| In recent years,spectrum resources scarcity is increasingly severe due to the explosive growth of the wireless communication devices.How to utilize the limited spectrum efficiently has drawed a large attention in academic research.Cognitive Radio(CR)has been proposed to enhance the spectrum efficiency(SE),which allows the unauthorized users to access the spectrum of the authorized users under the tolerant interference.Spectrum sensing is essential for the unauthorized users under the opportunistic access to use the primary spectrum,the accuracy of spectrum sensing has a significant influence on the performance of CR.However,the performance of CR is suffered from the poor sensing performance under low signal-to-noise ratio(SNR),severe channel fading and eavesdropping.To address these issues,intelligent reflecting surface(IRS)has been proposed as a promising enabling technology of the next generation network.In particular,the phase of the incident signals can be adjusted by the passive reflective elements of IRS according to the real-time channel conditions,which can intelligently reshape the channel and manipulate the end-to-end communication environment,thereby enhancing the desired signal power and suppress the interference and eavesdropping.Against this background,this thesis profoundly studies the performance analysis for spectrum sensing and resource allocation algorithms for IRS-assisted CR networks under the opportunistic access.The main contribution and innovation of the thesis are summarized as follows.Firstly,focusing on the poor spectrum sensing performance in low SNR,an IRS-enhanced singleuser spectrum sensing scenario is considered where the IRS is exploited to enhance the primary signal and increase the sensing SNR.In this regard,an IRS-enhanced energy detection algorithm is proposed for spectrum sensing.Considering whether the direct link between the primary user and the secondary user is available or not,the moment-matching method and the central limit theorem(CLT)are adopted to approximate the probability density function of the received SNR in each case,respectively,Moreover,the approximation for the Marcum-Q function is employed to derive the closed-form expression of the average probability of detection.Simulation results show that the proposed scheme can enhance the received SNR,increase and average probability of detection and improve the spectrum sensing performace in CR networks.To further enhance the SE of CR networks,the communication scenario of an IRS-assisted CR networks under opportunistic access is considered where the IRS is simultaneously exploited to enhance both the spectrum sensing accuracy and the secondary information transmission performance based on IRS-enhanced spectrum sensing.In this regard,an IRS-enhanced joint sensing and transmission resource allocation scheme is proposed,and a novel detection threshold is derived based on IRS-enhanced energy detection.The optimization problem is formulated to maximize the average achievable rate by jointly optimizing the sensing time,the beamforming of the secondary base station,the phase shifts of IRS in sensing stage and that in transmission stage.To solve the non-convex problem with coupled variables,a block coordinate descent(BCD)-based iterative algorithm is proposed,the optimization variables are fixed into several blocks and optimized alternately.The techniques of successive convex approximation(SCA)and semi-definite relaxation(SDR)are adopted to tackle the non-convex objective function and constraints.Then,the sub-optimal solutions can be obtained.Simulation results demonstrate that the BCD algorithm can converge fastly,and proposed scheme can lessen the required sensing time,significantly decrease the probability of false alarm in the low SNR,simultaneously improve the sensing accuracy and the transmission performance,and further improve the SE of CR networks.Moreover,since the spectrum band in CR networks is shared,the communication security issues become more complicated and can be caused by many conditions,e.g.,jamming and eavesdropping.To further improve the security of the network,the secure communications scenario of an IRSassisted CR networks under opportunistic access with multiple eavesdroppers is considered.In this regard,an IRS-enhnaced physical layer security(PLS)design is proposed and the aim is to maximize the average secrecy rate.To tackle this problem,a BCD-based iterative algorithm is proposed to optimize the sensing time,the beamforming and the IRS phase shifts alternately,a safe approximation for the Q-function is adopted to transform the probability of detection into a tractable expression.Moreover,to address semi-infinite constraints caused by the imperfect channel state information(CSI)of the eavesdropping links and primary links,the S-procedure is employed to transform the infinite non-convex constraints into linear matrix inequalities(LMIs).Simulation results show that IRS can greatly improve the secrecy performance and enhance the physical layer security. |
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