Performance analysis of Laser Communication Systems under atmospheric turbulence: a comparative study of channel models and modulation techniques

This study examines Probability Density Functions (PDFs) of several statistical models--Lognormal, Rayleigh, Gamma-Gamma, Nakagami-m, Rice, and Negative Exponential--in relation to irradiance under weak, moderate, and strong turbulence conditions. Each model exhibits unique characteristics crucial to Free-Space Optical (FSO) communication performance. Lognormal distribution suggests a high probability of low irradiance values, while Rayleigh and Rice show bell-shaped curves. Gamma-Gamma and Nakagami-m offer greater flexibility, displaying moderate peaks and gradual declines. Negative Exponential distribution shows a rapid decay, particularly in random scattering scenarios. Bit Error Rate (BER) performance is evaluated based on instantaneous signal-to-noise ratio (SNR(I)) for various modulation schemes. Among these, 16-Pulse Position Modulation (16-PPM) proves the most robust, followed by Binary Phase Shift Keying (BPSK) and 8-Phase Shift Keying (8-PSK), which also demonstrate strong performance. Differential Phase Shift Keying (DPSK) and 16-Quadrature Amplitude Modulation (16-QAM) offer a balance between performance and spectral efficiency, while 4-Pulse Amplitude Modulation (4-PAM) is highly sensitive to noise. The study reveals that Rayleigh and Rice distributions perform poorly in moderate and strong turbulence, while Nakagami-m and Gamma-Gamma perform better, with Gamma-Gamma excelling in weak and strong turbulence, and Nakagami-m in moderate conditions. At higher SNR(I) levels, BER performance converges across models, minimizing the impact of channel model on modulation scheme's performance.