS/CTS mechanism of collision avoidance and focus on the virtual jamming problem, which allows a malicious node to effectively jam a large fragment of a wireless network at a minimum expense of power. We propose a solution to this problem and provide experimental data illustrating the impact of virtual jamming and the effectiveness of our proposed solution
---
paper_title: Improving Spatial Reuse in Multihop Wireless Networks - A Survey

paper_content:

In multihop wireless ad-hoc networks, the medium access control (MAC) protocol plays a key role in coordinating the access to the shared medium among wireless nodes. Currently, the distributed coordination function (DCF) of the IEEE 802.11 is the dominant MAC protocol for both wireless LANs and wireless multihop ad hoc environment due to its simple implementation and distributed nature. The current access method of the IEEE 802.11 does not make efficient use of the shared channel due to its conservative approach in assessing the level of interference; this in turn affects the spatial reuse of the limited radio resources and highly affect the achieved throughput of a multihop wireless network. This paper surveys various methods that have been proposed in order to enhance the channel utilization by improving the spatial reuse.
---
paper_title: Differentiation mechanisms for IEEE 802.11

paper_content:

The IETF is currently working on service differentiation in the Internet. However, in wireless environments where bandwidth is scarce and channel conditions are variable, IP differentiated services are sub-optimal without lower layers' support. We present three service differentiation schemes for IEEE 802.11. The first one is based on scaling the contention window according to the priority of each flow or user. The second one assigns different inter-frame spacings to different users. Finally, the last one uses different maximum frame lengths for different users. We simulate and analyze the performance of each scheme with TCP and UDP flows.
---
paper_title: Analysis of the hidden terminal effect in multi-rate IEEE 802.11b networks

paper_content:

In this paper, we investigate the effect of hidden terminals and weak interferers in a multi-rate IEEE 802.11b system. We assume an ad-hoc network configuration, so that all the nodes operat e on the same portion of the ISM frequency band. Focusing on a pair of target nodes, we model the signal to noise and interference ratio (SNIR) at the intended receiver, as a function of the distance from the transmitter and from the potential interfering nodes. Hence, we derive the average nominal goodput in different conditions, both for the basic access and RTS/CTS modes. The present study opens a discussion on the efficacy of using RTS/CTS at high transmission rates. Keywords— WiFi, multi-rate, hidden node, interference, IEEE 802.11b.
---
paper_title: Revealing the problems with 802.11 medium access control protocol in multi-hop wireless ad hoc networks

paper_content:

The IEEE 802.11 medium access control (MAC) protocol is a standard for wireless LANs, it is also widely used in almost all test beds and simulations for the research in wireless mobile multi-hop ad hoc networks. However, this protocol was not designed for multi-hop networks. Although it can support some ad hoc network architecture, it is not intended to support the wireless mobile ad hoc network, in which multi-hop connectivity is one of the most prominent features. In this paper, we focus on the following question: can IEEE 802.11 MAC protocol function well in multihop networks? By presenting several serious problems encountered in transmission control protocol (TCP) connections in an IEEE 802.11 based multi-hop network, we show that the current TCP protocol does not work well above the current 802.11 MAC layer. The relevant problems include the TCP instability problem found in this kind of network, the severe unfairness problem, and the incompatibility problem. We illustrate that all these problems are rooted in the MAC layer. Furthermore, by revealing the in-depth cause of these problems, we conclude that the current version of this wireless LAN protocol does not function well in multi-hop ad hoc networks. We thus doubt whether the current WaveLAN based system is workable as a mobile multi-hop ad hoc test bed. All the results shown in this paper are based on NS2 simulations, and are compatible with the results from the OPNET simulations.
---
paper_title: Performance analysis of IEEE 802.11 CSMA/CA medium access control protocol

paper_content:

IEEE 802.11 (l-21 is thenew wireless network starndard which is going to be finished at the end of 1996. By use of two-dimension Markovian analysis, we derive the throughput and average delay of the MAC part of IEEE 802.11 to demostarte the efficiency of CA (collision avoidance) mechanism. Furthermore, we also focus on its performance with the existence of hidden terminals (9- lo).
---
paper_title: Wi-fi in ad hoc mode: a measurement study

paper_content:

In this paper we investigate the performance of IEEE 802.11b ad hoc networks by means of an experimental study. This analysis reveals several aspects that are usually neglected in simulation studies. Firstly, since different transmission rates are used for control and data frames, different transmission ranges and carrier-sensing ranges may exist at the same time in the network. In addition, the transmission ranges are in practice much shorter than usually assumed in simulation analysis, not constant but highly variable (even in the same session) and depends on several factors (i.e., mobile height, interference condition, etc.). Finally, exploiting our performance measurements, we present a channel model for an 802.11 network that indicates virtual carrier sensing is generally not necessary and the RTS/CTS mechanism only introduces additional overhead.
---
paper_title: Improving spatial reuse of IEEE 802.11 based ad hoc networks

paper_content:

In this paper, we evaluate and suggest methods to improve the performance of IEEE 802.11 based ad hoc networks from the perspective of spatial reuse. Since 802.11 employs virtual carrier sensing to reserve the medium prior to a packet transmission, the relative size of the spatial region it reserves for the impending traffic significantly affects the overall network performance. We show that the space reserved by 802.11 for a successful transmission is far from optimal and depending on the one hop distances between the sender and the receiver. We study three scenarios with very different spatial reuse characteristics. We also introduce a new quantitative measure, the spatial reuse index, to evaluate the efficiency of the medium reservation accomplished by 802.11 virtual carrier sensing. We also propose an improved virtual carrier sensing mechanism for wireless LAN scenarios and using analysis and simulation results, show that it can significantly increase the spatial reuse and network throughput.
---
paper_title: Tuning the carrier sensing range of IEEE 802.11 MAC

paper_content:

We investigate the effects of the carrier sensing range of the IEEE 802.11 multiple access control (MAC) scheme in this paper. Contrary to the simple and inaccurate cut-off circular collision model that is commonly used, we employ a more accurate collision model to realistically simulate MAC schemes in ad hoc networks. We argue that the carrier sensing range is a tunable parameter that can significantly affect the MAC performance in multihop ad hoc networks. An optimal carrier sensing range should balance the trade-off between the amount of spatial frequency reuse and the possibility of packet collisions. A reward formulation for the optimization of the carrier sensing range is presented. Extensive simulation results are provided to substantiate our study.
---
paper_title: Hidden Problems with the Hidden Node Problem

paper_content:

We discuss a few problems introduced by the RTS/CTS mechanism of collision avoidance and focus on the virtual jamming problem, which allows a malicious node to effectively jam a large fragment of a wireless network at a minimum expense of power. We propose a solution to this problem and provide experimental data illustrating the impact of virtual jamming and the effectiveness of our proposed solution
---
paper_title: Analysis of the hidden terminal effect in multi-rate IEEE 802.11b networks

paper_content:

In this paper, we investigate the effect of hidden terminals and weak interferers in a multi-rate IEEE 802.11b system. We assume an ad-hoc network configuration, so that all the nodes operat e on the same portion of the ISM frequency band. Focusing on a pair of target nodes, we model the signal to noise and interference ratio (SNIR) at the intended receiver, as a function of the distance from the transmitter and from the potential interfering nodes. Hence, we derive the average nominal goodput in different conditions, both for the basic access and RTS/CTS modes. The present study opens a discussion on the efficacy of using RTS/CTS at high transmission rates. Keywords— WiFi, multi-rate, hidden node, interference, IEEE 802.11b.
---
paper_title: Exploiting the capture effect for collision detection and recovery

paper_content:

In this paper we evaluate a technique to detect and recover messages from packet collisions by exploiting the capture effect. It can differentiate between collisions and packet loss and can identify the nodes involved in the collisions. This information is provided at virtually no extra cost and can produce significant improvements in existing collision mediation schemes. We characterize this technique using controlled collision experiments and evaluate it in real world flooding experiments on a 36-node sensor network.
---
paper_title: Exploiting the capture effect for collision detection and recovery

paper_content:

In this paper we evaluate a technique to detect and recover messages from packet collisions by exploiting the capture effect. It can differentiate between collisions and packet loss and can identify the nodes involved in the collisions. This information is provided at virtually no extra cost and can produce significant improvements in existing collision mediation schemes. We characterize this technique using controlled collision experiments and evaluate it in real world flooding experiments on a 36-node sensor network.
---
paper_title: Multi-Hop Ad Hoc Networks From Theory To Reality

paper_content:

We discuss ten years of research about multi-hop ad hoc networking. We try to summarize the main achievements and to point out the limits behind Mobile Ad hoc NETworks (MANET) research. This research has been carried out under the assumption that these networks are mainly used for large-scale general consumer applications, and nodes are ubiquitous, thus reasonably dense and active. Both assumptions are considerably far from reality and would certainly not be true in an initial phase of deployment. Lack of realism in MANET objectives coupled with a lack of realism during MANET design are the main causes that are running MANET towards a high risk of failing. Then we show that, while pure general-purpose MANETs do not yet happen in the real world, the multi-hop ad hoc networking paradigm has been successfully applied in several classes of networks that are penetrating the mass market. We take as examples mesh and opportunistic networks together with vehicular and sensor networks, where the multi-hop ad hoc paradigm is applied in a pragmatic way to extend the Internet, and/or to support well-defined application requirements. We oppose these successful areas of ad hoc networking to the lack of impacts of pure general-purpose MANETs, evidencing how a more pragmatic approach is winner.
---
paper_title: Leveraging multi-AP diversity for transmission resilience in wireless networks: architecture and performance analysis

paper_content:

With the increasing development of IEEE 802.11 based wireless local area network (WLAN) devices, large-scale multi-cell WLANs with a high density of users and access points (APs) have emerged widely in various hotspots. Providing resilient data transmission has been a primary challenge for scaling the WLANs because the high density of users and APs results in too many collisions. In this paper, we analyze and point out the defect of the single association mechanism defined in IEEE 802.11 on transmission reliability from a network perspective. Then, we propose a "multi-AP" architecture with which a MAC layer device called an AP controller (AC) is employed to enable each user to associate and cooperate with multiple APs. In this way, the users can benefit from the diversity effect of multipaths with independent collisions and transmission errors. This paper concentrates on the theoretical analysis of performance comparison between the proposed ldquoMulti-APrdquo architecture and that in IEEE 802.11. Extensive simulation results show that the proposed ldquomulti-APrdquo architecture can obtain much better performance in terms of the throughput per user and the total throughput, and the performance gain is position dependent. Moreover, the unfairness issue in traditional WLANs due to capture effect can be alleviated properly in the ldquomulti-APrdquo framework.
---
paper_title: Tuning the carrier sensing range of IEEE 802.11 MAC

paper_content:

We investigate the effects of the carrier sensing range of the IEEE 802.11 multiple access control (MAC) scheme in this paper. Contrary to the simple and inaccurate cut-off circular collision model that is commonly used, we employ a more accurate collision model to realistically simulate MAC schemes in ad hoc networks. We argue that the carrier sensing range is a tunable parameter that can significantly affect the MAC performance in multihop ad hoc networks. An optimal carrier sensing range should balance the trade-off between the amount of spatial frequency reuse and the possibility of packet collisions. A reward formulation for the optimization of the carrier sensing range is presented. Extensive simulation results are provided to substantiate our study.
---
paper_title: A stochastic model for optimizing physical carrier sensing and spatial reuse in wireless ad hoc networks

paper_content:

The choice of physical carrier sensing (PCS) threshold is key to the trade-off between the amount of spatial reuse and probability of packet collisions in a wireless ad hoc network. In this paper, we present a new analytical approach for optimizing the PCS threshold as measured by probability of packet collisions and the aggregate one-hop throughput. Our model simultaneously incorporates the impact of PCS threshold and the backoff mechanism via a suitable Markov chain model for saturation (i.e. all nodes always have a packet to send in their queues). Elaborate OPNET simulation results show the effectiveness of the analytical model
---
paper_title: Improving Spatial Reuse in Multihop Wireless Networks - A Survey

paper_content:

In multihop wireless ad-hoc networks, the medium access control (MAC) protocol plays a key role in coordinating the access to the shared medium among wireless nodes. Currently, the distributed coordination function (DCF) of the IEEE 802.11 is the dominant MAC protocol for both wireless LANs and wireless multihop ad hoc environment due to its simple implementation and distributed nature. The current access method of the IEEE 802.11 does not make efficient use of the shared channel due to its conservative approach in assessing the level of interference; this in turn affects the spatial reuse of the limited radio resources and highly affect the achieved throughput of a multihop wireless network. This paper surveys various methods that have been proposed in order to enhance the channel utilization by improving the spatial reuse.
---
paper_title: Performance analysis of CSMA and BTMA protocols in multihop networks: (I). Single channel case

paper_content:

Abstract Busy tone multiple access protocols have been used in multihop networks to reduce the effect of the hidden terminal problem. Due to complexity, the performance of these protocols for large networks has not been analyzed. In this paper, using a Markov chain model and an approximation, we are able to analyze and evaluate the throughput performance of the non-persistent CSMA protocol, the conservative busy tone multiple access (C-BTMA) protocol and the ideal destination-based busy tone multiple access (ID-BTMA) protocol for large networks. The throughput comparison of the protocols is given. The results show that in a large multihop network, the BTMA protocols have a better performance than the non-persistent CSMA protocol and the ID-BTMA protocol has a better performance than the C-BTMA protocol at light channel loads.
---
paper_title: Optimal Transmission Ranges for.Randomly Distributed Packet Radio Terminals

paper_content:

In multihop packet radio networks with randomly distributed terminals, the optimal transmission radii to maximize the expected progress of packets in desired directions are determined with a variety of transmission protocols and network configurations. It is shown that the FM capture phenomenon with slotted ALOHA greatly improves the expected progress over the system without capture due to the more limited area of possibly interfering terminals around the receiver. The (mini)slotted nonpersistent carrier-sense-multiple-access (CSMA) only slightly outperforms ALOHA, unlike the single-hop case (where a large improvement is available), because of a large area of "hidden" terminals and the long vulnerable period generated by them. As an example of an inhomogeneous terminal distribution, the effect of a gap in an otherwise randomly distributed terminal population on the expected progress of packets crossing the gap is considered. In this case, the disadvantage of using a large transmission radius is demonstrated.
---
paper_title: A scalable model for channel access protocols in multihop ad hoc networks

paper_content:

A new modeling framework is introduced for the analytical study of medium access control (MAC) protocols operating in multihop ad hoc networks. The model takes into account the effect of physical-layer parameters on the success of transmissions, the MAC protocol on the likelihood that nodes can access the channnel, and the connectivity of nodes in the network. A key feature of the model is that nodes can be modeled individually, i.e., it allows a per-node setup of many layer-specific parameters. Moreover, no spatial probability distribution or a particular arrangement of nodes is assumed; the model allows the computation of individual (per-node) performance metrics for any given network topology and radio channel model. To show the applicability of the modeling framework, we model multihop ad hoc networks using the IEEE 802.11 distributed coordination function and validate the results from the model with discrete-event simulations in Qualnet. The results show that our model predicts results that are very close to those attained by simulations, and requires seconds to complete compared to several hours of simulation time.
---
paper_title: Capacity Allocation in Multiple Access Networks

paper_content:

The theoretical issues of scheduling for capacity allocation in multiple access networks using in-band and out-of-band signaling are addressed. The analysis is carried out under the hypothesis that the "scheduling epochs" consist of bandwidth-dependent and bandwidthindependent components. The ultimate limitation on channel capacity rests with the bandwidth-independent component. It is shown that over the range of practical interest, scheduling using a separate side channel is superior to in-band scheduling in that, under similar conditions, the former attains a higher usable channel capacity.
---
paper_title: Opportunistic media access for multirate ad hoc networks

paper_content:

