Interference Alignment in Reduced-Rank MIMO Networks with Application to Dynamic TDD

In Dynamic Time Division Duplex (DynTDD), downlink/uplink (DL/UL) slot allocation is adaptive with traffic load. DynTDD systems have received a lot of attention for 5th generation (5G) mobile communication systems, as the spectrum efficiency of wireless communication networks is improved by the flexible and dynamic duplex operation. However when using DynTDD, a different DL/UL slot configuration is likely to be selected by neighboring cells, leading to Cross Link Interference (CLI) between the Base Stations (BS), which is known as BS-to-BS or DL-to-UL interference, and between User Equipment (UE) which is known as UE-to-UE or UL-to-DL interference. Rank deficient channels are frequently encountered in Multi-Input Multi-Output (MIMO) networks, due to poor scattering and keyhole effects, or when using Massive MIMO and moving to mmWave. While the implications of rank deficient channels are well understood for the single user (SU) point to point setting, less is known for interference networks. In this paper, we extend a MIMO Interference Alignment (IA) feasibility investigation framework to rank deficient channels and we investigate the IA feasibility in DynTDD considering rank deficient MIMO interfering channels, by establishing the simultaneously necessary and sufficient conditions. These conditions allow us to evaluate the Degrees-of-Freedom (DoF) of centralized designs more precisely compared to the loose proper conditions. We then compare the achievable DoF of centralized designs with those of various distributed designs, allowing us to get a better idea of the DoF price to pay for distributedness (but not accounting for the gain in information exchange reduction that distributed designs permit).