Hierarchical Precoding for the Network MIMO Downlink

Over the last few years, network MIMO or distributed MIMO has gained enormous interest in research and meanwhile is a candidate for future communication systems. However, existing techniques handling the multi-user interference, e.g., linear preequalization (LPE) or TomlinsonHarashima precoding (THP), assume full cooperation between the base stations. This might cause significant signaling overhead in the backhaul. In this paper, precoding strategies with reduced coordination effort are presented and assessed. We show how to optimize THP for a setting, where a hierarchy of increasing knowledge over the base stations is expected. A reduced-complexity variant is given. In addition, besides the usual sum power constraint, a per-antenna average power constraint is included in the optimization, which is of relevance in distributed MIMO. The performance of the proposed strategies is covered by means of numerical simulations.