  The theoretical analysis of downlink massive MIMO usually assumes the ideal
Gaussian channel matrix with asymp- totic orthogonality of channel vectors
associated with different users, since it can provide the favorable propagation
condition. 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 paper theoretically proves that
channel vectors associated with different users in massive MIMO over realistic
sparse channels satisfy the asymptotic orthogonality, which indicates that the
favorable propagation condition can also be provided. Moreover, the simulation
results confirm the theoretical analysis.
