Performance Analysis of a Multiuser Dual-Hop Amplify-and-Forward Relay System With FSO/RF Links

This paper studies the performance of dual-hop multiuser relay systems over mixed free-space optical/radio frequency (FSO/RF) links. We consider a system where a source node wants to communicate with multiple users through a single relay. The source node communicates with the relay point over an FSO link with heterodyne detection or direct detection, and the relay supports multiple users through the nonorthogonal multiple access technique over an RF link. One application scenario is in ultradense networks, where FSO links provide high-speed backhauling and RF links support multiple users. The FSO link is modeled by the M distribution in the presence of pointing errors, while the RF link is modeled by Nakagami-m fading. Utilizing amplify-and-forward fixed-gain relaying, we derive a novel closed-form expression for the outage probability in terms of the Fox H-function. Furthermore, a new asymptotic result for the outage probability in the large average signal-to-noise ratio regime is also presented in terms of simple functions. We then capitalize on these results to derive the diversity order of the system. Moreover, the ergodic capacity of the proposed system is also presented. Finally, Monte Carlo simulation results are presented to corroborate the derived results.