Asymptotic Orthogonality Analysis of Time-Domain Sparse Massive MIMO Channels

The theoretical analysis of downlink massive MIMO usually assumes the ideal Gaussian channel matrix with asymptotic orthogonality of channel vectors. Meanwhile, recent experiments have shown that massive MIMO channels between a certain user and massive base station antennas appear the spatial common sparsity (SCS) in both the time domain and angle domain. This motivates us to investigate whether realistic sparse massive MIMO channels could provide the favorable propagation condition, and reveal the capacity gap between massive MIMO systems over realistic sparse channels and that under the ideal Gaussian channel matrix assumption. This letter theoretically proves that channel vectors associated with different users in massive MIMO over sparse channels satisfy the asymptotic orthogonality. Moreover, the simulation results confirm the theoretical analysis.