Not Too Delayed CSIT Achieves the Optimal Degrees of Freedom


Namyoon Lee and Robert W. Heath, Jr.


50th Annual Allerton Conference on Communication, Control, and Computing, pp. 1262 -- 1269, Oct. 2012.


Channel state information at the transmitter (CSIT) aids interference management in many communication systems. Due to channel state information (CSI) feedback delay and time-variation in the wireless channel, perfect CSIT is not realistic. In this paper, the CSI feedback delay-DoF gain trade-off is characterized for the multi-user vector broadcast channel. A major insight is that it is possible to achieve the optimal degrees of freedom (DoF) gain if the delay is less than a certain fraction of the channel coherence time. This precisely characterizes the intuition that a small delay should be negligeable. To show this, a new transmission method called space-time interference alignment is proposed, which actively exploits both the current and past CSI.

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