Spectrum Sensing Methodologies for Cognitive Radio Systems: A Review <s> a) Centralized Cooperative Sensing <s> In cognitive radio systems, cooperative spectrum sensing is conducted among the cognitive users so as to detect the primary user accurately. However, when the number of cognitive users tends to be very large, the bandwidth for reporting their sensing results to the common receiver will be very huge. In this paper, the authors employ a censoring method with quantization to decrease the average number of sensing bits to the common receiver. By censoring the collected local observations, only the users with enough information will send their local one bit decisions (0 or 1) to the common receiver. The performance of spectrum sensing is investigated for both perfect and imperfect reporting channels. Numerical results show that the average number of sensing bits decreases greatly at the expense of a little sensing performance loss. <s> BIB001 </s> Spectrum Sensing Methodologies for Cognitive Radio Systems: A Review <s> a) Centralized Cooperative Sensing <s> In cognitive radio (CR) networks, secondary users can cooperatively sense the spectrum to detect the presence of primary users. In this paper, we propose a fully distributed and scalable cooperative spectrum-sensing scheme based on recent advances in consensus algorithms. In the proposed scheme, the secondary users can maintain coordination based on only local information exchange without a centralized common receiver. Unlike most of the existing decision rules, such as the or-rule or the 1-out-of-N rule, we use the consensus of secondary users to make the final decision. Simulation results show that the proposed consensus scheme can have significant lower missing detection probabilities and false alarm probabilities in CR networks. It is also demonstrated that the proposed scheme not only has proven sensitivity in detecting the primary user's presence but also has robustness in choosing a desirable decision threshold. <s> BIB002 </s> Spectrum Sensing Methodologies for Cognitive Radio Systems: A Review <s> a) Centralized Cooperative Sensing <s> A cooperative approach to the sensing task of wireless cognitive radio (CR) networks is introduced based on a basis expansion model of the power spectral density (PSD) map in space and frequency. Joint estimation of the model parameters enables identification of the (un)used frequency bands at arbitrary locations, and thus facilitates spatial frequency reuse. The novel scheme capitalizes on two forms of sparsity: the first one introduced by the narrow-band nature of transmit-PSDs relative to the broad swaths of usable spectrum; and the second one emerging from sparsely located active radios in the operational space. An estimator of the model coefficients is developed based on the Lasso algorithm to exploit these forms of sparsity and reveal the unknown positions of transmitting CRs. The resultant scheme can be implemented via distributed online iterations, which solve quadratic programs locally (one per radio), and are adaptive to changes in the system. Simulations corroborate that exploiting sparsity in CR sensing reduces spatial and frequency spectrum leakage by 15 dB relative to least-squares (LS) alternatives. <s> BIB003 </s> Spectrum Sensing Methodologies for Cognitive Radio Systems: A Review <s> a) Centralized Cooperative Sensing <s> A Cooperative Cognitive Radio (CCR) network, which integrates a conventional cooperative system and cognitive radios (CRs) into a holistic system, is a promising paradigm for the next generation mobile communication systems. The spectral efficiency, power efficiency, bandwidth reduction and system complexity in CCR networks are the fundamental parameters of our system design and optimization. In this thesis, we focus our attention on opportunistically exploiting the underutilized spectrum band in the CR network with the aid of cooperative protocols. Furthermore, we invoke channel coding schemes in our CCR system in order to improve the overall system throughput. In our CCR system, the overlay based cooperation scheme of Primary Users (PUs) and Cognitive Users (CUs) is considered, which has the potential of leading to a transmission power reduction and transmission rate improvement for both the PU and the CU. More explicitly, our cooperative protocol allows a group of CUs to serve as Relay Nodes (RNs) for relaying the signal of the PUs’ transmitters to the PUs’ intended destinations. To elaborate further, both one way relaying and two-way relaying schemes are used in our proposed system, so that the bandwidth requirement of the PUs is reduced. Alternatively, the freed bandwidth may be leased to a group of CUs for their secondary communications. Our numerical and simulation