On the Degrees of Freedom of MIMO X Networks With Non-Cooperation Transmitters

This paper proposes novel transmission schemes for a class of interference networks that can achieve new tradeoff regions between the sum of degrees of freedom (sum-DoF) and channel state information (CSI) feedback delay with distributed and temperately-delayed CSI at the transmitter (CSIT). A significant impact of the results is they reveal that distributed and temperately-delayed CSIT contributes to achieve better sum-DoF than that without CSIT in a certain class of interference networks. Specifically, a distributed space-time interference alignment (STIA) scheme is proposed for the two-user multiple-input multiple-output (MIMO) X channel via a novel precoding method called Cyclic Zero-padding. The achieved sum-DoFs herein for certain antenna configurations are greater than the best known sum-DoFs in literature with delayed CSIT. Furthermore, we propose a distributed retrospective interference alignment (RIA) scheme that achieves more than 1 sum-DoF for the K-user single-input single-output (SISO) X network. Finally, we extend the distributed STIA to the M×N user multiple-input single-output (MISO) X network, where each transmitter has N - 1 antennas and each receiver has a single antenna, yielding the same sum-DoF as that in the global and instantaneous CSIT case. The discussion and result of the MISO X network can be extended to the MIMO case due to the spatial scale invariance property.