Full Digital Transmit Beamforming with Low RF Complexity for Large-scale mmWave MIMO system

Conventional full digital transmit beamforming requires a separate Radio-Frequency (RF) chain per transmit antenna, which is not easy to realize for large-scale antenna systems due to the high hardware cost, system complexity and power consumption. In this paper, we propose a new scheme that enables full digital transmit beamforming with low RF complexity. We consider a multiple-input-multiple-output (MIMO) system with N transmit antennas and M receive antennas where N > M, and transmit antennas are connected to L RF chains, where L <; N. In our scheme, transmit antennas are dynamically divided into L groups based on the multipath channel spatial correlation profile. The beamforming gain is achieved by letting highly correlated antennas in the same group. Each group of antennas is connected to one out of L RF chains and each group transmits an independent data stream. RF chain is designed in such a way that it independently preserves baseband signals of a group of antennas in a time-division-multiplexing (TDM) way. Our scheme is evaluated in simulations by comparing with full RF complexity digital transmit beamforming. Simulation results demonstrate that, our proposal is suitable for doubly massive MIMO systems, particularly in line-of-sight (LOS) scenarios.