Condition Number Improvement of IRS-Aided Near-Field MIMO Channels

In near-field wireless propagation environment, line-of-sight (LoS) link becomes dominant, which leads to the signal energy focusing on the LoS path. In this paper, the concept of condition number is introduced to measure the spatial multiplexing gain available for the multiple-input multiple-output (MIMO) system. A criterion based on the minimized condition number of the MIMO channel is given out by deploying the emerging intelligent reconfigurable surfaces (IRS), which makes the energy of signals uniformly spread across the effective subchannels of the MIMO channel by adding reflective link and optimizing its phase shifters. Then, based on the improved MIMO channel, the optimal transmit and received beamforming matrices are obtained by using the singular value decomposition (SVD) approach to reach the upper bound of capacity for the IRS-aided MIMO channel. We demonstrate that deploying IRS and optimizing its phase can decrease the condition number of MIMO channel, leading to substantial multiplexing gains.