Performance analysis of adaptive modulation in cognitive relay networks with interference constraints

In this paper, we investigate the performance of adaptive modulation in spectrum sharing cognitive relay networks, where the secondary user (SU) transmits the message by one best relay with amplify and forward (AF) relaying scheme. Moreover, to guarantee the primary user (PU)´s transmission quality, the interferences caused by the SU transmitter and the selected relay are less than a predetermined interference threshold. The capacity of adaptive modulation with opportunistic AF relaying is firstly derived under independent Rayleigh fading channels. Then, with adaptive L-QAM modulation and fixed switching threshold, the outage probability, average spectral efficiency, and the average error bit rate (BER) of the SU are derived. Finally, simulation results validate our analysis and show that adaptive modulation can improve the SU´s performance significantly.