LTE Downlink 2X2 MIMO with Realistic CSI: Overview and Performance Evaluation

For downlink multiple input multiple output (MIMO) schemes, there is a tradeoff between downlink MIMO performance and uplink reporting cost. The mobile speed has an impact on this tradeoff since the reporting delays the measurements. This paper presents the major downlink MIMO modes in 3GPP Long Term Evolution (LTE) and their related channel state information (CSI) modes. The impact of realistic CSI is investigated through advanced system simulations, showing that the total performance degradation of realistic CSI reporting is at a surprisingly low 20% compared to full, recent channel knowledge. With realistic CSI reporting, a selected set of promising 2x2 MIMO applications are evaluated at different mobile speeds. The open-loop spatial multiplexing mode offers unexpectedly high cell throughput despite its limited feedback requirement. It performs well for a wide range of speeds, as it provides good robustness to delayed channel knowledge. Bitrates can only be further improved by closed-loop spatial multiplexing in stationary scenarios, where CSI delay does not hamper the precoding gain. These results confirm that there are MIMO modes suitable for both high and low speeds.