  Hybrid analog/digital architectures and receivers with low-resolution
analog-to-digital converters (ADCs) are two low power solutions for wireless
systems with large antenna arrays, such as millimeter wave and massive MIMO
systems. Most prior work represents two extreme cases in which either a small
number of RF chains with full-resolution ADCs, or low resolution ADC with a
number of RF chains equal to the number of antennas is assumed. In this paper,
a generalized hybrid architecture with a small number of RF chains and finite
number of ADC bits is proposed. For this architecture, achievable rates with
channel inversion and SVD based transmission methods are derived. Results show
that the achievable rate is comparable to that obtained by full-precision ADC
receivers at low and medium SNRs. A trade-off between the achievable rate and
power consumption for different numbers of bits and RF chains is devised. This
enables us to draw some conclusions on the number of ADC bits needed to
maximize the system energy efficiency. Numerical simulations show that coarse
ADC quantization is optimal under various system configurations. This means
that hybrid combining with coarse quantization achieves better energy-rate
trade-off compared to both hybrid combining with full-resolutions ADCs and
1-bit ADC combining.
