On greedy stream selection in MIMO BC

We consider the Broadcast Channel (BC), or in other words the multi-user (MU) downlink in a cell with a base station equipped with multiple antennas and mobile terminals equipped with a single antenna (MU-MISO case). It is well-known that user selection not only leads to multi-user diversity but also to decreased suboptimality of simple beamforming (BF) techniques compared to optimal Dirty Paper Coding (DPC) approaches. User selection by exhaustive search can be simplified to greedy approaches, in which one user gets added at a time. We review DPC-style and BF-style greedy user selection (GUS), which are developed with DPC or BF transmitters in mind. We introduce a new interpretation of the BF-style user selection criterion, which corresponds to a new approximate criterion that becomes more accurate for a large user pool. This new criterion allows interpretation and comparison of the respective roles of channel strength and orthogonality in DPC-style and BF-style selection.