  In this paper a novel multi-hop relay-assisted hybrid FSO / RF system is
presented. It is assumed that direct RF connection between mobile users and
source Base Station is impossible, therefore a relay connects RF users to the
source Base Station via a FSO link. Source and destination Base Stations are
connected via a multi-hop relay-assisted hybrid FSO / RF link. FSO link is
investigated over moderate to saturate regimes of atmospheric turbulence. Also
RF link is assumed to have Rayleigh fading. For the first time, new exact and
asymptotic expressions are derived in closed-form for Bit Error Rate (BER) and
the Outage Probability (P_out), of the proposed system. MATLAB simulations are
performed to validate obtained analytical results. Results indicate that the
proposed structure has low dependence on the number of users, therefore, the
proposed structure is suitable for cells which encounter different populations
during a day, because there is little performance difference between systems
with different number of users. Also the proposed structure, at Negative
exponential atmospheric turbulence has small dependence on the number of
relays, but this dependence is a bit more for Gamma-Gamma atmospheric
turbulence. Therefore, the proposed structure increases capacity whereas
maintaining performance of the system.
