Spatial-mode multiplexing with zero-forcing beamforming in free space optical communications

In this paper, the IM/DD spatial-mode multiplexing free-space optical communication systems are investigated. Different from those systems studied in the literature where mutually incoherent channels are assumed, we investigate a more practical SMM system with mutually coherent channels. Due to the nonlinear property of the square-law detectors, receiver side MIMO techniques such as decorrelator cannot be applied in such system to null out the crosstalk between subchannels. In order to improve the communication performance, we employ zero-forcing beamforming to eliminate the interchannel interference by using the channel state information at the transmitter. The channel estimation strategy is discussed and two power allocation methods are investigated, i.e., uniform power allocation and optimal power allocation. The average achievable rates of the system under different turbulence conditions are also presented.