From MU massive MISO to pathwise MU massive MIMO

The Interfering Broadcast Channel (IBC) applies to the downlink of cellular and heterogenous 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 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 with network size. Hence, designs that are optimal in terms of Degrees of Freedom (DoF) may not be so in terms of more relevant net DoF, accounting for CSI acquistion or at finite SNR. Also, the design of transmitters (Txs) and receivers (Rxs) is coupled and hence needs to be centralized or duplicated. Here we propose to take advantage of Massive MIMO simplifications, esp. for mmWave, by considering (multi-)path CSIT for crosslinks, which can be obtained without feedback. We consider a hierarchical cross/direct link beamformer design, maximizing Weighted Sum Rate (WSR) with partial CSIT at finite SNR, requiring on local CSIT. We also point out the use of receive antennas in genuine MU Massive MIMO.