Further results on the performance improvement in mixed RF-FSO systems using hybrid DF/AF (HDAF) relaying

This paper presents the performance of a dual-hop mixed radio frequency (RF)-free space optical (FSO) system employing a hybrid-decode/amplify-and-forward (HDAF) relaying protocol. The RF link forms the first hop and the FSO link is the second hop for the proposed system model. The assumption of the absence of the direct FSO and RF link between the source transmitter and the destination receiver ends is made for the analysis. The RF link is affected by short-term multipath fading and long-term shadowing phenomena and is characterized by the generalized-K fading distribution, whereas the FSO link is modeled by the gamma-gamma fading, representing moderate to strong turbulence scenarios. We assume pointing error-induced misalignment fading to be significantly present over the FSO link. We analyze the outage probability for the system employing amplify-and-forward, decode-and-forward, and HDAF relaying protocols and show the benefits of using HDAF relaying in mixed RF-FSO systems. The optimum relay position that results in the best outage performance is investigated. Furthermore, the bit error rate performance for a q-ary phase shift keying (q-PSK) modulated signal and the channel capacity for the proposed system employing HDAF relaying protocol are also evaluated. The expressions in closed form for the outage probability, bit error rate, and channel capacity are derived. The asymptotic outage and error performance at high signal-to-noise ratio is also evaluated. Furthermore, the performance of the system is demonstrated by numerical plots.