Multiuser MIMO systems using codebook tailored limited feedback protocol

While it is common understanding that feedback protocols are well developed for Multiuser MIMO systems surprisingly little has been done with respect to the metric used by the user terminals for selecting the quantization vectors. In particular, the Grassmanian chordal metric typically used in this context is widely unquestioned. However, engineering intuition suggests that e.g. the information about the transmit codebook can be beneficial. The aim of this paper is to make this intuition precise by introducing a new metric called the codebook oriented (CBO) metric. For the CBO metric it is shown that for any fixed transmit codebook a better scaling compared to Jindal's standard formula of the total throughput loss with respect to the number of feedback bits is attainable. Furthermore it is shown that for the CBO metric the relevant geometrical object for the underlying quantization problem is not the common Grassmanian manifold but instead a higher-dimensional simplex. The analysis is supported by standard-complient LTE system simulations showing performance gains of up to 50% compared to zeroforcing when using multiple antennas at both the base station and the terminal, and 70% when using a single antenna at the terminal.