Simultaneous Feedback Reduction and Sum Rate Maximization in Block-Diagonalized Space-Division Multiplexing

A major hindrance to the adoption of orthogonalized space-division multiplexing (SDM) via block diagonalization (BD) in multi-user MIMO downlinks is the need for channel state information (CSI) feedback from user terminals. Another drawback is lower than optimal sum rates, theoretically achievable with dirty paper coding (DPC). While multi-user diversity could be leveraged via user selection to narrow the sum- rate performance gap, it requires the presence of very large user pools. To help raise the practical feasibility of BD-SDM, we propose a scheme that jointly reduces CSI feedback while approaching optimal DPC sum rates with smaller user-pool sizes. Additionally, BD-SDM offers the flexibility for spatial mode allocation to cater for individual transmission rate requirements. This presents a challenging resource allocation problem because mode selection at one terminal affects the rates achieved at all other terminals and in turn, the overall sum rate. The proposed scheme offers a systematic means for resource allocation, while minimizing rate loss at the overall- and individual levels. It represents a streamlined process that simultaneously reduces CSI feedback while achieving sum rate maximization, user selection and systematic rate-loss minimizing resource allocation.