Soft Pilot Reuse and Multi-Cell Block Diagonalization Precoding for Massive MIMO Systems

摘要

The users at cell edge of a massive multiple-input multiple-output (MIMO)system suffer from severe pilot contamination, which leads to poor quality ofservice (QoS). In order to enhance the QoS for these edge users, soft pilotreuse (SPR) combined with multi-cell block diagonalization (MBD) precoding areproposed. Specifically, the users are divided into two groups according totheir large-scale fading coefficients, referred to as the center users, whoonly suffer from modest pilot contamination and the edge users, who suffer fromsevere pilot contamination. Based on this distinction, the SPR scheme isproposed for improving the QoS for the edge users, whereby a cell-center pilotgroup is reused for all cell-center users in all cells, while a cell-edge pilotgroup is applied for the edge users in the adjacent cells. By extending theclassical block diagonalization precoding to a multi-cell scenario, the MBDprecoding scheme projects the downlink transmit signal onto the null space ofthe subspace spanned by the inter-cell channels of the edge users in adjacentcells. Thus, the inter-cell interference contaminating the edge users' signalsin the adjacent cells can be efficiently mitigated and hence the QoS of theseedge users can be further enhanced. Our theoretical analysis and simulationresults demonstrate that both the uplink and downlink rates of the edge usersare significantly improved, albeit at the cost of the slightly decreased rateof center users.