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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.

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Considering the effect of imperfect channel state information (CSI), we study the performance of a cluster-based cognitive multihop wireless sensor network with decode-and-forward (DF) partial relay selection over Nakagami-m fading channels. The closedform expressions for the exact outage probability and bit error rate (BER) of the secondary system are derived and validated by simulations. Asymptotic outage analysis in high SNR regime reveals that the diversity order is determined by the minimum fading severity parameter of all the secondary transmission links, irrespective of the CSI imperfection. It is shown that the fading severity of the secondary transmission links has more influence on the outage performance than that of the interference links. We also conclude that, for secondary nodes whose transmit power is restricted by the interference constraint of the primary user, increasing the number of relaying hops is an effective way to improve their transmission performance. Besides, increasing the number of available relays in each relay cluster can mitigate the performance degradation caused by CSI imperfection.

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