  Future millimeter-wave (mmWave) systems, 5G cellular or WiFi, must rely on
highly directional links to overcome severe pathloss in these frequency bands.
Establishing such links requires the mutual discovery of the transmitter and
the receiver %in the angular domain potentially leading to a large latency and
high energy consumption. In this work, we show that both the discovery latency
and energy consumption can be significantly reduced by using fully digital
front-ends. In fact, we establish that by reducing the resolution of the
fully-digital front-ends we can achieve lower energy consumption compared to
both analog and high-resolution digital beamformers. Since beamforming through
analog front-ends allows sampling in only one direction at a time, the mobile
device is ''on'' for a longer time compared to a digital beamformer which can
get spatial samples from all directions in one shot. We show that the energy
consumed by the analog front-end can be four to six times more than that of the
digital front-ends, depending on the size of the employed antenna arrays. We
recognize, however, that using fully digital beamforming post beam discovery,
i.e., for data transmission, is not viable from a power consumption standpoint.
To address this issue, we propose the use of digital beamformers with
low-resolution analog to digital converters (4 bits). This reduction in
resolution brings the power consumption to the same level as analog beamforming
for data transmissions while benefiting from the spatial multiplexing
capabilities of fully digital beamforming, thus reducing initial discovery
latency and improving energy efficiency.
