Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> The unifying framework of the spectral-correlation theory of cyclostationary signals is used to present a broad treatment of weak, random signal detection for interception purposes. The relationships among a variety of previously proposed ad hoc detectors, optimum detectors, and newly proposed detectors are established. The spectral-correlation-plane approach to the interception problem is put forth as especially promising for detection, classification, and estimation in particularly difficult environments involving unknown and changing noise levels and interference activity. A fundamental drawback of the popular radiometric methods in such environments is explained. > <s> BIB001 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> One of the prime goals of a reconfigurable terminal is that it shall work on a great range of systems and standards. Amongst the many features that are required to support this inter-system roaming is the ability to detect, discover or identify alternative systems. Having located alternative systems, the terminal then needs the ability to connect to them and also monitor them in order to determine how viable the level of service offered by each one is. This paper is an introductory study of the problems that a software defined radio (SDR) terminal will face during its initial power-on in a geographical area where no a priori information about the available networks is provided to the terminal. <s> BIB002 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> Key technical aspects concerning the realization of a reconfigurable user terminal based on software-defined radio technology are discussed. These include functionalities such as mode monitoring, mode switching, adaptive baseband processing, and software download. The implications of these functionalities and the need for related entities within the network are also discussed. The concepts presented here form part of the 5th Framework IST European Research project Transparent Reconfigurable Ubiquitous Terminal (TRUST). It is the intention of this article to present the technical achievements of the TRUST project together with an overview of the potential complexity in the realization of a reconfigurable user terminal. <s> BIB003 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> Reconfigurable software radio equipment is seen as the key technology in the evolution of mobile communications. One of the most important properties of a software radio terminal is that it is capable of using a wide range of air interface standards, providing a seamless interoperability between different standards and an enhanced roaming capability. One of the key functions supporting this multimode operation is the air interface recognition. A Software radio terminal has to be capable of detecting, recognizing and monitoring the air interfaces available in the frequency environment. In our paper, we propose exploiting the distinct cyclostationary properties of different air interface signals as features for air interface recognition. <s> BIB004 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> There are new system implementation challenges involved in the design of cognitive radios, which have both the ability to sense the spectral environment and the flexibility to adapt transmission parameters to maximize system capacity while coexisting with legacy wireless networks. The critical design problem is the need to process multigigahertz wide bandwidth and reliably detect presence of primary users. This places severe requirements on sensitivity, linearity and dynamic range of the circuitry in the RF front-end. To improve radio sensitivity of the sensing function through processing gain we investigated three digital signal processing techniques: matched filtering, energy detection and cyclostationary feature detection. Our analysis shows that cyclostationary feature detection has advantages due to its ability to differentiate modulated signals, interference and noise in low signal to noise ratios. In addition, to further improve the sensing reliability, the advantage of a MAC protocol that exploits cooperation among many cognitive users is investigated. <s> BIB005 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> In this paper we consider the problem of detecting whether a frequency band is being used by a known primary user. We derive fundamental bounds on detection performance in low SNR in the presence of noise uncertainty - the noise is assumed to be white, but we know its distribution only to within a particular set. For clarity of analysis, we focus on primary transmissions that are BPSK-modulated random data without any pilot tones or training sequences. The results should all generalize to more general primary transmissions as long as no deterministic component is present. Specifically, we show that for every 'moment detector' there exists an SNR below which detection becomes impossible in the presence of noise uncertainty. In the neighborhood of that SNR wall, we show how the sample complexity of detection approaches infinity. We also show that if our radio has a finite dynamic range (upper and lower limits to the voltages we can quantize), then at low enough SNR, any detector can be rendered useless even under moderate noise uncertainty. <s> BIB006 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> In future applications, next generation spectrally agile software defined radios (SASDR) will be equipped with advanced radio frequency (RF) sensors that will enable them to sense their environment and adaptively select the best and most stable portions of the spectrum to establish communications. This paper presents a robust method for estimating locally the unused or underutilized regions of RF spectrum to optimize dynamic spectrum usage. These adaptive algorithms are described and results of "open spectrum" estimation performed on real RF spectrum measurements illustrate the robust performance of the approach. Simulation results are shown along with results from real measurements that describe the RF resources in rural, suburban and urban areas in and around Philadelphia in the VHF and UHF bands. The measurements represent a good dynamic data set including motion and spatio-temporal diversity. The