  This paper investigates performance of a relay assisted hybrid Free Space
Optical / Radio Frequency (FSO/RF) communication system. The proposed structure
is particularly recommended anywhere that direct RF communication between
mobile users and base station is not possible due to atmospheric conditions. In
this system, a multiuser RF link connects mobile users to relay and a FSO link
connects relay to the base station. It is the first time that effect of number
of users within the cell, on the performance of such structure, is
investigated. Also it is the first time that performance of a dual hop hybrid
FSO / RF system is investigated in Negative Exponential atmospheric turbulence.
Considering wide range of atmospheric turbulence regimes, from moderate to
saturated, for the first time, closed-form expressions are derived for Bit
Error Rate (BER) and outage probability (P_out) of the proposed structure.
MATLAB simulations verified accuracy of the derived expressions. Considering
fixed and adaptive gain amplify and forward protocols at relay, it is shown
that adaptive gain relay is less sensitive to the number of users within the
cell. It is also shown that fixed gain relay, despite its low complexity, has
better performance; because its gain is adjusted such that the performance be
favorable even at the worst case scenario.
