Breaking Spectrum Gridlock With Cognitive Radios: An Information Theoretic Perspective

Cognitive radios hold tremendous promise for increasing spectral efficiency in wireless systems. This paper surveys the fundamental capacity limits and associated transmission techniques for different wireless network design paradigms based on this promising technology. These paradigms are unified by the definition of a cognitive radio as an intelligent wireless communication device that exploits side information about its environment to improve spectrum utilization. This side information typically comprises knowledge about the activity, channels, codebooks, and/or messages of other nodes with which the cognitive node shares the spectrum. Based on the nature of the available side information as well as a priori rules about spectrum usage, cognitive radio systems seek to underlay, overlay, or interweave the cognitive radios' signals with the transmissions of noncognitive nodes. We provide a comprehensive summary of the known capacity characterizations in terms of upper and lower bounds for each of these three approaches. The increase in system degrees of freedom obtained through cognitive radios is also illuminated. This information-theoretic survey provides guidelines for the spectral efficiency gains possible through cognitive radios, as well as practical design ideas to mitigate the coexistence challenges in today's crowded spectrum.      

Capacity of Cognitive Interference Channels With and Without Secrecy

Like the conventional two-user interference channel, the cognitive interference channel consists of two transmitters whose signals interfere at two receivers. It is assumed that there is a common message (message 1) known to both transmitters, and an additional independent message (message 2) known only to the cognitive transmitter (transmitter 2). The cognitive receiver (receiver 2) needs to decode messages 1 and 2, while the noncognitive receiver (receiver 1) should decode only message 1. Furthermore, message 2 is assumed to be a confidential message which needs to be kept as secret as possible from receiver 1, which is viewed as an eavesdropper with regard to message 2. The level of secrecy is measured by the equivocation rate. In this paper, a single-letter expression for the capacity-equivocation region of the discrete memoryless cognitive interference channel is obtained. The capacity-equivocation region for the Gaussian cognitive interference channel is also obtained explicitly. Moreover, particularizing the capacity-equivocation region to the case without a secrecy constraint, the capacity region for the two-user cognitive interference channel is obtained, by providing a converse theorem.      

Available and Waiting Times for Cognitive Radios

Cognitive radios (CRs) have been recently proposed for the problem of spectrum scarcity. The principle of CRs?? operation is based on the opportunistic access to the frequency spectrum mainly dedicated to primary users (PUs). The statistical time pattern of PUs?? channel usage and arrival can affect the usability of specific frequency bands for CRs. In this note, the effect of the arrival rate and channel holding time of PUs on the available times for CRs is analyzed. To this end, first, based on Poissonian arrivals, the available time for CRs is calculated. Then, assuming a gamma distribution for the inter-arrival times and a uniform distribution of channel holding time of PU in these intervals, the probability density function and moments of the available time for CRs are derived. Next, the effect of PUs statistical parameters on the average number of packets and the average symbol rate that a CR can transmit is analyzed. Also, taking that CR needs at least T seconds, the average waiting time is calculated.     

Aggregate Interference of Equidistant Cognitive Radios

In spite of spectrum sensing, aggregate interference from cognitive radios (CRs) remains as a deterring factor to the implementation of spectrum sharing strategies. We provide a systematic approach of evaluating the aggregate interference (I _aggr) experienced at a victim primary receiver (PR). In our approach, we model the received power versus distance relations between a primary transmitter (PT), PR, and CRs. CRs can spatially reuse a channel and thus two adjacent CRs are separated by the co-channel reuse distance (R). Our analytical framework differs from the existing ones in that we have formulated I _aggr in terms of R and sensing inaccuracy. Energy detector is assumed for the purpose of spectrum sensing. I _aggr is expressed explicitly as a function of the number of energy samples collected (N) and the threshold SNR level used for comparison (SNR _ε ). This allows us to assess their impacts on I _aggr. A numerical example is constructed based on the scenario of spectrum sharing between DTV broadcast and IEEE 802.22 Wireless Regional Area Network. Our analysis demonstrates the extents of which I _aggr can be restricted, by increasing R or sensing accuracy (either by increasing N or decreasing SNR _ε ), and the amount of increment required. Conditioned on the example scenario, the critical {N, SNR _ε , R} values that fulfill certain regulatory requirement are revealed.     