effect on reducing the risk of interference by collaborating across multiple nodes that provides increased robustness of sensing of open spectrum through spatial diversity is also discussed and quantitative values for the interference mitigation under collaborative behaviors are also presented <s> BIB007 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> MB-OFDM ultra wideband radio will have to coexist with narrowband radio services that operate over the 3.1-10.6 GHz band. In order to coexist with these other radio systems without becoming a source of harmful interference, MB-OFDM system designers will have to develop methods to dynamically detect their presence so that the appropriate avoidance techniques can be implemented whenever it is determined that MB-OFDM transmissions will pose a source of harmful interference. This paper investigates the feasibility of detecting a downlink transmission from an indoor fixed service (FS) system when the MB-OFDM radio collects measurements over a number of consecutive OFDM symbol periods. The analysis shows that when the fixed service signal received by the MB-OFDM device is at a power level of -98 dBm, that it is possible to achieve a false alarm probability of 10-4 and a missed detection probability of ~10-6 when using the measurements that are obtained over 70 consecutive OFDM symbol periods (~22 us). Since the fixed service sub-channel time duration is 200 us, our results imply that the required measurement time is reasonable with respect to the duration of time that the fixed service signal is on the air <s> BIB008 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> Cognitive radio systems offer the opportunity to improve spectrum utilization by detecting unoccupied spectrum bands and adapting the transmission to those bands while avoiding the interference to primary users. This novel approach to spectrum access introduces unique functions at the physical layer: reliable detection of primary users and adaptive transmission over a wide bandwidth. In this paper, we address design issues involved in an implementation of these functions that could limit their performance or even make them infeasible. The critical design problem at the receiver is to achieve stringent requirements on radio sensitivity and perform signal processing to detect weak signals received by a wideband RF front-end with limited dynamic range. At the transmitter, wideband modulation schemes require adaptation to different frequency bands and power levels without creating interference to active primary users. We introduce algorithms and techniques whose implementation could meet these challenging requirements <s> BIB009 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> Today, the largest and most desirable portion of the radio spectrum is allocated to licensed services, which has resulted in the well-known profound scarcity of this resource for emerging applications. With the rapid growth of wireless technologies, current spectrum scarcity has become a serious problem as more and more wireless applications compete for very little spectrum. On the other hand, the licensed spectrum allocated to applications such as television, cellular telephony and public safety show very little usage over time at different geographical locations. This has, therefore, seriously impaired the evolution of newer technologies because of current regulatory constraints on the operation in licensed spectrum, such as TV bands, and is being addressed by FCC through recent rule makings. With the goal of ubiquitous communication in mind, we look into spectrum agile radios as a new technology enabled by such emerging regulatory rulings and study its advantages over conventional radios. Initially, we provide a simple mathematical modeling to understand the utilization that is achievable by spectrum agile radios. Next, we investigate several issues related to spectrum sensing, as it is one of the key pillars to realize spectrum agile radios. Through sensing, the spectrum agile radio identifies the so-called "white-spaces" in the spectrum and then decides whether to occupy those white spaces opportunistically to transmit data. We also discuss the concept of interference temperature introduced by the FCC, and propose a spectrum-aware sensor network as a way to address it Finally, we extend this spectrum-aware sensor networks to introduce a new sensing architecture to identify and locate white spaces in the spectrum <s> BIB010 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> Channel sensing and spectrum allocation has long been of interest as a prospective addition to cognitive radios for wireless communications systems occupying license-free bands. Conventional approaches to cyclic spectral analysis have been proposed as a method for classifying signals for applications where the carrier frequency and bandwidths are unknown, but is, however, computationally complex and requires a significant amount of observation time for adequate performance. Neural networks have been used for signal classification, but only for situations where the baseband signal is present. By combining these techniques a more efficient and reliable classifier can be developed where a significant amount of processing is performed offline, thus reducing online computation. In this paper we take a renewed look at signal classification using spectral coherence and neural networks, the performance of which is characterized by Monte Carlo simulations <s> BIB011 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> This paper investigates the performance of FFT- type feature TV detector of weak signals, which can be used in listen-before-talk (LBT) spectrum access algorithms. Using FFT plots applied to consecutive sets of measurements collected at multiple locations, we illustrate and characterize different types of spurious signal environments. Ultra sensitive TV detectors can significantly improve the overall system performance in terms of available TV channels versus transmit power. We show the system performance curves based on real measurements collected at multiple locations <s> BIB012 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> Recent study by FCC spectrum policy task force (SPTF) found that while the available spectrum becomes increasingly scarce, the