Towards improving LTE SU/MU-MIMO performance: Issues in channel estimation, interpolation and feedback

We consider improvements to the throughput performance of Single-User MIMO (SU-MIMO) and Multi-User MIMO (MU-MIMO) in the present 3GPP Long Term Evolution (LTE) design. The current design has been largely optimized for highly mobile macro-cellular scenarios. It is based on low-rate implicit Channel State Information (CSI) feedback, yielding large errors in the CSI at the transmitter (CSIT), thereby resulting in severely inter-stream interference limited operation for MU-MIMO. As a result much of the recent effort towards improving MU-MIMO performance has focused on increasing this feedback rate and, in particular, the number of bits used to describe each of the channels that are fed back. While such improvements are possible, they still tend to fall short of what should be expected by SU-MIMO and, in particular, MU-MIMO. As we show, in addition to CSI feedback rate (per channel fed back), other elements of the LTE design, such as the granularity of CSI feedback in frequency, and the pilot density in the OFDM plane, can play a critical role in enabling MU-MIMO. In particular, over some channel scenarios, such enhancements can improve the CSIT quality enough to enable significant improvements in the MU-MIMO performance. Detailed link-level evaluations of various feedback, pilot and interpolation designs are provided, demonstrating the above effects and illustrating the underlying practical and theoretical issues that are limiting MIMO performance improvements in LTE.