Interference mitigation and multiuser multiplexing with beam-steering antennas

We revisit a recently proposed framework for "massive MIMO" named Joint Space Division and Multiplexing (JSDM), that explicitly distinguishes between long-term channel statistics and instantaneous channel state information. In JSDM, the beamforming function is split into two components: prebeamforming and multiuser precoding. The pre-beamforming stage depends only on the channel covariance information, which is (locally) time-invariant and can be acquired without significant training overhead by averaging over time. Based on this information, the pre-beamforming stage separates the users into groups without significant mutual interference, irrespectively of their instantaneous random channel realization. Then, inside each user group, a standard multiuser MIMO precoding scheme based on instantaneous channel state information can be used in order to achieve further multiuser multiplexing gain. JSDM is naturally suited to a Hybrid Digital Analog (HDA) implementation, where the pre-beamforming function can be accomplished through a reconfigurable RF front-end. In this paper, we consider a low complexity HDA implementation of JSDM, where the canonical massive MIMO antenna array is replaced by a smaller number of programmable beam-steering antennas. We show by simulation that with this architecture it is possible to achieve both multiplexing gain and interference mitigation across different user groups.