An overview of spectrum occupancy models for cognitive radio networks
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paper_title: Spectrum Survey in Singapore: Occupancy Measurements and Analyses

paper_content:

We study the 24-hour spectrum usage pattern in Singapore in the frequency bands ranging from 80 MHz to 5850 MHz. The objectives are to find how the scarce radio spectrum allocated to different services is utilized in Singapore and identify the bands that could be accessed for future opportunistic use due to their low or no active utilization. The results from the spectrum measurements taken over 12 weekday periods reveal that a significant amount of spectrum in Singapore has very low occupancy all the time. The occupancy is quantified as the amount of spectrum detected above a certain received power threshold. The outcome of this study suggests that Singapore has a great potential for employing emerging spectrum sharing technology such as the cognitive radio technology to accommodate enormous demands for future wireless services. However, this study of spectrum survey is preliminary in its nature and future long term studies need to be performed to determine any potential secondary usage on those channels that have low or no active utilization.
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paper_title: NeXt generation/dynamic spectrum access/cognitive Radio Wireless Networks: A Survey

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Today's wireless networks are characterized by a fixed spectrum assignment policy. However, a large portion of the assigned spectrum is used sporadically and geographical variations in the utilization of assigned spectrum ranges from 15% to 85% with a high variance in time. The limited available spectrum and the inefficiency in the spectrum usage necessitate a new communication paradigm to exploit the existing wireless spectrum opportunistically. This new networking paradigm is referred to as NeXt Generation (xG) Networks as well as Dynamic Spectrum Access (DSA) and cognitive radio networks. The term xG networks is used throughout the paper. The novel functionalities and current research challenges of the xG networks are explained in detail. More specifically, a brief overview of the cognitive radio technology is provided and the xG network architecture is introduced. Moreover, the xG network functions such as spectrum management, spectrum mobility and spectrum sharing are explained in detail. The influence of these functions on the performance of the upper layer protocols such as routing and transport are investigated and open research issues in these areas are also outlined. Finally, the cross-layer design challenges in xG networks are discussed.
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paper_title: Evaluation of Spectrum Occupancy in Amsterdam Using Mobile Monitoring Vehicles

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Dynamic Spectrum Access (DSA) is an exciting new technology, which introduced a paradigm shift in spectrum access. As a result it also changes the role of the regulator. On one hand the scarce radio spectrum should be used in an optimal way, so that the society is best served. On the other hand interference between users and networks should be avoided. For that reason rules have to be defined for spectrum use, which are not only determined by technical parameters, also economical, legal, political and social constraints play an important role. This topic is called spectrum governance. For evaluation purposes a monitoring network is necessary, which is also supported by several papers, that argue the need for a monitoring network to assure compliance of (DSA) radio equipment with regulatory rules. Traditional networks of fixed monitoring sites are not qualified anymore, as modern communication use high frequencies, above the 800 MHz, and communicate mainly locally. In this paper, we propose to extent this monitoring network by a fleet of mobile monitoring vehicles.
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paper_title: Evaluation of Spectrum Occupancy in Indoor and Outdoor Scenario in the Context of Cognitive Radio

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Dynamic spectrum access is an integral part of the Cognitive Radio paradigm. However, efficient spectrum sensing techniques are crucial on the way towards systems, which use idle spectrum bands opportunistically and increase the overall spectral efficiency. Current spectrum occupancy was evaluated in few measurement campaigns at different locations mostly located in the USA. In this paper we report about an extensive measurement campaign conducted in Aachen, Germany, comparing indoor-and outdoor measurement results. The highly sensitive measurement system enabled us to measure also man-made or ambient noise. Since an energy detector cannot differentiate such noise from other primary user signals we determine a very high spectrum occupancy in the outdoor scenario in the band from 20 MHz up to 3 GHz. Considerably less occupation was measured in the indoor scenario also because of less ambient noise. Our measurements confirm that the spectrum band 3-6 GHz is rarely occupied. We further provide a case study how the amplitude probability distribution can be used together with detailed regulatory information to infer additional information about the spectral usage. Such information is beneficial in order to optimize the spectrum sensing process and identify candidate bands for further investigation and possible secondary usage.
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paper_title: A Quantitative Analysis of Spectral Occupancy Measurements for Cognitive Radio

