Performance Analysis of Adaptive Modulation in Cognitive Relay Network With Cooperative Spectrum Sensing

In practice, sensing results often affect the data transmission in cognitive radio systems. Therefore, a hybrid interweave/underlay cognitive radio system is presented by jointly considering spectrum sensing and data transmission. In particular, multiple relays are deployed for both the spectrum sensing and data transmission. The secondary user (SU) source and the relays cooperatively sense the spectrum with energy detection and decision fusion. According to the cooperative spectrum sensing results, the SU adaptively switches between interweave and underlay spectrum access mode. Moreover, to improve the spectral efficiency, an adaptive modulation technique is integrated into SU´s transmission. In this context, undertaking the interference from the primary user (PU), the upper bound for average spectral efficiency of the SU with continuous-rate adaptive modulation is derived over Rayleigh fading channels. Further, we study the performance of SU, where adaptive L-ary quadrature amplitude modulation (L-QAM) with fixed switching threshold is adopted. Simulation results verify our analysis and show that both hybrid interweave/underlay scheme and adaptive modulation can improve the SU´s performance significantly.