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Performance of 120 Gbps single and dual polarized 16-QAM coherent FSO systems under various turbulence regimes

Free space optical (FSO) communication can serve as a better alternative in metro and access networks. Subcarrier intensity modulation systems have been designed in free space optical (FSO) communication systems to improve the bandwidth efficiency. In this work we study the comparative performance of SP/DP (single polarization/dual polarization) coherent detection 16-QAM DSP integrated FSO system for two cases. Firstly, the system performance is evaluated for slow fading channels with the index refraction structure ranging from 1e−15 to 5 e−11 m−2/3. The fading model considered over here are log normal (LN) and gamma gamma (GG) turbulence model. Secondly the system is evaluated for various channel attenuation ranging from clear weather (0.11 dB/km) to dense fog (22 dB/km) conditions. From the analysis, it has been noted that at a fixed length of 100 m and under clear weather condition, the system can attain a respective rate of 518 Gbps and 1000 Gbps for SP/DP system. For a dense fog condition, at a fixed length of 100 m a system can attain a respective rate of 498 Gbps and 996 Gbps for SP/DP system. It has been analyzed that for error free condition the optimum range for clear weather condition is 1.709 km and 2.540 km for SP/DP systems respectively. For dense fog condition the optimum range comes out to be 494 m and 558 m at zero bit error rate (BER) for SP/DP systems respectively. System performance is evaluated using bit error rate, error vectored magnitude, electrical constellation diagram and polarization analyzer. It has been noted that DP systems maintain their state of polarization (SOP) throughout the link while it gets distracted for SP systems. The spectral efficiency of SP and DP system is measured as 5.333 bits/s/Hz and 10.667 bits/s/Hz respectively. The importance of integration of DSP system at receiver is evaluated in the work and the working process of DSP system is also highlighted.