Joint Two-Level Spatial Multiplexing and Beamforming in Terahertz Ultra-Massive MIMO Systems

In this paper, an analysis of the joint two-level spatial multiplexing and beamforming (J2SMB) in the ultra-massive MIMO system is presented in the Terahertz (THz) band. The spatial multiplexing gain in the J2SMB system is proved as the multiplication of the number of subarrays k and the number of propagation paths Np, which is k times that of the classic fully-connected hybrid beamforming (FC-HB) system. Then the intractable J2SMB problem is decomposed into k tractable subproblems, which can be solved by a low-complexity column-by-column (CBC) algorithm. Simulation results show that the capacity of the J2SMB system is significantly larger than the FC-HB and the Line-of-Sight (LoS) MIMO systems. The performance of the low-complexity CBC algorithm on spectral efficiency is fairly close to the capacity of the J2SMB system.