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ABSTRACT: Cognitive radio (CR) is the enabling technology for supporting dynamic spectrum access: the policy that addresses the spectrum scarcity problem that is encountered in many countries. Thus, CR is widely regarded as one of the most promising technologies for future wireless communications. To make radios and wireless networks truly cognitive, however, is by no means a simple task, and it requires collaborative effort from various research communities, including communications theory, networking engineering, signal processing, game theory, software-hardware joint design, and reconfigurable antenna and radio-frequency design. In this paper, we provide a systematic overview on CR networking and communications by looking at the key functions of the physical (PHY), medium access control (MAC), and network layers involved in a CR design and how these layers are crossly related. In particular, for the PHY layer, we will address signal processing techniques for spectrum sensing, cooperative spectrum sensing, and transceiver design for cognitive spectrum access. For the MAC layer, we review sensing scheduling schemes, sensing-access tradeoff design, spectrum-aware access MAC, and CR MAC protocols. In the network layer, cognitive radio network (CRN) tomography, spectrum-aware routing, and quality-of-service (QoS) control will be addressed. Emerging CRNs that are actively developed by various standardization committees and spectrum-sharing economics will also be reviewed. Finally, we point out several open questions and challenges that are related to the CRN design.
IEEE Transactions on Vehicular Technology 10/2011; · 1.92 Impact Factor
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ABSTRACT: In this paper, we consider the medium access control (MAC) protocol design for cognitive radio networks. An opportunistic spectrum access protocol named Slotted CR-ALOHA is proposed, and its performances in terms of normalized throughput and average packet delay are evaluated. Simulation results show that for various frame lengths and number of SUs, the optimal performance can be achieved at an appropriate spectrum sensing time, and there also exists a tradeoff between the achievable performance of secondary network and the protection effect on primary network.
Communications (ICC), 2011 IEEE International Conference on; 07/2011
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ABSTRACT: Analytical formulations of the throughput of cognitive multi-channel MACs with perfect and imperfect sensing are presented. Both imperfect concurrent sensing and imperfect sequential sensing schemes are considered. A discrete time Markov chain is used to model the number of communicating node pairs in the MAC protocols. The throughput of the MAC protocol with perfect sensing is expressed as a function of the number of available data channels, the channel transmission rate, the average utilization per channel, the steady state probability of having a number of available data channels, while the throughput of the MAC protocol with imperfect sensing is expressed as a function of the number of available data channels, the channel transmission rate, the average utilization per channel, the steady state probability of having a number of available data channels, probability of false alarm and probability of misdetection. The results also clearly demonstrate the advantage of our proposed MAC protocol with imperfect concurrent sensing having low probability of misdetection but high probability of false alarm.
Communications (ICC), 2011 IEEE International Conference on; 07/2011
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ABSTRACT: Based on the Neyman-Pearson theorem, the optimal cooperative sensing for distributed sensors with time independent signals is derived. It is shown that the optimal scheme is simply a linearly combined energy detection and the combining coefficient is a simple function of the signal to noise ratio (SNR). To reduce the required information at the fusion center and simplify the decision-making process and threshold setting, an approximated optimal cooperative sensing is proposed and compared with some other sub-optimal methods. Finally the impact of decoding error in the reported results is analyzed. Based on the closed-form expression for the performance, it is proved that the impact of decoding error is equivalent to the reduction of sensing time. Simulations are provided to support the results.
