Partial Relay Selection With Outdated Channel State Estimation in Mixed RF/FSO Systems

We study a dual-hop relaying system, composed of mixed radio frequency (RF)/free-space optical (FSO) links. Partial amplify-and-forward relay selection is assumed, which is based on outdated channel state information (CSI). The RF link is subject to Rayleigh fading, while the FSO link is affected by atmospheric turbulence, modeled by Gamma-Gamma distribution. Novel analytical expressions for the outage probability and average bit error rate (BER) are derived. Numerical results are presented and validated by Monte-Carlo simulations. The outage probability and average BER dependence on turbulence strength, average signalto-noise ratio (SNR) over RF hop, average electrical SNR over FSO hop, number of relays, and rank of selected relay, are also investigated, while the effect of time correlation between current source-relay channel and its outdated CSI, is considered. The results illustrate that the outdated CSI used for the relay selection has a strong effect on the system performance, especially in weak turbulence conditions of the FSO hop. Furthermore, the improvement of the system performance with increase of the number of the relays is highly dependent on the FSO channel state.