Interference mitigation in dynamic TDD MIMO interference channels

Dynamic Time Division Duplexing (DynTDD) is one of the key features that enable the dynamic or flexible uplink and downlink transmission and reception for a specific sub-frame in SG mobile networks. However, the advantages of the DynTDD system are difficult to fully utilize due to the cross-link interference (CLI) arising from neighboring cells using different transmission directions on the same or partially- overlapping time-frequency resources. There are two types of cross-link interference; 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 deficiency of channel matrices is an important aspect of Multi-Input Multi-Output (MIMO) wireless systems. Poor scattering and the presence of single or very few direct paths are some reasons for rank deficiency in wireless channels. 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 give a sufficient condition for a full rank MIMO interfering channel, that outperforms the current state of the art, we provide also tighter necessary conditions for Interference Alignment (IA) feasibility, that are very close to the necessary and sufficient condition, which gives all the feasible cases. Then we extend a MIMO Interference Alignment (IA) feasibility framework to rank deficient channels, by investigating our published theorem for the necessary and sufficient condition in this case.