Performance Analysis and Optimization of Multiuser Mixed FSO/RF Cognitive Radio DF Relay Network

This paper analyzes the outage probability and ergodic channel capacity of dual-hop multiuser mixed free space optical (FSO)/radio frequency (RF) relay network that uses underlay secondary user as a relay for sending the source message to the destination. We consider decode-and-forward (DF) relaying scheme with cognitive radio scenario under two constraints, maximum power and interference threshold. In this paper, we consider Gamma–Gamma channel model that considers pointing error effect to represent the FSO link at the first hop with both input independent and cost-dependent intensity modulation-direct detection (IM-DD) receivers. The RF links and the cognitive interference channels follow the Rayleigh distribution fading model. A closed-form expression is derived for the outage probability, in addition to studying the system performance at high signal-to-noise ratio (SNR) values, where approximate asymptotic expressions for the outage probability and the bit error rate (BER) are derived. Also, the diversity order and coding gain of the system are obtained and analyzed in this region. The system ergodic channel capacity was derived and a power optimization analysis is performed to provide the optimal power allocation of the system. The main results reflect the advantage of the proposed power optimization in enhancing the system performance.