  At millimeter wave (mmWave) frequencies, the higher cost and power
consumption of hardware components in multiple-input multiple output (MIMO)
systems do not allow beamforming entirely at the baseband with a separate radio
frequency (RF) chain for each antenna. In such scenarios, to enable spatial
multiplexing, hybrid beamforming, which uses phase shifters to connect a fewer
number of RF chains to a large number of antennas is a cost effective and
energy-saving alternative. This paper describes our research on fully adaptive
transceivers that adapt their behaviour on a frame-by-frame basis, so that a
mmWave hybrid MIMO system always operates in the most energy efficient manner.
Exhaustive search based brute force approach is computationally intensive, so
we study fractional programming as a low-cost alternative to solve the problem
which maximizes energy efficiency. The performance results indicate that the
resulting mmWave hybrid MIMO transceiver achieves significantly improved energy
efficiency results compared to the baseline cases involving analogue-only or
digital-only signal processing solutions, and shows performance trade-offs with
the brute force approach.
