PERFORMANCE ANALYSIS OF FSO COMMUNICATION SYSTEMS UNDER DIFFERENT IMPAIRMENTS

Abstract

Free-space optical (FSO) communication is emerged as a promising technology for high-data- rate wireless communication due to its license-free spectrum, high security, and ease of deployment. However, FSO systems are highly susceptible to atmospheric turbulence, path loss, and pointing errors, which can significantly impact their reliability. To enhance system performance and mitigate these impairments, hybrid RF/FSO communication systems are proposed, leveraging the advantages of both RF and optical wireless channels. In a dual-hop RF/FSO system, the RF link ensures robust connectivity in adverse weather conditions, while the FSO link provides high-speed data transmission under favorable conditions. To accurately model channel fading effects, advanced statistical distributions are required, providing deeper insights into system performance under realistic conditions. This study investigates a dual-hop RF/FSO system by employing a Mixture Gamma (MG) distribution to model the RF channel and a Double Generalized Gamma (DGG) distribution for the FSO channel, analyzing key performance metrics such as outage probability, ergodic capacity, average capacity, and symbol error rate (SER) under varying turbulence conditions. Considering a Dual Hop RF/FSO communication system, one hop is used as RF Signal and another as an FSO channel. Here, it uses Mixture Gamma Distribution to model the RF channel and a Double Generalized Gamma distribution to model the FSO channel. Also, as a special case, of Mixture Gamma, Rayleigh and Nakagami-m distribution is modelled at RF channel. In the first work, the mathematical and graphical analysis for different turbulence conditions are performed. The system's performance is analyzed in terms of Ergodic Capacity with different threshold SNR. The analytical closed-form expression for the Outage and Ergodic Capacity is derived for the IM/DD detection approach to analyze the system's performance, The study finds that, given a particular threshold, the outage capacity decreases when the average electrical SNR rises and increases with the outage rate value rises. Also, it is noticed that as the average electrical SNR increases, Ergodic capacity increases for a specific threshold. For the ergodic capacity, it is found that when the path loss increases, the ergodic capacity reduces for a given electrical SNR. Next, the performance improvement of FSO system is analyzed in terms of AC and SER. Specifically, analytical expression of AC for Double Generalized Gamma fading iv distribution under Equal Gain Combining (EGC) is presented. In addition, MGF based analytical expression of Symbol Error Rate (SER) of M-PSK and M-QAM for Double Generalized Gamma fading distribution under EGC scheme in terms of Fox-H is presented. Various special cases of proposed expressions are deduced for different fading distributions such as Double Weibull and Gamma-Gamma. It is evident from the results that increasing the diversity branches improve the performance in terms of AC and SER. However, the capacity improvement is more as it increases from L=1 to L=2 than L=2 to L=3 which is found in agreement with the literature. Further, at SNR of 30 dB, SER of QPSK are 3.7x10-19, 1.2x10-4 and 1.1x10-3 over weak, moderate and strong turbulence conditions respectively. Similarly, SER of BPSK, 4QAM and 8QAM is analyzed over weak, moderate and strong turbulence conditions. This implies that irrespective of fading scenario and/modulation scheme, the effect of moderate turbulence lies in between strong and weak turbulence conditions. This shows perfect agreement with the theoretical background. Finally, License free spectrum, high data rates and ease of installation make Radio on Free space optical (Ro-FSO) ubiquitous for upcoming generation of communication system. However, atmospheric turbulence induced fading and/or pointing error are the challenging issues of Ro-FSO system. The effects of these issues on performance of Ro-FSO is thoroughly analyzed by presenting closed form expressions under different turbulence conditions for different schemes. Specifically, BER of M-QAM and K-PSK modulation schemes over Double Generalized Gamma Fading distribution, under weak, moderate, and strong turbulence conditions, for OFDM based Ro-FSO system is analyzed by presenting closed form expressions for respective measures. Further, closed form expressions and in turn the presented analysis is based on with and without pointing errors. Furthermore, presented expressions are deduced over Gamma-Gamma and double Weibull fading as a special case of Double Generalized Gamma Fading distribution. BER of M-QAM and K-PSK modulation schemes for different values of K/M, different values of turbulence conditions and with or without pointing errors is observed numerically. Observation shows that irrespective of severity of turbulence be it strong or be it moderate an increase in order of modulation causes in an increase in BER. Also, pointing errors deteriorate the BER irrespective of severity and/or order of modulation schemes. Finally, it is observed that the effect of the order of modulation scheme on BER is more severe under moderate turbulence condition than under strong turbulence condition. Whereas, the effect of pointing error on BER is more under weak turbulence than under strong turbulence conditions.