Multi-user MIMO scheduling in the fourth generation cellular uplink

In this paper, we consider Multi-User MIMO (MU-MIMO) scheduling in the 3GPP LTE-Advanced (3GPP LTE-A) cellular uplink. The 3GPP LTE-A cellular uplink allows for precoded multi-stream (precoded MIMO) transmission from each user scheduled in the uplink along with flexible multi-user scheduling, wherein multiple users can be assigned the same time-frequency resource. However, optimally exploiting these features is challenging since certain practical constraints have to be satisfied, such as the one which demands that the number of users co-scheduled on any time-frequency resource must not exceed a specified number. We show that while the resulting resource allocation problem is NP-hard, it can be formulated as the maximization of a non-negative monotonic submodular set function subject to one matroid and multiple column-sparse knapsack constraints. We then provide low complexity approximation algorithms and demonstrate their superior performance via simulations.