The IEEE 802.11 wireless media access standard supports multiple data rates at the physical layer. Moreover, various auto rate adaptation mechanisms at the medium access layer have been proposed to utilize this multi-rate capability by automatically adapting the transmission rate to best match the channel conditions. In this paper, we introduce the Opportunistic Auto Rate (OAR) protocol to better exploit durations of high-quality channels conditions. The key mechanism of the OAR protocol is to opportunistically send multiple back-to-back data packets whenever the channel quality is good. As channel coherence times typically exceed multiple packet transmission times for both mobile and non-mobile users, OAR achieves significant throughput gains as compared to state-of-the-art auto-rate adaptation mechanisms. Moreover, over longer time scales, OAR ensures that all nodes are granted channel access for the same time-shares as achieved by single-rate IEEE 802.11. We describe mechanisms to implement OAR on top of any existing auto-rate adaptation scheme in a nearly IEEE 802.11 compliant manner. We also analytically study OAR and characterize the gains in throughput as a function of the channel conditions. Finally, we perform an extensive set of ns-2 simulations to study the impact of such factors as node velocity, channel conditions, and topology on the throughput of OAR.
---
paper_title: Differentiation mechanisms for IEEE 802.11

paper_content:

The IETF is currently working on service differentiation in the Internet. However, in wireless environments where bandwidth is scarce and channel conditions are variable, IP differentiated services are sub-optimal without lower layers' support. We present three service differentiation schemes for IEEE 802.11. The first one is based on scaling the contention window according to the priority of each flow or user. The second one assigns different inter-frame spacings to different users. Finally, the last one uses different maximum frame lengths for different users. We simulate and analyze the performance of each scheme with TCP and UDP flows.
---
paper_title: Transmission of information

paper_content:

A quantitative measure of “information” is developed which is based on physical as contrasted with psychological considerations. How the rate of transmission of this information over a system is limited by the distortion resulting from storage of energy is discussed from the transient viewpoint. The relation between the transient and steady state viewpoints is reviewed. It is shown that when the storage of energy is used to restrict the steady state transmission to a limited range of frequencies the amount of information that can be transmitted is proportional to the product of the width of the frequency-range by the time it is available. Several illustrations of the application of this principle to practical systems are included. In the case of picture transmission and television the spacial variation of intensity is analyzed by a steady state method analogous to that commonly used for variations with time.
---
paper_title: On physical carrier sensing in wireless ad hoc networks

paper_content:

The aggregate throughput of a wireless ad hoc network depends on the channel capacity, channel utilization (i.e., the fraction of channel capacity used for generating good put), and the concurrent transmissions allowed in the network. While channel utilization is determined by MAC overhead, physical carrier sense has been used as an effective way to avoid interference and exploit spatial reuse. Prior research has attempted to identify the optimal carrier sense range that can maximize the aggregate throughput. However, the impact of MAC overhead has been ignored. In this paper, we use both an analytical model and simulation results to show that MAC overhead has significant impact on the choice of optimal carrier sense range. If MAC overhead is not taken into account properly in determining the optimal carrier sense range, the aggregate throughput can suffer a significant loss.
---
paper_title: A unified interference-collision analysis for power-aware adhoc networks

paper_content:

In this paper we address the issue of controlling transmission power in power-aware ad hoc networks. Previous work that minimizes the transmission power does not consider both the energy consumed in collision resolution and the energy disbursed to overcome the interference resulting from neighboring nodes. We investigate the basic transmission power control for the 802.11 MAC protocols in which the control frames and the data frames can be transmitted at different power levels. A collision model together with an interference model of a uniformly distributed network is constructed. Based on these models, the end-to-end network throughput and the total energy consumption of the network are examined. For a network with a given node density, our results show the optimal transmission power for control messages and for data messages that will yield maximum throughput and minimum energy consumption per message.
---
paper_title: Improving spatial reuse of IEEE 802.11 based ad hoc networks

paper_content:

In this paper, we evaluate and suggest methods to improve the performance of IEEE 802.11 based ad hoc networks from the perspective of spatial reuse. Since 802.11 employs virtual carrier sensing to reserve the medium prior to a packet transmission, the relative size of the spatial region it reserves for the impending traffic significantly affects the overall network performance. We show that the space reserved by 802.11 for a successful transmission is far from optimal and depending on the one hop distances between the sender and the receiver. We study three scenarios with very different spatial reuse characteristics. We also introduce a new quantitative measure, the spatial reuse index, to evaluate the efficiency of the medium reservation accomplished by 802.11 virtual carrier sensing. We also propose an improved virtual carrier sensing mechanism for wireless LAN scenarios and using analysis and simulation results, show that it can significantly increase the spatial reuse and network throughput.
---
paper_title: Does the IEEE 802.11 MAC protocol work well in multihop wireless ad hoc networks

paper_content:

The IEEE 802.11 MAC protocol is the standard for wireless LANs; it is widely used in testbeds and simulations for wireless multihop ad hoc networks. However, this protocol was not designed for multihop networks. Although it can support some ad hoc network architecture, it is not intended to support the wireless mobile ad hoc network, in which multihop connectivity is one of the most prominent features. In this article we focus on the following question: can the IEEE 802.11 MAC protocol function well in multihop networks? By presenting several serious problems encountered in an IEEE 802.11-based multihop network and revealing the in-depth cause of these problems, we conclude that the current version of this wireless LAN protocol does not function well in multihop ad hoc networks. We thus doubt whether the WaveLAN-based system is workable as a mobile ad hoc testbed.
---
paper_title: Adapting Physical Carrier Sensing to Maximize Spatial Reuse in 802.11 Mesh Networks

paper_content:

Spatial reuse in a mesh network can allow multiple communications to proceed simultaneously, hence proportionally improve the overall network throughput. To maximize spatial reuse, the MAC protocol must enable simultaneous transmitters to maintain the minimal separation distance that is sufficient to avoid interference. This paper demonstrates that physical carrier sensing enhanced with a tunable sensing threshold is effective at avoiding interference in 802.11 mesh networks without requiring the use of virtual carrier sensing. We present an analytical model for deriving the optimal sensing threshold given network topology, reception power and data rate. A distributed adaptive scheme is also presented to dynamically adjust the physical carrier sensing threshold based on periodic estimation of channel conditions in the network. Simulation results are shown for large-scale 802.1 1b and 802.11a networks to validate both the analytical model and the adaptation scheme. It is demonstrated that the enhanced physical carrier sensing mechanism effectively improves network throughput by maximizing the potential of spatial reuse. With dynamically tuned physical carrier sensing, the end to end throughput approaches 90% of the predicted theoretical upper-bound assuming a perfect MAC protocol, for a regular chain topology of 90 nodes.
---
paper_title: Tuning the carrier sensing range of IEEE 802.11 MAC

paper_content:

We investigate the effects of the carrier sensing range of the IEEE 802.11 multiple access control (MAC) scheme in this paper. Contrary to the simple and inaccurate cut-off circular collision model that is commonly used, we employ a more accurate collision model to realistically simulate MAC schemes in ad hoc networks. We argue that the carrier sensing range is a tunable parameter that can significantly affect the MAC performance in multihop ad hoc networks. An optimal carrier sensing range should balance the trade-off between the amount of spatial frequency reuse and the possibility of packet collisions. A reward formulation for the optimization of the carrier sensing range is presented. Extensive simulation results are provided to substantiate our study.
---
paper_title: Understanding the real-world performance of carrier sense

paper_content:

Carrier sense is a fundamental part of most wireless networking stacks in wireless local area- and sensor networks. As increasing numbers of users and more demanding applications push wireless networks to their capacity limits, the efficacy of the carrier sense mechanism becomes a key factor in determining wireless network capacity.We describe how carrier sense works, point out its limitations, and advocate an experimental approach to studying carrier sense. We describe our current testbed setup, and then present preliminary experimental results from both a 60-node sensor network deployment and a small-scale 802.11 deployment. Our preliminary results evaluate how well carrier sense works and expose its limitations.
---
paper_title: Adapting Physical Carrier Sensing to Maximize Spatial Reuse in 802.11 Mesh Networks

paper_content:

Spatial reuse in a mesh network can allow multiple communications to proceed simultaneously, hence proportionally improve the overall network throughput. To maximize spatial reuse, the MAC protocol must enable simultaneous transmitters to maintain the minimal separation distance that is sufficient to avoid interference. This paper demonstrates that physical carrier sensing enhanced with a tunable sensing threshold is effective at avoiding interference in 802.11 mesh networks without requiring the use of virtual carrier sensing. We present an analytical model for deriving the optimal sensing threshold given network topology, reception power and data rate. A distributed adaptive scheme is also presented to dynamically adjust the physical carrier sensing threshold based on periodic estimation of channel conditions in the network. Simulation results are shown for large-scale 802.1 1b and 802.11a networks to validate both the analytical model and the adaptation scheme. It is demonstrated that the enhanced physical carrier sensing mechanism effectively improves network throughput by maximizing the potential of spatial reuse. With dynamically tuned physical carrier sensing, the end to end throughput approaches 90% of the predicted theoretical upper-bound assuming a perfect MAC protocol, for a regular chain topology of 90 nodes.
---
paper_title: Spatial reuse in wireless ad-hoc networks

paper_content:

When investigating co-channel spatial reuse, typical studies on ad-hoc networks have adopted static, device-specific transmission/interference ranges regardless of the runtime environment. In this paper, we present an analytical framework to investigate co-channel spatial reuse in dense wireless ad-hoc networks based on RF propagation models for some common network topologies. We derive the minimum separation between simultaneous co-channel transmitters while maintaining desirable signal-noise-interference-ratio at receivers. Spatial reuse is characterized by the ratio between co-channel transmitter (T-T) and transmit-receiver (T-R) distances. Our results show that increasing transmission power improves spatial reuse in ambient noise dominated environments. However, in co-channel interference limited scenarios, increasing transmission power has little effect on spatial reuse. On the other hand, more simultaneous transmissions can be physically possible by decreasing the average T-R separation, independent of transmission power. Our results confirm that the popular practice of using static transmission/interference ranges leads to an oversimplified model that is unable to accurately characterize the spatial reuse (and consequently aggregate network capacity) in ad-hoc networks.
---
paper_title: Leveraging spatial reuse in 802.11 mesh networks with enhanced physical carrier sensing

paper_content:

Spatial reuse in a wireless network can allow multiple communications to proceed simultaneously, hence proportionally improve the overall network throughput. To maximize spatial reuse, the MAC protocol must enable simultaneous transmitters to maintain the minimal separation distance to avoid interference. This paper demonstrates that physical carrier sensing enhanced with tunable sensing threshold is effective at avoiding interference in 802.11 mesh networks without requiring the use of virtual carrier sensing through RTS/CTS. We present analytical model that demonstrates how to derive the optimal sensing threshold given reception power, data rate and network topology. Simulation results are shown for large-scale 802.11b networks to demonstrate that physical carrier sensing with the optimally tuned threshold improves network throughput by maximizing the potential of spatial reuse. In the case of a regular chain topology of 90 nodes, with tuned physical carrier sensing, the end-to-end throughput approaches 90% of the theoretical upper bound that assumes a perfect MAC protocol. Hence, without modifying the 802.11 MAC protocol, our enhanced physical carrier sensing mechanism effectively maximizes the potential of improving network throughput with spatial reuse.
---
paper_title: Adapting Physical Carrier Sensing to Maximize Spatial Reuse in 802.11 Mesh Networks

paper_content:

Spatial reuse in a mesh network can allow multiple communications to proceed simultaneously, hence proportionally improve the overall network throughput. To maximize spatial reuse, the MAC protocol must enable simultaneous transmitters to maintain the minimal separation distance that is sufficient to avoid interference. This paper demonstrates that physical carrier sensing enhanced with a tunable sensing threshold is effective at avoiding interference in 802.11 mesh networks without requiring the use of virtual carrier sensing. We present an analytical model for deriving the optimal sensing threshold given network topology, reception power and data rate. A distributed adaptive scheme is also presented to dynamically adjust the physical carrier sensing threshold based on periodic estimation of channel conditions in the network. Simulation results are shown for large-scale 802.1 1b and 802.11a networks to validate both the analytical model and the adaptation scheme. It is demonstrated that the enhanced physical carrier sensing mechanism effectively improves network throughput by maximizing the potential of spatial reuse. With dynamically tuned physical carrier sensing, the end to end throughput approaches 90% of the predicted theoretical upper-bound assuming a perfect MAC protocol, for a regular chain topology of 90 nodes.
---
paper_title: The capacity of wireless networks

paper_content:

When n identical randomly located nodes, each capable of transmitting at W bits per second and using a fixed range, form a wireless network, the throughput /spl lambda/(n) obtainable by each node for a randomly chosen destination is /spl Theta/(W//spl radic/(nlogn)) bits per second under a noninterference protocol. If the nodes are optimally placed in a disk of unit area, traffic patterns are optimally assigned, and each transmission's range is optimally chosen, the bit-distance product that can be transported by the network per second is /spl Theta/(W/spl radic/An) bit-meters per second. Thus even under optimal circumstances, the throughput is only /spl Theta/(W//spl radic/n) bits per second for each node for a destination nonvanishingly far away. Similar results also hold under an alternate physical model where a required signal-to-interference ratio is specified for successful receptions. Fundamentally, it is the need for every node all over the domain to share whatever portion of the channel it is utilizing with nodes in its local neighborhood that is the reason for the constriction in capacity. Splitting the channel into several subchannels does not change any of the results. Some implications may be worth considering by designers. Since the throughput furnished to each user diminishes to zero as the number of users is increased, perhaps networks connecting smaller numbers of users, or featuring connections mostly with nearby neighbors, may be more likely to be find acceptance.
---
paper_title: A stochastic model for optimizing physical carrier sensing and spatial reuse in wireless ad hoc networks

paper_content:

The choice of physical carrier sensing (PCS) threshold is key to the trade-off between the amount of spatial reuse and probability of packet collisions in a wireless ad hoc network. In this paper, we present a new analytical approach for optimizing the PCS threshold as measured by probability of packet collisions and the aggregate one-hop throughput. Our model simultaneously incorporates the impact of PCS threshold and the backoff mechanism via a suitable Markov chain model for saturation (i.e. all nodes always have a packet to send in their queues). Elaborate OPNET simulation results show the effectiveness of the analytical model
---
paper_title: Enhancing spatial reuse in ad hoc networks by carrier sense adaptation

paper_content:

The optimal choice of the carrier sense threshold for throughput maximization in a Carrier Sense Multiple Access ad hoc network is investigated. We show that the optimal threshold is linearly proportional to a function of the number of nodes and of the network size. When the path loss exponent is 2 (free space) the optimal threshold is linearly proportional to the node density. Given this finding, we develop a modification of the IEEE 802.11 protocol that performs a local and fully distributed adaptation of the threshold that yields about a 40% improvement in throughput, and also outperforms state-of-the-art protocols for CS adaptation.
---
paper_title: On Optimal Physical Carrier Sensing: Theoretical Analysis and Protocol Design

paper_content:

Traditional Physical Carrier Sensing (PCS), which aims at eliminating hidden terminals in wireless networks, causes too many exposed terminals and deteriorates the throughput per user seriously. Some existing work has proven that an aggressive PCS can improve the throughput by balancing the tradeoff between hidden terminals and exposed terminals. However, little work has been conducted to optimize the PCS according to the network conditions. In this paper, we develop an analytical model to study the behaviors of a user in the aggressive PCS scenario, with which the optimal PCS threshold can be derived. Then, to avoid the complicated computation, we present a heuristic algorithm, in which the parameters required for PCS tuning are estimated by the carrier sensing mechanism employed in IEEE 802.11. Simulation results show that the proposed heuristic algorithm can make the PCS threshold approach to the theoretical optimal one. Moreover, the proposed heuristic algorithm can obtain significant throughput gain compared to the traditional PCS threshold setting solutions.
---
paper_title: QoS-aware Adaptive Physical Carrier Sensing for Wireless Networks

paper_content:

