Performance analysis of a DF based dual hop mixed RF-FSO system with a direct RF link

In this paper, the error performance analysis is performed for the decode-and-forward (DF) based dual hop mixed radio frequency-free space optical (RF-FSO) communication system with a direct RF link. The direct link between the source-destination (S-D) and the link between source-relay (S-R) are Nakagami-m distributed asymmetric radio frequency (RF) links and the R-D link is a free space optical (FSO) link characterized by the path loss, Gamma-Gamma distributed turbulence, and pointing errors. Selection combining diversity scheme is used to evaluate the performance of this dual branch cooperative system. For the considered system, novel closed-form mathematical expressions of cumulative distribution function and probability density function of instantaneous signal-to-noise ratio are derived. Using these, new mathematical expressions are obtained for the outage probability and the average bit error rate (BER) for various modulation techniques. The effects of fading, turbulence, and pointing errors are observed on the outage probability and the BER of the considered system. These performance metrics are also compared with those of a DF based dual hop mixed RF-FSO system without direct link. The improvement in the performance is observed due to the diversity introduced by addition of RF link.