  In this manuscript, two novel multi-hop relay-assisted hybrid Free Space
Optical / Radio Frequency (FSO / RF) communication systems are presented and
compared. In these structures, RF and FSO links, at each hop, are parallel and
send data simultaneously. This is the first time that in a multihop hybrid FSO
/ RF structure, Detect and Forward protocol is used. In the first structure, at
each hop, received signals with higher Signal to Noise Ratio (SNR) is selected.
But in the second structure, at each hop, received FSO and RF signals are
separately detected and forwarded and selection is done only at the last hop.
Considering FSO link in Negative Exponential atmospheric turbulence and RF link
in Rayleigh fading, for the first time, closed-form expressions are derived for
Outage Probability (P_out) and Bit Error Rate (BER) of the proposed structures.
MATLAB simulations are provided to verify derived expressions. The main
motivation of this work is to answer this question that how much is the
difference of selection at each hop and selection at the last hop. Results
indicate that the structure with selection at each hop has better performance
than the structure with selection at the last hop. At different target Outage
Probability, selection at the last hop consumes about two times (~3dB) more
power than selection at each hop. Both structures are particularly suitable for
long-range communications. However, selection at each hop, in the cost of more
complexity, is recommended for applications which have problem with supplying
the required power for communication.
