Exploiting multiuser diversity for fair scheduling in MIMO downlink networks with imperfect channel state information

In this paper, we consider a time-slotted MIMO point-to-multipoint network, which, for example, could represent either a cellular downlink, or a cluster in a wireless mesh network. The transmitter sends an independent data stream from each transmit antenna to communicate simultaneously to a subset of the receivers in each slot. The transmitter sends training symbols at the beginning of each slot so that each receiver can estimate the channel and determine the data rate it could potentially receive from each transmit antenna after configuring its antenna weights appropriately. For this purpose, we develop a maximum-likelihood estimator of the signal to interference plus noise ratio (SINR) from the received training symbols. The transmitter decides which receivers to serve in each slot to maximize the minimum normalized average data rate realized by each receiver. This scheduling decision is made based on estimated SINR information fed back by receivers. Through some numerical examples, we discuss some general issues with regard to exploitation of multi-user diversity in the context of imperfect channel state information. For example, the scheduler introduces a bias for positive SINR estimation errors, which is important to take into account when the receivers declare a potential data rate in each slot.