assigned spectrum is significantly underutilized. Cognitive radio technology holds the key promise to solve such problems. In this paper, we investigate some physical layer issues of wide-band cognitive radio systems. Specifically, we discuss the physical layer signal structure to support flexible operations required by wide-band cognitive radios. We analyze the performances of generalized power sensing and waveform sensing. And we study the adjacent channel interference mechanisms focusing on the interference caused by time-domain signal truncation <s> BIB013 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> Spectrum sensing is one of the most challenging problems in cognitive radio systems. The spectrum of interest needs to be characterized and unused frequencies should be identified for possible exploitation. This process, however, should be computationally simple and fast in order to catch up with the changing transmission parameters. This paper proposes a sensing method for identifying the unused spectrum for opportunistic transmission by estimating the RF transmission parameters of primary users. The primary users are identified by matching the a priory information about their transmission characteristics to the features extracted from the received signal. The application of the proposed sensing method to WiMAX mobile stations for finding the active channels during initial network entry is also discussed as a case study. <s> BIB014 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> Cognitive Radio (CR) is a promising technology that can alleviate the spectrum shortage problem by enabling unlicensed users equipped with CRs to coexist with incumbent users in licensed spectrum bands without inducing interference to incumbent communications. Spectrum sensing is one of the essential mechanisms of CRs that has attracted great attention from researhers recently. Although the operational aspects of spectrum sensing are being investigated actively, its security aspects have garnered little attention. In this paper, we describe an attack that poses a great threat to spectrum sensing. In this attack, which is called the primary user emulation (PUE) attack, an adversary's CR transmits signals whose characteristics emulate those of incumbent signals. The highly flexible, software-based air interface of CRs makes such an attack possible. Our investigation shows that a PUE attack can severely interfere with the spectrum sensing process and significantly reduce the channel resources available to legitimate unlicensed users. As a way of countering this threat, we propose a transmitter verification procedure that can be integrated into the spectrum sensing mechanism. The transmitter verification procedure employs a location verification scheme to distinguish incumbent signals from unlicensed signals masquerading as incumbent signals. Two alternative techniques are proposed to realize location verification: Distance Ratio Test and Distance Difference Test. We provide simulation results of the two techniques as well as analyses of their security in the paper. <s> BIB015 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> Under the current system of spectrum allocation, rigid partitioning has resulted in vastly underutilized spectrum bands, even in urban locales. Cognitive radios have been proposed as a way to reuse this underutilized spectrum in an opportunistic manner. To achieve this reuse while guaranteeing non-interference with the primary user, cognitive radios must detect very weak primary signals. However, uncertainties in the noise+interference impose a limit on how low of a primary signal can be robustly detected. ::: In this paper, we show that the presence/absence of possible interference from other opportunistic spectrum users represents a major component of the uncertainty limiting the ability of a cognitive radio network to reclaim a band for its use. Coordination among nearby cognitive radios is required to control this uncertainty. While this coordination can take a form similar to a traditional MAC protocol for data communication, its role is different in that it aims to reduce the uncertainty about interference rather than just reducing the interference itself. ::: We show how the degree of coordination required can vary based on the coherence times and bandwidths involved, as well as the complexity of the detectors themselves. The simplest sensing strategies end up needing the most coordination, while more complex strategies involving adaptive coherent processing and interference prediction can be individually more robust and thereby reduce the need for coordination across different networks. We also show the existence of a coordination radius wall which limits secondary user densities that can be supported irrespective of coordination involved. Furthermore, local cooperation among cognitive radios for collective decision making can reduce the fading margins we need to budget for. This cooperation benefits from increased secondary user densities and hence induces a minima in the power-coordination tradeoff. <s> BIB016 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> In this paper we present an experimental study that comprehensively evaluates the performance of three different detection methods proposed for sensing of primary user signals in cognitive radios. For pilot and energy detection, our measurement results confirmed the theoretical expectations on sensing time performance. However, a physical implementation of these detectors in the presence of real noise uncertainties, analog impairments and interference allowed us to establish practical bounds on the detectable signal levels. In the case of collaborative detection, our analysis of experimental data collected in indoor environments identified the design parameters that can significantly improve the sensing gain: adaptive threshold, spatial separation and multiple antennas. <s> BIB017 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> Opportunistic spectrum usage would enable enhancing the efficiency of existing and emerging wireless communication systems. One of the key issues related to those systems is spectrum opportunity estimation. In this paper, we are using a technique utilized earlier in narrowband signal detection, namely the localization