Line-of-Sight MIMO Systems: DoF Analysis and Hybrid Beamforming Design

A millimeter wave (mmWave) line-of-sight (LOS) multi-input-multioutput (MIMO) system is studied, where spatial multiplexing is achieved through LOS transmissions in near-field areas. A comprehensive analysis of the achievable rate in mmWave LOS MIMO system with respect to the rotation of uniform linear arrays (ULAs) is provided for both full-rank orthogonal and rank-deficient LOS MIMO channels. 1) For full-rank orthogonal scenarios, the necessary and sufficient condition for the LOS MIMO system to have the maximum achievable rate is established. 2) For rank-deficient scenarios, a closed-form expression for the Degrees of Freedom (DoF) of mmWave LOS MIMO channels is derived. Subsequently, the alternating phase approximation-based (APA-based) hybrid precoding algorithm is proposed, where the number of radio frequency (RF) chains is determined by the DoF of mmWave LOS MIMO channels and the initial phase of iterative optimizations is determined according to the optimal digital precoder. Our numerical results confirm the effectiveness of our analysis and proposed algorithm. It is also unveiled that 1) the DoF can be maximized through a specific angle design and 2) compared to existing hybrid precoding algorithms, our proposed APA-based hybrid precoding algorithm exhibits superior robustness against the angular rotation of ULAs.