Review of spatial mode multiplexed FSO transmission in the presence of atmospheric turbulence

Free-space optical (FSO) communication has recently attracted more attention as an enabler for several 6G applications, such as backhaul and fronthaul connectivity, non-terrestrial networks (NTNs) or satellite communications, data centre interconnects, and quantum-secured communications. In scenarios where the geometry of FSO system lies in the near-field regime (e.g., backhaul/fronthaul), spatial mode multiplexing (SMM) can be implemented. However, atmospheric turbulence distorts the amplitude and phase of propagating spatial modes of light, leading to power leakage among orthogonal modes. In this paper, the impact of atmospheric turbulence on signal-carrying spatial modes of light and the resulting crosstalk among originally orthogonal spatial modes are investigated. Different implementations of SMM FSO systems are studied considering cases with mutually coherent and mutually incoherent channels. The performance of different turbulence mitigation techniques such as spatial mode diversity, zero-forcing beamforming, and mode selection optimization is reviewed and compared with benchmark FSO systems.