Mixed RF/FSO Communications With Outdated-CSI-Based Relay Selection Under Double Generalized Gamma Turbulence, Generalized Pointing Errors, and Nakagami-m Fading

This paper investigates the performance of dual-hop amplify-and-forward (AF) mixed radio frequency (RF)/free-space optical (FSO) transmissions with partial relay selection (PRS) based on outdated channel state information (CSI) estimates. Turbulence-induced fading, path loss, and pointing errors, are all considered in the FSO channel modeling. Both the fading and the path loss are described through general models which encompass the commonly used models. Novel expressions for the cumulative distribution function, probability density function, moment generating function, and moments of the end-to-end signal-to-noise ratio (SNR) are obtained in closed-form. Thereafter, novel closed-form expressions for key performance metrics, namely, outage probability, average bit error probability and spectral efficiency, are derived. The analysis is unified, applying to both types of detection techniques, intensity modulation/direct direction and heterodyne detection. Asymptotic analysis is further conducted, which open the door to additional important results. Monte Carlo simulation results confirm the effectiveness of the proposed analysis, and attest that in AF RF/FSO systems with PRS based on outdated CSI estimates, where implementing higher number of relays is highly difficult and costly, the low SNR regime may be prohibitive because it hinders the diversity advantages promised by the use of more relays in PRS systems.