Antenna combiners for block-diagonalization based multi-user MIMO with limited feedback

In this paper, we consider multi-user MIMO (MU-MIMO) based on block-diagonalization (BD) precoding with limited feedback in the downlink of a cellular network. Non-codebook based MU-MIMO is already considered in LTE Advanced (LTE Rel. 10) as an important technology to improve the average cell spectral efficiency. Still, channel state information (CSI) feedback and downlink signaling are mainly designed for single-user MIMO in LTE, with simple modifications to support MU-MIMO in LTE-A. Thus, MU-MIMO improvements are a hot topic for LTE Rel. 12 and beyond. With BD precoding, the eNodeB transmits multiple data streams interference-free to multiple user-equipments (UEs) in parallel. When BD is based on limited feedback CSI, residual multi-user interference cannot be avoided. If the number of receive antennas per UE is larger than the number of data streams per UE, CSI feedback can be optimized to minimize the effect of multi-user interference. Additionally, the users can exploit the excess antennas to cancel residual multi-user interference. Assuming varying degrees of knowledge about the interference at the UEs, we propose different receive antenna combiners and CSI feedback algorithms, and compare their performance by means of link-level simulations.