MIMO downlink scheduling in LTE systems

Scheduling plays a vital role in LTE downlink systems with Multiple Input and Multiple Output (MIMO) antennas. We consider the MIMO downlink scheduling problem at the base station (BS) in LTE networks under several practical constraints mandated by the 3GPP standards. We Define a new construct called transmission mode, which denotes a particular choice of MIMO operational mode, precoding matrix, transmission rank, as well as the modulation and coding schemes (MCSs) of up to two codewords and show that both LTE systems require that each scheduled user be served using only one transmission mode in every subframe. We prove that the resulting scheduling problems are NP-hard under both backlogged and finite queue traffic models, and then develop a unified low-complexity greedy algorithm that yields solutions guaranteed to be within 1/2 of the respective optima. Extensive performance evaluation in realistic settings reveals near-optimal performance of our proposed algorithm and that it significantly outperforms the state of the art, especially under the more practical, finite queue model.