This paper investigates the performance of a multi-user relay-assisted hybrid Free Space Optical/Radio Frequency (FSO/RF) communication system. The proposed dual-hop structure is particularly suitable for areas, in which direct RF communication between mobile users and base station is not possible due to bad channel conditions. In this system, mobile users are connected to the base station with the help of a fixed gain or adaptive gain amplify and forward relay. It is the first time that effect of the number of users on the performance of a dual-hop hybrid FSO/RF structure is investigated. Also, it is the first time that the performance of a dual-hop hybrid FSO/RF is investigated in Negative Exponential atmospheric turbulence (saturate atmospheric turbulence regime). Considering FSO link at a wide range of atmospheric turbulence regimes, from moderate to saturate, closed-form expressions are derived for Bit Error Rate (BER) and outage probability (Pout) of the proposed structure. The MATLAB simulations verified the accuracy of the derived expressions. It is shown that the performance of adaptive gain relaying is less sensitive to the number of users; therefore, this structure is suitable for areas with variable population density. Fixed gain relaying is shown to perform better than adaptive gain relaying. This is because although the fixed gain scheme has low complexity, it assumes much more power, which causes favorable even in the worst case scenario. Therefore, this structure is suitable for the situation that the relay could not deserve much complexity. Accordingly and due to favorable performance at a wide range of atmospheric turbulence, the proposed structure is a special case for low cost, low complexity, and reliable communication.

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