Distributed Space-Time Interference Alignment
Channel state information at the transmitter (CSIT) plays an important role in interference management in wireless systems. Interference networks with global and instantaneous CSIT provide a great improvement of performance. In practice, however, obtaining global and instantaneous CSIT for transmitter cooperation is especially challenging, when the transmitters are distributed and the mobility of wireless nodes increases. In an extreme case where the channel coherence time is shorter than the CSI feedback delay, it is infeasible to acquire instantaneous CSIT in wireless systems. Obtaining global knowledge of CSIT is another obstacle for realizing transmitter cooperation when the backhaul or feedback link capacity is very limited for CSIT sharing between the distributed transmitters. Therefore, we propose a fundamental question: is it still possible to obtain benefits in increasing the scaling law of the rate, i.e degress-of-freedom (DoF), for interference networks under these two practical constraints?
Motivated by this question, WNCG PhD. Candidate Namyoon Lee, Virginia Tech Research Professor Ravi Tandon, and WNCG Professor Robert Heath propose a distributed interference alignment algorithm exploiting local and moderately-delayed CSIT. The proposed method is a structured space-time repetition transmission technique that exploits both current and outdated CSIT jointly to align interference signals at unintended receivers in a distributed way. With this algorithm, they characterize trade-off regions between the sum of degrees of freedom (sum-DoF) and feedback delay in both the X channel and three-user interference channel to reveal the impact on how the CSI feedback delay affects the sum-DoF of the interference networks.
Their key finding is that distributed and moderately-delayed 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, they illustrate 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, they characterize the ergodic sum-capacity of the two-user X channel within a constant bit. This paper was submitted to IEEE Transaction on Wireless Communications.
This research funded by the Intel 5G Program.