  In this paper, we propose a two-stage analog combining architecture for
millimeter wave (mmWave) communications with hybrid analog/digital beamforming
and low-resolution analog-to-digital converters (ADCs). We first derive a
two-stage combining solution by solving a mutual information (MI) maximization
problem without a constant modulus constraint on analog combiners. With the
derived solution, the proposed receiver architecture splits the analog
combining into a channel gain aggregation stage followed by a spreading stage
to maximize the MI by effectively managing quantization error. We show that the
derived two-stage combiner achieves the optimal scaling law with respect to the
number of radio frequency (RF) chains and maximizes the MI for homogeneous
singular values of a MIMO channel. Then, we develop a two-stage analog
combining algorithm to implement the derived solution under a constant modulus
constraint for mmWave channels. Simulation results validate the algorithm
performance in terms of MI.
