Sum Rate Analysis of Massive MIMO Downlink With Hybrid Beamforming

The achievable sum rate of multi-cell multi-user massive multiple-input multiple-output (MIMO) downlink is investigated. Hybrid beamformers, which are constructed by using estimated/imperfect channel state information, are employed at the base-station. Thus, the massive MIMO base-station in each of the L cells is enabled with a typical power-intensive digital precoder and a quantized analog beamformer. This setup can significantly reduce the number of radio frequency (RF) chains required for the digital precoder, and thereby, the power consumption in the digital signal processing and the complexity of circuitry at the massive MIMO base-station. The achievable asymptotic sum rate expressions are derived for linear digital precoders namely, (i) zero-forcing transmission and (ii) maximum ratio transmission cascaded with a phase-shifting analog beamformer. Thereby, the asymptotic sum rate degradation due to the hybrid beamforming is quantified and compared against the full-dimensional digital beamforming. This sum rate loss is a function of the number of phase quantization levels and cannot be canceled completely even in the asymptotic base-station antenna regime. Nevertheless, our analysis reveals that the detrimental effects of phase quantization and reduced number of RF chains of the hybrid beamforming can be mitigated in the limit of increasingly many base-station antennas when the receiver thermal noise power is negligibly smaller than the residual interference due to pilot contamination.