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result is that the active exploitation of not only the current CSI but also the outdated CSI obtained from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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