Ultrahigh capacity up to 1.12 Tbps WDM-MIMO free-space optical system using PAM-3 and PAM-6 under real atmospheric turbulence conditions

The proposed work investigates the performance of a wavelength division multiplexed (WDM) combined with a MIMO FSO communication system designed based on advanced pulse amplitude electrical modulation formats (PAM-3 and PAM-6) and electro-absorption optical modulator with data rates of 80 Gbps and 140 Gbps, utilizing 640 Gbps and 1.12 Tbps on 8 port WDM evaluated under combined factors, including; atmospheric attenuation, divergence, and both strong and weak atmospheric turbulence conditions modeled by Gamma–Gamma (GG) and Log-Normal (LN) distributions. Measurements show that MIMO FSO channel loss is mainly dominated by weather attenuation and divergence spreading, whereas, the scintillation turbulence and pointing error have minor contribution. At the intermediate range of 1.4 km a very low BER values is reported, particularly, for the PAM-3 modulation under weak turbulence GG and strong turbulence LN models with almost zero BER, whereas, BER values of 10 −130 , 10 −150 and 10 −170 are reported for the PAM-6 modulation scheme under strong turbulence GG, LN and weak turbulence GG, respectively. However, as the FSO range extends to approximately 1.8 km, the PAM-6 BER deteriorated to 10 −25 with a slightly higher OSNR compared to, PAM-3 that showed competitive BER in all turbulence conditions and across entire transmission range.