On the Performance of Cooperative-Diversity Networks with the Nth Best-Relay Selection Scheme

In this letter, we consider the adaptive decode-and-forward (DF) and amplify-and-forward (AF) cooperative-diversity systems with the N
^th
best-relay selection scheme. In the best-relay selection scheme, from the set of M relays the best relay only forwards the source signal to the destination. However, the best relay might be unavailable; hence we might resort to the second, third or generally the Nth best relay. We derive closed-form expressions for the symbol error probability, outage probability and asymptotic error probability. In particular, we derive a closed-form expression for the probability density function (PDF) of the signal-to-noise ratio (SNR) of the relayed signal at the destination node. Then, we find a closed-form expression for the moment generating function (MGF) of the output SNR at the destination. This MGF is used to derive the closed-form expressions of the performance metrics. All these expressions are derived over identical and non-identical Rayleigh fading channels. Results show that with the N
^th
best relay the diversity order is equal to (M - N + 2) where M is the number of relays.