Decoupled, rank reduced, massive and frequency-selective aspects in MIMO interfering broadcast channels

The Interfering Broadcast Channel (IBC) applies to the downlink of cellular and heterogenous networks, which are limited by multi-user 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 and requires little diversity. 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. Also, the design of transmitters (Txs) and receivers (RXs) is coupled and hence needs to be centralized or duplicated. Recently, a number of (usually suboptimal in terms of DoF) approaches with reduced, incomplete or local CSIT requirements have been introduced requiring less CSI acquisition overhead and allowing decoupled Tx/Rx designs. Network decomposition is aided by the for finite SNR more relevant topological IBC scenario, in which also reduced rank MIMO channels may appear. The transition to Massive MIMO furthermore introduces a reduced rank in the covariance CSIT and allows network decomposition. We will also highlight that any scenario allowing decomposition is favorable for the design of asynchronous frequency-selective networks.