Communications (ICC), 2011 IEEE International Conference on; 07/2011
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ABSTRACT: In this paper, we consider medium access control (MAC) protocol design for random-access cognitive radio (CR) networks. A two-level opportunistic spectrum access strategy is proposed to optimize the system performance of the secondary network and to adequately protect the operation of the primary network. At the first level, secondary users (SUs) maintain a sufficient detection probability to avoid interference with primary users (PUs), and the spectrum sensing time is optimized to control the total traffic rate of the secondary network allowed for random access when the channel is detected to be available. At the second level, two MAC protocols called the slotted cognitive radio ALOHA (CR-ALOHA) and cognitive-radio-based carrier-sensing multiple access (CR-CSMA) are developed to deal with the packet scheduling of the secondary network. We employ normalized throughput and average packet delay as the network metrics and derive closed-form expressions to evaluate the performance of the secondary network for our proposed protocols. Moreover, we use the interference and agility factors as the performance parameters to measure the protection effects on the primary network. For various frame lengths and numbers of SUs, the optimal performance of throughput and delay can be achieved at the same spectrum sensing time, and there also exists a tradeoff between the achievable performance of the secondary network and the effects of protection on the primary network. Simulation results show that the CR-CSMA protocol outperforms the slotted CR-ALOHA protocol and that the PUs' activities have an influence on the performance of SUs for both the slotted CR-ALOHA and CR-CSMA.
IEEE Transactions on Vehicular Technology 07/2011; · 1.92 Impact Factor
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ABSTRACT: Since a cognitive radio does not have fixed spectra, it may need to sense a very large frequency range to find an available band. The sensed aggregate bandwidth could be as large as several GHz. This is especially challenging if the center frequencies and bandwidths of the sensed signals are unknown and need to be detected. In this paper, an edge based wideband sensing is proposed. The method first uses the product of wavelet transforms at different scales to detect the edges (sharp changing points) of the power spectral density (PSD) of the received signal. It then forms the possible bands based on the detected edges. Thereafter, it applies a multi-band detection scheme to classify the bands as occupied or vacant. Finally, the signal to noise ratio (SNR) of each occupied band is estimated. Performance evaluation is also a complicated issue for wideband sensing. Other than the conventional metrics as probability of detection and probability of false alarm, three new criteria are proposed to evaluate the performance of a wideband sensing. Simulations are provided to verify the methods.
New Frontiers in Dynamic Spectrum Access Networks (DySPAN), 2011 IEEE Symposium on; 06/2011
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ABSTRACT: In this paper, we address the issue of optimal transmitter design to achieve physical layer security for a multiple-input single-output (MISO) cognitive radio network (CRN), in which a secondary user transmitter (SU-Tx) sends confidential information to a SU receiver (SU-Rx) on the same frequency band with a primary user (PU) in the presence of an eavesdropper receiver (ED-Rx). It is assumed that all the channel state information (CSI) of the secondary, primary and eavesdropper channels is not perfectly known at the SU-Tx. The optimal transmitter design, under the restriction of Gaussian signaling without preprocessing of information, involves a nonconvex semiinfinite optimization problem which maximizes the rate of the secondary link while avoiding harmful interference to the PU and keeping the eavesdropper totally ignorant of the messages sent regardless of the uncertainties in the CSI. We propose two approaches to solve this challenging optimization problem. The first one relates the original problem to a sequence of semiinfinite capacity-achieving transmitter design problems in an auxiliary CRN without any eavesdropper, which can then be solved through transformations and using convex semidefinite programs (SDPs). The second approach explores the hidden convexity of the problem and hence transforms it into a single SDP, which significantly reduces the computational complexity. Furthermore, a few heuristic beamforming solutions for the ease of implementation are also introduced. Finally, simulation results are presented to evaluate the performance of the proposed optimal and suboptimal solutions.
IEEE Transactions on Signal Processing 05/2011; · 2.63 Impact Factor
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ABSTRACT: The problem of nonlinear transceiver optimization in a multi-tier Multiple-Input Multiple-Output (MIMO) network in Cognitive Radio Network (CR-Net) configuration is studied. The employed transmission schemes are based on the Matrix Decision Feedback Equalizer (DFE) and Tomlinson-Harashima Precoder (THP). It is assumed that the Channel State Information (CSI) is not known perfectly. The former type of uncertainty is modeled using Stochastic Error (SE) model while the latter one is modeled using Norm Bounded Error (NBE) model. The performance measure used for optimizing the network is based on the sum Mean Square Error (MSE) of symbol estimation in the system. The design problem is constrained by the transmit power of the Secondary Users (SU's) as well as the maximum allowed interfering power to the Primary Users (PU's). The design problem is not jointly convex in its design variables and has infinitely many constraints. To overcome this, a suboptimal iterative procedure is proposed. Based on the chosen model for uncertainty, the two resultant problems are Semidefinite Programs (SDP). These two problems are solved numerically. Finally simulations results are provided to assess the performance of the system.
