Comparison between a hybrid digital and analog beamforming system and a fully digital Massive MIMO system with adaptive beamsteering receivers in millimeter-Wave transmissions

Massive MIMO systems are known to be a very promising solution for future 5G systems. The attractiveness of using such a system for millimeter Wave (mmWave) transmissions comes partly from the reduced size of a many antenna base station. Furthermore, the beamforming gains that they allow is highly suited to combat the high path losses experienced at such wavelengths. Unfortunately the complexity to implement a mmWave Massive MIMO system comes with the difficulty to have a high number of stacked Radio Frequency (RF) chains. Hybrid Analog and Digital Beamforming systems have then emerged to retain a high number of antennas without as many RF chains in order to keep high beamforming gains. In this paper we first describe a User Equipment (UE) solution allowing the system to form a beam that adapts to its own movement so that it always focuses its energy toward the base station, using an on board analog array and an Inertial Measurement Unit. Then we compare the performance of the known Hybrid solution with a fully digital Massive MIMO system, having as many RF chains as the Hybrid system, but serving UEs with beamforming abilities. Mostly we emphasize how such a system allows for great flexibility and evolution, both traits being invaluable features in many future networks.