Optimizing Multi-Cell Massive MIMO for Spectral Efficiency: How Many Users Should Be Scheduled?

TL;DR

This paper investigates the optimal number of users to schedule in massive MIMO systems to maximize spectral efficiency, analyzing the relationship between user count, antenna numbers, and system parameters.

Contribution

It derives a closed-form expression for the optimal number of users in large antenna regimes and explores finite-size effects through simulations.

Findings

Optimal user number depends on system parameters and antenna count.

The traditional rule-of-thumb of N much larger than K is not always optimal.

Simulation results show variations in optimal K under different interference scenarios.

Abstract

Massive MIMO is a promising technique to increase the spectral efficiency of cellular networks, by deploying antenna arrays with hundreds or thousands of active elements at the base stations and performing coherent beamforming. A common rule-of-thumb is that these systems should have an order of magnitude more antennas, $N$, than scheduled users, $K$, because the users' channels are then likely to be quasi-orthogonal. However, it has not been proved that this rule-of-thumb actually maximizes the spectral efficiency. In this paper, we analyze how the optimal number of scheduled users, $K^\star$, depends on $N$ and other system parameters. The value of $K^\star$ in the large-$N$ regime is derived in closed form, while simulations are used to show what happens at finite $N$, in different interference scenarios, and for different beamforming.