Power Allocation and Performance of Multiuser Mixed RF/FSO Relay Networks With Opportunistic Scheduling and Outdated Channel Information

Abstract

This paper studies the performance of a multiuser mixed radio frequency (RF)/free space optical (FSO) relay network with transmit opportunistic scheduling. The outdated channel information (OCI) on the first relaying hop and its effect on the system performance is also studied in this paper. Furthermore, a power allocation scheme is proposed for optimizing the overall system performance. The considered system includes $K$ sources (users), one amplify-and-froward relay and one destination. The users are connected with the relay node through RF links and the relay is connected with the destination through an FSO link. In the analysis, the first hop channels are assumed to follow Rayleigh fading model and the second hop channel is assumed to follow Gamma–Gamma fading model with considering the effect of pointing errors. Closed-form expressions are derived for the outage probability, average symbol error probability, and ergodic channel capacity. Furthermore, the system performance is studied at high signal-to-noise ratio regime, where the diversity order and coding gain are derived and analyzed. Using the asymptotic results, the power of users and relay are determined to minimize the system outage probability under a total power constraint. Monte–Carlo simulations are provided to validate the achieved exact and asymptotic results. Main results show that under weak atmospheric turbulence conditions, the system performance is dominated by the RF channels and a diversity order of $K$ is achieved by the system, whereas under severe atmospheric turbulence conditions, the system is dominated by the FSO channel and the diversity order is determined by the minimum value of the turbulence fading and pointing error parameters.