  For 5G it will be important to leverage the available millimeter wave
spectrum. To achieve an approximately omni- directional coverage with a similar
effective antenna aperture compared to state of the art cellular systems, an
antenna array is required at both the mobile and basestation. Due to the large
bandwidth and inefficient amplifiers available in CMOS for mmWave, the analog
front-end of the receiver with a large number of antennas becomes especially
power hungry. Two main solutions exist to reduce the power consumption: Hybrid
BeamForming (HBF) and Digital BeamForming (DBF) with low resolution ADC. Hybrid
beamforming can also be combined with low resolution ADCs. This paper compares
the spectral and energy efficiency based on the chosen RF-frontend
configuration. A channel with multipath propagation is used. In contrast to
previous publication, we take the spatial correlation of the quantization noise
into account. We show that the low resolution ADC are robust to small Automatic
Gain Control (AGC) imperfections. We showed that in the low SNR regime the
performance of DBF even with 1-2 bit resolution outperforms HBF. If we consider
the relationship of spectral and energy efficiency then DBF with 3-5 bits
resolution achieves the best ratio of spectral efficiency per power consumption
of the RF receiver frontend over a very wide SNR region. The power consumption
model is based on components reported in literature.