Traditional physical carrier sensing (PCS) threshold setting, which aims at eliminating hidden terminals completely in wireless networks, brings too many exposed terminals and degrades the throughput per user seriously. Some existing work has proven that an aggressive PCS threshold can improve the throughput by balancing the tradeoff between the existence of hidden terminals and exposed terminals. However, the aggressive PCS results in high packet loss, which degrades the quality-of-service (QoS) seriously. To address this issue, in this paper we develop a QoS aware aggressive PCS threshold tuning algorithm, in which the PCS threshold is tuned dynamically according to the varying network conditions. One important feature of the proposed algorithm is that it can bound the packet loss in the region defined by QoS requirement. Extensive simulation results show that the proposed algorithm can bound the packet loss in the QoS requirement and at the same time obtain remarkable throughput gain compared to the traditional PCS threshold setting solutions.
---
paper_title: Kalman filter estimation of the number of competing terminals in an IEEE 802.11 network

paper_content:

Throughput performance of the IEEE 802.11 distributed coordination function (DCF) is very sensitive to the number n of competing stations. The contribute of this paper is threefold. First, we show that n can be expressed as function of the collision probability encountered on the channel; hence, it can be estimated based on run-time measurements. Second, we show that the estimation of n, based on exponential smoothing of the measured collision probability (specifically, an ARMA filter), results to be a biased estimation, with poor performance in terms of accuracy/tracking trade-offs. Third, we propose a methodology to estimate n, based on an extended Kalman filter coupled with a change detection mechanism. This approach shows both high accuracy as well as prompt reactivity to changes in the network occupancy status. Numerical results show that, although devised in the assumption of saturated terminals, our proposed approach results effective also in non-saturated conditions, and specifically in tracking the average number of competing terminals.
---
paper_title: Enhancing spatial reuse in ad hoc networks by carrier sense adaptation

paper_content:

The optimal choice of the carrier sense threshold for throughput maximization in a Carrier Sense Multiple Access ad hoc network is investigated. We show that the optimal threshold is linearly proportional to a function of the number of nodes and of the network size. When the path loss exponent is 2 (free space) the optimal threshold is linearly proportional to the node density. Given this finding, we develop a modification of the IEEE 802.11 protocol that performs a local and fully distributed adaptation of the threshold that yields about a 40% improvement in throughput, and also outperforms state-of-the-art protocols for CS adaptation.
---
paper_title: Throughput performance in multihop networks using adaptive carrier sensing threshold

paper_content:

Understanding the capacity of end-to-end throughput limits with any given wireless ad hoc network is critical to its deployment but very challenging task. This paper aims at conducting a quantitative analysis to get an expression for the upper bound for the throughput limits for regular network topologies like chain and grid networks. Factors such as carrier sense threshold that optimizes the throughput of such networks are examined. The results from the analysis will then be extrapolated to estimate closed-form solution for the throughput capacities of complex and random topologies.
---
paper_title: Kalman filter estimation of the number of competing terminals in an IEEE 802.11 network

paper_content:

Throughput performance of the IEEE 802.11 distributed coordination function (DCF) is very sensitive to the number n of competing stations. The contribute of this paper is threefold. First, we show that n can be expressed as function of the collision probability encountered on the channel; hence, it can be estimated based on run-time measurements. Second, we show that the estimation of n, based on exponential smoothing of the measured collision probability (specifically, an ARMA filter), results to be a biased estimation, with poor performance in terms of accuracy/tracking trade-offs. Third, we propose a methodology to estimate n, based on an extended Kalman filter coupled with a change detection mechanism. This approach shows both high accuracy as well as prompt reactivity to changes in the network occupancy status. Numerical results show that, although devised in the assumption of saturated terminals, our proposed approach results effective also in non-saturated conditions, and specifically in tracking the average number of competing terminals.
---
paper_title: Adaptive CSMA for Scalable Network Capacity in High-Density WLAN: A Hardware Prototyping Approach

paper_content:

Fast proliferation of 802.11 devices has led to the emergence of High-Density WLAN where 100s-1000s clients being serviced by 10s-100s of APs in a multi-cell environment. Scalable network capacity becomes a primary design challenge. In this paper, we investigate the impact of hidden/exposed terminal and the lack of capture effect on today's 802.11 devices on network throughput. We argue that the CSMA/CA parameters in 802.11 MAC protocol need to be adaptive to the changing network topology, traffic and interference. We propose a combination of Receiving-Sensitivity adaptation to reduce stronger-last collision and a Clear-Channel-Assessment adaptation to balance the hidden/exposed terminal problems. We take a hardware-centric approach to modify the 802.11 MAC protocol and prototype adaptation algorithms on Intel Centrino TM laptops. We demonstrate through testbed measure- ment that Receiving-Signal-Strength-Indicator and Packet-Error- Rate are effective and readily available metric on today's 802.11 hardware for the two adaptation algorithms. Our results show that the adaptation algorithms are able to react to the changing network environment and quickly converge to the new optimal value setting. Up to 300% throughput increase has been observed with the adaptation algorithms.
---
paper_title: CSMA Self-Adaptation Based on Interference Differentiation

paper_content:

This paper addresses the design challenge of interference mitigation in the emerging high density (HD) wireless LAN. It is proposed to differentiate interference according to their energy and timing relative to desired signal, and measure packet error rate (PER) locally at transmitter for each type of interference. Then, self-adaptation algorithms are designed to adjust a) clear channel assessment (CCA) threshold, aka physical carrier sensing threshold, to leverage spatial reuse for achieving higher aggregate throughput, and b) transmit power (TP) to compensate location difference among links, and prevent individual links from starving. Compared to an end-to-end (E2E) feedback loop, ours has negligible complexity and zero over-the-air overhead. Extensive OPNET simulations are used to compare the performance of our solutions against the legacy and the ideal.
---
paper_title: Simple and Effective Carrier Sensing Adaptation for Multi Rate Ad-Hoc MESH Networks

paper_content:

Adaptive Physical Carrier Sensing(PCS) based on tuning the PCS threshold has been shown to be an effective michanism for improving aggregate network throughput. However, earlier work [3] assumed a single link rate and a common PCS threshold for the entire network, as appropriate for a regular 2-D lattice grid of nodes with constant link (1-hop) distances. In an adhoc network topology, the 1-hop link distances vary significantly and a single PCS threshold is no longer suitable. Because IEEE 802.11 a/b/g networks provide multiple data rates over any link, joint tuning of the link rate and PCS threshold is thus desirable for achieving optimal aggregate throughput for ad-hoc networks. In this work, we first propose a simple yet effective principle for the above optimization. Next, we use the intuition offered by these formulations to suggest run-time adaptive solutions in OPNET simulations. We restrict ourselves to 1-dimensional random linear networks primarily to corroborate analysis with simulations and defer results with other topologies for future work.
---
paper_title: Optimizing Throughput with Carrier Sensing Adaptation for IEEE 802.11 Mesh Networks Based on Loss Differentiation

paper_content:

In high density (HD) mesh networks, packet losses can occur due to co-channel interference (asynchronous interference) or collisions (synchronous interference). In this paper, we first propose a novel method of estimating the probability of collision and interference statistically. Further, we integrate this differentiation method with physical carrier sensing adaptation in a novel centralized algorithm to improve the aggregate throughput in HD mesh network. Extensive simulations results show that the on-line algorithm approaches the optimal throughput predicted by analytical models.
---
paper_title: Optimizing 802.11 Wireless Mesh Networks Based on Physical Carrier Sensing

paper_content:

Multi-hop ad hoc networks suffer from the "hidden" and "exposed" node problems which diminish aggregate network throughput. While there are various approaches to mitigating these problems, in this work we focus exclusively on the role of physical carrier sensing (PCS). Specifically, tuning the PCS threshold leads to a trade-off between the hidden and exposed cases; reducing one typically increases the other, implying the existence of an optimal PCS threshold setting maximizes the aggregate network throughput. The contributions of this work are two-fold: i. We develop an analytical model to determine the optimal PCS threshold for a homogeneous network with constant link distances and show that setting the carrier sensing range close to the interference range is a robust close-to-optimal setting for network optimization in many scenarios. As an extension to more pragmatic network topologies with non-uniform link distances, a rate-to-link allocation scheme is proposed based on rendering the interference range equal for all links that allows a single carrier sense range to be used for the whole network, ii. The above suggests the need for on-line adaptation of tunable PCS threshold in general. The proposed algorithm is based on the key concept of loss differentiation (LD), which disambiguates the cause of packet loss event due to link layer interference (hidden terminals) and collisions respectively. Extensive simulation results show that the proposed PCS adaptations make the PCS threshold converge to its optimal value and thus outperform schemes without PCS adaptation.
---
paper_title: A Distributed Procedure for Carrier Sensing Threshold Adaptation in CSMA-Based Mobile Ad Hoc Networks

paper_content:

This paper discusses the problem of determining the carrier sense threshold (CSth) of nodes in a CSMA-based Mobile Ad Hoc Network (MANET) and proposes a practical, fully distributed, CSth adaptation procedure. The procedure enables a node to select its CSth in order to improve the data throughput of the system by maximizing the number of successful transmissions in its neighborhood. Simulation results show that the procedure is able to improve the data throughput in more than 85% of random topologies. The average throughput improvement over all topologies were 11% and 18% in two different scenarios, with some topologies showing up to 75% data throughput gains.
---
paper_title: Revealing the problems with 802.11 medium access control protocol in multi-hop wireless ad hoc networks

paper_content:

The IEEE 802.11 medium access control (MAC) protocol is a standard for wireless LANs, it is also widely used in almost all test beds and simulations for the research in wireless mobile multi-hop ad hoc networks. However, this protocol was not designed for multi-hop networks. Although it can support some ad hoc network architecture, it is not intended to support the wireless mobile ad hoc network, in which multi-hop connectivity is one of the most prominent features. In this paper, we focus on the following question: can IEEE 802.11 MAC protocol function well in multihop networks? By presenting several serious problems encountered in transmission control protocol (TCP) connections in an IEEE 802.11 based multi-hop network, we show that the current TCP protocol does not work well above the current 802.11 MAC layer. The relevant problems include the TCP instability problem found in this kind of network, the severe unfairness problem, and the incompatibility problem. We illustrate that all these problems are rooted in the MAC layer. Furthermore, by revealing the in-depth cause of these problems, we conclude that the current version of this wireless LAN protocol does not function well in multi-hop ad hoc networks. We thus doubt whether the current WaveLAN based system is workable as a mobile multi-hop ad hoc test bed. All the results shown in this paper are based on NS2 simulations, and are compatible with the results from the OPNET simulations.
---
paper_title: Adaptive Physical Carrier Sense in Topology-Controlled Wireless Networks

paper_content:

Transmit power and carrier sense threshold are key MAC/PHY parameters in carrier sense multiple access (CSMA) wireless networks. Transmit power control has been extensively studied in the context of topology control. However, the effect of carrier sense threshold on topology control has not been properly investigated in spite of its crucial role. Our key motivation is that the performance of a topology-controlled network may become worse than that of a network without any topology control unless carrier sense threshold is properly chosen. In order to remedy this deficiency of conventional topology control, we present a framework on how to incorporate physical carrier sense into topology control. We identify that joint control of transmit power and carrier sense threshold can be efficiently divided into topology control and carrier sense adaptation. We devise a distributed carrier sense update algorithm (DCUA), by which each node drives its carrier sense threshold toward a desirable operating point in a fully distributed manner. We derive a sufficient condition for the convergence of DCUA. To demonstrate the utility of integrating physical carrier sense into topology control, we equip a localized topology control algorithm, LMST, with the capability of DCUA. Simulation studies show that LMST-DCUA significantly outperforms LMST and the standard CSMA protocol.
---
paper_title: Design and analysis of an MST-based topology control algorithm

paper_content:

In this paper, we present a minimum spanning tree (MST) based topology control algorithm, called local minimum spanning tree (LMST), for wireless multi-hop networks. In this algorithm, each node builds its local minimum spanning tree independently and only keeps on-tree nodes that are one-hop away as its neighbors in the final topology. We analytically prove several important properties of LMST: (1) the topology derived under LMST preserves the network connectivity; (2) the node degree of any node in the resulting topology is bounded by 6; and (3) the topology can be transformed into one with bidirectional links (without impairing the network connectivity) after removal of all uni-directional links. These results are corroborated in the simulation study.
---
paper_title: Embracing wireless interference: analog network coding

paper_content:

Traditionally, interference is considered harmful. Wireless networks strive to avoid scheduling multiple transmissions at the same time in order to prevent interference. This paper adopts the opposite approach; it encourages strategically picked senders to interfere. Instead of forwarding packets, routers forward the interfering signals. The destination leverages network-level information to cancel the interference and recover the signal destined to it. The result is analog network coding because it mixes signals not bits. So, what if wireless routers forward signals instead of packets? Theoretically, such an approach doubles the capacity of the canonical 2-way relay network. Surprisingly, it is also practical. We implement our design using software radios and show that it achieves significantly higher throughput than both traditional wireless routing and prior work on wireless network coding.
---
paper_title: Combating Hidden and Exposed Terminal Problems in Wireless Networks

paper_content:

The hidden terminal problem is known to degrade the throughput of wireless networks due to collisions, while the exposed terminal problem results in poor performance by wasting valuable transmission opportunities. As a result, extensive research has been conducted to solve these two problems, such as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). However, CSMA-like protocols cannot solve both of these two problems at once. The fundamental reason lies in the fact that they cannot obtain accurate Channel Usage Information (CUI, who is transmitting or receiving nearby) with a low cost. To obtain additional CUI in a cost-efficient way, we propose a cross layer design, FAST (Full-duplex Attachment System). FAST contains a PHY layer Attachment Coding, which transmits control information independently on the air, without degrading the effective throughput of the original data traffic, and a MAC layer Attachment Sense, which utilizes the PHY layer control information to identify the hidden and exposed nodes in real time. We theoretically analyze the feasibility of the Attachment Coding, and then implement it on a GNU Radio testbed consisting of eight USRP2 nodes. We also conduct extensive simulations to evaluate the performance of FAST, and the experimental results show that FAST can effectively solve both the hidden and the exposed terminal problems, and improve the average throughput by up to 200% over CSMA in practical ad-hoc networks.
---
paper_title: Chip Error Pattern Analysis in IEEE 802.15.4

paper_content:

IEEE 802.15.4 standard specifies physical layer (PHY) and medium access control (MAC) sublayer protocols for low-rate and low-power communication applications. In this protocol, every 4-bit symbol is encoded into a sequence of 32 chips that are actually transmitted over the air. The 32 chips as a whole is also called a pseudonoise code (PN-Code). Due to complex channel conditions such as attenuation and interference, the transmitted PN-Code will often be received with some PN-Code chips corrupted. In this paper, we conduct a systematic analysis on these errors occurring at chip level. We find that there are notable error patterns corresponding to different cases. We then show that recognizing these patterns enables us to identify the channel condition in great details. We believe that understanding what happened to the transmission in our way can potentially bring benefit to channel coding, routing, and error correction protocol design. Finally, we propose Simple Rule, a simple yet effective method based on the chip error patterns to infer the link condition with an accuracy of over 96 percent in our evaluations.
---
paper_title: Fine-grained channel access in wireless LAN

paper_content:

Modern communication technologies are steadily advancing the physical layer (PHY) data rate in wireless LANs, from hundreds of Mbps in current 802.11n to over Gbps in the near future. As PHY data rates increase, however, the overhead of media access control (MAC) progressively degrades data throughput efficiency. This trend reflects a fundamental aspect of the current MAC protocol, which allocates the channel as a single resource at a time. This paper argues that, in a high data rate WLAN, the channel should be divided into separate subchannels whose width is commensurate with PHY data rate and typical frame size. Multiple stations can then contend for and use subchannels simultaneously according to their traffic demands, thereby increasing overall efficiency. We introduce FICA, a fine-grained channel access method that embodies this approach to media access using two novel techniques. First, it proposes a new PHY architecture based on OFDM that retains orthogonality among subchannels while relying solely on the coordination mechanisms in existing WLAN, carrier-sensing and broadcasting. Second, FICA employs a frequency-domain contention method that uses physical layer RTS/CTS signaling and frequency domain backoff to efficiently coordinate subchannel access. We have implemented FICA, both MAC and PHY layers, using a software radio platform, and our experiments demonstrate the feasibility of the FICA design. Further, our simulation results suggest FICA can improve the efficiency ratio of WLANs by up to 400% compared to existing 802.11.
---
paper_title: Adaptive Carrier Sensing and Packet Sending - An Alternative to Boost the Performance in Directional Communications

paper_content:

Despite the efforts to leverage the network performance through the use of Directional Communications, most protocols still rely on the basic mechanisms employed by the IEEE802.11 standards. While this might be desirable to facilitate compatibility, nodes empowered with directional antennas should be able to explore the benefits of directional communications whenever possible. With that in mind, we propose two mechanisms, termed Adaptive Carrier Sensing and Adaptive Packet Sending, aiming to allow nodes equipped with directional antennas to exploit the advantages of such systems to improve network performance.
---
paper_title: An adaptive MAC protocol for wireless ad hoc community network (WACNet) using electronically steerable passive array radiator antenna

paper_content:

In the context of ad hoc networks, use of directional antennas can largely reduce the radio interference, thereby improving the utilization of wireless medium and consequently the network throughput. However, in order to fully exploit the capability of directional antennas, whenever a source and a destination engage in a communication, all the neighbors of source and destination nodes should know the direction of communication so that they can initiate new communication in other directions, thus preventing interference with on-going data communication. In our adaptive MAC protocol, each node keeps certain neighborhood information dynamically so that each node can keep a track of the direction of communication events going on in its neighborhood at that instant of time. Moreover, appropriate mechanisms for null steering of directional antennas in user terminals can help exchange the neighborhood information in the presence of on-going communication and can drastically improve the medium utilization through overlapping communications in different directions.
---
paper_title: Medium access control protocols using directional antennas in ad hoc networks

paper_content:

Using directional antennas can be beneficial for wireless ad hoc networks consisting of a collection of wireless hosts. To best utilize directional antennas, a suitable medium access control (MAC) protocol must be designed. Current MAC protocols, such as the IEEE 802.11 standard, do not benefit when using directional antennas, because these protocols have been designed for omnidirectional antennas. In this paper, we attempt to design new MAC protocols suitable for ad hoc networks based on directional antennas.
---
paper_title: A MAC protocol for mobile ad hoc networks using directional antennas

paper_content:

We propose a medium access control (MAC) protocol for an ad hoc network of mobile wireless terminals that are equipped with multiple directional antennas. Use of directional antennas in ad hoc networks can largely reduce the radio interference, thereby improving the packet throughput. However, the main problem of using directional antennas in such networks is due to the dynamic nature of the network caused by frequent node movements. This gives rise to problems such as locating and tracking during random channel access. The MAC protocol presented in this paper proposes a solution to these problems without the help of additional hardware. Results obtained from detailed computer simulations demonstrate the performance improvement obtained with the proposed scheme.
---
paper_title: Delivering the Benefits of Directional Communications for Ad Hoc Networks Through an Efficient Directional MAC Protocol

paper_content:

It is known that wireless ad hoc networks employing omnidirectional communications suffer from poor network throughput and inefficient spatial reuse. Although directional communications are expected to provide significant improvements in this respect, efficient Medium Access Control (MAC) protocols capable of harvesting the benefits of directional communications in the context of ad hoc networks are yet to be seen. In this work we propose a MAC scheme aiming to provide efficient means to explore the transmission range extension and improved spatial division provided by directional antennas. In order to measure the potentiality of our MAC protocol, we have extensively evaluated its performance in terms of average throughput and end-to-end delay under single and multi-hop communication scenarios. The performance evaluation results indicate that our protocol is highly efficient, particularly with increasing number of communications and data rates.
---
paper_title: Deafness: a MAC problem in ad hoc networks when using directional antennas

paper_content:

This work addresses deafness - a problem that appears when MAC protocols are designed using directional antennas. Briefly, deafness is caused when a transmitter fails to communicate to its intended receiver, because the receiver is beamformed towards a direction away from the transmitter. Existing CSMA/CA protocols rely on the assumption that congestion is the predominant cause of communication failure, and adopt backoff schemes to handle congestion. While this may be appropriate for omnidirectional antennas, for directional antennas, both deafness and congestion can be the reason for communication failures. An appropriate directional MAC protocol needs to classify the actual cause of failure, and react accordingly. This paper quantifies the impact of deafness on directional medium access control, and proposes a tone-based mechanism as one way of addressing deafness. The tone-based mechanism, ToneDMAC, assumes congestion as the default reason for communication failures, and applies a corrective measure whenever the cause is deafness. Simulation results indicate that ToneDMAC can alleviate deafness, and perform better than existing directional MAC protocols.
---
paper_title: A busy-tone based directional MAC protocol for ad hoc networks

paper_content:

In mobile wireless ad hoc networking environments, such as the future combat system (FCS), the shared wireless communication medium is an inherently limited resource and is collision prone. In this paper, we propose to adapt the dual busy tone multiple access (DBTMA) protocol for use with directional antennas, which further increases effective channel capacity. In contrast to other directional antenna based MAC protocols, our protocol, termed DBTMA/DA, is capable of reserving channel capacity in finer grain without relying on extra locationing support. A simulation study is performed to demonstrate the better network performance of DBTMA/DA over DBTMA and the IEEE 802.11a MAC protocols.
---
paper_title: Adapting Physical Carrier Sensing to Maximize Spatial Reuse in 802.11 Mesh Networks

paper_content:

Spatial reuse in a mesh network can allow multiple communications to proceed simultaneously, hence proportionally improve the overall network throughput. To maximize spatial reuse, the MAC protocol must enable simultaneous transmitters to maintain the minimal separation distance that is sufficient to avoid interference. This paper demonstrates that physical carrier sensing enhanced with a tunable sensing threshold is effective at avoiding interference in 802.11 mesh networks without requiring the use of virtual carrier sensing. We present an analytical model for deriving the optimal sensing threshold given network topology, reception power and data rate. A distributed adaptive scheme is also presented to dynamically adjust the physical carrier sensing threshold based on periodic estimation of channel conditions in the network. Simulation results are shown for large-scale 802.1 1b and 802.11a networks to validate both the analytical model and the adaptation scheme. It is demonstrated that the enhanced physical carrier sensing mechanism effectively improves network throughput by maximizing the potential of spatial reuse. With dynamically tuned physical carrier sensing, the end to end throughput approaches 90% of the predicted theoretical upper-bound assuming a perfect MAC protocol, for a regular chain topology of 90 nodes.
---
paper_title: IEEE802.11k enabled adaptive physical carrier sense mechanism for wireless networks (K-APCS)

paper_content:

The existence of hidden and exposed nodes can have a significantly negative impact on the performance of IEEE802.11 networks. Such nodes can increase the probability of collisions and limit the spatial reuse on the channel. The value of the Physical Carrier Sensing Threshold (PCST) is instrumental to the trade-off between the number of hidden and exposed nodes in a system. This paper presents a new adaptive physical carrier sensing mechanism for wireless networks. The K-APCS algorithm maximizes the aggregate system throughput by optimizing the balance between the spatial reuse and the collision rate of a system. It incorporates 802.11k radio resource measurements and a frame loss bound on each node to enable the on-line tuning of the PCST. Simulations were performed on five variants of carrier sensing mechanisms; results show that K-APCS achieves the maximum spatial reuse with a throughput gain of 28% and the minimum collision rate with a decrease of 0.01%.
---
paper_title: Adaptive CSMA for Scalable Network Capacity in High-Density WLAN: A Hardware Prototyping Approach

paper_content:

Fast proliferation of 802.11 devices has led to the emergence of High-Density WLAN where 100s-1000s clients being serviced by 10s-100s of APs in a multi-cell environment. Scalable network capacity becomes a primary design challenge. In this paper, we investigate the impact of hidden/exposed terminal and the lack of capture effect on today's 802.11 devices on network throughput. We argue that the CSMA/CA parameters in 802.11 MAC protocol need to be adaptive to the changing network topology, traffic and interference. We propose a combination of Receiving-Sensitivity adaptation to reduce stronger-last collision and a Clear-Channel-Assessment adaptation to balance the hidden/exposed terminal problems. We take a hardware-centric approach to modify the 802.11 MAC protocol and prototype adaptation algorithms on Intel Centrino TM laptops. We demonstrate through testbed measure- ment that Receiving-Signal-Strength-Indicator and Packet-Error- Rate are effective and readily available metric on today's 802.11 hardware for the two adaptation algorithms. Our results show that the adaptation algorithms are able to react to the changing network environment and quickly converge to the new optimal value setting. Up to 300% throughput increase has been observed with the adaptation algorithms.
---
paper_title: Adaptive Carrier-Sensing for Throughput Improvement in IEEE 802.11 Networks

paper_content:

As a Carrier Sense Multiple Access (CSMA) network, the performance of IEEE 802.11 networks highly depends on the accuracy of the carrier sensing procedure. However, conventional carrier sensing approaches suffer from the well known hidden and exposed node problems, adversely affecting aggregate throughput of the IEEE 802.11 networks. In this paper, we propose a novel scheme through which each station can adaptively select its Carrier Sense Threshold (CST) in order to mitigate the hidden/exposed node problems. The basic idea behind our approach is for the Access Point (AP) to periodically transmit a Busy/Idle (BI) signal to all the stations. Individual stations then use the BI signal from the AP together with their own local BI signal in order to adjust their CST. We use NS-2 simulations to show that our approach can enhance the aggregate throughput by as much as 50%.
---
paper_title: Improving spatial reuse of IEEE 802.11 based ad hoc networks

paper_content:

In this paper, we evaluate and suggest methods to improve the performance of IEEE 802.11 based ad hoc networks from the perspective of spatial reuse. Since 802.11 employs virtual carrier sensing to reserve the medium prior to a packet transmission, the relative size of the spatial region it reserves for the impending traffic significantly affects the overall network performance. We show that the space reserved by 802.11 for a successful transmission is far from optimal and depending on the one hop distances between the sender and the receiver. We study three scenarios with very different spatial reuse characteristics. We also introduce a new quantitative measure, the spatial reuse index, to evaluate the efficiency of the medium reservation accomplished by 802.11 virtual carrier sensing. We also propose an improved virtual carrier sensing mechanism for wireless LAN scenarios and using analysis and simulation results, show that it can significantly increase the spatial reuse and network throughput.
---
paper_title: IEEE802.11k enabled adaptive carrier sense management mechanism (KAPCS2)

paper_content:

The existence of hidden and exposed nodes can have a significant negative impact on the performance of IEEE802.11 networks. Such nodes can increase the probability of collisions and limit the spatial reuse on the channel. The value of the Physical Carrier Sensing Threshold (PCST) is instrumental in the presence of such nodes in a system. This paper presents a new adaptive physical carrier sensing management algorithm for wireless networks. Our KAPCS2 management system maximizes the aggregate system throughput by identifying and addressing problem nodes. It incorporates 802.11k Radio Resource Measurements (RRM) on each node to enable on-line tuning of the PCST.
---
paper_title: How Physical Carrier Sense Affects System Throughput in IEEE 802.11 Wireless Networks

paper_content:

In recent years, wireless ad hoc networks have become increasingly popular in both military and civilian applications due to their capability of building networks without the need for a pre-existing infrastructure. While a number of studies have been carried out to study the performance of IEEE 802.11 DCF in single-cell wireless LANs, the analysis of IEEE 802.11 DCF in multi-hop wireless networks with consideration of the effects of physical carrier sense, SINR, and collision caused by accumulative interference has been lacking. In this paper, we substantially extend Cali's work and rigorously model, with all these effects considered, channel activities governed by IEEE 802.11 DCF in multi-hop wireless networks. We show that as in single-cell WLANs, the choice of the contention window size can impact the system throughput in multi-hop wireless networks. However, the optimal value of the contention window size is quite different. Moreover, the optimal attempt probability p (the optimal window size) derived for multi-hop wireless networks with respect to maximizing the system throughput is larger (smaller) than that for single-cell WLANs. We also show that, given the minimal SINR threshold beta, the optimal carrier sense range is smaller than the conventional value used (provided that the contention window size is tuned accordingly).
---
paper_title: Local estimation of collision probabilities in 802.11 WLANs: An experimental study

paper_content:

Current 802.11 networks do not typically achieve the maximum potential throughput despite link adaptation and cross-layer optimization techniques designed to alleviate many causes of packet loss. A primary contributing factor is the difficulty in distinguishing between various causes of packet loss, including collisions caused by high network use, co-channel interference from neighboring networks, and errors due to poor channel conditions. In previous work, we used NS-2 simulations to show that estimating various components of loss probability such as direct collisions, staggered collisions, and physical layer errors, can be used to improve the throughput of 802.11 networks via link adaptation, carrier sense threshold adaptation, and MAC layer packet length adaptation. We have also proposed a method to estimate the various components of loss probability by comparing channel occupancy at a station with that of its access point. In this paper, we use Ath5k open source wireless card driver in an experimental testbed in order to experimentally verify the accuracy of our previously proposed approach to estimating collision probability. We show that our proposed methodology accurately estimates overall collision probability to within 5%. This experimental verification demonstrates the feasibility of our collision probability estimation approach and the resulting throughput gains in practice.
---
paper_title: Local Estimation of Probabilities of Direct and Staggered Collisions in 802.11 WLANs

paper_content:

Current 802.11 networks do not typically achieve the maximum potential throughput despite link adaptation and cross-layer optimization techniques designed to alleviate many causes of packet loss. A primary contributing factor is the difficulty in distinguishing between various causes of packet loss, including collisions caused by high network use, co-channel interference from neighboring networks, and errors due to poor channel conditions. In this paper, we propose a novel method for estimating various collision type probabilities locally at a given node of an 802.11 network. Our approach is based on combining locally observable quantities with information observed and broadcast by the access point (AP) in order to obtain partial spatial information about the network traffic. We provide a systematic assessment and definition of the different types of collision, and show how to approximate each of them using only local and AP information. Additionally, we show how to approximate the sensitivity of these probabilities to key related configuration parameters including carrier sense threshold and packet length. We verify our methods through NS-2 simulations, and characterize estimation accuracy of each of the considered collision types.
---
paper_title: Distributed Physical Carrier Sensing Adaptation Scheme in Cooperative MAP WLAN

paper_content:

Recently a multiple access point (MAP) architecture is proposed for wireless local area networks (WLAN) to exploit the spatial diversity by permitting each user to associate with multiple APs. Benefited from multiple association, the promising cooperative communication technology can be utilized to further enhance the spatial diversity gain. However, cooperative transmission results in low spatial reuse efficiency with traditional physical carrier sensing (PCS) mechanism. That is because multiple cooperative nodes transmit simultaneously, which causes interference to the adjacent nodes and prevents their transmissions. Therefore the PCS range should be decreased aggressively to support more parallel cooperative transmissions. In this paper, a distributed PCS tuning scheme is proposed to enhance the spatial reuse efficiency in cooperative MAP WLAN. A cooperative node selection scheme is jointly designed to avoid the excessive interference caused by cooperative nodes transmission. Simulation results show that the proposed scheme significantly improves the throughput of cooperative MAP WLAN.
---
paper_title: A cooperative MAC protocol with virtual-antenna array support in a multi-AP WLAN system

paper_content:

In this paper we propose a cooperative-aware medium access control (MAC) protocol in the newly emerged multiple access point (MAP) WLAN system, where each user can associate with multiple APs. Leveraging the multiple association feature, the spectrum efficient virtual-antenna array (VA) cooperative strategy can be utilized. However, the APs selected for VA transmission cannot serve packet transmission individually, which leads to spatial reuse inefficiency. To balance the tradeoff between spatial reuse efficiency and cooperative gain, an interference model that describes interference among cooperative transmissions should be first established. In this paper, a virtual link model is proposed, where each virtual link is composed of one end user, one combination of VA APs and one data rate to represent one cooperative transmission. Based on this model, a multi-cell virtual link scheduling problem is formulated to achieve optimal system performance. Combined with local clique searching procedure and physical carrier sensing adaptation scheme, the dual decomposition of the scheduling problem can distributively achieve the near optimal performance. Finally a VA-based cooperative MAC (V-MAC) protocol is proposed to implement the cooperative scheduling scheme. Simulation result shows V-MAC significantly improves the system throughput meanwhile guarantees the system fairness.
---
paper_title: A Framework for Distributed Spatio-Temporal Communications in Mobile Ad Hoc Networks

paper_content:

Space-time communications can help combat fading and hence can significantly increase the capacity of ad hoc networks. Cooperative diversity or virtual antenna arrays facilitate spatio-temporal communications without actually requiring the deployment of physical antenna arrays. Virtual MISO entails the simultaneous transmission of appropriately encoded information by multiple nodes to effectively emulate a transmission on an antenna array. We present a novel multi-layer approach for exploiting virtual MISO links in ad hoc networks. The approach spans the physical, medium access control and routing layers and provides: (a) a significant improvement in the end-to-end performance in terms of throughput and delay and, (b) robustness to mobility and interference induced link failures. The key physical layer property that we exploit is an increased transmission range due to achieved the diversity gain. Except for space-time signal processing capabilities, our design does not require any additional hardware. We perform extensive simulations to quantify the benefits of our approach using virtual MISO links. As compared to using only SISO links, we achieve an increase of up to 150% in terms of the end-to-end throughput and a decrease of up to 75% in the incurred end-to-end delay. Our results also demonstrate a reduction in the route discovery attempts due to link failures by up to 60%, a direct consequence of the robustness that our approach provides to link failures.
---
paper_title: Broadcast reception rates and effects of priority access in 802.11-based vehicular ad-hoc networks

paper_content:

One key usage of VANET is to support vehicle safety applications. This use case is characterized by the prominence of broadcasts in scaled settings. In this context, we try to answer the following questions: i) what is the probability of reception of a broadcast message by another car depending on its distance to the sender, ii) how to give priority access and an improved reception rate for important warnings, e.g., sent out in an emergency situation, and iii) how are the above two results affected by signal strength fluctuations caused by radio channel fading? We quantify via simulation the probability of reception for the two-ray-ground propagation model as well as for the Nakagami distribution in saturated environments. By making use of some IEEE 802.11e EDCA mechanisms for priority access, we do not only quantify how channel access times can be reduced but also demonstrate how improved reception rates can be achieved. Our results show that the mechanisms for priority access are successful under the two-way-ground model. However, with a non-deterministic radio propagation model like Nakagami's distribution the benefit is still obvious but the general level of probability of reception is much smaller compared to two-ray-ground model. The results indicate that -- particularly for safety-critical and sensor network type of applications -- the proper design of repetition or multi-hop retransmission strategies represents an important aspect of future work for robustness and network stability of vehicular ad hoc networks.
---
paper_title: Effects of wireless physical layer modeling in mobile ad hoc networks

paper_content:

In most studies on mobile ad hoc networks (MANET), simulation models are used for the evaluation of devices and protocols. Typically, such simulations focus on the specific higher layer protocols that are being proposed, and tend to ignore details of models at other layers, particularly the interactions with physical layer models. In this paper, we present the set of factors at the physical layer that are relevant to the performance evaluations of higher layer protocols. Such factors include signal reception, path loss, fading, interference and noise computation, and preamble length. We start the discussion with the comparisions of physical layer models in ns-2 and GloMoSim, two commonly used simulators for MANET studies, and then quantify the impact of the preceding factors under typical scenarios used for the performance evaluation of wireless ad hoc routing protocols. Our experimental results show that the factors at the physical layer not only affect the absolute performance of a protocol, but because their impact on different protocols is non-uniform, it can even change the relative ranking among protocols for the same scenari
---
paper_title: Experimental evaluation of wireless simulation assumptions

paper_content:

All analytical and simulation research on ad~hoc wireless networks must necessarily model radio propagation using simplifying assumptions. We provide a comprehensive review of six assumptions that are still part of many ad hoc network simulation studies, despite increasing awareness of the need to represent more realistic features, including hills, obstacles, link asymmetries, and unpredictable fading. We use an extensive set of measurements from a large outdoor routing experiment to demonstrate the weakness of these assumptions, and show how these assumptions cause simulation results to differ significantly from experimental results. We close with a series of recommendations for researchers, whether they develop protocols, analytic models, or simulators for ad~hoc wireless networks.
---
paper_title: A framework for investigating the performances of wireless ad hoc networks

paper_content:

In previous studies, unrealistic interference models have been taken to investigate the performances of wireless ad hoc networks. In this paper, based on realistic interference model, we propose a framework to investigate the performances of wireless ad hoc networks. This framework regards the given network and the settings as input and models the interferences in wireless networks using transmission relation graph (TRG). Then the framework can compute a precise performance value for the given network. This framework can be used to investigate the optimum carrier sensing range, the spatial usage, the optimum power level, and so on.
---
paper_title: On the accuracy of MANET simulators

paper_content:

The deployment of wireless applications or protocols in the context of Mobile Ad-hoc NETworks (MANETs), often requires to step through a simulation phase. For the results of the simulation to be meaningful, it is important that the model on which is based the simulator matches as closely as possible the reality. In this paper we present the simulation results of a straightforward algorithm using several popular simulators (OPNET Modeler, NS-2, GloMoSim). The results tend to show that significant divergences exist between the simulators. This can be explained partly by the mismatching of the modelisation of each simulator and also by the different levels of detail provided to implement and configure the simulated scenarios.
---
paper_title: A stochastic model for optimizing physical carrier sensing and spatial reuse in wireless ad hoc networks

paper_content:

The choice of physical carrier sensing (PCS) threshold is key to the trade-off between the amount of spatial reuse and probability of packet collisions in a wireless ad hoc network. In this paper, we present a new analytical approach for optimizing the PCS threshold as measured by probability of packet collisions and the aggregate one-hop throughput. Our model simultaneously incorporates the impact of PCS threshold and the backoff mechanism via a suitable Markov chain model for saturation (i.e. all nodes always have a packet to send in their queues). Elaborate OPNET simulation results show the effectiveness of the analytical model
---
paper_title: Effects of wireless physical layer modeling in mobile ad hoc networks

paper_content:

In most studies on mobile ad hoc networks (MANET), simulation models are used for the evaluation of devices and protocols. Typically, such simulations focus on the specific higher layer protocols that are being proposed, and tend to ignore details of models at other layers, particularly the interactions with physical layer models. In this paper, we present the set of factors at the physical layer that are relevant to the performance evaluations of higher layer protocols. Such factors include signal reception, path loss, fading, interference and noise computation, and preamble length. We start the discussion with the comparisions of physical layer models in ns-2 and GloMoSim, two commonly used simulators for MANET studies, and then quantify the impact of the preceding factors under typical scenarios used for the performance evaluation of wireless ad hoc routing protocols. Our experimental results show that the factors at the physical layer not only affect the absolute performance of a protocol, but because their impact on different protocols is non-uniform, it can even change the relative ranking among protocols for the same scenari
---
paper_title: Simulation validation using direct execution of wireless Ad-Hoc routing protocols

paper_content:

Computer simulation is the most common approach to studying wireless ad-hoc routing algorithms. The results, however, are only as good as the models the simulation uses. One should not underestimate the importance of validation, as inaccurate models can lead to wrong conclusions. In this paper, we use direct-execution simulation to validate radio models used by ad-hoc routing protocols, against real-world experiments. This paper documents a common testbed that supports direct execution of a set of ad-hoc routing protocol implementations in a wireless network simulator. The testbed reads traces generated from real experiments, and uses them to drive direct-execution implementations of the routing protocols. Doing so we reproduce the same network conditions as in real experiments. By comparing routing behavior measured in real experiments with behavior computed by the simulation, we are able to validate the models of radio behavior upon which protocol behavior depends. We conclude that it is possible to have fairly accurate results using a simple radio model, but the routing behavior is quite sensitive to one of this model's parameters. The implication is that one should: i) use a more complex radio model that explicitly models point-to-point path loss; or ii) use measurements from an environment typical of the one of interest; or iii) study behavior over a range of environments to identify sensitivities.
---
paper_title: Performance modeling of asynchronous data transfer methods of IEEE 802.11 MAC protocol

paper_content:

To satisfy the needs of wireless data networking, study group 802.11 was formed under IEEE project 802 to recommend an international standard for Wireless Local Area Networks (WLANs). A key part of standard are the Medium Access Control (MAC) protocol needed to support asynchronous and time bounded delivery of data frames. It has been proposed that unslotted Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) be the basis for the IEEE 802.11 WLAN MAC protocols. We conduct performance evaluation of the asynchronous data transfer protocols that are a part of the proposed IEEE 802.11 standard taking into account the decentralized nature of communication between stations, the possibility of “capture”, and presence of “hidden” stations. We compute system throughput and evaluate fairness properties of the proposed MAC protocols. Further, the impact of spatial characteristics on the performance of the system and that observed by individual stations is determined. A comprehensive comparison of the access methods provided by the 802.11 MAC protocol is done and observations are made as to when each should be employed. Extensive numerical and simulation results are presented to help understand the issues involved.
---
paper_title: Adaptive Physical Carrier Sense in Topology-Controlled Wireless Networks

paper_content:

Transmit power and carrier sense threshold are key MAC/PHY parameters in carrier sense multiple access (CSMA) wireless networks. Transmit power control has been extensively studied in the context of topology control. However, the effect of carrier sense threshold on topology control has not been properly investigated in spite of its crucial role. Our key motivation is that the performance of a topology-controlled network may become worse than that of a network without any topology control unless carrier sense threshold is properly chosen. In order to remedy this deficiency of conventional topology control, we present a framework on how to incorporate physical carrier sense into topology control. We identify that joint control of transmit power and carrier sense threshold can be efficiently divided into topology control and carrier sense adaptation. We devise a distributed carrier sense update algorithm (DCUA), by which each node drives its carrier sense threshold toward a desirable operating point in a fully distributed manner. We derive a sufficient condition for the convergence of DCUA. To demonstrate the utility of integrating physical carrier sense into topology control, we equip a localized topology control algorithm, LMST, with the capability of DCUA. Simulation studies show that LMST-DCUA significantly outperforms LMST and the standard CSMA protocol.
---
paper_title: Leveraging spatial reuse in 802.11 mesh networks with enhanced physical carrier sensing

paper_content:

Spatial reuse in a wireless network can allow multiple communications to proceed simultaneously, hence proportionally improve the overall network throughput. To maximize spatial reuse, the MAC protocol must enable simultaneous transmitters to maintain the minimal separation distance to avoid interference. This paper demonstrates that physical carrier sensing enhanced with tunable sensing threshold is effective at avoiding interference in 802.11 mesh networks without requiring the use of virtual carrier sensing through RTS/CTS. We present analytical model that demonstrates how to derive the optimal sensing threshold given reception power, data rate and network topology. Simulation results are shown for large-scale 802.11b networks to demonstrate that physical carrier sensing with the optimally tuned threshold improves network throughput by maximizing the potential of spatial reuse. In the case of a regular chain topology of 90 nodes, with tuned physical carrier sensing, the end-to-end throughput approaches 90% of the theoretical upper bound that assumes a perfect MAC protocol. Hence, without modifying the 802.11 MAC protocol, our enhanced physical carrier sensing mechanism effectively maximizes the potential of improving network throughput with spatial reuse.
---
paper_title: Physical Carrier Sensing and Spatial Reuse in Multirate and Multihop Wireless Ad Hoc Networks

paper_content:

Physical carrier sensing is an effective mechanism of medium access control (MAC) protocols to reduce collisions in wireless networks, and the size of the carrier sensing range has a great impact on the system performance. Previous studies have shown that the MAC layer overhead plays an important role in determining the optimal carrier sensing range. However, variable transmission ranges and receiver sensitivities for different channel rates and the impact of multihop forwarding have been ignored. In this paper, we investigate the impacts of these factors as well as several other important factors, such as SINR (signal to interference plus noise ratio), node topology, hidden/exposed terminal problems and bidirectional handshakes, on determining the optimum carrier sensing range to maximize the throughput through both analysis and simulations. The results show that if any one of these factors is not addressed properly, the system performance may suffer a significant degradation. Furthermore, considering both multirate capability and carrier sensing ranges, we propose to use bandwidth distance product as a routing metric, which improves end-to-end throughput by up to 27% in the simulated scenario.
---
paper_title: On physical carrier sensing in wireless ad hoc networks

paper_content:

The aggregate throughput of a wireless ad hoc network depends on the channel capacity, channel utilization (i.e., the fraction of channel capacity used for generating good put), and the concurrent transmissions allowed in the network. While channel utilization is determined by MAC overhead, physical carrier sense has been used as an effective way to avoid interference and exploit spatial reuse. Prior research has attempted to identify the optimal carrier sense range that can maximize the aggregate throughput. However, the impact of MAC overhead has been ignored. In this paper, we use both an analytical model and simulation results to show that MAC overhead has significant impact on the choice of optimal carrier sense range. If MAC overhead is not taken into account properly in determining the optimal carrier sense range, the aggregate throughput can suffer a significant loss.
---
paper_title: Tuning the carrier sensing range of IEEE 802.11 MAC

paper_content:

We investigate the effects of the carrier sensing range of the IEEE 802.11 multiple access control (MAC) scheme in this paper. Contrary to the simple and inaccurate cut-off circular collision model that is commonly used, we employ a more accurate collision model to realistically simulate MAC schemes in ad hoc networks. We argue that the carrier sensing range is a tunable parameter that can significantly affect the MAC performance in multihop ad hoc networks. An optimal carrier sensing range should balance the trade-off between the amount of spatial frequency reuse and the possibility of packet collisions. A reward formulation for the optimization of the carrier sensing range is presented. Extensive simulation results are provided to substantiate our study.
---
paper_title: Interplay of Spatial Reuse and SINR-Determined Data Rates in CSMA/CA-Based, Multi-Hop, Multi-Rate Wireless Networks

paper_content:

In CSMA/CA-based, multi-hop, multi-rate wireless networks, spatial reuse can be increased by tuning the carrier-sensing threshold (Tcs) to reduce the carrier sense range (dcs). While reducing dcs enables more concurrent transmissions, the transmission quality suffers from the increased accumulative interference contributed by concurrent transmissions outside dcs. As a result, the data rate at which the transmission can sustain may decrease. How to balance the interplay of spatial reuse and transmission quality (and hence the sustainable data rate) so as to achieve high network capacity is thus an important issue. In this paper, we investigate this issue by extending Cali's model and devising an analytical model that characterizes the transmission activities as governed by IEEE 802.11 DCF in a single-channel, multi-rate, multi-hop wireless network. The systems throughput is derived as a function of Tcs, SINR, beta, and other PHY/MAC systems parameters. We incorporate the effect of varying the degree of spatial reuse by tuning the Tcs. Based on the physical radio propagation model, we theoretically estimate the potential accumulated interference contributed by concurrent transmissions and the corresponding SINR. For a given SINR value, we then determine an appropriate data rate at which a transmission can sustain. To the best of our knowledge, this is perhaps the first effort that considers tuning of PHY characteristics (transmit power and data rates) and MAC parameters (contention backoff timer) jointly in an unified framework in order to optimize the overall network throughput. Analytical results indicate that the systems throughput is not a monotonically increasing/decreasing function of Tcs, but instead exhibits transitional points where several possible choices of Tcs can be made. In addition, the network capacity can be further improved by choosing the backoff timer values appropriately.
---
paper_title: IEEE 802.11 protocol: design and performance evaluation of an adaptive backoff mechanism

paper_content:

In WLANs, the medium access control (MAC) protocol is the main element that determines the efficiency of sharing the limited communication bandwidth of the wireless channel. The fraction of channel bandwidth used by successfully transmitted messages gives a good indication of the protocol efficiency, and its maximum value is referred to as protocol capacity. In a previous paper we have derived the theoretical limit of the IEEE 802.11 MAC protocol capacity. In addition, we showed that if a station has an exact knowledge of the network status, it is possible to tune its backoff algorithm to achieve a protocol capacity very close to its theoretical bound. Unfortunately, in a real case, a station does not have an exact knowledge of the network and load configurations (i.e., number of active stations and length of the message transmitted on the channel) but it can only estimate it. In this work we analytically study the performance of the IEEE 802.11 protocol with a dynamically tuned backoff based on the estimation of the network status. Results obtained indicate that under stationary traffic and network configurations (i.e., constant average message length and fixed number of active stations), the capacity of the enhanced protocol approaches the theoretical limits in all the configurations analyzed. In addition, by exploiting the analytical model, we investigate the protocol performance in transient conditions (i.e., when the number of active stations sharply changes).
---
paper_title: Experimental evaluation of wireless simulation assumptions

paper_content:

All analytical and simulation research on ad~hoc wireless networks must necessarily model radio propagation using simplifying assumptions. We provide a comprehensive review of six assumptions that are still part of many ad hoc network simulation studies, despite increasing awareness of the need to represent more realistic features, including hills, obstacles, link asymmetries, and unpredictable fading. We use an extensive set of measurements from a large outdoor routing experiment to demonstrate the weakness of these assumptions, and show how these assumptions cause simulation results to differ significantly from experimental results. We close with a series of recommendations for researchers, whether they develop protocols, analytic models, or simulators for ad~hoc wireless networks.
---
paper_title: Improving spatial reuse through tuning transmit power, carrier sense threshold, and data rate in multihop wireless networks

paper_content:

The importance of spatial reuse in wireless ad-hoc networks has been long recognized as a key to improving the network capacity. One can increase the level of spatial reuse by either reducing the transmit power or increasing the carrier sense threshold (thereby reducing the carrier sense range). On the other hand, as the transmit power decreases or the carrier sense threshold increases, the SINR decreases as a result of the smaller received signal or the increased interference level. Consequently, the data rate sustained by each transmission may decrease. This leads naturally to the following questions:(1)How can the trade-off between the increased level of spatial reuse and the decreased data rate each node can sustain be quantified? In other words,is there an optimal range of transmit power/carrier sense threshold in which the network capacity is maximized? (2)What is the relation between the transmit power and the carrier sense threshold.In this paper, we study both problems, and show that (i)in the case that the achievable channel rate follows the Shannon capacity, spatial reuse depends only on the ratio of the transmit power to the carrier sense threshold; and (ii) in the case that only a set of discrete data rates are available, tuning the transmit power offers several advantages that tuning the carrier sense threshold cannot, provided that there is a sufficient number of power levels available. Based on the findings, we then propose a decentralized power and rate control algorithm to enable each node to adjust, based on its signal interference level, its transmit power and data rate. The transmit power is so determined that the transmitter can sustain a high data rate, while keeping the adverse interference effect on the other neighboring concurrent transmissions minimal. Simulation results have shown that, as compared to existing carrier sense threshold tuning algorithms, the proposed power and rate control algorithm yields higher network capacity.
---
paper_title: Performance Analysis of IEEE 802.11 MAC Protocols in Wireless LANs

paper_content:

Summary IEEE 802.11 MAC protocol is the de facto standard for wireless local area networks (LANs), and has also been implemented in many network simulation packages for wireless multi-hop ad hoc networks. However, it is well known that, as the number of active stations increases, the performance of IEEE 802.11 MAC in terms of delay and throughput degrades dramatically, especially when each station’s load approaches its saturation state. To explore the inherent problems in this protocol, it is important to characterize the probability distribution of the packet service time at the MAC layer. In this paper, by modeling the exponential backoff process as a Markov chain, we can use the signal transfer function of the generalized state transition diagram to derive an approximate probability distribution of the MAC layer service time. We then present the discrete probability distribution for MAC layer packet service time, which is shown to accurately match the simulation data from network simulations. Based on the probability model for the MAC layer service time, we can analyze a few performance metrics of the wireless LAN and give better explanation to the performance degradation in delay and throughput at various traffic loads. Furthermore, we demonstrate that the exponential distribution is a good approximation model for the MAC layer service time for the queueing analysis, and the presented queueing models can accurately match the simulation data obtained from ns-2 when the arrival process at MAC layer is Poissonian. Copyright # 2004 John Wiley & Sons, Ltd.
---
paper_title: Does the IEEE 802.11 MAC protocol work well in multihop wireless ad hoc networks

paper_content:

The IEEE 802.11 MAC protocol is the standard for wireless LANs; it is widely used in testbeds and simulations for wireless multihop ad hoc networks. However, this protocol was not designed for multihop networks. Although it can support some ad hoc network architecture, it is not intended to support the wireless mobile ad hoc network, in which multihop connectivity is one of the most prominent features. In this article we focus on the following question: can the IEEE 802.11 MAC protocol function well in multihop networks? By presenting several serious problems encountered in an IEEE 802.11-based multihop network and revealing the in-depth cause of these problems, we conclude that the current version of this wireless LAN protocol does not function well in multihop ad hoc networks. We thus doubt whether the WaveLAN-based system is workable as a mobile ad hoc testbed.
---
paper_title: Adapting Physical Carrier Sensing to Maximize Spatial Reuse in 802.11 Mesh Networks

paper_content:

Spatial reuse in a mesh network can allow multiple communications to proceed simultaneously, hence proportionally improve the overall network throughput. To maximize spatial reuse, the MAC protocol must enable simultaneous transmitters to maintain the minimal separation distance that is sufficient to avoid interference. This paper demonstrates that physical carrier sensing enhanced with a tunable sensing threshold is effective at avoiding interference in 802.11 mesh networks without requiring the use of virtual carrier sensing. We present an analytical model for deriving the optimal sensing threshold given network topology, reception power and data rate. A distributed adaptive scheme is also presented to dynamically adjust the physical carrier sensing threshold based on periodic estimation of channel conditions in the network. Simulation results are shown for large-scale 802.1 1b and 802.11a networks to validate both the analytical model and the adaptation scheme. It is demonstrated that the enhanced physical carrier sensing mechanism effectively improves network throughput by maximizing the potential of spatial reuse. With dynamically tuned physical carrier sensing, the end to end throughput approaches 90% of the predicted theoretical upper-bound assuming a perfect MAC protocol, for a regular chain topology of 90 nodes.
---
paper_title: A scalable model for channel access protocols in multihop ad hoc networks

paper_content:

A new modeling framework is introduced for the analytical study of medium access control (MAC) protocols operating in multihop ad hoc networks. The model takes into account the effect of physical-layer parameters on the success of transmissions, the MAC protocol on the likelihood that nodes can access the channnel, and the connectivity of nodes in the network. A key feature of the model is that nodes can be modeled individually, i.e., it allows a per-node setup of many layer-specific parameters. Moreover, no spatial probability distribution or a particular arrangement of nodes is assumed; the model allows the computation of individual (per-node) performance metrics for any given network topology and radio channel model. To show the applicability of the modeling framework, we model multihop ad hoc networks using the IEEE 802.11 distributed coordination function and validate the results from the model with discrete-event simulations in Qualnet. The results show that our model predicts results that are very close to those attained by simulations, and requires seconds to complete compared to several hours of simulation time.
---
paper_title: Spatial reuse in wireless ad-hoc networks

paper_content:

When investigating co-channel spatial reuse, typical studies on ad-hoc networks have adopted static, device-specific transmission/interference ranges regardless of the runtime environment. In this paper, we present an analytical framework to investigate co-channel spatial reuse in dense wireless ad-hoc networks based on RF propagation models for some common network topologies. We derive the minimum separation between simultaneous co-channel transmitters while maintaining desirable signal-noise-interference-ratio at receivers. Spatial reuse is characterized by the ratio between co-channel transmitter (T-T) and transmit-receiver (T-R) distances. Our results show that increasing transmission power improves spatial reuse in ambient noise dominated environments. However, in co-channel interference limited scenarios, increasing transmission power has little effect on spatial reuse. On the other hand, more simultaneous transmissions can be physically possible by decreasing the average T-R separation, independent of transmission power. Our results confirm that the popular practice of using static transmission/interference ranges leads to an oversimplified model that is unable to accurately characterize the spatial reuse (and consequently aggregate network capacity) in ad-hoc networks.
---
paper_title: Selecting transmit powers and carrier sense thresholds in CSMA protocols for wireless ad hoc networks

paper_content:

Carrier sense multiple access (CSMA) protocols, such as the IEEE 802.11 protocol, are widely used in wireless ad hoc networks. In this paper, we consider how the parameters of such protocols should be selected. Our primary goal is an increase in spatial reuse. By analyzing conditions for collision prevention, we argue that the product of the transmit power and the carrier sense threshold should remain constant. We then discuss the practical implementation of this principle within the framework of the IEEE 802.11 protocol. Finally, we provide simulation results that compare our scheme to other methods of selecting the protocol parameters. These results indicate that an increase in spatial reuse can indeed be achieved.
---
paper_title: Opportunistic media access for multirate ad hoc networks

paper_content:

The IEEE 802.11 wireless media access standard supports multiple data rates at the physical layer. Moreover, various auto rate adaptation mechanisms at the medium access layer have been proposed to utilize this multi-rate capability by automatically adapting the transmission rate to best match the channel conditions. In this paper, we introduce the Opportunistic Auto Rate (OAR) protocol to better exploit durations of high-quality channels conditions. The key mechanism of the OAR protocol is to opportunistically send multiple back-to-back data packets whenever the channel quality is good. As channel coherence times typically exceed multiple packet transmission times for both mobile and non-mobile users, OAR achieves significant throughput gains as compared to state-of-the-art auto-rate adaptation mechanisms. Moreover, over longer time scales, OAR ensures that all nodes are granted channel access for the same time-shares as achieved by single-rate IEEE 802.11. We describe mechanisms to implement OAR on top of any existing auto-rate adaptation scheme in a nearly IEEE 802.11 compliant manner. We also analytically study OAR and characterize the gains in throughput as a function of the channel conditions. Finally, we perform an extensive set of ns-2 simulations to study the impact of such factors as node velocity, channel conditions, and topology on the throughput of OAR.
---
paper_title: Leveraging spatial reuse in 802.11 mesh networks with enhanced physical carrier sensing

paper_content:

Spatial reuse in a wireless network can allow multiple communications to proceed simultaneously, hence proportionally improve the overall network throughput. To maximize spatial reuse, the MAC protocol must enable simultaneous transmitters to maintain the minimal separation distance to avoid interference. This paper demonstrates that physical carrier sensing enhanced with tunable sensing threshold is effective at avoiding interference in 802.11 mesh networks without requiring the use of virtual carrier sensing through RTS/CTS. We present analytical model that demonstrates how to derive the optimal sensing threshold given reception power, data rate and network topology. Simulation results are shown for large-scale 802.11b networks to demonstrate that physical carrier sensing with the optimally tuned threshold improves network throughput by maximizing the potential of spatial reuse. In the case of a regular chain topology of 90 nodes, with tuned physical carrier sensing, the end-to-end throughput approaches 90% of the theoretical upper bound that assumes a perfect MAC protocol. Hence, without modifying the 802.11 MAC protocol, our enhanced physical carrier sensing mechanism effectively maximizes the potential of improving network throughput with spatial reuse.
---
paper_title: On Optimal Physical Carrier Sensing: Theoretical Analysis and Protocol Design

paper_content:

Traditional Physical Carrier Sensing (PCS), which aims at eliminating hidden terminals in wireless networks, causes too many exposed terminals and deteriorates the throughput per user seriously. Some existing work has proven that an aggressive PCS can improve the throughput by balancing the tradeoff between hidden terminals and exposed terminals. However, little work has been conducted to optimize the PCS according to the network conditions. In this paper, we develop an analytical model to study the behaviors of a user in the aggressive PCS scenario, with which the optimal PCS threshold can be derived. Then, to avoid the complicated computation, we present a heuristic algorithm, in which the parameters required for PCS tuning are estimated by the carrier sensing mechanism employed in IEEE 802.11. Simulation results show that the proposed heuristic algorithm can make the PCS threshold approach to the theoretical optimal one. Moreover, the proposed heuristic algorithm can obtain significant throughput gain compared to the traditional PCS threshold setting solutions.
---
paper_title: On physical carrier sensing in wireless ad hoc networks

paper_content:

The aggregate throughput of a wireless ad hoc network depends on the channel capacity, channel utilization (i.e., the fraction of channel capacity used for generating good put), and the concurrent transmissions allowed in the network. While channel utilization is determined by MAC overhead, physical carrier sense has been used as an effective way to avoid interference and exploit spatial reuse. Prior research has attempted to identify the optimal carrier sense range that can maximize the aggregate throughput. However, the impact of MAC overhead has been ignored. In this paper, we use both an analytical model and simulation results to show that MAC overhead has significant impact on the choice of optimal carrier sense range. If MAC overhead is not taken into account properly in determining the optimal carrier sense range, the aggregate throughput can suffer a significant loss.
---
paper_title: Interplay of Spatial Reuse and SINR-Determined Data Rates in CSMA/CA-Based, Multi-Hop, Multi-Rate Wireless Networks

paper_content:

In CSMA/CA-based, multi-hop, multi-rate wireless networks, spatial reuse can be increased by tuning the carrier-sensing threshold (Tcs) to reduce the carrier sense range (dcs). While reducing dcs enables more concurrent transmissions, the transmission quality suffers from the increased accumulative interference contributed by concurrent transmissions outside dcs. As a result, the data rate at which the transmission can sustain may decrease. How to balance the interplay of spatial reuse and transmission quality (and hence the sustainable data rate) so as to achieve high network capacity is thus an important issue. In this paper, we investigate this issue by extending Cali's model and devising an analytical model that characterizes the transmission activities as governed by IEEE 802.11 DCF in a single-channel, multi-rate, multi-hop wireless network. The systems throughput is derived as a function of Tcs, SINR, beta, and other PHY/MAC systems parameters. We incorporate the effect of varying the degree of spatial reuse by tuning the Tcs. Based on the physical radio propagation model, we theoretically estimate the potential accumulated interference contributed by concurrent transmissions and the corresponding SINR. For a given SINR value, we then determine an appropriate data rate at which a transmission can sustain. To the best of our knowledge, this is perhaps the first effort that considers tuning of PHY characteristics (transmit power and data rates) and MAC parameters (contention backoff timer) jointly in an unified framework in order to optimize the overall network throughput. Analytical results indicate that the systems throughput is not a monotonically increasing/decreasing function of Tcs, but instead exhibits transitional points where several possible choices of Tcs can be made. In addition, the network capacity can be further improved by choosing the backoff timer values appropriately.
---
paper_title: A unified interference-collision analysis for power-aware adhoc networks

paper_content:

In this paper we address the issue of controlling transmission power in power-aware ad hoc networks. Previous work that minimizes the transmission power does not consider both the energy consumed in collision resolution and the energy disbursed to overcome the interference resulting from neighboring nodes. We investigate the basic transmission power control for the 802.11 MAC protocols in which the control frames and the data frames can be transmitted at different power levels. A collision model together with an interference model of a uniformly distributed network is constructed. Based on these models, the end-to-end network throughput and the total energy consumption of the network are examined. For a network with a given node density, our results show the optimal transmission power for control messages and for data messages that will yield maximum throughput and minimum energy consumption per message.
---
paper_title: Distributed Physical Carrier Sensing Adaptation Scheme in Cooperative MAP WLAN

paper_content:

Recently a multiple access point (MAP) architecture is proposed for wireless local area networks (WLAN) to exploit the spatial diversity by permitting each user to associate with multiple APs. Benefited from multiple association, the promising cooperative communication technology can be utilized to further enhance the spatial diversity gain. However, cooperative transmission results in low spatial reuse efficiency with traditional physical carrier sensing (PCS) mechanism. That is because multiple cooperative nodes transmit simultaneously, which causes interference to the adjacent nodes and prevents their transmissions. Therefore the PCS range should be decreased aggressively to support more parallel cooperative transmissions. In this paper, a distributed PCS tuning scheme is proposed to enhance the spatial reuse efficiency in cooperative MAP WLAN. A cooperative node selection scheme is jointly designed to avoid the excessive interference caused by cooperative nodes transmission. Simulation results show that the proposed scheme significantly improves the throughput of cooperative MAP WLAN.
---
paper_title: Adapting Physical Carrier Sensing to Maximize Spatial Reuse in 802.11 Mesh Networks

paper_content:

Spatial reuse in a mesh network can allow multiple communications to proceed simultaneously, hence proportionally improve the overall network throughput. To maximize spatial reuse, the MAC protocol must enable simultaneous transmitters to maintain the minimal separation distance that is sufficient to avoid interference. This paper demonstrates that physical carrier sensing enhanced with a tunable sensing threshold is effective at avoiding interference in 802.11 mesh networks without requiring the use of virtual carrier sensing. We present an analytical model for deriving the optimal sensing threshold given network topology, reception power and data rate. A distributed adaptive scheme is also presented to dynamically adjust the physical carrier sensing threshold based on periodic estimation of channel conditions in the network. Simulation results are shown for large-scale 802.1 1b and 802.11a networks to validate both the analytical model and the adaptation scheme. It is demonstrated that the enhanced physical carrier sensing mechanism effectively improves network throughput by maximizing the potential of spatial reuse. With dynamically tuned physical carrier sensing, the end to end throughput approaches 90% of the predicted theoretical upper-bound assuming a perfect MAC protocol, for a regular chain topology of 90 nodes.
---
paper_title: A cooperative MAC protocol with virtual-antenna array support in a multi-AP WLAN system

paper_content:

In this paper we propose a cooperative-aware medium access control (MAC) protocol in the newly emerged multiple access point (MAP) WLAN system, where each user can associate with multiple APs. Leveraging the multiple association feature, the spectrum efficient virtual-antenna array (VA) cooperative strategy can be utilized. However, the APs selected for VA transmission cannot serve packet transmission individually, which leads to spatial reuse inefficiency. To balance the tradeoff between spatial reuse efficiency and cooperative gain, an interference model that describes interference among cooperative transmissions should be first established. In this paper, a virtual link model is proposed, where each virtual link is composed of one end user, one combination of VA APs and one data rate to represent one cooperative transmission. Based on this model, a multi-cell virtual link scheduling problem is formulated to achieve optimal system performance. Combined with local clique searching procedure and physical carrier sensing adaptation scheme, the dual decomposition of the scheduling problem can distributively achieve the near optimal performance. Finally a VA-based cooperative MAC (V-MAC) protocol is proposed to implement the cooperative scheduling scheme. Simulation result shows V-MAC significantly improves the system throughput meanwhile guarantees the system fairness.
---
paper_title: IEEE802.11k enabled adaptive physical carrier sense mechanism for wireless networks (K-APCS)

paper_content:

The existence of hidden and exposed nodes can have a significantly negative impact on the performance of IEEE802.11 networks. Such nodes can increase the probability of collisions and limit the spatial reuse on the channel. The value of the Physical Carrier Sensing Threshold (PCST) is instrumental to the trade-off between the number of hidden and exposed nodes in a system. This paper presents a new adaptive physical carrier sensing mechanism for wireless networks. The K-APCS algorithm maximizes the aggregate system throughput by optimizing the balance between the spatial reuse and the collision rate of a system. It incorporates 802.11k radio resource measurements and a frame loss bound on each node to enable the on-line tuning of the PCST. Simulations were performed on five variants of carrier sensing mechanisms; results show that K-APCS achieves the maximum spatial reuse with a throughput gain of 28% and the minimum collision rate with a decrease of 0.01%.
---
paper_title: Adaptive CSMA for Scalable Network Capacity in High-Density WLAN: A Hardware Prototyping Approach

paper_content:

Fast proliferation of 802.11 devices has led to the emergence of High-Density WLAN where 100s-1000s clients being serviced by 10s-100s of APs in a multi-cell environment. Scalable network capacity becomes a primary design challenge. In this paper, we investigate the impact of hidden/exposed terminal and the lack of capture effect on today's 802.11 devices on network throughput. We argue that the CSMA/CA parameters in 802.11 MAC protocol need to be adaptive to the changing network topology, traffic and interference. We propose a combination of Receiving-Sensitivity adaptation to reduce stronger-last collision and a Clear-Channel-Assessment adaptation to balance the hidden/exposed terminal problems. We take a hardware-centric approach to modify the 802.11 MAC protocol and prototype adaptation algorithms on Intel Centrino TM laptops. We demonstrate through testbed measure- ment that Receiving-Signal-Strength-Indicator and Packet-Error- Rate are effective and readily available metric on today's 802.11 hardware for the two adaptation algorithms. Our results show that the adaptation algorithms are able to react to the changing network environment and quickly converge to the new optimal value setting. Up to 300% throughput increase has been observed with the adaptation algorithms.
---
paper_title: Optimal tradeoff between exposed and hidden nodes in large wireless networks

paper_content:

Wireless networks equipped with the CSMA protocol are subject to collisions due to interference. For a given interference range we investigate the tradeoff between collisions (hidden nodes) and unused capacity (exposed nodes). We show that the sensing range that maximizes throughput critically depends on the activation rate of nodes. For infinite line networks, we prove the existence of a threshold: When the activation rate is below this threshold the optimal sensing range is small (to maximize spatial reuse). When the activation rate is above the threshold the optimal sensing range is just large enough to preclude all collisions. Simulations suggest that this threshold policy extends to more complex linear and non-linear topologies.
---
paper_title: A stochastic model for optimizing physical carrier sensing and spatial reuse in wireless ad hoc networks

paper_content:

The choice of physical carrier sensing (PCS) threshold is key to the trade-off between the amount of spatial reuse and probability of packet collisions in a wireless ad hoc network. In this paper, we present a new analytical approach for optimizing the PCS threshold as measured by probability of packet collisions and the aggregate one-hop throughput. Our model simultaneously incorporates the impact of PCS threshold and the backoff mechanism via a suitable Markov chain model for saturation (i.e. all nodes always have a packet to send in their queues). Elaborate OPNET simulation results show the effectiveness of the analytical model
---
paper_title: Optimizing Throughput with Carrier Sensing Adaptation for IEEE 802.11 Mesh Networks Based on Loss Differentiation

paper_content:

In high density (HD) mesh networks, packet losses can occur due to co-channel interference (asynchronous interference) or collisions (synchronous interference). In this paper, we first propose a novel method of estimating the probability of collision and interference statistically. Further, we integrate this differentiation method with physical carrier sensing adaptation in a novel centralized algorithm to improve the aggregate throughput in HD mesh network. Extensive simulations results show that the on-line algorithm approaches the optimal throughput predicted by analytical models.
---
paper_title: Adaptive Physical Carrier Sense in Topology-Controlled Wireless Networks

paper_content:

Transmit power and carrier sense threshold are key MAC/PHY parameters in carrier sense multiple access (CSMA) wireless networks. Transmit power control has been extensively studied in the context of topology control. However, the effect of carrier sense threshold on topology control has not been properly investigated in spite of its crucial role. Our key motivation is that the performance of a topology-controlled network may become worse than that of a network without any topology control unless carrier sense threshold is properly chosen. In order to remedy this deficiency of conventional topology control, we present a framework on how to incorporate physical carrier sense into topology control. We identify that joint control of transmit power and carrier sense threshold can be efficiently divided into topology control and carrier sense adaptation. We devise a distributed carrier sense update algorithm (DCUA), by which each node drives its carrier sense threshold toward a desirable operating point in a fully distributed manner. We derive a sufficient condition for the convergence of DCUA. To demonstrate the utility of integrating physical carrier sense into topology control, we equip a localized topology control algorithm, LMST, with the capability of DCUA. Simulation studies show that LMST-DCUA significantly outperforms LMST and the standard CSMA protocol.
---
paper_title: Leveraging spatial reuse in 802.11 mesh networks with enhanced physical carrier sensing

paper_content:

Spatial reuse in a wireless network can allow multiple communications to proceed simultaneously, hence proportionally improve the overall network throughput. To maximize spatial reuse, the MAC protocol must enable simultaneous transmitters to maintain the minimal separation distance to avoid interference. This paper demonstrates that physical carrier sensing enhanced with tunable sensing threshold is effective at avoiding interference in 802.11 mesh networks without requiring the use of virtual carrier sensing through RTS/CTS. We present analytical model that demonstrates how to derive the optimal sensing threshold given reception power, data rate and network topology. Simulation results are shown for large-scale 802.11b networks to demonstrate that physical carrier sensing with the optimally tuned threshold improves network throughput by maximizing the potential of spatial reuse. In the case of a regular chain topology of 90 nodes, with tuned physical carrier sensing, the end-to-end throughput approaches 90% of the theoretical upper bound that assumes a perfect MAC protocol. Hence, without modifying the 802.11 MAC protocol, our enhanced physical carrier sensing mechanism effectively maximizes the potential of improving network throughput with spatial reuse.
---
paper_title: On Optimal Physical Carrier Sensing: Theoretical Analysis and Protocol Design

paper_content:

Traditional Physical Carrier Sensing (PCS), which aims at eliminating hidden terminals in wireless networks, causes too many exposed terminals and deteriorates the throughput per user seriously. Some existing work has proven that an aggressive PCS can improve the throughput by balancing the tradeoff between hidden terminals and exposed terminals. However, little work has been conducted to optimize the PCS according to the network conditions. In this paper, we develop an analytical model to study the behaviors of a user in the aggressive PCS scenario, with which the optimal PCS threshold can be derived. Then, to avoid the complicated computation, we present a heuristic algorithm, in which the parameters required for PCS tuning are estimated by the carrier sensing mechanism employed in IEEE 802.11. Simulation results show that the proposed heuristic algorithm can make the PCS threshold approach to the theoretical optimal one. Moreover, the proposed heuristic algorithm can obtain significant throughput gain compared to the traditional PCS threshold setting solutions.
---
paper_title: IEEE802.11k enabled adaptive carrier sense management mechanism (KAPCS2)

paper_content:

The existence of hidden and exposed nodes can have a significant negative impact on the performance of IEEE802.11 networks. Such nodes can increase the probability of collisions and limit the spatial reuse on the channel. The value of the Physical Carrier Sensing Threshold (PCST) is instrumental in the presence of such nodes in a system. This paper presents a new adaptive physical carrier sensing management algorithm for wireless networks. Our KAPCS2 management system maximizes the aggregate system throughput by identifying and addressing problem nodes. It incorporates 802.11k Radio Resource Measurements (RRM) on each node to enable on-line tuning of the PCST.
---
paper_title: On physical carrier sensing in wireless ad hoc networks

paper_content:

The aggregate throughput of a wireless ad hoc network depends on the channel capacity, channel utilization (i.e., the fraction of channel capacity used for generating good put), and the concurrent transmissions allowed in the network. While channel utilization is determined by MAC overhead, physical carrier sense has been used as an effective way to avoid interference and exploit spatial reuse. Prior research has attempted to identify the optimal carrier sense range that can maximize the aggregate throughput. However, the impact of MAC overhead has been ignored. In this paper, we use both an analytical model and simulation results to show that MAC overhead has significant impact on the choice of optimal carrier sense range. If MAC overhead is not taken into account properly in determining the optimal carrier sense range, the aggregate throughput can suffer a significant loss.
---
paper_title: Optimizing 802.11 Wireless Mesh Network Performance Using Physical Carrier Sensing

paper_content:

Multi-hop ad-hoc networks suffer from the presence of hidden and exposed nodes that diminish aggregate network throughput. While there are various approaches to mitigating these problems, in this work we focus exclusively on the role of physical carrier sensing (PCS). Specifically, tuning the PCS threshold leads to a trade-off between the ‘exposed’ and ‘hidden’ areas. The contributions of this work are (at-least) two-fold: i. we develop an analytical formulation (assuming constant rate nodes) to determine the optimal PCS threshold. It is shown that choosing the carrier sensing range equal to the interference rangeis the appropriate optimality principle for PCS based network optimization. An important component of this analysis is an improved link layer model in contrast to much of the existing literature, that leads to significantly new and different insights into enhancing 802.11 network performance. Further, ii. in attempting to support the analysis via simulations with OPNET, several problems with various aspects of OPNET’s implementations of the 802.11 link and MAC layers were exposed. Our description of these issues as well as fixes are likely to be of interest to the broader research community interested in using OPNET for their own investigations in 802.11 network performance 1 .
---
paper_title: Adapting Physical Carrier Sensing to Maximize Spatial Reuse in 802.11 Mesh Networks

paper_content:

Spatial reuse in a mesh network can allow multiple communications to proceed simultaneously, hence proportionally improve the overall network throughput. To maximize spatial reuse, the MAC protocol must enable simultaneous transmitters to maintain the minimal separation distance that is sufficient to avoid interference. This paper demonstrates that physical carrier sensing enhanced with a tunable sensing threshold is effective at avoiding interference in 802.11 mesh networks without requiring the use of virtual carrier sensing. We present an analytical model for deriving the optimal sensing threshold given network topology, reception power and data rate. A distributed adaptive scheme is also presented to dynamically adjust the physical carrier sensing threshold based on periodic estimation of channel conditions in the network. Simulation results are shown for large-scale 802.1 1b and 802.11a networks to validate both the analytical model and the adaptation scheme. It is demonstrated that the enhanced physical carrier sensing mechanism effectively improves network throughput by maximizing the potential of spatial reuse. With dynamically tuned physical carrier sensing, the end to end throughput approaches 90% of the predicted theoretical upper-bound assuming a perfect MAC protocol, for a regular chain topology of 90 nodes.
---
paper_title: IEEE802.11k enabled adaptive physical carrier sense mechanism for wireless networks (K-APCS)

paper_content:

The existence of hidden and exposed nodes can have a significantly negative impact on the performance of IEEE802.11 networks. Such nodes can increase the probability of collisions and limit the spatial reuse on the channel. The value of the Physical Carrier Sensing Threshold (PCST) is instrumental to the trade-off between the number of hidden and exposed nodes in a system. This paper presents a new adaptive physical carrier sensing mechanism for wireless networks. The K-APCS algorithm maximizes the aggregate system throughput by optimizing the balance between the spatial reuse and the collision rate of a system. It incorporates 802.11k radio resource measurements and a frame loss bound on each node to enable the on-line tuning of the PCST. Simulations were performed on five variants of carrier sensing mechanisms; results show that K-APCS achieves the maximum spatial reuse with a throughput gain of 28% and the minimum collision rate with a decrease of 0.01%.
---
paper_title: Adaptive Carrier Sensing and Packet Sending - An Alternative to Boost the Performance in Directional Communications

paper_content:

Despite the efforts to leverage the network performance through the use of Directional Communications, most protocols still rely on the basic mechanisms employed by the IEEE802.11 standards. While this might be desirable to facilitate compatibility, nodes empowered with directional antennas should be able to explore the benefits of directional communications whenever possible. With that in mind, we propose two mechanisms, termed Adaptive Carrier Sensing and Adaptive Packet Sending, aiming to allow nodes equipped with directional antennas to exploit the advantages of such systems to improve network performance.
---
paper_title: A Distributed Procedure for Carrier Sensing Threshold Adaptation in CSMA-Based Mobile Ad Hoc Networks

paper_content:

This paper discusses the problem of determining the carrier sense threshold (CSth) of nodes in a CSMA-based Mobile Ad Hoc Network (MANET) and proposes a practical, fully distributed, CSth adaptation procedure. The procedure enables a node to select its CSth in order to improve the data throughput of the system by maximizing the number of successful transmissions in its neighborhood. Simulation results show that the procedure is able to improve the data throughput in more than 85% of random topologies. The average throughput improvement over all topologies were 11% and 18% in two different scenarios, with some topologies showing up to 75% data throughput gains.
---
paper_title: Simple and Effective Carrier Sensing Adaptation for Multi Rate Ad-Hoc MESH Networks

paper_content:

Adaptive Physical Carrier Sensing(PCS) based on tuning the PCS threshold has been shown to be an effective michanism for improving aggregate network throughput. However, earlier work [3] assumed a single link rate and a common PCS threshold for the entire network, as appropriate for a regular 2-D lattice grid of nodes with constant link (1-hop) distances. In an adhoc network topology, the 1-hop link distances vary significantly and a single PCS threshold is no longer suitable. Because IEEE 802.11 a/b/g networks provide multiple data rates over any link, joint tuning of the link rate and PCS threshold is thus desirable for achieving optimal aggregate throughput for ad-hoc networks. In this work, we first propose a simple yet effective principle for the above optimization. Next, we use the intuition offered by these formulations to suggest run-time adaptive solutions in OPNET simulations. We restrict ourselves to 1-dimensional random linear networks primarily to corroborate analysis with simulations and defer results with other topologies for future work.
---
paper_title: IEEE802.11k enabled adaptive carrier sense management mechanism (KAPCS2)

paper_content:

The existence of hidden and exposed nodes can have a significant negative impact on the performance of IEEE802.11 networks. Such nodes can increase the probability of collisions and limit the spatial reuse on the channel. The value of the Physical Carrier Sensing Threshold (PCST) is instrumental in the presence of such nodes in a system. This paper presents a new adaptive physical carrier sensing management algorithm for wireless networks. Our KAPCS2 management system maximizes the aggregate system throughput by identifying and addressing problem nodes. It incorporates 802.11k Radio Resource Measurements (RRM) on each node to enable on-line tuning of the PCST.
---
paper_title: Analysis of a cone-based distributed topology control algorithm for wireless multi-hop networks

paper_content:

The topology of a wireless multi-hop network can be controlled by varying the transmission power at each node. In this paper, we give a detailed analysis of a cone-based distributed topology control algorithm. This algorithm, introduced in [16], does not assume that nodes have GPS information available; rather it depends only on directional information. Roughly speaking, the basic idea of the algorithm is that a node u transmits with the minimum power p u, α required to ensure that in every cone of degree α around u, there is some node that u can reach with power p u, α . We show that taking α = 5π/6 is a necessary and sufficient condition to guarantee that network connectivity is preserved. More precisely, if there is a path from s to t when every node communicates at maximum power then, if α l 5π/6, there is still a path in the smallest symmetric graph G α containing all edges (u, v) such that u can communicate with v using power p u, α . On the other hand, if α > 5π/6, connectivity is not necessarily preserved. We also propose a set of optimizations that further reduce power consumption and prove that they retain network connectivity. Dynamic reconfiguration in the presence of failures and mobility is also discussed. Simulation results are presented to demonstrate the effectiveness of the algorithm and the optimizations.
---
paper_title: Distributed topology control for power efficient operation in multihop wireless ad hoc networks

paper_content:

The topology of wireless multihop ad hoc networks can be controlled by varying the transmission power of each node. We propose a simple distributed algorithm where each node makes local decisions about its transmission power and these local decisions collectively guarantee global connectivity. Specifically, based on the directional information, a node grows it transmission power until it finds a neighbor node in every direction. The resulting network topology increases the network lifetime by reducing the transmission power and reduces traffic interference by having low node degrees. Moreover, we show that the routes in the multihop network are efficient in power consumption. We give an approximation scheme in which the power consumption of each route can be made arbitrarily close to the optimal by carefully choosing the parameters. Simulation results demonstrate significant performance improvements.
---
paper_title: Power controlled dual channel (PCDC) medium access protocol for wireless ad hoc networks

paper_content:

In this paper, we propose a comprehensive solution for power control in mobile ad hoc networks (MANETs). Our solution emphasizes the interplay between the MAC and network layers, whereby the MAC layer indirectly influences the selection of the next-hop by properly adjusting the power of route request packets. This is done while maintaining network connectivity. Directional and channel-gain information obtained mainly from overheard RTS and CTS packets is used to dynamically construct the network topology. By properly estimating the required transmission power for data packets, our protocol allows for interference-limited simultaneous transmissions to take place in the neighborhood of a receiving node. Simulation results indicate that compared to the IEEE 802.11 approach, the proposed protocol achieves a significant increase in the channel utilization and end-to-end network throughput, and a significant decrease in the total energy consumption.
---
paper_title: Power Control in Ad-Hoc Networks: Theory, Architecture, Algorithm and Implementation of the COMPOW Protocol

paper_content:

We present a new protocol for power control in ad hoc networks. We describe the issues in conceptualizing the power control problem, and provide an architecturally simple as well as theoretically well founded solution. The solution is shown to simultaneously satisfy the three objectives of maximizing the traffic carrying capacity of the entire network, extending battery life through providing low power routes, and reducing the contention at the MAC layer. Further, the protocol has the plug and play feature that it can be employed in conjunction with any routing protocol that pro-actively maintains a routing table. The protocol, called COMPOW, has been implemented in the Linux kernel and we describe the software architecture and implementation details.
---
paper_title: Self-management in chaotic wireless deployments

paper_content:

Over the past few years, wireless networking technologies have made vast forays into our daily lives. Today, one can find 802.11 hardware and other personal wireless technology employed at homes, shopping malls, coffee shops and airports. Present-day wireless network deployments bear two important properties: they are unplanned, with most access points (APs) deployed by users in a spontaneous manner, resulting in highly variable AP densities; and they are unmanaged, since manually configuring and managing a wireless network is very complicated. We refer to such wireless deployments as being chaotic.In this paper, we present a study of the impact of interference in chaotic 802.11 deployments on end-client performance. First, using large-scale measurement data from several cities, we show that it is not uncommon to have tens of APs deployed in close proximity of each other. Moreover, most APs are not configured to minimize interference with their neighbors. We then perform trace-driven simulations to show that the performance of end-clients could suffer significantly in chaotic deployments. We argue that end-client experience could be significantly improved by making chaotic wireless networks self-managing. We design and evaluate automated power control and rate adaptation algorithms to minimize interference among neighboring APs, while ensuring robust end-client performance.
---
paper_title: Topology control of multihop wireless networks using transmit power adjustment

paper_content:

We consider the problem of adjusting the transmit powers of nodes in a multihop wireless network (also called an ad hoc network) to create a desired topology. We formulate it as a constrained optimization problem with two constraints-connectivity and biconnectivity, and one optimization objective-maximum power used. We present two centralized algorithms for use in static networks, and prove their optimality. For mobile networks, we present two distributed heuristics that adaptively adjust node transmit powers in response to topological changes and attempt to maintain a connected topology using minimum power. We analyze the throughput, delay, and power consumption of our algorithms using a prototype software implementation, an emulation of a power-controllable radio, and a detailed channel model. Our results show that the performance of multihop wireless networks in practice can be substantially increased with topology control.
---
paper_title: Improving Spatial Reuse in Multihop Wireless Networks - A Survey

paper_content:

In multihop wireless ad-hoc networks, the medium access control (MAC) protocol plays a key role in coordinating the access to the shared medium among wireless nodes. Currently, the distributed coordination function (DCF) of the IEEE 802.11 is the dominant MAC protocol for both wireless LANs and wireless multihop ad hoc environment due to its simple implementation and distributed nature. The current access method of the IEEE 802.11 does not make efficient use of the shared channel due to its conservative approach in assessing the level of interference; this in turn affects the spatial reuse of the limited radio resources and highly affect the achieved throughput of a multihop wireless network. This paper surveys various methods that have been proposed in order to enhance the channel utilization by improving the spatial reuse.
---
paper_title: A single-channel solution for transmission power control in wireless ad hoc networks

paper_content:

Transmission power control (TPC) has a great potential to increase the throughput of a mobile ad hoc network (MANET). Existing TPC schemes achieve this goal by using additional hardware (e.g., multiple transceivers), by compromising the collision avoidance property of the channel access scheme, or by imposing impractical requirements on the operation of the MAC protocol. In this paper, we present a novel power control MAC protocol, known as POWMAC, for MANETs that enjoys the same simple single-channel, single-transceiver design of the IEEE 802.11 ad hoc MAC protocol, but that achieves a significant throughput improvement over the 802.11 scheme. Collision avoidance is integrated into the design of POWMAC. Instead of alternating between the transmission of control (RTS/CTS)and data packets, as done in the 802.11 scheme, POWMAC uses an access window (AW) to allow for a series of RTS/CTS exchanges to take place before multiple, concurrent data packet transmissions can commence. The length of the AW is dynamically adjusted (based on local traffic load information) to allow for concurrent interference-limited transmissions to take place in the same vicinity of a receiving node. Collision avoidance informat on is inserted into the CTS packet and is used to bound the transmission powers of potential nterferers, rather than to silence such nodes. Simulation results for "random-grid" and "clustered" topologies are used to demonstrate the signicant throughput and energy gains that can be obtained under the POWMAC protocol.
---
paper_title: A power controlled multiple access protocol for wireless packet networks

paper_content:

Multiple access-based collision avoidance MAC protocols have typically used fixed transmission power, and have not considered power control mechanisms based on the distance of the transmitter and receiver in order to improve spatial channel reuse. This work proposes PCMA, a power controlled multiple access wireless MAC protocol within the collision avoidance framework. PCMA generalizes the transmit-or-defer "on/off" collision avoidance model of current protocols to a more flexible "variable bounded power" collision suppression model. The algorithm is provisioned for ad hoc networks and does not require the presence of base stations to manage transmission power (i.e. it is decentralized). The advantage of implementing a power controlled protocol in an ad-hoc network is that source-destination pairs can be more tightly packed into the network allowing a greater number of simultaneous transmissions (spectral reuse). Our initial simulation results show that the PCMA can improve the throughput performance of the non-power controlled IEEE 802.11 by a factor of 2 with potential for additional scalability as source-destination pairs become more localized, thus providing a compelling reason for migrating to a new power controlled multiple access wireless MAC protocol standard.
---
paper_title: Improving spatial reuse through tuning transmit power, carrier sense threshold, and data rate in multihop wireless networks

paper_content:

The importance of spatial reuse in wireless ad-hoc networks has been long recognized as a key to improving the network capacity. One can increase the level of spatial reuse by either reducing the transmit power or increasing the carrier sense threshold (thereby reducing the carrier sense range). On the other hand, as the transmit power decreases or the carrier sense threshold increases, the SINR decreases as a result of the smaller received signal or the increased interference level. Consequently, the data rate sustained by each transmission may decrease. This leads naturally to the following questions:(1)How can the trade-off between the increased level of spatial reuse and the decreased data rate each node can sustain be quantified? In other words,is there an optimal range of transmit power/carrier sense threshold in which the network capacity is maximized? (2)What is the relation between the transmit power and the carrier sense threshold.In this paper, we study both problems, and show that (i)in the case that the achievable channel rate follows the Shannon capacity, spatial reuse depends only on the ratio of the transmit power to the carrier sense threshold; and (ii) in the case that only a set of discrete data rates are available, tuning the transmit power offers several advantages that tuning the carrier sense threshold cannot, provided that there is a sufficient number of power levels available. Based on the findings, we then propose a decentralized power and rate control algorithm to enable each node to adjust, based on its signal interference level, its transmit power and data rate. The transmit power is so determined that the transmitter can sustain a high data rate, while keeping the adverse interference effect on the other neighboring concurrent transmissions minimal. Simulation results have shown that, as compared to existing carrier sense threshold tuning algorithms, the proposed power and rate control algorithm yields higher network capacity.
---
paper_title: Measuring Transmission Opportunities in 802.11 Links

paper_content:

We propose a powerful MAC/PHY cross-layer approach to measuring IEEE 802.11 transmission opportunities in WLAN networks on a per-link basis. Our estimator can operate at a single station and it is able to: 1) classify losses caused by noise, collisions, and hidden nodes; and 2) distinguish between these losses and the unfairness caused by both exposed nodes and channel capture. Our estimator provides quantitative measures of the different causes of lost transmission opportunities, requiring only local measures at the 802.11 transmitter and no modification to the 802.11 protocol or in other stations. Our approach is suited to implementation on commodity hardware, and we demonstrate our prototype implementation via experimental assessments. We finally show how our estimator can help the WLAN station to improve its local performance.
---

Title: A Survey of Adaptive Carrier Sensing Mechanisms for IEEE 802.11 Wireless Networks

Section 1: INTRODUCTION
Description 1: Introduce the problem domain, discuss the significance of adaptive carrier sensing in IEEE 802.11 networks, and provide an overview of what will be covered in the paper.

Section 2: BACKGROUND
Description 2: Provide background information on traditional IEEE 802.11 carrier sensing, hidden and exposed nodes, the relationship between interference range and physical carrier sensing range, capture effect, and various IEEE 802.11 architectures.

Section 3: OVERVIEW OF RESEARCH AREA
Description 3: Overview the research area of adaptive PCS, describe the benefits, and offer a categorization of PCS adaptation schemes based on the target wireless architecture.

Section 4: OPTIMAL PHYSICAL CARRIER SENSING
Description 4: Discuss initial investigations into the optimum PCS threshold, evaluate its impact on system performance, and various analytical models developed for this purpose.

Section 5: ADAPTIVE PHYSICAL CARRIER SENSING IN IEEE 802.11 WIRELESS AD HOC NETWORKS
Description 5: Review adaptive PCS mechanisms and protocols designed for ad-hoc networks, focusing on approaches for optimizing PCS in multi-hop networks, differentiating interference, and enhancing spatial reuse.

Section 6: ADAPTIVE PHYSICAL CARRIER SENSING IN IEEE 802.11 INFRASTRUCTURE NETWORKS
Description 6: Detail PCS mechanisms developed for infrastructure-based networks, focusing on single and multiple access point systems, and leveraging centralized control for optimization.

Section 7: COMPARATIVE ANALYSIS
Description7: Perform a comparative analysis of various adaptive PCS mechanisms, discussing performance, advantages, disadvantages, limitations, and complexity of different models.

Section 8: FUTURE DIRECTIONS
Description 8: Discuss potential areas for future research, emphasize the importance of experimental testing, and suggest improvements to existing models for better practical application.

Section 9: CONCLUSION
Description 9: Summarize the key findings of the survey, highlight the contributions of various adaptive PCS mechanisms, and conclude the importance of continued research in this area.