  User selection (US) with Zero-forcing beamforming is considered in fast
fading Gaussian vector broadcast channels with perfect channel state
information (CSI) at the transmitter. A novel criterion for US is proposed,
which depends on both CSI and the data symbols, while conventional criteria
only depend on CSI. Since the optimization of US based on the proposed
criterion is infeasible, a greedy algorithm of data-dependent US is proposed to
perform the optimization approximately. An overhead issue arises in fast fading
channels: On every update of US, the transmitter might inform each user whether
he/she has been selected, using a certain fraction of resources. This overhead
results in a significant rate loss for fast fading channels. In order to
circumvent this overhead issue, iterative detection and decoding schemes are
proposed on the basis of belief propagation. The proposed iterative schemes
require no information about whether each user has been selected. The proposed
US scheme is compared to a data-independent US scheme. The complexity of the
two schemes is comparable to each other for fast fading channels. Numerical
simulations show that the proposed scheme can outperform the data-independent
scheme for fast fading channels in terms of energy efficiency, bit error rate,
and achievable sum rate.
