Opportunistic Two-Way Full-Duplex Relay Selection in Underlay Cognitive Networks

The performance of secondary user in underlay cognitive networks is analyzed by considering opportunistic selection of two-way full-duplex (FD)-based amplify-and-forward (AF) relays. The closed-form expressions for the distribution of end-to-end signal-to-noise ratio (SNR) are first derived, with independent and identically distributed (i.i.d.) Rayleigh fading channels assumed in each wireless link. After that, the theoretical closed-form expressions for various performance metrics, including the outage probability of each link, lower bounds of both the derived outage probability and the symbol error probability (SEP), and upper bound of the average/outage channel capacity, are derived. Numerical results show that the performance of the secondary users can be substantially influenced by parameters such as the number of relays, the average SNR, the primary user's tolerable interference threshold lambda, and the power of residual self-interference in the FD relays after performance self-interference cancellation, etc. Furthermore, in the presence of multiple candidate relays, an optimum relay selection in terms of the outage probability, the SEP, the average channel capacity, and/or the outage capacity can always be attained as long as the average SNR of each relaying link approaches lambda.