  Millimeter-wave (mm-wave) communication is a key technology for future
wireless networks. To combat significant path loss and exploit the abundant
mm-wave spectrum, effective beamforming is crucial. Nevertheless, conventional
fully digital beamforming techniques are inapplicable, as they demand a
separate radio frequency (RF) chain for each antenna element, which is costly
and consumes too much energy. Hybrid beamforming is a cost-effective
alternative, which can significantly reduce the hardware cost and power
consumption by employing a small number of RF chains. This paper presents a
holistic view on hybrid beamforming for 5G and beyond mm-wave systems, based on
a new taxonomy for different hardware structures. We take a pragmatic approach
and compare different proposals from three key aspects: 1) hardware efficiency,
i.e., the required hardware components; 2) computational efficiency of the
associated beamforming algorithm; and 3) achievable spectral efficiency, a main
performance indicator. Through systematic comparisons, the interplay and
trade-off among these three design aspects are demonstrated, and promising
candidates for hybrid beamforming in future wireless networks are identified.
