TABLE OF CONTENTS Multiuser diversity takes advantage of the fact that, in a system with many users whose channels fade independently, at any given time some users will have better channels than others. By transmitting only to users with the best channels at any given time, system resources are allocated to the users that can best exploit them, which leads to improved system capacity and/or performance. Multiuser diversity was first explored in [53] as a means to increase throughput and reduce error probability in uplink channels, and the same ideas can be applied to downlink channels. The multiuser diversity concept is an extension of the single-user diversity concepts described in Chapter 7. In single-user diversity systems, a point-to-point link consists of multiple independent channels whose signals can be combined to improve performance. In multiuser diversity the multiple channels are associated with different users, and the system typically uses selection diversity to select the user with the best channel in any given fading state. The multiuser diversity gain relies on disparate channels between users and so, the larger the dynamic range of the fading, the higher the multiuser diversity gain. In addition, as with any diversity technique, performance improves with the number of independent channels. Thus, multiuser diversity is most effective in systems with a large number of users.
From Section 14.5, we have seen that the total throughput (sum-rate capacity) of the fading downlink is maximized by allocating the full system bandwidth to the user with the best channel in...
