Secrecy Performance Analysis of Opportunistic Relay Selection Systems with Outdated CSI

This paper investigates the secrecy performance of opportunistic decodeandforward (DF) relay selection in secure cooperative systems with maximal ratio combining (MRC) at the destination and the eavesdropper over Nakagamim fading channels.The optimal legitimate relay is selected based on the outdated channel state information (CSI) of legitimates channels due to practical delayed feedback.To evaluate the capability of the opportunistic relay selection in improving the secrecy performance,the expressions for exact probability of positive secrecy capacity and exact secrecy outage probability (SOP) are derived in closed form.Furthermore,simple asymptotic expressions are derived for two kinds of cases,explicitly showing the secrecy diversity order and the secrecy array gain.Analytical analysis and numerical simulations show that,the SOP performance can be improved by increasing whether the number of relays or the number of antennas at the destination.Furthermore,when the CSI is outdated,the secrecy diversity order of the system is independent of the number of relays.