Global Telecommunications Conference (GLOBECOM 2010), 2010 IEEE; 01/2011
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ABSTRACT: A new approach to spectrum sensing in cognitive radio systems based on the Kolmogorov-Smirnov (K-S) test is proposed. The K-S test is a non-parametric method to measure the goodness of fit. The basic procedure involves computing the empirical cumulative distribution function (ECDF) of some decision statistic obtained from the received signal, and comparing it with the ECDF of the channel noise samples. A sequential version of the K-S-based spectrum sensing technique is also proposed. Extensive simulation results demonstrate that compared with the existing spectrum detection methods, such as the energy detector and the eigenvalue-based detector, the proposed K-S detectors offer superior detection performance and faster detection, and is more robust to channel uncertainty and non-Gaussian noise.
IEEE Transactions on Communications 01/2011; · 1.68 Impact Factor
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ABSTRACT: In cognitive radio networks, joint optimization of sensing time and cooperative fusion scheme has been studied in the past in terms of sensing-throughput tradeoff design. In this paper, different from previous studies, we consider the case that the secondary users have different detection signal-to-noise ratios (SNRs) and their decisions are weighted based on the likelihood-ratio test at the fusion center. We consider three scenarios. In Scenario I, we optimize individual secondary users' thresholds together with the fusion rule's threshold at the fusion center. In Scenario II, all the secondary users' thresholds are constrained to be the same and we seek the optimal threshold jointly with the fusion rule's threshold at the fusion center. In Scenario III, each secondary user computes its own threshold while the fusion center optimizes the fusion rule's threshold based on the secondary users' threshold results. Solutions are provided for the three different scenarios and computer simulations are presented to compare their performances.
IEEE Transactions on Wireless Communications 01/2011; · 2.59 Impact Factor
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ABSTRACT: In this paper the linear beamforming design of a point to point relay system having multiple antennas at the transmitter, the relay and the receive side is addressed. The Channel State Information (CSI) is assumed to be perfectly known for the link from the source to the relay station. However, it is assumed that CSI from the relay station to the destination is imperfectly known. This uncertainty is described using stochastic and deterministic models known as Stochastic Error (SE) and Norm Bounded Error (NBE) models. The problem formulation is based on the Mean Square Error (MSE) of the signal at both ends of the transmission channel. This problem is constrained to satisfy the transmit power limitation of the relay station. It is shown that both the MSE of the system and the transmit power have a Second Order Cone (SOC) structure. The design problem for the SE model, is recast as a Second Order Cone Program (SOCP) while for the NBE model, the same design problem is a Semidefinite Program (SDP). Using the SE model, the average performance and using the NBE model, the worst-case related performance measure is guaranteed. Both problems are solved numerically and simulation results are provided.
Personal Indoor and Mobile Radio Communications (PIMRC), 2010 IEEE 21st International Symposium on; 10/2010
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ABSTRACT: Cyclostationary detection is regarded as a major method for spectrum sensing in cognitive radio and other applications as well. The rationale behind the detection is that the second order statistic of the interested signal is periodical. The period is therefore used as the critical feature for detection. In practice, due to clock error or oscillator error or other errors, the detector is hardly able to know the exact period of the signal. This causes a cyclic frequency mismatch in the detection. In this paper, the origin of the mismatch and the impact of it are analyzed. Theoretic analysis and simulations are presented to show that the cyclostationary detection is actually very sensitive to the mismatch. The theoretic analysis on the test statistics matches very well with simulations and can be used for predicting the detection performances and designing the detection parameters.
