  We consider Multi-User MIMO (MU-MIMO) scheduling in the 3GPP LTE-Advanced
(3GPP LTE-A) cellular uplink. The 3GPP LTE-A uplink allows for precoded
multi-stream (precoded MIMO) transmission from each scheduled user and also
allows flexible multi-user (MU) scheduling wherein multiple users can be
assigned the same time-frequency resource. However, exploiting these features
is made challenging by certain practical constraints that have been imposed in
order to maintain a low signaling overhead. We show that while the scheduling
problem in the 3GPP LTE-A cellular uplink is NP-hard, it can be formulated as
the maximization of a submodular set function subject to one matroid and
multiple knapsack constraints. We then propose constant-factor polynomial-time
approximation algorithms and demonstrate their superior performance via
simulations.