A Statistical Method for Reconfiguration of Cognitive Radios

Recent developments in computer technology have enabled radio developers to accomplish in software what traditionally was performed with application-specific integrated circuits. A radio that has the core of its functionality implemented in software is called a software-defined radio. When an SDR has the capability to sense, reason, and dynamically adapt to requirements and environmental change, we call this more capable device a cognitive radio. Many private and public agencies are investing in the promise of CR to improve the utilization of radio frequency spectrum. They envision devices that can sense frequency vacancies and dynamically reconfigure to utilize idle channels. The promise of CR depends on the capability of a radio to change operating frequencies, power, and/or modulation schemes (physical layer flexibility). In addition to this physical layer flexibility, there are a large number of opportunities to capitalize on the interplay of the CR physical layer configuration and other parameters in the radio network protocol stack. At the core of CR functionality is the ability to select from thousands of potential configurations to maximize performance-be it in terms of spectrum use, throughput, or reliability. In this article, we describe a method for selecting from a number of potential configurations to fulfill the communication requirements of a CR network. By using accepted statistical methods, we show how parameters at the physical, data link, network, and application layers interact to affect performance. We build upon this parametric insight with our presentation of a technique for predicting radio performance.     

免疫遗传算法及认知无线电参数优化决策

根据感知的频谱环境变化及时优化并调整无线电参数是认知无线电的关键技术之一,也是一个复杂的非线性多目标优化决策问题。遗传算法是最适合优化问题的,但当遗传算法应用于优化问题时存在过早收敛问题。提出了基于遗传算法和人工免疫系统相结合的免疫遗传算法（IGA）来克服以上问题。由于在GA算法中引入了免疫系统中抗体和抗原的概念并在每一次迭代中丢弃亲和力较大的抗体,有效地防止了GA中过早收敛现象。最后,用免疫遗传算法来解决认知无线电的参数优化问题。仿真结果表明,免疫遗传算法可以迅速达到最优决策。     

认知无线电中频谱检测技术研究

认知无线电技术是为了解决频谱资源匮乏的问题而提出的一种无线通信技术。首先简述了认知无线电的背景和概念,然后详细研究了认知无线电中的各种检测技术,分析了其优缺点。     

Distributed Scheduling and Resource Allocation for Cognitive OFDMA Radios

Scheduling spectrum access and allocating power and rate resources are tasks affecting critically the performance of wireless cognitive radio (CR) networks. The present contribution develops a primal-dual optimization framework to schedule any-to-any CR communications based on orthogonal frequency division multiple access and allocate power so as to maximize the weighted average sum-rate of all users. Fairness is ensured among CR communicators and possible hierarchies are respected by guaranteeing minimum rate requirements for primary users while allowing secondary users to access the spectrum opportunistically. The framework leads to an iterative channel-adaptive distributed algorithm whereby nodes rely only on local information exchanges with their neighbors to attain global optimality. Simulations confirm that the distributed online algorithm does not require knowledge of the underlying fading channel distribution and converges to the optimum almost surely from any initialization.

认知无线电中基于无限次重复博弈的功率控制算法

通过博弈来实现认知无线电中的功率控制。当两用户功率控制博弈时,用户通过功率的迭代注水法实现最大化自身速率,达到纳什均衡。功率分配的纳什均衡点构成囚徒困境,但这种囚徒困境的均衡点并非全局最优。应用两用户功率控制的无限次重复博弈算法,通过选择严厉的触发策略,当折扣因子σ足够接近于1,使两用户在无限次重复博弈中一直合作使囚徒走出了困境,最终达到了功率分配的帕雷托最优的均衡结果。     

Autocorrelation-Based Decentralized Sequential Detection of OFDM Signals in Cognitive Radios

