Abstract
We consider a multi-antenna broadcast channel with more single-antenna receivers than
transmit antennas and partial channel state information at the transmitter (CSIT). We
propose a novel type of CSIT representation for the purpose of user selection, coined as
ranking-based feedback. Each user calculates and feeds back the rank, an integer between 1
and W + 1, of its instantaneous channel quality information (CQI) among a set of W
past CQI measurements. Apart from reducing significantly the required feedback load,
ranking-based feedback enables the transmitter to select users that are on the highest peak
(quantile) with respect to their own channel distribution, independently of the distribution
of other users. It can also be shown that this feedback metric can restore temporal fairness
in heterogeneous networks, in which users' channels are not identically distributed and
mobile terminals experience different average signal-to-noise ratio (SNR). The performance of a system that
performs user selection using ranking-based CSIT in the context of random opportunistic
beamforming is analyzed, and we provide design guidelines on the number of required past
CSIT samples and the impact of finite W on average throughput. Simulation results
show that feedback reduction of order of 40‐50% can be achieved with negligible decrease
in system throughput.