How Much Feedback Is Required for TDDMulti-Antenna Broadcast Channels With User Selection?

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. The authors 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. They provide two transmission strategies through which the BS and the users get necessary CSI. They 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.