Quasi-Orthogonal Beamspace Modulation in Near-Field LoS MIMO Channel

This paper proposes a novel quasi-orthogonal (QO) beamspace modulation (BM) scheme to enhance data transmission in a near-field line-of-sight (LoS) ultra-massive multiple-input multiple-output (MIMO) system with limited radio frequency (RF) chains. Leveraging the characteristics of near-field spherical wavefronts, we construct a set of QO beams focusing on distinct receiver (Rx) antennas. The QO beamforming vectors are constructed with constant-magnitude entries, and thus can be realized by only using phase shifters. The transmitter (Tx) then delivers information via beam indexing, as well as via spatial multiplexing over the selected beams. Compared with the existing singular value decomposition (SVD) based BM, the proposed QO-BM scheme simplifies the implementation in hybrid array architectures, and achieves higher spectrum efficiency with reduced computational complexity.