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Cognitive radio (CR) is viewed as a possible solution to the problem of radio spectrum congestion. The CR technique utilizes a temporarily unoccupied licensed band. Before investigating the technical and political implications of CR, it is necessary to know to what extent the licensed bands are temporally unoccupied. This paper describes a spectral occupancy measurement campaign conducted in the frequency range between 806 MHz and 2750 MHz in urban Auckland, New Zealand. The purpose of the measurement is to identify potential spectral opportunities for CR. Statistical analysis of the measurement results are presented in the form of noise distributions, signal amplitude probability distributions, and spectral occupancy rates as percentages of time. These analyses indicate that, on average, the actual spectral usage in this band is only about 6.2%. Pint-to-point links and some mobile uplink channels are identified as the most probable candidates for future CR operations. These results suggest that CR could bring significant gains in spectral usage. These results also imply that, in order to provide a reliable detection of the primary signal, cooperative sensing techniques may be necessary to mitigate various wireless channel effects.
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paper_title: Spectral occupation measurements and blind standard recognition sensor for cognitive radio networks

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Cognitive radio has been claimed to be a hopeful solution to the existing conflicts between spectrum demand growth and spectrum underutilization. The basic underlying idea of cognitive radio is to allow unlicensed users to access in an opportunistic and non-interfering manner some licensed bands temporarily unoccupied by licensed users. The cognitive radio concept relies on two basic premises: the current spectrum underutilization, which has been demonstrated in some spectrum measurements campaigns, and the ability of unlicensed users to effectively detect and identify the presence of different licensed technologies in order not to cause harmful interference. In this context, this paper reports the joint work on these two areas that is currently being carried out in the framework of the FP7 Network of Excellence in Wireless COMmunications (NEWCOM++). Concretely, this paper presents spectrum occupancy measurements conducted in the frequency range from 75 MHz to 7075 MHz that demonstrate the low degree to which spectrum is currently used in an urban outdoor environment and also describes the blind standard recognition sensor concept, a sensor embedded in a cognitive radio equipment to enable the identification of many commercial wireless standards without the need to connect to any network. The joint research in both areas is a key step in promoting and validating the idea of dynamic spectrum usage.
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paper_title: A framework for statistical wireless spectrum occupancy modeling

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In this paper, we propose a novel spectrum occupancy model designed to generate accurate temporal and frequency behavior of various wireless transmissions. Our proposed work builds upon existing concepts in open literature in order to develop a more accurate time-varying spectrum occupancy model. This model can be employed by wireless researchers for evaluating new wireless communication and networking algorithms and techniques designed to perform dynamic spectrum access (DSA). Using statistical characteristics extracted from actual radio frequency measurements, first- and second-order parameters are employed in a statistical spectrum occupancy model based on a combination of several different probability density functions (PDFs) defining various features of a specific spectrum band with several concurrent transmissions. To assess the accuracy of the model, the output characteristics of the proposed spectrum occupancy model are compared with realtime radio frequency measurements in the television and paging bands.
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paper_title: Discrete-time spectrum occupancy model based on Markov Chain and duty cycle models

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This paper presents an empirical time-dimension model of spectrum use in the context of dynamic spectrum access. Concretely, a discrete-time two-state Markov chain with novel duty cycle models is proposed as an adequate mean to accurately describe spectrum occupancy in the time domain. The validity and accuracy of the proposed model is assessed and corroborated with extensive empirical data from a multi-band spectrum measurement campaign. The obtained results demonstrate that the proposed approach is able to capture and reproduce with significant accuracy the statistical properties of spectrum use observed in real channels of various technologies.
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paper_title: Modeling and simulation of time-correlation properties of spectrum use in cognitive radio

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The development of the cognitive radio technology can enormously benefit from the availability of realistic and accurate spectrum occupancy models. Time-dimension models of spectrum use proposed in the literature so far are able to capture and reproduce essential temporal characteristics such as the mean occupancy level (duty cycle) and the statistical distribution of busy/idle-period lengths. However, more sophisticated aspects of spectrum use, such as time-correlation properties, have not been properly addressed yet. In this context, this paper studies and analyzes, based on field measurements of various radio technologies, the time-correlation properties of spectrum use. Appropriate models and simulation techniques are proposed in order to accurately capture and reproduce such features.
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paper_title: Modelling Primary System Activity in Dynamic Spectrum Access Networks by Aggregated ON/OFF-Processes

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Accurate modelling of primary system activity is key to reliable performance evaluation of dynamic spectrum access (DSA) technologies. Measurements on spectrum use show that in some cases the lengths of the vacant periods in a given frequency band can be correlated, a feature commonly used semi-Markov ON/OFF-models cannot reproduce. In this paper we study the stochastic properties of a simple primary system activity model constructed by aggregating the realisations of several semi-Markov ON/OFF-models. We show that despite its simplicity, our model does result in the desired correlations provided heavy-tailed distributions are used for the period length distributions of the individual processes. We also study in detail the statistical behaviour of our model as parameters such as number of processes being aggregated and distributions of the period lengths are changed. The results show that by suitable parameter selection wide range of behaviours can be modelled, making the model of interest in performance evaluation of algorithms and protocols for DSA networks.
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paper_title: Spatial duty cycle model for Cognitive Radio

