Multiuser Diversity in Downlink Channels: When Does the Feedback Cost Outweigh the Spectral Efficiency Benefit?

In this paper, we do a cost-benefit analysis of multiuser diversity in single antenna broadcast channels. It is well known that the multiuser diversity can be beneficial but there is a significant cost in terms of system resources, bandwidth and power associated with acquiring instantaneous CSI. We work out a cost-benefit analysis of multiuser diversity for 2 types of CSI feedback methods, dedicated feedback and SNR dependent feedback, quantifying how many users should feedback CSI i.e the amount of available multiuser diversity that should be used from a net throughput perspective. Dedicated feedback, in which orthogonal resources are allocated to each user, has significant feedback cost and this limits the amount of available multiuser diversity that can be used. SNR dependent feedback method, in which only users with SNR above a threshold attempt to feedback, has relatively much smaller feedback cost and this allows for all of the available multiuser diversity to be used. Next, we study the effect of single user multiantenna techniques, which reduce the SNR variation, on the number of feedback users. It is seen that a broadcast channel using single user multiantenna techniques should reduce the number of feedback users with the spatial dimension.