Hybrid structure in massive MIMO: Achieving large sum rate with fewer RF chains

Massive multiple-input multiple-output (MIMO) systems can attain a high channel capacity and spectral efficiency by using a very large antenna array at the base station (BS). However, the cost of having one radio frequency (RF) chain behind every antenna element can be prohibitive. In addition, the overall power consumption of the RF hardware can be excessively high. A hybrid analog-digital structure can be utilized to reduce the required number of RF chains at the BS. In this paper, we present achievable rates of hybrid beamforming in multi-user MIMO (MU-MIMO) when employing only one RF chain per user. The analysis and simulation results show that the asymptotic signal-plus-interference-to-noise ratio (SINR) of hybrid beamforming is reduced by a factor of π/4 compared to conventional beamforming methods, and the resulting achievable sum-rate degradation can be compensated by simply employing 27% more transmit antennas.