Advanced Reliability Enhancement Techniques in Free-Space Optical Communication Systems: A Review of WDM, QAM, PDM, OAM, and Hybrid RF/FSO Architectures

Abstract

Free-space optical (FSO) communication is an attractive candidate for high-capacity terrestrial backhaul, fronthaul, last-mile access, high-altitude-platform, unmannedaerial-vehicle and satellite feeder links because it offers fiber-like optical bandwidth without laying fiber. The same narrow optical beam that enables spectrum reuse and physical-layer security also makes FSO vulnerable to fog, turbulence-induced scintillation, beam wander, pointing errors, background light and weatherdriven outages. This review examines reliability enhancement in FSO systems through a physical-layer methodology centered on wavelength-division multiplexing (WDM), quadrature amplitude modulation (QAM), polarization-division multiplexing (PDM), orbital-angular-momentum (OAM) modes and hybrid radiofrequency/free-space-optical (RF/FSO) diversity. Recent experimental and survey literature is synthesized to compare how each technique improves throughput, outage probability, bit-error rate (BER), spectral efficiency and availability. The review finds that no single technique is sufficient for high-reliability nextgeneration links: robust designs combine adaptive modulation and coding, coherent detection, WDM/PDM scaling, OAM or spatial diversity, beam alignment, adaptive optics, link-layer protection and RF backup. A layered design framework is therefore proposed for journal- and conference-ready FSO deployments in which reliability is improved by matching each impairment to the appropriate mitigation mechanism.