Massive MIMO for mmWave systems

The large bandwidths available at the millimeter wave (mmWave) carrier frequencies (e.g., 30-100 GHz) have sparked significant interest in developing cellular systems in those bands to meet the ever-increasing demand for high data rates. Large-scale antenna arrays with tens or hundreds of antennas are envisioned to be a prerequisite for operating in the mmWave bands due to the poor path loss conditions in those bands. Previous studies for 72GHz carrier frequencies have shown how extremely high data rates can be achieved in ultra-dense small cell deployments through simple single-user MIMO techniques mainly by virtue of the high system bandwidth (on the order of 1-2GHz). In this paper, we extend the prior work on single-user MIMO (SU-MIMO) for mmWave bands and examine the question of whether Multi-User MIMO (MU-MIMO) is a useful approach for mmWave bands. We show that there are definite cases where MU-MIMO can provide significant system capacity gains over SU-MIMO in the mmWave bands, which is in contrast to the expectation that the poor path loss conditions necessitate simple high gain beamforming techniques. We show that in many cases, a large-scale array provides sufficient SINR gain that can enable further gains from multi-user spatial multiplexing. We show how those gains depend on a variety of factors such as the user density and the transmission strategy.