algorithm based on double-thresholding (LAD), for sensing the existence of primary user signals in a cognitive radio systems. The LAD method requires no a priori information on the primary user statistics and it has a low computational complexity that will enable a low-cost real-time implementation. The LAD method is able to estimate the number of narrowband signals and their characteristics, including bandwidth and power. A simplified version of the LAD method which uses normalized thresholds (NT) as well as an enhancement of the scheme that uses adjacent cluster combining (ACC) are proposed. Simulation results show that the simplified version of the LAD method is useful in the considered situations, and the enhanced version of the LAD method improves the performance of the LAD and LAD NT methods significantly. <s> BIB018 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> Cognitive radios sense the radio spectrum in order to find unused frequency bands and use them in an agile manner. Transmission by the primary user must be detected reliably even in the low signal-to-noise ratio (SNR) regime and in the face of shadowing and fading. Communication signals are typically cyclostationary, and have many periodic statistical properties related to the symbol rate, the coding and modulation schemes as well as the guard periods, for example. These properties can be exploited in designing a detector, and for distinguishing between the primary and secondary users' signals. In this paper, a generalized likelihood ratio test (GLRT) for detecting the presence of cyclostationarity using multiple cyclic frequencies is proposed. Distributed decision making is employed by combining the quantized local test statistics from many secondary users. User cooperation allows for mitigating the effects of shadowing and provides a larger footprint for the cognitive radio system. Simulation examples demonstrate the resulting performance gains in the low SNR regime and the benefits of cooperative detection. <s> BIB019 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> In this paper, detection of digital TV signals is studied since there is strong motivation to better utilize the idle TV channels to improve spectral efficiency, as seen in the efforts of IEEE 802.22 Wireless Regional Area Networks (WRAN) international standard activities. Cyclostationary feature of DVB signals is explored so that the robust single-cycle detector (SCD) can be employed to handle the problem of noise uncertainty, to which the widely used energy detector (ED) is susceptible. Due to complexity concerns for real-time implementation, a blind twin peak (BTP) detector is proposed to approximate the performance of SCD while keep the complexity as low as ED. <s> BIB020 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> This paper presents cyclostationarity-inducing transmission techniques that enable the receiver to distinguish among different systems employing OFDM (Orthogonal Frequency Division Multiplexing) as a common air interface. Several systems using different air interfaces may have different properties of cyclostationarity that can be utilized to distinguish among them. However, this is usually not possible when the same air interface is used. To address this problem for systems using OFDM as an air interface, we consider methods that induce different properties of cyclostationarity in the transmitter of different systems. Specifically, we configure the OFDM signal before transmission such that its cyclic autocorrelation function (CAF) has peaks at certain pre-chosen cycle frequencies. The difference in the cycle frequencies, at which the CAF peaks occur for the configured OFDM signals, is utilized to distinguish among several systems even when OFDM is used as a common air interface. Two configuration methods for the OFDM signal are proposed. The first proposed method inserts a specific preamble at the head of each OFDM frame. Each preamble is configured such that only a selected subset of subcarriers is used for transmission. A different subset of subcarriers results in the occurrence of CAF peaks at different cycle frequencies for the OFDM signal. The second proposed method is based on dedicating a few subcarriers in an OFDM frame to the transmission of specific signals so that the whole frame exhibits cyclostationarity at pre-chosen cycle frequencies. For this method, we introduce a method for generating signals on the dedicated subcarriers and describe their relation to the cycle frequencies of the configured OFDM frame. Both proposed methods are evaluated by computer simulation in a multipath Rayleigh fading environment. Simulation results show that system recognition based on the proposed methods exhibits excellent detection probability. <s> BIB021 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> Cognitive radio-based Open Spectrum systems offer a solution to the issue of spectrum scarcity by allowing wireless networks to dynamically access spectrum while coordinating to co-exist and avoid the creation of harmful interference. However, before a practical open spectrum system may be implemented, a number of significant technical and policy challenges must be overcome. One such technical challenge is the distributed coordination of operating frequencies and bandwidths between co-existing systems. Cyclostationary signatures have been shown to be a powerful tool in overcoming this challenge. A cyclostationary signature is a unique identifier or watermark which may be embedded in the physical properties of a communications signal. Such signatures may be used to aid peer devices in performing a number of critical tasks, including signal detection, classification and frequency acquisition. A key limitation of cyclostationary signatures when implemented in orthogonal frequency division multiplex (OFDM)-based systems is the sensitivity exhibited in time-variant multipath Rayleigh fading environments. Although OFDM-based systems offer robust performance under multipath conditions, detection of cyclostationary signatures can be severely degraded. As signature detection is adversely affected, the ability of Open Spectrum Systems to coordinate and coexist is seriously undermined. This paper therefore presents techniques for effectively over-coming the issue of multipath