A new scenario of triple-hop mixed RF/FSO/RF relay network with generalized order user scheduling and power allocation

This paper proposes and evaluates the performance of multiuser (MU) triple-hop mixed radio frequency (RF)/free-space optical (FSO) relay network with generalized order user scheduling. An important example on the applicability of this scenario is in cellular networks where two sets of various users are communicating with their own base stations (BSs) over RF links and these BSs are connected together via an FSO link. The considered system includes K₁ sources or users, two decode-and-forward (DF) relays, and K₂ destinations or users. The sources and destinations are connected with their relay nodes through RF links, and the relays are connected with each other through an FSO link. To achieve MU diversity, the generalized order user scheduling is used on the RF hops to select among sources and destinations. In the analysis, the RF channels are assumed to follow the Rayleigh fading model and the FSO channel is assumed to follow the Gamma-Gamma fading model including the effect of pointing errors. Closed-form expressions are derived for the outage probability, average symbol error probability (ASEP), and ergodic channel capacity. Moreover, in order to gain more insight onto the system behavior, the system is studied at the signal-to-noise ratio (SNR) regime whereby the diversity order and coding gain are provided and studied. The asymptotic results are used to obtain the optimum transmission power of the system. Monte Carlo simulations are given to validate the achieved exact and asymptotic results. The results show that the diversity order and coding of the proposed scenario are determined by the worst link among the three links. Also, results illustrate the effectiveness of the proposed power allocation algorithm in enhancing the system performance compared to the case with no power allocation.