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Spectrum occupancy modeling in the context of Dynamic Spectrum Access/Cognitive Radio (DSA/CR) constitutes a rather unexplored research area that still requires much more effort. This paper addresses the problem of modeling spectrum occupancy in the spatial domain by proposing a novel theoretical approach that enables modeling the occupancy level perceived at any geographical location based on the knowledge of some simple primary signal parameters. The validity of the theoretical model is verified with extensive empirical measurement results. Some examples of its potential applicability are discussed as well.
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paper_title: Statistical Prediction of Spectrum Occupancy Perception in Dynamic Spectrum Access Networks

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Cognitive radio nodes of a dynamic spectrum access network monitor the spectrum around them in order to detect and opportunistically exploit temporarily unoccupied spectrum gaps. The overall behavior of the whole network depends on the spectrum occupancy perceived by each node at its local environment. An appropriate understanding of the local perception of cognitive radio nodes in various scenarios and locations as well as its dependence on the propagation environment can provide valuable information for the design, dimensioning and performance evaluation of dynamic spectrum access networks. In this context, this work presents a novel and practical approach to predict the spectrum occupancy that would be perceived by cognitive radio nodes in realistic scenarios. Based on such approach, we explore the dependence of the spectrum occupancy perception with the considered scenario and location as well as the impact of particular parameters of the radio environment.
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paper_title: Spatial statistics and models of spectrum use

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In order to opportunistically exploit unused radio spectrum nodes of dynamic spectrum access (DSA) networks monitor the spectrum around them. Such cognitive radios can greatly benefit from a spatial characterization of spectrum use. However, there is need to find an efficient way to describe spatial use, something which has not been studied in details so far. In this paper, we introduce spatial statistics techniques as promising methods to describe spectrum use and enable optimization of DSA networks. We discuss two approaches to spatial modelling of spectrum, namely a deterministic approach based on a system model of the complete radio environment and an empirical approach that exploits passive measurements of the spectrum use. We elaborate on the impact of different network properties on the models and provide realistic parameter sets for generation of simulation scenarios. Additionally, we investigate cooperative sensing as a use case for spatial statistics based runtime optimization of the network configuration.
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paper_title: Lessons learned from an extensive spectrum occupancy measurement campaign and a stochastic duty cycle model

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Several measurement campaigns have shown that numerous spectrum bands are vacant although licenses have been issued by the regulatory agencies. Dynamic spectrum access (DSA) has been proposed in order to alleviate this problem and increase the spectral utilization. In this paper we present our spectrum measurement setup and discuss lessons learned during our measurement activities. We compare measurement results gathered at three locations and show differences in the background noise processes. Additionally, we introduce a new model for the duty cycle distribution that has multiple applications in the DSA research. We point out that fully loaded and completely vacant channels should be modelled explicitly and discuss the impact of duty cycle correlation in the frequency domain. Finally, we evaluate the efficiency of an adaptive spectrum sensing process as an example for applications of the introduced model.
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Title: An Overview of Spectrum Occupancy Models for Cognitive Radio Networks

Section 1: Introduction
Description 1: Provide background on wireless communication systems, spectrum management policies, and the emergence of Dynamic Spectrum Access (DSA) and Cognitive Radio (CR). Explain the importance of modeling spectrum occupancy patterns.

Section 2: Time-dimension models
Description 2: Discuss the use of time-dimension models to represent spectrum usage, focusing on two-state Markov chains, discrete-time models, continuous-time models, and time-correlation models. Analyze the merits and limitations of these models.

Section 3: Discrete-time models
Description 3: Explain the functioning and application of Discrete-Time Markov Chain (DTMC) models in representing spectrum occupancy. Describe transition probabilities and duty cycle (DC) calculations.

Section 4: Continuous-time models
Description 4: Detail the Continuous-Time Markov Chain (CTMC) models and their ability to represent idle and busy periods accurately. Discuss mean sojourn times and non-stationary models.

Section 5: Time-correlation models
Description 5: Describe how time-correlation models account for the correlation between busy and idle periods. Explain the autocorrelation functions and modeling approaches for periodic and non-periodic behaviors.

Section 6: Frequency-dimension models
Description 6: Review models that describe statistical properties of spectrum usage in the frequency domain. Mention the distribution of channel occupancy levels and statistical fits like the beta and Kumaraswamy distributions.

Section 7: Space-dimension models
Description 7: Explore models characterizing spectrum occupancy patterns in the spatial domain. Discuss spatial statistics, random fields, semivariograms, and how model-based estimations of radio frequency propagation inform occupancy perceptions.

Section 8: Conclusions
Description 8: Summarize the importance of accurate spectrum usage models for the development of DSA/CR technology. Highlight the broad categorization of models and their advantage for analytical and simulation-based studies while listing their respective advantages and shortcomings.