Personal Indoor and Mobile Radio Communications (PIMRC), 2010 IEEE 21st International Symposium on; 10/2010
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ABSTRACT: Spectrum sensing is a critical step in cognitive radio to learn the radio environment. Despite its long history, in recent years spectrum sensing has attracted substantial interests from the cognitive radio community as well as other areas. Although there have been many methods, most of them need some idealistic assumptions and are hardly applicable in real cognitive radio practice. In this paper, we first discuss the possible hostile environment facing the spectrum sensing in cognitive radio. We then investigate some methods which could survive under such hostile environment. These methods include eigenvalue/covariance based detections, cooperative sensing, and cyclostationary detections. The robustness of these methods are discussed in detail through theoretic analysis and simulations.
Personal, Indoor and Mobile Radio Communications Workshops (PIMRC Workshops), 2010 IEEE 21st International Symposium on; 10/2010
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ABSTRACT: This paper studies the problem of robust downlink beamforming design in a multiuser multiple-input-single-output (MISO) cognitive radio network (CR-Net) in which multiple secondary users (SUs) coexist with multiple primary users (PUs) of a single-cell primary radio network (PR-Net). It is assumed that the channel-state information (CSI) for all relevant channels is imperfectly known, and the imperfectness of the CSI is modeled using a Euclidean ball-shaped uncertainty set. Our design objective is to minimize the transmit power of the SU-Transmitter (SU-Tx) while simultaneously achieving a lower bound on the received signal-to-interference-plus-noise ratio (SINR) for the SUs and imposing an upper limit on the interference power (IP) at the PUs. The design parameters at the SU-Tx are the beamforming weights, and the proposed methodology to solve the problem is based on the worst-case design scenario through which the performance metrics of the design are immune to variations in the channels. The original problem is a separable homogeneous quadratically constrained quadratic problem (QCQP), which is an NP-hard problem, even for uncertain CSI. We reformulate our original design problem to a relaxed semidefinite program (SDP) and then investigate three different approaches based on convex programming. Finally, simulation results are provided to validate the robustness of the proposed methods.
IEEE Transactions on Vehicular Technology 08/2010; · 1.92 Impact Factor
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ABSTRACT: This paper examines the IEEE 802.16j multihop relay (MR) technology that improves vehicle-to-infrastructure communications. We study the selection of optimal relay station (RS) for a vehicular subscriber station (SS) that maximizes the end-to-end capacity, assuming that the locations of vehicular SSs are known. By incorporating a highway mobility model, the problem can be formulated into a nonlinear optimization problem and solved for the optimal locations of RSs that guarantee maximal end-to-end capacities to SSs. Numerical results from a case study show that the selection of the corresponding optimal RS increases the expected end-to-end capacity for individual SSs by 50%, as compared with the method without using relays.
IEEE Transactions on Vehicular Technology 07/2010; · 1.92 Impact Factor
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ABSTRACT: The problem of robust linear transceiver design in Multiple-Input Multiple-Output (MIMO) ad hoc Cognitive Radio Networks (CR-Nets) is studied in this paper. In this problem multiple interfering MIMO links are active within the service range of a Primary Radio Network (PR-Net) for which, unlike in conventional design problems, the Channel State Information (CSI) is not known perfectly. The imperfection in CSI is modeled using norm-bounded uncertainty matrices. The design problem is formulated to provide the minimum Sum-Mean Square Error (SMSE) of all the links while the transmit power of Secondary Users (SUs) is limited and the amount of interfering power toward the Primary users (PUs) is controlled. This problem is not convex, because its objective function is non-convex in nature and it has semi-infinite robust constraints. To overcome these limitations an iterative solution, which is based on the relaxed version of this problem, is provided. The problem is then solved numerically. Finally simulation results are provided to demonstrate the performance of this method.
