Outage performance analysis of cognitive relay networks with BRS and MMRS

In this paper, we consider a spectrum sharing system where a secondary user (SU) transmitter (SU-Tx) communicates with an SU receiver (SU-Rx) with the help of multiple secondary relays (SRs) in the presence of a single primary network link. In particular, the SU-Tx and SR are subject to their maximum transmit power limits and the timeout constraint of the primary user (PU). Moreover, we assume that the relay selection is based on either best relay selection (BRS) or max-max relay selection (MMRS). In BRS, a single relay having the best end-to-end channel condition among the SRs is selected for cooperation between the source and the destination. However, the selected relay may not experience the best source-relay channel and relay-destination channel at the same time. To overcome the limitation of selecting the same relay for both reception and transmission, MMRS uses the best relay of the SU-Tx→SR link for reception at the relay and the best relay of the SR→SU-Rx link for transmission to the destination. Specifically, we compare the performance for BRS and MMRS in terms of the outage probability, and study the impact of the number of SRs, PU transmit power and different channel mean powers of interference links on the secondary network performance. Furthermore, based on the performance of the MMRS and assuming that automatic repeat request (ARQ) is implemented for packet retransmission, we investigate the outage probability for delay limited applications. The numerical results indicate that the MMRS outperforms BRS when more than one SRs are deployed. The secondary network outage probability is reduced when the number of SRs and the number of retransmissions become large. It is also shown that the interference from the PU has a significant impact on the secondary network performance.