  Large antenna arrays will be needed in future millimeter wave (mmWave)
cellular networks, enabling a large number of different possible antenna
architectures and multiple-input multiple-output (MIMO) techniques. It is still
unclear which MIMO technique is most desirable as a function of different
network parameters. This paper, therefore, compares the coverage and rate
performance of hybrid beamforming enabled multi-user (MU) MIMO and single-user
spatial multiplexing (SM) with single-user analog beamforming (SU-BF). A
stochastic geometry model for coverage and rate analysis is proposed for
MU-MIMO mmWave cellular networks, taking into account important mmWave-specific
hardware constraints for hybrid analog/digital precoders and combiners, and a
blockage-dependent channel model which is sparse in angular domain. The
analytical results highlight the coverage, rate and power consumption tradeoffs
in multiuser mmWave networks. With perfect channel state information at the
transmitter and round robin scheduling, MU-MIMO is usually a better choice than
SM or SU-BF in mmWave cellular networks. This observation, however, neglects
any overhead due to channel acquisition or computational complexity.
Incorporating the impact of such overheads, our results can be re-interpreted
so as to quantify the minimum allowable efficiency of MU-MIMO to provide higher
rates than SM or SU-BF.
