Multiuser Diversity in Downlink Channels: When does the Feedback Cost Outweigh the Spectral Efficiency Gain?

TL;DR

This paper analyzes the trade-offs between feedback costs and spectral efficiency gains in multiuser diversity for downlink channels, comparing dedicated and SNR-dependent feedback methods, and considering multiantenna techniques.

Contribution

It provides a quantitative cost-benefit analysis of feedback strategies and examines how multiantenna techniques influence feedback requirements in multiuser diversity.

Findings

SNR-dependent feedback has lower costs and allows full utilization of multiuser diversity.

Dedicated feedback incurs high costs, limiting the number of feedback users.

Multiantenna techniques reduce the need for feedback users by stabilizing SNR variations.

Abstract

In this paper, we perform a cost-benefit analysis of multiuser diversity in single antenna broadcast channels. It is well known that multiuser diversity can be beneficial but there is a significant cost associated with acquiring instantaneous CSI. We perform 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 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 neccessary. It is seen that a broadcast channel using single user multiantenna techniques should reduce the number of feedback users with the spatial dimension.