Distributed Space-Time Interference Alignment

This paper proposes an interference alignment method with distributed and delayed channel state information at the transmitter (CSIT) for a class of interference networks. The core idea of the proposed method is to align interference signals over time at the unintended receivers in a distributed manner. With the proposed method, trade-off regions between the sum of degrees of freedom (sum-DoF) and feedback delay of CSI are characterized in both the X channel and three-user interference channel to reveal the impact on on how the CSI feedback delay affects the sum-DoF of the interference networks. A major implication of derived results is that distributed and moderatelydelayed CSIT is useful to obtain strictly better the sum-DoF over the case of no CSI at the transmitter in a certain class of interference networks. For a class of X channels, the results show how to optimally use distributed and moderately-delayed CSIT to yield the same sum-DoF as instantaneous and global CSIT. Further, leveraging the proposed transmission method and the known outer bound results, the ergodic sum-capacity of the twouser X channel is characterized within a constant bit.