A. Interference Management with Delayed and Distributed CSIT
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, one of fundamental questions is that 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, I have proposed interference alignment algorithms 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 The proposed method is a structured space-time repetition transmission technique that exploits both current and outdated CSIT jointly to align interference signals at unintended receivers in a distributed way.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 vector broadcast channels, the X channels, and a three-user interference channel to reveal the impact on how the CSI feedback delay affects the sum-DoF of the interference networks.
The key finding from this work 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 some classes of vector broadcast channels and X channels, I have illustrated how to optimally use distributed and moderately-delayed CSIT to yield the same sum-DoF as instantaneous and global CSIT.
d. Kwang-Won Lee, Namyoon Lee, and Inkyu Lee, “Achievable Degrees of Freedom on MIMO Two-way Relay Interference Channels,” IEEE Transaction on Wireless Communications, vol. 12, no. 4, pp. 1472-1480, April. 2013.
Interference management is complicated in the multi-hop networks because relay nodes between the source-destination pairs propagate the mixture of interference signals as well as desired signals on the network. This complicates the selection and design of relay strategies as it is not clear the extent to which a relay should forward, cancel, align, or otherwise manage interference. In this research direction, I have proposed interference-aware relay transmission techniques exploiting the concept of aligned interference neutralization for the multiple-input-multiple-output (MIMO) two-hop interference channels to characterize the scaling law of network sum-capacity.
b. Junil Choi, Bruno Clerckx, Namyoon Lee, and Gil Kim, “A New Design of Polar-Cap Differential Codebook for Temporally/Spatially Correlated MISO Channels,” IEEE Transaction on Wireless Communications, vol. 11, pp. 703-711, Feb. 2012.
c. Namyoon Lee, Wonjae Shin, Robert W. Heath and Bruno Clerckx, “Interference Alignment with Limited Feedback on Two-cell Interfering MIMO-MAC,” IEEE International Symposium on Wireless Communication Systems (ISWCS), Aug. 2012. (Invited)