Sub-Connected Hybrid Precoding Architectures in Massive MIMO Systems

Hybrid Precoding (HP) has been introduced to reduce the complexity/costs due to a large number of RF chains in the fully-digital massive MIMO precoding. In a fully-connected (FC) HP, each RF chain is connected to all available antenna elements to exploit the full beamforming capability of the antenna array at the expense of large connectivity. To further reduce costs/complexity associated with this large connectivity, sub-connected (SC) HP considers that each RF chain connected to a subset of selected antenna elements of the antenna array at the costs of inferior performance. This paper aims to study the complexity and performance of both FC-HP and SC-HP. For comparison, we consider a common 2-stage HP scheme with the RF-beamforming (BF) stage designed via the slow time-varying angle-of-departure (AoD) information, using both orthogonal and non-orthogonal BF approaches, while the baseband-precoding stage uses a regularized zero-forcing (RZF) technique. This common HP scheme is used by a base-station equipped with a uniform rectangular large-scale antenna-array to serve multiple single-antenna users clustered in multiple groups. Three sub-array configurations (vertical, horizontal, square) are considered for SC- HP. Illustrative simulation results are provided to compare the performance of FC-HP and SC-HP in various scenarios and indicate that for a 64-element URA and 4 RF chains, SC-HP can achieve 91.46% sum-rate performance of FC-HP with only 25% complexity.