Joint Uplink-Downlink Fronthaul Bit Allocation in Fronthaul-Limited Massive MU-MIMO Systems

TL;DR

This paper addresses the challenge of optimally allocating fronthaul bits between uplink CSI and downlink precoding in massive MU-MIMO systems with limited fronthaul capacity, enhancing spectral efficiency.

Contribution

It formulates a sum spectral efficiency maximization problem, derives a closed-form SE expression for maximum ratio transmission, and proposes an exact algorithm for bit split optimization.

Findings

Optimal bit allocation varies with SNR levels.

Closed-form SE expression for maximum ratio transmission.

Proposed algorithm effectively balances fronthaul resource distribution.

Abstract

This paper optimizes the fronthaul bit allocation in massive multi-user multiple-input multiple-output (MU-MIMO) systems operating with limited-capacity fronthaul links. We consider an advanced antenna system (AAS) controlled by a centralized baseband unit (BBU). In the AAS, multiple antenna elements together with their radio units are integrated into a single unit. In this setup, a key challenge is allocating fronthaul bits between uplink channel state information (CSI) quantization and downlink precoding matrix quantization. We formulate the problem of maximizing the sum spectral efficiency (SE) for a given fronthaul capacity. We develop an SE expression for this scenario based on the hardening bound. We compute the expression in closed form for maximum ratio transmission, which reveals the relative impact of the two types of quantization distortion. We then formulate a bit split optimization problem and propose an algorithm that exactly solves it. Numerical results demonstrate how the relative importance of assigning bits to CSI and precoding varies depending on the signal-to-noise ratio.