A low-complexity relay selection scheme based on switch-and-examine diversity combining for AF relay systems

In this study, the authours present a low-complexity relay selection scheme for channel-state information (CSI)-assisted dual-hop amplify-and-forward cooperative systems. The scheme is mainly based on the switch-and-examine diversity combining (SEC) and SEC post-selection (SECps) techniques in which a relay out of multiple relays is selected to forward the source signal to destination. The selection process is performed such that the selected relay signal-to-noise ratio (SNR) satisfies a predetermined switching threshold instead of best relay. Such a relay that satisfies this threshold will be chosen instead of the best relay. In this study, the authorsuse an upper bound on the end-to-end (e2e) SNR of the selection scheme and derive the probability density function and the cumulative distribution function of this SNR assuming the Rayleigh fading channels. These statistics are then used to derive accurate approximations for both the e2e outage probability and bit error probability, where the direct link in considered. The authours assume that maximal-ratio combining is used at the destination to combine the signals through the relay and the direct link. To obtain more about system insights, the outage performance is studied at high SNR regime, where approximate expressions for the outage probability as well as the diversity order and coding gain are derived and analysed. The Monte-Carlo simulations are provided to illustrate the validity of the analytical results and to show the tightness of the used SNR bound. Results illustrate the effectiveness of the proposed relaying schemes in reducing the required number of channel estimations and hence, reducing the system complexity compared with the opportunistic relaying. Furthermore, results show the gain achieved in the system performance; especially, at low-to-medium SNR values when the SECps selection scheme is used compared with the conventional SEC relaying. Finally, findings show that the system with the SEC and the SECps relaying schemes has the same diversity order of two and the same coding gain.