Error Performance of Opportunistic Relaying with Outdated Channel State Information

Relay selection for cooperative communications has been proposed as a technique to provide remarkable gains in multi-relay cooperative networks. Opportunistic relaying (OR) is a relay selection scheme used in dual-hop decode-and-forward cooperative networks, shown to be globally outage- optimal under an aggregate power constraint. However, due to channel fluctuations, the channel state information (CSI) used in the selection process may become outdated and differ from the CSI during the actual transmission of data. In this work, we analyze the effect of outdated CSI on the performance of OR over independent but not necessarily identically distributed Rayleigh fading channels. Particularly, we derive an exact expression for the average symbol error probability (SEP) of OR with outdated CSI. An approximate but tight closed-form SEP expression is also provided. We further study the asymptotic behavior of the SEP and show that the diversity order of OR reduces to one due to the outdated CSI. Finally, we perform Monte Carlo simulations to verify the analytical results.