  Due to the heavy reliance of millimeter-wave (mmWave) wireless systems on
directional links, beamforming (BF) with high-dimensional arrays is essential
for cellular systems in these frequencies. How to perform the array processing
in a power efficient manner is a fundamental challenge. Analog and hybrid BF
require fewer analog-to-digital and digital-to-analog converters (ADCs and
DACs), but can only communicate in a small number of directions at a time,
limiting directional search, spatial multiplexing and control signaling.
Digital BF enables flexible spatial processing, but must be operated at a low
quantization resolution to stay within reasonable power levels. This decrease
in quantizer resolution introduces noise in the received signal and degrades
the quality of the transmitted signal. To assess the effect of low-resolution
quantization on cellular system, we present a simple additive white Gaussian
noise (AWGN) model for quantization noise. Simulations with this model reveal
that at moderate resolutions (3-4 bits per ADC), there is negligible loss in
downlink cellular capacity from quantization. In essence, the low-resolution
ADCs limit the high SNR, where cellular systems typically do not operate. For
the transmitter, it is shown that DACs with 4 or more bits of resolution do not
violate the adjacent carrier leakage limit set by 3-rd Generation Partnership
Project (3GPP) New Radio (NR) standards for cellular operations. Further, this
work studies the effect of low resolution quantization on the error vector
magnitude (EVM) of the transmitted signal.In fact, our findings suggests that
low-resolution fully digital BF architectures can be a power efficient
alternative to analog or hybrid beamforming for both transmitters and receivers
at millimeter wave.
