Performance investigation of FSO transmission system based on combining MDM and PDM techniques

In this study, we propose a potential solution by employing free space optical (FSO) system based on combining polarization division multiplexing (PDM) with multimode division multiplexing (MDM). The proposed system uses six channels to transmit data, in which data in each channel is modulated using 4-quadrature amplitude modulation (4QAM) format at 20 Gsymbol/s symbol rate. Sequentially, the modulated signals are multiplexed using PDM and MDM schemes to form the 4QAM PDM-MDM signal and then it send through few-mode fiber and FSO link. The proposed system performance is investigated and compared under varying strengths of clear and foggy weather conditions with single-mode transmission to determine the influence of modes interaction on the system performance. Our results show that our proposed system has a higher transmission capacity-distance product (TC-DP), where it achieves TC-DP of 5280 Gbps/km under clear weather and 268.8 Gbps/km under light fog. On the other hand, single-mode propagation accomplishes TC-DP of 2080 Gbps/km and 136 Gbps/km for clear and light fog conditions, respectively. In addition, at SNR of ~ 12 dB, the transmission reaches 1740 m, 760 m, and 268 m for light, moderate, and heavy fog weathers, respectively. This means that the transmission distance of multimode transmission is degraded by 74% of the single-mode transmission at SNR of ~ 12 dB.