Polarization-division multiplexed 112 Gb/s 16-QAM hybrid SMF-FSO link over Indian metropolitan atmospheres: performance evaluation

Abstract

In this paper, the design and simulation of the polarization-division multiplexed 16-level quadrature amplitude modulation (PDM-16QAM) based hybrid single-mode fiber-free-space optical (SMF-FSO) communication system is presented. It uses a fixed 50-km SMF segment and a variable-range FSO hop (from 2.25 km to 20 km) in four different climate zones of India (Chennai, Jodhpur, Mumbai, and Srinagar) with attenuation coefficient of the atmosphere obtained from the visibility data from India Meteorological Department for the year 2025 with Kim model of 1.13, 4.696, 4.974, and 13.59 dB/km, respectively. The performance is evaluated based on the FSO link range, logarithmic bit error rate (log(BER)), error vector magnitude (EVM%), and received optical power (in dBm) corresponding to three values of beam divergence angle 0.20, 0.25, and 0.30 mrad. To compensate for channel impairments, a 2 × 2 MIMO-based digital signal processing chain for coherent homodyne receiver is included that includes IQ imbalance compensation, adaptive equalization, frequency offset estimation, and carrier phase estimation. The results show that the worst case city, Srinagar with a maximum attenuation of 13.59 dB/km, can achieve a maximum FSO range of only 2.4 km up to the FEC threshold of log(BER) = −2.42, whereas the best case city, Chennai, with a minimum attenuation of 1.13 dB/km, can achieve a maximum FSO range of 15.5 km under the same conditions.