Power minimization in parallel vector broadcast channels with zero-forcing beamforming

We consider the problem of power minimization under per-user quality of service (QoS) constraints (expressed in terms of rates) in parallel multiple-input single-output (MISO) broadcast channels employing linear zero-forcing precoding. Solving the arising scheduling problem by an exhaustive search is prohibitively complex due to its combinatorial nature so that the use of successive user allocation schemes has been proposed. We show that existing schemes lead to strongly suboptimal solutions in systems with a low number of degrees of freedom and that better performance can be achieved with a new scheduling criterion. By introducing additional correction steps, we end up with an efficient close-to-optimum algorithm.