Performance Analysis and Enhancement of ROFSO Communication System

Abstract

Radio over Free Space Optics (RoFSO) communication is a promising technology addressing the escalating demands for high bandwidth and rapid data transfer, capitalizing on its inherent capacity to transcend conventional radio transmission capabilities. The features of RoFSO includes huge bandwidth, license free spectrum, low power consumption and immunity from electromagnetic interference. However, RoFSO’s complete potential is only attainable when it effectively mitigates the adverse influences of the atmospheric channel, encompassing scattering, absorption, turbulence, and the pervasive issue of pointing errors. In this thesis, we have introduced mitigation techniques Multiple Input Multiple Output (MIMO), Reed Solomon (RS) and Bose-Chaudhuri-Hocquenghem (BCH) error correcting codes, and Reconfigurable Intelligent Surface (RIS) to overcome these challenges. In our first research work, we harnessed spatial diversity at transmission and reception ends to bolster RoFSO performance across diverse turbulence and meteorological scenarios. Employing the Malaga distribution for modelling atmospheric turbulence, we examined configurations spanning Single Input Single Output (SISO), Single Input Multiple Output (SIMO), Multiple Input Single Output (MISO), and MIMO. We derived closed-form expressions for the average bit error rate (ABER) within this framework. We also explored two combining techniques, Optimal Combining and Equal Gain Combining, to further improve system performance. In our second research work, we introduced error-correcting codes RS and BCH for binary phase shift keying (BPSK) orthogonal frequency division multiplexing (OFDM) based RoFSO for 5G applications. This pioneering approach yielded impressive results, with an ABER of 10−6 achieved at carrier to noise plus distortion ratio (CNDR) of 40 dB, 17 dB, and 4 dB for the uncoded, RS-coded, and BCH-coded systems, respectively, under conditions of weak turbulence. RoFSO’s inherent line of sight (LOS) nature led to the emergence of skip zone challenge in targeted areas ; to address this obstacle, our third research work introduced RIS to facilitate LOS connections, extending comv munication channel coverage in a smart and controllable manner. Robust performance analysis, incorporating outage probability, ergodic channel capacity, and ABER assessment with heterodyne detection, substantiated the merits of RIS in resolving LOS challenges. Our fourth research work unveiled a multi-RIS-assisted RoFSO system operating within the dynamic framework of the Malaga distributed atmospheric turbulence model. This comprehensive exploration yielded precise closed-form expressions for performance metrics, encompassing outage probability, ergodic channel capacity, and ABER. The inquiry embraced heterodyne detection and scrutinized two modulation schemes, namely differential binary phase-shift keying (DBPSK) and M-ary quadrature amplitude modulation (M-QAM). Comparisons were drawn across different turbulence conditions, link lengths, and scattering error conditions. The findings underscored the advantages of deploying a multi-RIS-assisted RoFSO system, particularly in vehicular communication scenarios.