Achievable Rates of Multi-User Millimeter Wave Systems with Hybrid Precoding
Ahmed Alkhateeb, Robert W. Heath, Jr., and Geert Leus
in Proc. of the IEEE Int. Conf. on Communications Workshops, London, UK, June 8-12, 2015.
Millimeter wave (mmWave) systems will likely employ
large antenna arrays at both the transmitters and receivers.
A natural application of antenna arrays is simultaneous transmission
to multiple users, which requires multi-user precoding at the
transmitter. Hardware constraints, however, make it difficult to
apply conventional lower frequency MIMO precoding techniques
at mmWave. This paper proposes and analyzes a low complexity
hybrid analog/digital precoding algorithm for downlink multiuser
mmWave systems. Hybrid precoding involves a combination
of analog and digital processing that is motivated by the
requirement to reduce the power consumption of the 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. For this algorithm, we derive a lower bound
on the achievable rate for the case of single-path channels, show
its asymptotic optimality at large numbers of antennas, and make
useful insights for more general cases. Simulation results show
that the proposed algorithm offers higher sum rates compared
with analog-only beamforming, and approaches the performance
of the unconstrained digital precoding solutions.