results show that the bandwidth reduction attained by the proposed two-way relaying based CR scheme may approach as much as 80% of the PU’s bandwidth. Moreover, an Adaptive Dynamic Network Coding (ADNC) scheme is also conceived for this overlay CCR system, which is designed for supporting communications between multiple PUs and a common Base Station (BS). More particularly, the near-instantaneously Adaptive Turbo Trellis Coded Modulation (ATTCM) is employed for appropriately adjusting both the modulation mode as well as the channel coding rate and the network coding rate, according to the near-instantaneous channel conditions. In order to facilitate the recovery of the source information at the BS, the CUs invoke the ADNC technique, which is assisted by our cooperative protocol operating by exchanging the CCR-based control information between the near-instantaneously ATTCM and network coding codec as well as between the CUs and the BS. Additionally, the network encoder may also be activated in its adaptive mode for supporting the CUs, depending on the Boolean value of the feedback flags generated based on the success/failure of the ATTCMdecoder and of the network decoder, which is evaluated and fed back by the BS. Quantitatively, it was found that the joint holistic design of our ATTCM-ADNC-CCR scheme is either capable of freeing up an approximately 40% of the PU’s bandwidth in comparison to its non-cooperative counterpart, or increasing the attainable throughput by as much as 2 bit/symbol. Furthermore, a Pragmatic Distributed Algorithm (PDA) is proposed for supporting the efficient spectral access of multiple PUs and CUs in CCR networks. The novelty of our PDA is that the Pus negotiate with the CUs concerning the specific amount of relaying and transmission time, and the CU will decide either to accept or to decline this offer. These CUs relay the signal received from the PUs to the PUs’ receiver, but only when both the PUs’ and the CUs’ minimum rate requirements are satisfied. Moreover, we show that the cooperative spectral access based on our PDA reaches an equilibrium, when it is repeated for a sufficiently long duration. These benefits are achieved, because the PUs are motivated to cooperate by the incentive of achieving a higher PU rate, whilst defecting from cooperation can be discouraged with the aid of a limited-duration punishment. Therefore, our proposed PDA outperforms the benchmark, despite its significantly lower overhead and lower complexity. Finally, we present the joint design of coding, modulation, user-cooperation and CR techniques, which may lead to significant mutual benefits for both the PUs and the CUs. <s> BIB004
In the centralized cooperative sensing, the central identity also known as fusion center (FC)  controls the three steps of cooperative sensing process. The first stage of the process, FC then chooses a channel or frequency band for sensing then delegates all cooperating cognitive radio to individually perform local sensing  . In the second process, all cooperating cognitive radio reports the sensing results through the control channel. In the third process the FC combines the received local sensing information, then determines the presence of primary users and then passes the decision to the cooperating cognitive radio users. All cognitive radio users are tuned to the selected channel or frequency band where a wireless point to point link between the primary user (PU) transmitter and each cooperating radio also known as sensing channel is used for observing the PU and for data reporting all cognitive users are tuned to a control called a reporting channel. From figure 1 , the Fusion center (FC) and CR1-CR5 performing local sensing and reporting back to the FC. In centralized system, the cognitive radio base station is the FC. But in cognitive Radio ad hoc network (CRAHNs), the cognitive radio base station is not present; hence any cognitive radio can then act as a FC to coordinate the sensing activities and then combines the sensing information from the cooperating neighbors. In a situation where there are large numbers, the required bandwidth for reporting results becomes huge. The reduction techniques of the sharing bandwidth and local observation are discussed in BIB001 . Only cognitive radios with reliable information are required to send decision to the fusion center BIB002 . BIB002 BIB003 BIB004 , based on these algorithm each cognitive radio user transmits its own data to other users, then combines its results with the received data and then decides if a primary user is present by using local criterion  ,Distributed sensing is more advantageous than the centralized sensing because it does not require fusion center (FC) for cooperative decision thereby reducing cost.