Performance Analysis and Power Allocation for Two-Way Multi-user Mixed RF/FSO Relay Networks

This paper studies the performance of two-way multiuser mixed radio frequency/free space optical (RF/FSO) relay networks with opportunistic user scheduling' and asymmetric fading channels. The considered system consists of multiple users communicating with a destination node through a decode-and-forward (DF) relay in a two-way fashion. The links between the users and relay are assumed to be Rayleigh distributed RF channels, while the link between the relay and destination is Gamma-Gamma fading FSO channel. First, exact closed-form and asymptotic (high signal-to-noise ratio (SNR)) expressions are derived for the outage probability. Then, the asymptotic results are used to conduct a power optimization algorithm where exact expressions for the optimal transmission powers are provided. Additionally, performance comparisons between the considered two-way relaying (TWR) and one-way relaying (OWR) schemes under different network parameters are provided and discussed. The results show that the opportunistic user scheduling in TWR networks does not affect the network diversity order, but it enhances the system coding gain. Additionally, it is found that severe pointing error may result in a total service blockage. Additionally, the proposed power allocation scheme is found to enhance the network outage performance significantly compared to the scheme of equal power allocation.