This paper introduces a simple and computationally efficient spectrum sensing scheme for Orthogonal Frequency Division Multiplexing (OFDM) based primary user signal using its autocorrelation coefficient. Further, it is shown that the log likelihood ratio test (LLRT) statistic is the maximum likelihood estimate of the autocorrelation coefficient in the low signal-to-noise ratio (SNR) regime. Performance of the local detector is studied for the additive white Gaussian noise (AWGN) and multipath channels using theoretical analysis. Obtained results are verified in simulation. The performance of the local detector in the face of shadowing is studied by simulations. A sequential detection (SD) scheme where many secondary users cooperate to detect the same primary user is proposed. User cooperation provides diversity gains as well as facilitates using simpler local detectors. The sequential detection reduces the delay and the amount of data needed in identification of the underutilized spectrum. The decision statistics from individual detectors are combined at the fusion center (FC). The statistical properties of the decision statistics are established. The performance of the scheme is studied through theory and validated by simulations. A comparison of the SD scheme with the Neyman-Pearson fixed sample size (FSS) test for the same false alarm and missed detection probabilities is also carried out.     

Intelligent Improvement in Throughput of Cognitive Radio

Under the energy detection scheme based cognitive radio (CR) system, the process of spectrum sensing is of high importance. The sensing performance of CR primarily depends on two important parameters namely, the sensing time τ and the reference threshold λ. In order to achieve a goal where the CR system obtains a high value of throughput and simultaneously ensures a sufficient level of protection to the licensed users, the values of these parameters can neither be too high nor too low, so proper settings of their values is of prime concern. However, under these constraints on choosing a particular value of τ and λ, it is challenging for CR to fulfill this goal. In this paper we propose a CR system which operates under the scheme of double threshold to ensure a sufficient protection required by the licensed users and also makes an efficient utilization of the confusion region to improve its achievable throughput. It is observed that, under the proposed approach, the CR system achieves better throughput than the CR system based on the single threshold and also to the conventional double threshold based CR system where confusion region is used based on the results of sensing performed in the next sensing rounds. We further study the problem of optimizing the sensing duration to maximize the throughput of the proposed CR system. We formulate the sensing-throughput tradeoff problem mathematically and prove that, the formulated problem indeed has an optimal sensing duration where throughput of the CR system is maximized.     

Performance Analysis for Two-Way Relaying Networks with and Without Relay Selection

In this paper, we consider a two-way relaying network where two users communicate with each other with the help of multiple relays. Specifically, we first investigate the performance of a all-relay participating network with the amplify-and-forward relaying protocol. Based on the moment generating function-based method, we derive some tight bounds for the outage probability and bit-error rate performance. For comparison purpose, we then present a performance analysis for the relay selection scheme. Finally, numerical results are provided to verify our analysis.     

Secure Relay Selection of Cognitive Two-Way Denoise-and-Forward Relaying Networks

In this paper, secrecy performance of a cognitive two-way denoise-and-forward relaying network consisting of two primary user (PT and PD) nodes, two secondary source (SA and SB) nodes, multiple secondary relay (\({\textit{SR}}_i\)) nodes and an eavesdropper (E) node is considered, where SA and SB exchange their messages with the help of one of the relays using a two-way relaying scheme. The eavesdropper tries to wiretap the information transmitted between SA and SB. To improve secrecy performance of the network, two relay selection schemes called maximum sum rate and maximum secrecy capacity based relay selection (MSRRS and MSCRS) are proposed and analyzed in terms of intercept probability. It is proved that the MSRRS and MSCRS schemes have the same secrecy performance. Two parameters called average number gain and average cost gain are proposed to show the performance of the proposed relay selection schemes. Numerical results demonstrated that with 10 relay nodes, the proposed relay selection schemes can achieve, respectively, 3.7 dB and 1.9 dB’s improvements in terms of the reduced intercept probability and the enhanced secrecy capacity compared to the traditional round-robin scheme.     

