Massive MIMO inspired 2-stage design of the multi-cell multi-user MIMO downlink

The Interfering Broadcast Channel (IBC) applies to the downlink of (cellular and/or heterogeneous) multi-cell networks, which are limited by multi-user (MU) interference. The interference alignment (IA) concept has shown that interference does not need to be inevitable. In particular spatial IA in the MIMO IBC allows for low latency. However, IA requires perfect and typically global Channel State Information at the Transmitter(s) (CSIT), whose acquisition does not scale well with network size. Also, the design of transmitters (Txs) and receivers (Rxs) is coupled and hence needs to be centralized (cloud) or duplicated (distributed approach). CSIT, which is crucial in multi-user systems, is always imperfect in practice, especially for the intercell links. We consider mean and covariance Gaussian partial CSIT. We focus on the optimization of beamformers for the expected weighted sum rate (EWSR) under per BS power constraints. We apply a perfect CSI technique, based on a difference of convex functions approach, to a number of deterministic approximations of the EWSR, involving the Massive MIMO (MaMIMO) limit (large number of transmit antennas), and the large MIMO limit (both large transmit and receive antenna numbers). We then focus on distributed techniques that exploit local CSIT, feedback of a limited number of scalars and

vectors, and only one or few iterations.