Vehicular Technology Conference (VTC 2010-Spring), 2010 IEEE 71st; 06/2010
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ABSTRACT: This article provides an overview of the state-of-art results on communication resource allocation over space, time, and frequency for emerging cognitive radio (CR) wireless networks. Focusing on the interference-power/interference-temperature (IT) constraint approach for CRs to protect primary radio transmissions, many new and challenging problems regarding the design of CR systems are formulated, and some of the corresponding solutions are shown to be obtainable by restructuring some classic results known for traditional (non-CR) wireless networks. It is demonstrated that convex optimization plays an essential role in solving these problems, in a both rigorous and efficient way. Promising research directions on interference management for CR and other related multiuser communication systems are discussed.
IEEE Signal Processing Magazine 06/2010; · 4.07 Impact Factor
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ABSTRACT: This paper addresses the design issues of the multiantenna-based cognitive radio (CR) system that is able to concurrently operate with the licensed primary-radio (PR) system. We propose a practical CR transmission strategy consisting of three major stages, namely, environment learning, channel training, and data transmission. In the environment-learning stage, the CR transceivers both listen to the PR transmission and apply blind algorithms to estimate the spaces that are orthogonal to the channels from the PR. Assuming time-division duplex (TDD)-based transmission for the PR, cognitive beamforming is then designed and applied at CR transceivers to restrict the interference to/from the PR during the subsequent channel-training and data-transmission stages. In the channel-training stage, the CR transmitter sends training signals to the CR receiver, which applies the linear-minimum-mean-square-error (LMMSE)-based estimator to estimate the effective channel. Considering imperfect estimations in both learning and training stages, we derive a lower bound on the ergodic capacity that is achievable for the CR in the data-transmission stage. From this capacity lower bound, we observe a general learning/training/throughput tradeoff associated with the proposed scheme, pertinent to transmit power allocation between the training and transmission stages, as well as time allocation among the learning, training, and transmission stages. We characterize the aforementioned tradeoff by optimizing the associated power and time allocation to maximize the CR ergodic capacity.
IEEE Transactions on Vehicular Technology 06/2010; · 1.92 Impact Factor
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ABSTRACT: This paper is concerned with a spectrum sharing cognitive radio network. In particular, the individual outage capacity region for a M-user fading cognitive multiple access network sharing the same spectrum with an existing primary network is first characterized. The primary network's transmission is assumed to be protected by the interference power constraint. Then, under the interference power constraint and the individual transmit power constraint of each user, the individual outage capacity region is implicitly obtained by characterizing the boundary points on the individual usage probability region for a given rate vector. The optimal power allocation and decoding strategy is then derived by the Lagrange dual decomposition method. It is proved that the optimal decoding strategy is the successive decoding strategy, and the decoding order is determined by dual variables together with the channel fading gains of the primary and secondary links. Finally, several numerical examples are given to validate the proposed studies.
Wireless Communications and Networking Conference (WCNC), 2010 IEEE; 05/2010
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ABSTRACT: This paper is interested in spectrum sensing using multiple antennas under spatially and temporally correlated noise environments. We exploit cyclostationary features of the primary user's signal in terms of cyclic spectral coherence function and the proposed modified cyclic spectral density function, which has less computational complexity. Two types of detectors are proposed: pre-combining and post-combining detectors. For pre-combining method, a blind maximum ratio combining technique is considered. All detectors are designed to handle noise uncertainty and also be effective in both white noise and colored noise scenarios. Numerical results are given to illustrate the performance of all detectors and verify their efficiency against the noise correlation effect. With the use of estimated channels, pre-combining detectors are superior to post-combining detectors, which do not require channel information. It is also shown that the modified cyclic spectral density function achieves comparable performance to the cyclic spectral coherence function.
New Frontiers in Dynamic Spectrum, 2010 IEEE Symposium on; 05/2010
