Performance comparison of M-QAM and DQPSK modulation schemes in a 2 × 20 Gbit/s–40 GHz hybrid MDM–OFDM-based radio over FSO transmission system

This work is focused on the modeling and performance investigation of a 2 × 20 Gbit/s–40 GHz hybrid mode division multiplexing–orthogonal frequency division multiplexing-based radio over free space optics (RoFSO) transmission system under the influence of different weather conditions. The performance of the proposed system has been compared for 4-quadrature amplitude modulation (QAM), differential quadrature phase-shift keying, 16-QAM, and 32-QAM modulation schemes using error vector magnitude, optical signal-to-noise ratio requirement, and maximum link reach as the performance metrics. The results show that 4-QAM scheme demonstrates the best performance. The proposed RoFSO transmission system incorporating 4-QAM modulation demonstrates a successful transmission of 2 × 20 Gbit/s–40 GHz information over 104 km link range under clear weather conditions. Also, the maximum link range using the proposed system is reported as 4.52 km under light fog, 2.78 km under moderate fog, and 2.11 km under heavy fog conditions. Further, the performance of the proposed system has been compared with the previously reported literature which shows that the proposed system has a better figure of merit (information rate × transmission distance). The presented work can be used to implement a spectrum efficient, high-speed, long-haul information transmission system for future wireless networks.