Performance investigation of dual-hop relaying for hybrid FSO/RF system based on optimum threshold SNR

A less-complex and efficient time-hysteresis switching algorithm assisted hybrid free-space optics (FSO)/millimeter wave (mm)-radio frequency (RF) system is presented in the paper. A time-hysteresis switching algorithm is utilized for link transient for selection of FSO or RF links. For improving the long-haul communication efficiency and coverage a selective decode-and-forward relay scheme is blended with hybrid FSO/RF system along with maximal-ratio-combining (MRC) for signal reconstruction at the receiver end. However, the pointing errors due to misalign apertures and building sway, which have the potential to suppress the benefits of the relay assisted hybrid system. The impact of pointing errors and path loss attenuation are modeled with Gamma-Gamma distribution and generalised Nakagami-m is used to model the RF channel fading. The exact closed-form expressions of average symbol-error-rate (ASER) and outage-probability (OP) have been derived for the performance evaluation of the proposed system with and without pointing errors. We also present an asymptotic-analysis of the ASER and outage with lower computational-complexity for the proposed system and obtain the diversity order. To achieve target ASER value for the proposed switching scheme, the optimum-threshold signal-to-noise ratio (SNR) value has been obtained numerically in this work. The derived theoretical outcomes have been validated with Monte-Carlo simulation. In addition, we present comparative results of ASER and OP performance for proposed system with Gamma-Gamma and generalised Malaga distribution under various adverse weather conditions. It is observed that the proposed hybrid FSO/RF system outperforms a cooperative single FSO system as well as single-hop hybrid FSO/RF system on account of diversity as well as switching technique benefits. The presented hybrid FSO/RF system achieves ASER of 10(-7) in the lower SNR regimes.