Space-Time Interference Alignment and Degrees of Freedom for the MIMO Broadcast Channel with Periodic CSI Feedback


Namyoon Lee and Robert W. Heath, Jr.


IEEE Transaction on Information Theory, vol 60, no. 1, pp. 515-528, Jan. 2014.


This paper characterizes the degrees of freedom (DoF) gain for the multi-antenna broadcast channel with periodic channel state information (CSI) feedback. As a part of the characterization, a new transmission method called space-time interference alignment is proposed. One consequence of the result is that the active exploitation of not only the current CSI but also the outdated CSI obtained from the periodic feedback mechanism provides an unbounded sum-rate performance gain, i.e., DoF gain, compared to when only the current CSI is periodically used for the multiple-antenna broadcast channel. The gain comes from aligning the inter-user interference signals between the past observed and the current arriving by using the proposed interference alignment technique, thereby enabling each user to use its signal dimensions more efficiently in the both space and time perspective. Under a block fading channel assumption, an achievable DoF for three user vector broadcast channel is studied when the CSI feedback delay time is the half of the channel coherence time, called the moderate CSI feedback delay regime. Using the derived DoF result, a simple ˇ°CSI feedback delay-DoF gain trade-off ˇ± for this channel is characterized, which provides an insight into the interplay between CSI feedback delay and DoF gain perspective.

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