  Antenna arrays will be an important ingredient in millimeter wave (mmWave)
cellular systems. A natural application of antenna arrays is simultaneous
transmission to multiple users. Unfortunately, the hardware constraints in
mmWave systems make it difficult to apply conventional lower frequency
multiuser MIMO precoding techniques at mmWave. This paper develops low
complexity hybrid analog/digital precoding for downlink multiuser mmWave
systems. Hybrid precoding involves a combination of analog and digital
processing that is inspired by the power consumption of complete radio
frequency and mixed signal hardware. The proposed algorithm configures hybrid
precoders at the transmitter and analog combiners at multiple receivers with a
small training and feedback overhead. The performance of the proposed algorithm
is analyzed in the large dimensional regime and in single path channels. When
the analog and digital precoding vectors are selected from quantized codebooks,
the rate loss due to the joint quantization is characterized and insights are
given into the performance of hybrid beamforming compared with analog-only
beamforming solutions. Analytical and simulation results show that the proposed
techniques offer higher sum rates compared with analog-only beamforming
solutions, and approach the performance of the unconstrained digital
beamforming with relatively small codebooks.
