  In this paper, we investigate the quantization and the feedback of downlink
spatial covariance matrix for massive multiple-input multiple-output (MIMO)
systems with cascaded precoding. Massive MIMO has gained a lot of attention
recently because of its ability to significantly improve the network
performance. To reduce the overhead of downlink channel estimation and uplink
feedback in frequency-division duplex massive MIMO systems, cascaded precoding
has been proposed, where the outer precoder is implemented using traditional
limited feedback while the inner precoder is determined by the spatial
covariance matrix of the channels. In massive MIMO systems, it is difficult to
quantize the spatial covariance matrix because of its large size caused by the
huge number of antennas. In this paper, we propose a spatial spectrum based
approach for the quantization and the feedback of the spatial covariance
matrix. The proposed inner precoder can be viewed as modulated discrete prolate
spheroidal sequences and thus achieve much smaller spatial leakage than the
traditional discrete Fourier transform submatrix based precoding. Practical
issues for the application of the proposed approach are also addressed in this
paper.
