In the current study of cognitive relay networks, most related works focus on the effect of interference from secondary users (SUs) to primary receivers (PRs), while neglecting the links from primary transmitters (PTs) to SUs. In this paper, the interference both from SUs to PRs and from PTs to SUs is considered in the analysis of cognitive two-way relay networks with opportunistic relay selection. The exact closed-form expression for the outage probability of the secondary system is derived over Rayleigh fading channels, which is verified through various Monte Carlo simulations. Meanwhile, an asymptotic expression and diversity order are also derived to reveal additional insights into the effect of the mutual interference between the primary and secondary systems on the diversity. Above all, based on the analysis, the effects of the positions of the PT, the PR, and secondary relays on the outage performance of the secondary system are studied.
Our results reveal that network placement planning is desperately necessary to achieve a better outage performance. It is shown that the position of relays has a strong impact on the performance, particularly when the number of relays is large. However, when the relative position between the primary system and secondary systems is fixed, the positions of the PT have a very slight impact on the outage performance, which can be neglected. Hence, in practical deployment, when performing cognitive two-way relaying with opportunistic relay selection, proper network placement planning should be carefully addressed.