Rayleigh fading in the detection of cyclostationary signatures for Open Spectrum systems. Approaches for the generation and detection of signatures in OFDM-based waveforms are outlined and improvements in detection performance are illustrated using simulation results. <s> BIB022 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> Spectrum sensing is crucial for dynamic spectrum management systems. In this paper, we propose a scheme that utilizes the guard interval of OFDM symbol at the transmitter for spectrum sensing. The cyclic prefix is not inserted in the guard interval at the transmitter, whereas the circulant convolution is secured at the OFDM receiver through the proposed mechanism. Simulation results show that the scheme can be implemented with no impact on the BER under various channel conditions, and detection of incumbent DTV signal is possible in the OFDM guard interval. In addition, we develop enhancements to the circulant convolution preserving mechanism for handling the transceiver imperfections in practice. <s> BIB023 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> In a cognitive radio network, the secondary users are allowed to utilize the frequency bands of primary users when these bands are not currently being used. To support this spectrum reuse functionality, the secondary users are required to sense the radio frequency environment, and once the primary users are found to be active, the secondary users are required to vacate the channel within a certain amount of time. Therefore, spectrum sensing is of significant importance in cognitive radio networks. There are two parameters associated with spectrum sensing: probability of detection and probability of false alarm. The higher the probability of detection, the better the primary users are protected. However, from the secondary users' perspective, the lower the probability of false alarm, the more chances the channel can be reused when it is available, thus the higher the achievable throughput for the secondary network. In this paper, we study the problem of designing the sensing duration to maximize the achievable throughput for the secondary network under the constraint that the primary users are sufficiently protected. We formulate the sensing-throughput tradeoff problem mathematically, and use energy detection sensing scheme to prove that the formulated problem indeed has one optimal sensing time which yields the highest throughput for the secondary network. Cooperative sensing using multiple mini-slots or multiple secondary users are also studied using the methodology proposed in this paper. Computer simulations have shown that for a 6 MHz channel, when the frame duration is 100 ms, and the signal-to-noise ratio of primary user at the secondary receiver is -20 dB, the optimal sensing time achieving the highest throughput while maintaining 90% detection probability is 14.2 ms. This optimal sensing time decreases when distributed spectrum sensing is applied. <s> BIB024 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> Spectrum sensing is an essential enabling functionality for cognitive radio networks to detect spectrum holes and opportunistically use the under-utilized frequency bands without causing harmful interference to legacy networks. This paper introduces a novel wideband spectrum sensing technique, called multiband joint detection, which jointly detects the signal energy levels over multiple frequency bands rather than consider one band at a time. The proposed strategy is efficient in improving the dynamic spectrum utilization and reducing interference to the primary users. The spectrum sensing problem is formulated as a class of optimization problems in interference limited cognitive radio networks. By exploiting the hidden convexity in the seemingly non-convex problem formulations, optimal solutions for multiband joint detection are obtained under practical conditions. Simulation results show that the proposed spectrum sensing schemes can considerably improve the system performance. This paper establishes important principles for the design of wideband spectrum sensing algorithms in cognitive radio networks. <s> BIB025 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> This paper considers the detection of the presence/absence of signals in uncertain low SNR environments. Small modeling uncertainties are unavoidable in any practical system and so robustness to them is a fundamental performance metric. The impact of these modeling uncertainties can be quantified by the position of the "SNR wall" below which a detector will fail to be robust, no matter how long it can observe the channel. We propose simple mathematical models for the uncertainty in the noise and fading processes. These are used to show what aspects of the model lead to SNR walls for differing levels of knowledge of the signal to be detected. These results have implications for wireless spectrum regulators. The context is opportunistically sharing spectrum with primary users that must be detected in order to avoid causing harmful interference on a channel. Ideally, a secondary system would be able to detect primaries robustly without having to know much about their signaling strategies. We argue that the tension between primary and secondary users is captured by the technical question of computing the optimal tradeoff between the primary user's capacity and the secondary user's sensing robustness as quantified by the SNR wall. This is an open problem, but we compute this tradeoff for some simple detectors. <s> BIB026 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> Merging the fundamental principles of resource allocation with the state-of-the-art in research and application examples, Han and Liu present a novel and comprehensive perspective for improving wireless systems performance. Cross-layer multiuser optimization in wireless networks is described systematically. Starting from the basic principles, such as power control and multiple access, coverage moves to the optimization techniques for resource allocation, including formulation and analysis, and game theory. Advanced topics such as dynamic resource allocation and resource allocation in antenna array processing, and in cooperative, sensor, personal area, and ultrawideband networks, are then discussed. Unique