  For a massive multiple-input multiple-output (MIMO) system, restricting the
number of RF chains to far less than the number of antenna elements can
significantly reduce the implementation cost compared to the full complexity RF
chain configuration. In this paper, we consider the downlink communication of a
massive multiuser MIMO (MU-MIMO) system and propose a low-complexity hybrid
block diagonalization (Hy-BD) scheme to approach the capacity performance of
the traditional BD processing method. We aim to harvest the large array gain
through the phase-only RF precoding and combining and then digital BD
processing is performed on the equivalent baseband channel. The proposed Hy-BD
scheme is examined in both the large Rayleigh fading channels and millimeter
wave (mmWave) channels. A performance analysis is further conducted for
single-path channels and large number of transmit and receive antennas.
Finally, simulation results demonstrate that our Hy-BD scheme, with a lower
implementation and computational complexity, achieves a capacity performance
that is close to (sometimes even higher than) that of the traditional
high-dimensional BD processing.
