Efficient Multi-User MIMO Transmissions in the LTE-A Uplink

Single-User Multiple-Input Multiple-Output (MIMO) transmissions are possible since Rel. 10 of 3GPP LTE-A. The Closed Loop Spatial Multiplexing (CLSM) transmission mode enables up to a four fold increase in spectral efficiency compared to Rel. 8, yet it requires complex User Equipment (UE) hardware supporting more than one transmit antenna. The closed loop spatial multiplexing transmission mode enables up to a four fold increase in spectral efficiency compared to Rel. 8, yet it requires complex UE hardware supporting more than one transmit antenna. As it is likely that the Base Station (BS) has more receive antennas than the UE transmit antennas, employing Multi-User (MU) MIMO transmissions in the uplink can lead to significant improvement in cell throughput. We formulate MU MIMO transmissions with linear receivers as a compact matrix system model. Based on our model we present a post-equalization SINR equation, allowing the calculation of a quantized feedback parameter, the Channel Quality Indicator (CQI) value. Within this framework we reveal possible MU gains through known scheduling algorithms although the frequency selective channel is approximated by its arithmetic mean.