Coded-Optical Improved Quadrature Spatial Modulation Over Gamma–Gamma Atmospheric Strong Turbulence Channels With Pointing Errors

Despite their multitude of benefits, free-space optical (FSO) communication systems experience severe performance degradation due to the diverse atmospheric conditions commonly present in their deployment areas. Optical signals transmitted through the atmosphere are subject to substantial scintillation losses caused by fluctuations in the refractive index even under clear weather conditions. To mitigate these challenges, in this study, the gamma-gamma distribution model was employed to statistically analyze the channel characteristics and irradiance fluctuations. The impact of pointing errors in a strong turbulence regime under both clear and light–fog weather conditions was also considered. The proposed Coded-Optical Improved Quadrature Spatial Modulation over Gamma-Gamma Atmospheric Turbulence Channels (COIQSM-GG) offers an effective solution for enhancing the reliability and resilience of FSO systems against such atmospheric turbulences. By integrating low-density parity check (LDPC) coding and the OIQSM technique and adapting them to the gamma-gamma channel model, COIQSM-GG improves spectral efficiency, data rates, link distance extension, and communication reliability. In addition, a comprehensive performance assessment that primarily focuses on the bit error rate (BER) pertaining to FSO links using an intensity modulation and direct detection (IM/DD) scheme is presented and discussed in detail. The results indicate that the proposed solution achieves an approximately 20.4 dB reduction in optical transmission power at a slight error rate (BER) of 10−5 for clear weather and a 20.53 dB reduction for light fog weather at a distance of 2 km with 1/2 LDPC.