How Many Users should be Turned On in a Multi-Antenna Broadcast Channel?

TL;DR

This paper analyzes how to optimally select the number of active users in a multi-antenna broadcast channel, using asymptotic analysis to improve throughput and feedback strategies in large systems with heterogeneous users.

Contribution

It introduces an asymptotic framework to determine the optimal number of on-users based on system parameters, improving upon prior fixed-s approaches.

Findings

Optimal number of on-users depends on SNR and system parameters.

Asymptotic analysis guides feedback and user selection strategies.

Scheme achieves significant gains over fixed-s methods.

Abstract

This paper considers broadcast channels with L antennas at the base station and m single-antenna users, where each user has perfect channel knowledge and the base station obtains channel information through a finite rate feedback. The key observation of this paper is that the optimal number of on-users (users turned on), say s, is a function of signal-to-noise ratio (SNR) and other system parameters. Towards this observation, we use asymptotic analysis to guide the design of feedback and transmission strategies. As L, m and the feedback rates approach infinity linearly, we derive the asymptotic optimal feedback strategy and a realistic criterion to decide which users should be turned on. Define the corresponding asymptotic throughput per antenna as the spatial efficiency. It is a function of the number of on-users s, and therefore, s should be appropriately chosen. Based on the above asymptotic results, we also develop a scheme for a system with finite many antennas and users. Compared with other works where s is presumed constant, our scheme achieves a significant gain by choosing the appropriate s. Furthermore, our analysis and scheme is valid for heterogeneous systems where different users may have different path loss coefficients and feedback rates.