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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