Outage and diversity of cognitive relaying systems under spectrum sharing environments in nakagami-m fading

In this letter, the outage performance of dual-hop cooperative spectrum sharing systems with a direct link is investigated. A selection combining receiver is employed at the destination in order to combine the signals received from the decode-and-forward (DF) relay and from the source. Assuming independent non-identically distributed Nakagami-m fading channels, exact and asymptotic closed-form expressions are derived for the outage probability. Our results reveal that the diversity order of the considered system is solely determined by the fading severity parameters of the secondary network, being equal to min(m₁,m₂)+m₀, where m₀, m₁, and m₂ represent the fading severity parameters of the secondary nodes i.e., source→destination, source→relay, and relay→destination links, respectively.