A Comparison of Energy Detectability Models for Cognitive Radios in Fading Environments

In this work, we first analyze the accuracy of different energy detector models in approximating the exact solution in AWGN. These models motivate us to develop approximation analysis to address energy detection for fading channels. Our analysis develops approximation that has almost the same performance as the exact solution in Rayleigh channels. Our new model is simple enough to derive the relationship between the required number of samples (N) and the signal-to-noise ratio for a single Rayleigh channel similar to the one obtained for AWGN channels. We also define a fading margin for link budget calculations that relates N in fading channels to AWGN channels. Furthermore, we analyze the impact of multiple antennas for cognitive radios considering two receiver diversity schemes and quantify the improvement in performance regarding this margin. All the analytical results derived in this paper are verified by simulations. Finally, we have implemented and verified energy detection models in our multiple antenna wireless testbed.     

Performance Evaluation of Cognitive Radios: Metrics, Utility Functions, and Methodology

Performance evaluation of cognitive radio (CR) networks is an important problem but has received relatively limited attention from the CR community. Unlike traditional radios, a cognitive radio may change its objectives as radio scenarios vary. Because of the dynamic pairing of objectives and contexts, it is imperative for cognitive radio network designers to have a firm understanding of the interrelationships among goals, performance metrics, utility functions, link/network performance, and operating environments. In this paper, we first overview various performance metrics at the node, network, and application levels. From a game-theoretic viewpoint, we then show that the performance evaluation of cognitive radio networks exhibits the interdependent nature of actions, goals, decisions, observations, and context. We discuss the interrelationships among metrics, utility functions, cognitive engine algorithms, and achieved performance, as well as various testing scenarios. We propose the radio environment map-based scenario-driven testing (REM-SDT) for thorough performance evaluation of cognitive radios. An IEEE 802.22 WRAN cognitive engine testbed is presented to provide further insights into this important problem area.     

Differential Modulation and Relay Selection With Detect-and-Forward Cooperative Relaying

In this paper, we consider the bit error rate (BER) performance analysis of a cooperative relay communication system with differential modulation and selective relaying, where the best relay is selected to forward the signals from the source to the destination. The advantage of differential modulation is that it allows noncoherent detection without requiring the full channel-state information (CSI), as obtaining CSI in a cooperative communication system at each node is a complicated and costly task. We consider a simple detect-and-forward (DetF) relay system and take into account the effect of imperfect detection at the relay. We derive the average bit error probability of the DetF relay scheme by using the closed-form relay link signal-to-noise ratio (SNR). Based on the analysis, we show that the proposed system with $N$ relay nodes and the simple relay selection can achieve a total diversity gain of $N + 1$. The BER performance of the system is also evaluated by simulation. It is shown that the analysis matches well with the simulation results.      

Single-User and Multicast OFDM Power Loading With Nonregenerative Relaying

In this paper, we derive power-loading strategies for single-user and multicast orthogonal frequency-division multiplexing (OFDM) wireless links in the presence of a nonregenerative relay node. Our approach is based on the minimization of the vector error rate at the destination subject to individual power constraints at the source and the relay. For the single-user case, we propose a joint max–min power-loading strategy that is optimal at large signal-to-noise ratios (SNRs), and in doing so, we show how the original problem may be recast into an effective (no-relay) OFDM power-loading problem with a previously known solution. Using bounds on effective SNRs, we propose disjoint power-loading strategies that require channel state information either at the source or at the relay but not at both. For the multicast scenario, we propose a max–min equalizing solution that yields a minimum effective SNR at each user terminal. We also formulate several linear solutions based on prioritizing the users in the network. Our single-user simulations show considerable error-rate performance gains for the proposed joint max–min solution and gives hints as to where disjoint power loading is the most effective. Our multicast simulations show that the max–min equalizing solution is best suited for networks with random user locations, whereas the linear prioritizing solutions work best for networks with fixed user locations.      

Security Performance of Underlay Cognitive Relaying Networks with Energy Harvesting

Wireless Personal Communications - An unlicensed relay is located between an unlicensed sender and an unlicensed recipient for securing the sender’s message against a wire-tapper in underlay...