Unified Performance Analysis for Multiuser Mixed $\eta $ - $\mu $ and $\mathcal {M}$ - Distribution Dual-Hop RF/FSO Systems

A mixed free-space optical (FSO) communication scheme is considered with the help of a fixed gain relay. We assume that the links from the source to the relay are radio frequency links with multiuser diversity operating in η-μ fading channels, while the link between the relay and the destination is a FSO link with M-distribution. We first derive expressions for the cumulative distribution function of the end-to-end signal-tonoise ratio for the considered system with pointing errors, and then use them to derive some exact closed-form expressions for the outage probability, bit-error rate performance, and ergodic capacity. To investigate the diversity order and effects of different system and channel parameters on the system performance, we further present the asymptotic performance expressions. On the other hand, we also present the performance analysis for a mixed RF/FSO system with variable-gain relaying and multiuser diversity. Results show that the system diversity orders for both fixed and variable gains systems are min{2Kμ, g
²
/2, α/2, k/2}, where K is the number of users, g represents the pointing error, and α and k are the channel parameters. Finally, some selected numerical results are given to corroborate the derived analytical results.