Simultaneous transmission of multiple data streams
from a multiple antenna base station (BS) to multiple single
antenna users gives significant gain in spectral efficiency as
compared to when a single such user is being served. This
simultaneous transmission to multiple users is realizable only if
BS knows the forward channels linking its transmitting antennas
to these users which requires channel feedback from these users.
This feedback overhead could be prohibitively large especially in
large user systems, limiting the multi-user transmission gains.
Exploiting the channel reciprocity in a time-division duplexed
(TDD) broadcast channel, we give a simple transmission strategy,
where users feedback independent of their channel realizations.
We analyze the sum rate of this multi-user system when the
channel acquisition load is completely accounted for. We derive
a novel lower bound of the sum rate which allows us to
optimize over how many users should inform the BS about their
channel information, solving the intriguing trade-off of multiuser
diversity, interference cancellation and feedback overhead.
