Cellular and Network MIMO architectures: MU-MIMO spectral efficiency and costs of channel state information

Cooperative Multi-User MIMO signaling across cells, also known as Network MIMO, has recently received a great deal of attention as a potential physical layer technique for future high-throughput wireless systems. Indeed, cooperative signaling is able to reduce inter-cell interference within groups of coordinated cells thereby improving system throughput. Such benefits, however, are often assessed ignoring inherent overheads such as those required to obtain channel state information (CSI) to enable downlink signaling by MU-MIMO. In fact CSI estimation does take physical layer resources from data and thus should be accounted for in assessing the net benefits of any cooperative signalling strategy. The paper characterizes some elements of this overhead and its effect on CSI estimation error and MU-MIMO efficiency. The results show that, when CSI overheads are taken into account, conventional cellular architectures, with no coordination across cells, can be quite attractive relative to Network MIMO.