A dual-beam FSO communication system using hybrid OQPSK/AM modulation and LDPC coding for severe weather conditions

This study builds a Free Space Optics (FSO) system around hybrid OQPSK/AM modulation and LDPC coding, running 10 Gbps through attenuation up to 300 dB/km. Two amplifier chains, cascaded EDFA-EDFA and hybrid SOA-EDFA, were tested in single-beam and dual-beam configurations in OptiSystem. SOA-EDFA pushed the usable range from 250 m to 750 m. The single-beam link started at a Q-factor of 23 and fell to 14 by 675 m; the dual-beam link started lower, near 13, easing to 11 over the same stretch, but overtook the single-beam scheme past 700-800 m. EDFA-EDFA never exceeded a Q-factor of 5. SOA-EDFA held a cleaner SNR throughout, peaking near 16 at 150 m. Raising the PM phase shift from 50° to 150° lifted the SOA-EDFA Q-factor from 2 to 6, and LDPC coding gave a clear improvement between 725 and 800 m.