Research On Signals Detection And Cooperative Communications In Cognitive Radio System

In recent years,with the rapid development of Internet and the increase of wireless communication users,rational utilization of spectrum resources has been one of most critical researches for current communications technology.Because wireless signals in wireless channel would be affected by multipath fading and shadow at the same time,hence,in order to better dynamically manage and utilize wireless spectrum resources,it is significant to study technology of signal detection and cooperative communications in real communications scenairos.At the same time,to further improve communication performance of secondary users(SUs)in cognitive radios networks,this thesis mainly studies signals detection and cooperative communications in cognitive radios system,main contributions are listed as follows:1 Aiming at disadvantage in signal-to-noise ratio(SNR)wall and noise uncertainty in wireless communication scenarios,we present enhanced energy detection(ED)with cooperative spectrum sensing under non-identical Gaussian background noise in cognitive radios networks.The probability of detection and the probability of false alarm have been derived for enhanced energy detection under low SNR condition in cooperative spectrum sensing,and the number of optimized collaboration users has been deduced for the minimum total error rate.Theoretical analysis and numerical results show that proposed schemes have better detection performance than other energy detection plans for solving noise,and have moderate complexity.2 In order to overcome the problem of ignoring the sensing information between the two fixed thresholds for double-thresholds energy detection algorithm,the sequential cooperative sensing algorithm based on dynamic adaptive double-threshold energy detection is proposed.Since the soft decision between the two dynamic adaptive double thresholds is used for optimizing the probability of detection,it takes sequential cooperative sensing algorithm to model the adaptive double-threshold regions.Furthermore,the proposed algorithm can adaptively adjust the thresholds values and the number of collaborative users for each decision region to optimize the probability of detection and receiver operating characteristics curve.Simulation results show that the proposed algorithm has better performance for the detection capability,compared with other double thresholds sensing algorithms.3 Since energy detection has the advantages of fast implementation,no requirement of priori received information and low complexity,we analyze the unified performance of energy detection of spectrum sensing(SS)over generalized fading channels in cognitive radios.The detection probability with Marcum-Q function has been derived and the close-form expressions with moment generating function(MGF)method are deduced,and exact closed-form analytic expressions for average area under the receiver operating characteristics curve also have been deduced to analyze the performance characteristics of ED over ?-j-? fading channels.Simulation results show that the detection capacity of ED will be evidently affected by ?-?-? fading channels,but appropriate channel parameters can improve sensing performance.In addition,the established ED-fading pattern is approved by simulations,and it can significantly enhance the detection performance of proposed algorithms.4 As in wireless communication the composite fading distributions have been investigated to show multipath and shadow phenomena for fading channels,the generalized product models ?-?/?-? have been proposed to approximate generalized composite multipath shadowing channels,which describes the intertwined effects of line-of-sight(LOS)short-term fading ?-? distribution and long-term fading distribution that is substituted by ?-? distribution,exact series product model ?-?/?-? for two independent-non-identical-distribution(i.n.i.d.)RVs with fading distributions are derived,and ECC in composite multipath-shadowing fading channels ?-?/?-? is derived.Simulations show that nonlinear fading parameter plays the most important role and higher a will evidently improve channel capacity in CR networks more effectively.5 we propose generalized optimal cloud-based region relays subsets paring algorithm in dual-hop cognitive networks,considering cloud servers are deployed more closer to relays than sender and receiver of secondary users in cloud-based relays networks as close as possible,unified relay selection models have been presented and each hop is respectively optimized,algorithm flaws of relays subsets pairing and replacement are proposed to creatively solve the problem that close form expressions are hard to obtain with ideology of subsets match.Simulations show that proposed generalized algorithm enormously has 2-6 higher orders of magnitude than other schemes,and can better guarantee communication reliability.