in its scope, timeliness, and innovative author insights, this invaluable work will help graduate students and researchers to understand the basics of wireless resource allocation whilst highlighting modern research topics, and will help industrial engineers to improve system optimization. <s> BIB027 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> In the dynamic spectrum access (DSA) type of cognitive radios, secondary users need to detect the signals from the primary system prior to communicating in the sharing band. Hence, spectrum sensing is an important function for DSA. Key requirements for spectrum sensing in realistic radio environments are stable performance in detecting the primary signals as well as robustness against noise uncertainty at the secondary device or interference signals from other secondary systems. This paper proposes a novel spectrum-sensing method based on maximum cyclic autocorrelation selection (MCAS), which exhibits good detection performance and robustness against noise uncertainty and interference with low computational complexity. Our MCAS-based spectrum-sensing method is used to detect whether the primary signal is present or not, by comparing the peak and non-peak values of the cyclic autocorrelation function (CAF). Our MCAS-based spectrum-sensing method does not require noise variance estimation. Furthermore, it is robust against noise uncertainty and interference signals. Through computer simulations, we found that our method performs as well as or better than conventional sensing methods and is robust against noise uncertainty and interference signals. Therefore, it could be a practical candidate in realistic radio environments. <s> BIB028 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> In this paper we consider the problem of robustly detecting the presence/absence of signals in uncertain low SNR environments. Our previous results have shown the existence of fundamental SNR thresholds called SNR walls below which robust detection is impossible. For signals with narrowband pilots, we had shown earlier that a simple modification to the matched filter provides significant robustness gains. This technique is called runtime noise calibration. In this paper we show that runtime noise calibration can also be applied to improve the robustness of other feature detectors. We use a 50% duty-cycle pulse-amplitude-modulated signal as an example to illustrate the gains from noise calibration. Robustness results for this example give us important insights which also apply in the general case of cyclostationary signals. Our results suggests that for cyclostationary signals, frequency-selective fading and uncertain noise color are the main reasons for detector non-robustness. Furthermore, we introduce the notion of delay coherence for frequency-selective channels. We also characterize the location of the SNR wall as a function of the delay-coherence time of the channel. <s> BIB029 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> In this paper, we investigate an optimization of threshold level with energy detection to improve the spectrum sensing performance. Determining threshold level to minimize spectrum sensing error both reduces collision probability with primary user and enhances usage level of vacant spectrum, resulting in improving total spectrum efficiency. However, when determining threshold level, spectrum sensing constraint should also be satisfied since it guarantees minimum required protection level of primary user and usage level of vacant spectrum. To minimize spectrum sensing error for given spectrum sensing constraint, we derive an optimal adaptive threshold level by utilizing the spectrum sensing error function and constraint which is given by inequality condition. Simulation results show that the proposed scheme provides better spectrum sensing performance compared to conventional schemes. <s> BIB030 </s> Advances in cognitive radio networks: A survey <s> 1) Energy Detector: <s> Spectrum sensing is one of the enabling functionalities for cognitive radio (CR) systems to operate in the spectrum white space. To protect the primary incumbent users from interference, the CR is required to detect incumbent signals at very low signal-to-noise ratio (SNR). In this paper, we present a spectrum sensing technique based on correlating spectra for detection of television (TV) broadcasting signals. The basic strategy is to correlate the periodogram of the received signal with the a priori known spectral features of the primary signal. We show that according to the Neyman-Pearson criterion, this spectra correlation-based sensing technique is asymptotically optimal at very low SNR and with a large sensing time. From the system design perspective, we analyze the effect of the spectral features on the spectrum sensing performance. Through the optimization analysis, we obtain useful insights on how to choose effective spectral features to achieve reliable sensing. Simulation results show that the proposed sensing technique can reliably detect analog and digital TV signals at SNR as low as -20 dB. <s> BIB031
Energy detection is the most common type of spectrum sensing because it is easy to implement and requires no prior knowledge about the primary signal. Assume the hypothesis model of the received signal is (2) where is the primary user's signal to be detected at the local receiver of a secondary user, is the additive white Gaussian noise, and is the channel gain from the primary user's transmitter to the secondary user's receiver. is a null hypothesis, meaning there is no primary user present in the band, while means the primary user's presence. The detection statistics of the energy detector can be defined as the average (or total) energy of observed samples (3) The decision on whether the spectrum is being occupied by the primary user is made by comparing the detection statistics with a predetermined threshold . The performance of the detector is characterized by two probabilities: the probability of false alarm and the probability of detection . denotes the probability that the hypothesis test decides while it is actually , i.e., denotes the probability that the test correctly decides , i.e., (5) A good detector should ensure a high detection probability and a low false alarm , or