Adaptive Modulation Technique In Cognitive Radio

Cognitive radio (CR) can resolve the problem of spectrum scarcity by sharing the precious radio spectrum resources. Subject to a target bit error rate (BER) constraint, adaptive modulation technique can maximize the spectral efficiency of communication systems by adaptively changing the parameters, such as transmission power and rate, according to channel state information (CSI). As a powerful way to combat channel fading, antenna receive diversity can be used to enhance spectral efficiency or transmission reliability of communication systems. Cooperative diversity not only can combat the effect of channel fading, but also can overcome the limitation of communication systems’service coverage. Therefore, this thesis investigates the secondary user (SU) link’s performance when SU of CR communication system employs maximal ratio combining (MRC) receive diversity and adaptive continuous-rate (or discrete-rate) M-ary quadrature amplitude modulation (MQAM) over fading channels. Three communication systems that we consider are peak interference power constraint CR communication system with multiple primary users (PU) and single SU, average interference power constraint CR communication system with single PU and single SU, and spectrum underlay CR communication system based on decode-and-forward (DF) dual-hop relaying, respectively.Chapter1presents a brief review of spectrum underlay CR, adaptive modulation technique in CR and cooperative diversity in spectrum underlay CR.Chapter2investigates SU link’s spectral efficiency when MRC and variable-rate and variable-power (VRVP)/truncated channel inversion with fixed rate (TIFR) adaptive continuous-rate MQAM are employed, and derives the closed-form expressions of SU link’s outage probability, average spectral efficiency, and average BER when MRC and adaptive discrete-rate MQAM are employed in peak interference power constraint CR communication system with multiple PUs and single SU. Numerical and simulation results show that the SU link’s performance is deteriorated when there are multiple PUs.Chapter3investigates the SU link’s performance when adaptive continuous-rate (or discrete-rate) MQAM is employed in average interference power constraint CR communication system with single PU and single SU. Firstly, the closed-form expressions of SU link’s spectral efficiency, outage probability, and average BER are derived when MRC is adopted by both primary receiver (PR) and secondary receiver (SR). Secondly, SU link’s effective capacity is investigated when MRC is adopted under delay quality of service (QoS) guarantee. Finally, the closed-form expressions of SU link’s spectral efficiency and average BER are derived when MRC is adopted with imperfect CSI of the interference link. Numerical and simulation results show that SU link’s performance is improved when MRC is adopted by SU link.Chapter4investigates SU link’s performance when MRC is adopted by both the relay node and the destination node in adaptive CR communication system based on DF dual-hop relaying, and derives the closed-form expressions of SU link’s outage probability and its effective capacity under delay QoS guarantee. Numerical and simulation results show that SU’s performance is improved when MRC is adopted by both the relay node and the destination node.Chapter5is the conclusion of this thesis.