Abstract
The enormous gains in a multi-antenna transmitter broadcast channel require the Channel State Information at the Transmitter (CSIT). Although the fundamental question How much feedback is required for a broadcast channel? has been treated in the literature to some extent, a more comprehensive treatment is certainly desirable. We study the time-division duplex broadcast channel with initial assumption of channel state information (CSI) neither at the base station (BS) nor at the users' side. We provide two transmission strategies through which the BS and the users get necessary CSI. We derive novel lower and upper bounds for the sum rate reflecting the rate loss compared to a perfect CSIT system. Corresponding approximate sum rate expressions are also developed for both schemes. These expressions fully capture the benefits of the CSIT feedback, allowing multi-user diversity gain and better inter-user interference cancellation, and the cost of exchange of information required. These expressions can be optimized for any set of system parameters to unveil the trade-off between the cost and the gains associated to feedback. Thus they allow to characterize the optimal amount of feedback which maximizes the sum rate of the broadcast channel, a well-accepted metric of system performance at the physical layer.
Highlights
We provide here a novel similar lower bound in which the channel distribution corresponding to the perfect Channel State Information at the Transmitter (CSIT) is that of the estimated channels, providing a much tighter lower bound compared to the one in [15], where the usual perfect CSIT channel distribution is that of the true channel
We studied the problem of determining the optimal amount of feedback/training for the sum rate maximization of the broadcast channel with no initial assumption of channel state information (CSI)
We introduced two transmission strategies for providing the CSIT to the base station (BS) and derived a novel tight lower bound which clearly shows the rate loss w.r.t. a perfect CSI system
Summary
Under the restriction of a fixed number of feedback bits, they conclude that accurate channel information is more important than having multiuser diversity In another recent work [18] treating essentially UL systems, the authors considered chunk size optimization (amount of feedback) taking UL and DL of a SU MIMO channel into account. Our analysis is for systems with a larger number of users than BS transmit antennas because this setting is certainly more practical than its opposite counterpart In both of our transmission strategies (oblivious and informed users), the users are explicitly trained about their effective channel after precoding. The cardinality of a set S is expressed as |S|
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