it should optimize the spectrum usage efficiency (e.g., QoS of a secondary user network) while guaranteeing a certain level of primary user protection. To this end, various approaches have been proposed to improve the efficiency of energy detector based spectrum sensing. Since the detection performance is very sensitive to the noise power estimate error BIB006 , an adaptive noise level estimation approach is proposed in BIB007 , where multiple signal classification algorithm is used to decouple the noise and signal subspaces and estimate the noise floor. A constant false alarm rate threshold is further computed to study the spectrum occupancy and its statistics. A well chosen detection threshold can minimize spectrum sensing error, provide the primary user with enough protection, and fully enhance spectrum utilization. In  , the detection threshold is optimized iteratively to satisfy the requirement on false alarm probability. Threshold optimization subject to spectrum sensing constraints is investigated in BIB030 , where an optimal adaptive threshold level is developed by utilizing the spectrum sensing error function. In  , forward methods for energy detection are proposed, where the noise power is unknown and is adaptively estimated. In order to find and localize narrowband signals, a localization algorithm based on double-thresholding (LAD) is proposed in BIB018 , where the usage of two thresholds can provide signal separation and localization. The LAD method is a blind narrowband signal detection, and no information about the noise level nor narrowband signals are required. The LAD method with normalized thresholds can reduce computational complexity without performance loss, and the estimation of the number of narrowband signals becomes more accurately by adjacent cluster combining. The sensing throughput tradeoff of energy detection is studied in BIB024 , where the sensing period duration in a time slot is optimized to maximize the achievable throughput for the secondary users under the constraint that the primary users are sufficiently protected. A novel wideband spectrum sensing technique based on energy detection is introduced in BIB025 , which jointly detects the signal energy levels over multiple frequency bands in order to improve the opportunistic throughput of CRs and reduce their interference to the primary systems. The analysis in BIB008 shows that detection of narrowband transmission using energy detection over multi-band orthogonal frequency-division multiplexing (OFDM) is feasible, and can be further extended to cover more complex systems. Besides its low computational and implementation complexity and short detection time, there also exist some challenges in designing a good energy detector. First, the detection threshold depends on the noise power, which may change over time and hence is difficult to measure precisely in real time. In low signal-to-noise ratio (SNR) regimes where the noise power is very high, reliable identification of a primary user is even impossible BIB026 . Moreover, an energy detector can only decide the primary user's presence by comparing the received signal energy with a threshold; thus, it cannot differentiate the primary user from other unknown signal sources. As such, it can trigger false alarm frequently. 2) Feature Detector: There are specific features associated with the information transmission of a primary user. For instance, the statistics of the transmitted signals in many communication paradigms are periodic because of the inherent periodicities such as the modulation rate, carrier frequency, etc. Such features are usually viewed as the cyclostationary features, based on which a detector can distinguish cyclostationary signals from stationary noise. In a more general sense, features can refer to any intrinsic characteristics associated with a primary user's transmission, as well as the cyclostationary features. For example, center frequencies and bandwidths BIB014 extracted from energy detection can also be used as reference features for classification and determining a primary user's presence. In this section, we will introduce the cyclostationary feature detection followed by a generalized feature detection. Cyclostationary feature detection was first introduced in BIB001 . As in most communication systems, the transmitted signals are modulated signals coupled with sine wave carriers, pulse trains, hopping sequences, or cyclic prefixes, while the additive noise is generally wide-sense stationary (WSS) with no correlation, cyclostationary feature detectors can be utilized to differentiate noise from primary users' signal BIB009 - BIB010 and distinguish among different types of transmissions and primary systems BIB019 . Different from an energy detector which uses time-domain signal energy as test statistics, a cyclostationary feature detector performs a transformation from the time-domain into the frequency feature domain and then conducts a hypothesis test in the new domain. Specifically, define the cyclic autocorrelation function (CAF) of the received signal by BIB027 where is the expectation operation, denotes complex conjugation, and is the cyclic frequency. Since periodicity is a common property of wireless modulated signals, while noise is WSS, the CAF of the received signal also demonstrates periodicity when the primary signal is present. Thus, we can represent the CAF using its Fourier series expansion, called the cyclic spectrum density (CSD) function, expressed as BIB001 (7) The CSD function have peaks when the cyclic frequency equals to the fundamental frequencies of the transmitted signal , i.e., with being the period of . Under hypothesis , the CSD function does not have any peaks since the noise is non-cyclostationary signals. A peak detector BIB020 or a generalized likelihood ratio test BIB004 , BIB019 can be further used to distinguish among the two hypothesis. Different primary communication systems using different air interfaces (modulation, multiplexing, coding, etc.) can also be differentiated by their different properties of cyclostationarity. However, when OFDM becomes the air interface as suggested by several wireless communication standards, identification of different systems may become problematic, since the features due to the nature of OFDM signaling