Multiuser diversity with limited feedback

Multiple antennas at the base station can be employed to serve multiple single-antenna users in the downlink of a system. When the number of active users K is larger than the number of transmit antennas M, the base station can exploit multiuser diversity by cleverly scheduling a set of M users. For this purpose, the base station needs to know the downlink channel state information of all the users in the system. In a frequency division duplex (FDD) system, this is attained with limited feedback from the users. Albeit most of the literature assumes a per-user limited feedback, a system limited feedback is actually more appropriate. Given a constraint on the total amount of airtime available for feedback, in this work we are interested in determining the optimum number of users that should feed back. We observe a tradeoff between the attainable degree of multiuser diversity and the feedback quality. The users are scheduled based on minimizing the sum of the mean square error (MSE) for the selected set of users.