This letter presents a unified framework for the outage performance analysis of cognitive opportunistic amplify- and-forward relay networks with a direct link using maximum ratio combining (MRC) and selection combining (SC) schemes in the presence of multiple primary receivers. Specifically, we derive unified closed-form expressions of outage probability and asymptotic outage behavior for both the schemes under Nakagami-m fading.

We manifest analytically that the primary network affects only the coding gains of these schemes, and not the diversity gains that are found to be same in both cases. Moreover, we provide an analytical high signal-to-noise ratio (SNR) gap expression between the two schemes, which is shown to be independent of the primary network parameters. Numerical and simulation results validate our theoretical analysis.

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