On the performance of underlay cognitive relay networks with multiple primary users

In this paper, with an adaptive modulation technique integrated into a secondary user´s (SU´s) transmission, the performance of a SU in underlay cognitive relay networks with multiple primary users (PUs) available is investigated. In order to guarantee the transmission quality of the PU, the interference from the SU transmitter and the selected relay should be less than a predetermined interference threshold. Under the condition of satisfying the interference constraint, the SU transmits the message through the relay with the largest end-to-end signal to noise ratio (SNR). In practice, whether there is a direct link between a SU transmitter and a SU receiver depends on the real transmission environment. Hence, with or without a direct link and adopting an adaptive L-ary quadrature amplitude modulation (L-QAM) with a fixed switching threshold, we study the outage probability and average bit error rate (BER) of the SU over Rayleigh fading channels with independent identical distribution (i.i.d.). Finally, the simulation results verify our analysis and we find that the adaptive modulation with selection combining (SC) has a better performance than that without a direct link.