Joint Power Allocation and User Association Optimization for Massive MIMO Systems

TL;DR

This paper presents a low-complexity optimization framework for joint power allocation and user association in Massive MIMO systems, improving spectral efficiency and power consumption while balancing load across base stations.

Contribution

It derives universal spectral efficiency bounds, formulates solvable power minimization and fairness problems, and reveals that typically only one base station serves each user at optimality.

Findings

Proposed methods reduce transmit power compared to small-scale systems.

Optimal user association often involves only a single base station per user.

The framework effectively balances load among base stations.

Abstract

This paper investigates the joint power allocation and user association problem in multi-cell Massive MIMO (multiple-input multiple-output) downlink (DL) systems. The target is to minimize the total transmit power consumption when each user is served by an optimized subset of the base stations (BSs), using non-coherent joint transmission. We first derive a lower bound on the ergodic spectral efficiency (SE), which is applicable for any channel distribution and precoding scheme. Closed-form expressions are obtained for Rayleigh fading channels with either maximum ratio transmission (MRT) or zero forcing (ZF) precoding. From these bounds, we further formulate the DL power minimization problems with fixed SE constraints for the users. These problems are proved to be solvable as linear programs, giving the optimal power allocation and BS-user association with low complexity. Furthermore, we formulate a max-min fairness problem which maximizes the worst SE among the users, and we show that it can be solved as a quasi-linear program. Simulations manifest that the proposed methods provide good SE for the users using less transmit power than in small-scale systems and the optimal user association can effectively balance the load between BSs when needed. Even though our framework allows the joint transmission from multiple BSs, there is an overwhelming probability that only one BS is associated with each user at the optimal solution.