are likely to be close or even identical. To address this issue, particular features need to be introduced to OFDM-based communications. In BIB021 , methods that induce different properties of cyclostationarity to different systems are considered. The OFDM signal is configured before transmission so that its CAF outputs peaks at certain pre-chosen cycle frequencies, and the difference in these frequencies is used to distinguish among several systems under the same OFDM air interface. A similar approach is considered in BIB022 . Compared to energy detectors that are prone to high false alarm probability due to noise uncertainty and cannot detect weak signals in noise, cyclostationary detectors become good alternatives because they can differentiate noise from primary users' signal and have better detection robustness in low SNR regime. A spectrum sensing method based on maximum cyclic autocorrelation selection has been proposed in BIB028 , where the peak and non-peak values of the cyclic autocorrelation function are compared to determine whether the primary signal is present or not. This method does not require noise variance estimation, and is robust against noise uncertainty and interference signals. Frequency-selective fading and uncertain noise impair the robustness of cyclostationary signal detection in low SNR environments. Run time noise calibration has been considered in BIB026 and BIB029 , in order to improve detector robustness. The method exploits the in-band measurements at frequencies where a pilot is absent to calibrate the noise statistics at the pilot frequencies. By combining neural network for signal classification with cyclic spectral analysis, a more efficient and reliable classifier is developed in BIB011 . Since a large amount of processing is performed offline using neural networks, the online computation for signal classification is greatly reduced. Generalized feature detection refers to detection and classification that extracts more feature information other than the cyclostationarity due to the modulated primary signals, such as the transmission technologies used by a primary user, the amount of energy and its distribution across different frequencies BIB002 , BIB003 , channel bandwidth and its shape BIB014 ,  , power spectrum density BIB031 , center frequency BIB014 , idle guard interval of OFDM BIB023 , FFT-type feature BIB012 , etc. By matching the features extracted from the received signal to the a priori information about primary users' transmission characteristics, primary users can be identified. Location information of the primary signal is also an important feature that can be used to distinguish a primary user from other signal sources. Under primary user emulation attack, a malicious secondary user transmits signals whose characteristics emulate those of the primary signals. A transmitter verification scheme is proposed in BIB015 to secure trustworthy spectrum sensing based on the location verification of the primary user. 3) Matched Filtering and Coherent Detection: If secondary users know information about a primary user' signal a priori, then the optimal detection method is the matched filtering  , since a matched filter can correlate the already known primary signal with the received signal to detect the presence of the primary user and thus maximize the SNR in the presence of additive stochastic noise. The merit of matched filtering is the short time it requires to achieve a certain detection performance such as a low probability of missed detection and false alarm  , since a matched filter needs less received signal samples. However, the required number of signal samples also grows as the received SNR decreases, so there exists a SNR wall BIB026 for a matched filter. In addition, its implementation complexity and power consumption is too high BIB005 , because the matched filter needs receivers for all types of signals and corresponding receiver algorithms to be executed. Matched filtering requires perfect knowledge of the primary user's signal, such as the operating frequency, bandwidth, modulation type and order, pulse shape, packet format, etc. If wrong information is used for matched filtering, the detection performance will be degraded a lot. On the other hand, most wireless communication systems exhibit certain patterns, such as pilot tones, preambles, midambles, spreading codes, and etc., which are used to assist control, equalization, synchronization, continuity, or reference purposes. Even though perfect information of a primary user's signal may not be attainable, if a certain pattern is known from the received signals, coherent detection (a.k.a. waveform-based sensing) can be used to decide whether a primary user is transmitting or not BIB016 . As an example, the procedure of coherent detection using pilot pattern is explained as follows BIB016 . There are two hypothesis in the coherent detection: where is a known pilot tone, is the fraction of energy allocated to the pilot tone, is the desired signals assumed to be orthogonal to the pilot tone, and is additive white noise. The test statistics of the coherent detection is defined as the projected received signal in the pilot direction, i.e.,  with is a normalized unit vector in the direction of the pilot tone. As increases, the test statistics under hypothesis is much greater than that under . By comparing with a pre-determined detection threshold, one can decide the presence of a primary user. Coherent detection can also be performed in frequency domain BIB031 . One can express the binary hypothesis test using the power spectrum density of the received signal , and distinguish between and by exploiting the unique spectral signature exhibited in . Coherent detection is shown to be robust to noise uncertainty, and not limited by the SNR wall BIB016 as is large enough. Moreover, coherent detection outperforms energy detection in the sensing convergence time BIB017 , BIB013 , because the sensing time of energy detection increases quadratically with the SNR reduction, while that of coherent detection only increases linearly BIB013 . However, information about the waveform patterns is a prerequisite for implementing coherent detection; the more precise information a coherent detector has, the better the sensing performance will be.