MIMO Broadcast Channels with Digital Channel Feedback

This paper summarizes recent results on the multiple antenna broadcast (downlink) channel in which limited- rate digital feedback is used to convey channel state information from each mobile (receiver), each of which is assumed to have perfect channel information, to the transmitter. The transmitter employs linear precoding based on this feedback, and the effect of imperfect channel state information is multi-user interference, which can significantly degrade throughput if the channel information is not sufficiently accurate. Feedback requirements are studied for systems with single antenna mobiles in both the small system regime, i.e., number of mobiles equal to the number of transmit antennas, as well as in the large system regime, where user selection is performed on the basis of digital feedback. In addition, two different methods for utilizing multiple mobile antennas are described and compared in the small system regime.