  In this paper, we propose a hybrid analog-digital beamforming architecture
with resolution-adaptive ADCs for millimeter wave (mmWave) receivers with large
antenna arrays. We adopt array response vectors for the analog combiners and
derive ADC bit-allocation (BA) solutions in closed form. The BA solutions
reveal that the optimal number of ADC bits is logarithmically proportional to
the RF chain's signal-to-noise ratio raised to the 1/3 power. Using the
solutions, two proposed BA algorithms minimize the mean square quantization
error of received analog signals under a total ADC power constraint.
Contributions of this paper include 1) ADC bit-allocation algorithms to improve
communication performance of a hybrid MIMO receiver, 2) approximation of the
capacity with the BA algorithm as a function of channels, and 3) a worst-case
analysis of the ergodic rate of the proposed MIMO receiver that quantifies
system tradeoffs and serves as the lower bound. Simulation results demonstrate
that the BA algorithms outperform a fixed-ADC approach in both spectral and
energy efficiency, and validate the capacity and ergodic rate formula. For a
power constraint equivalent to that of fixed 4-bit ADCs, the revised BA
algorithm makes the quantization error negligible while achieving 22% better
energy efficiency. Having negligible quantization error allows existing
state-of-the-art digital beamformers to be readily applied to the proposed
system.
