  In this paper, a multi-user multi-hop hybrid FSO / RF system is presented.
This structure is consisted of two main parts. The main motivation of
presenting this structure is communication in long-range impassable links or
some specific atmospheric conditions under which RF connection becomes easily
disrupted. Although these effects could be mitigated by consuming more power or
adding processing complexity, but a small user mobile phone cannot deserve more
complexity or power supply. The fact that FSO and RF links are complementary of
each other brings a new solution in mind; an access point that amplifies
received signal via short-range RF link and forwards it to via long-range FSO
link, could solve the mentioned problem. This scenario is exactly implemented
at the first part of the proposed structure. At the second part, a multi-hop
hybrid parallel FSO / RF link is implemented to connect source and destination
Base Stations. It is the first time that in a multi-hop FSO / RF system,
multi-user scheme, signal selection at each hop, known and un-known CSI in
amplify and forward relaying, and saturate atmospheric turbulence with the
effect of pointing error are considered. New expressions are derived in
closed-form for Bit Error Rate (BER) and Outage Probability of the proposed
structure and verified by MATLAB simulations. The proposed structure has
advantages of FSO, RF, relay-assisted, and multi-user systems at the same time.
Results indicate that it has low dependence on number of users and number of
relays. Therefore, it is suitable for areas with varying population and
long-range links. This structure offers independent performance without
additional power consumption, processing latency, and complexity.
