Utility-based Downlink Pilot Assignment in Cell-Free Massive MIMO

TL;DR

This paper introduces a utility-based downlink pilot assignment strategy in cell-free massive MIMO systems that optimizes pilot allocation based on user channel conditions, improving throughput and support for high-mobility users.

Contribution

It proposes a novel utility metric for pilot assignment that dynamically allocates pilots based on user needs, enhancing system performance over existing methods.

Findings

Higher downlink net sum throughput achieved.

Improved support for high-speed users.

Better performance in shorter coherent intervals.

Abstract

We propose a strategy for orthogonal downlink pilot assignment in cell-free massive MIMO (multiple-input multiple-output) that exploits knowledge of the channel state information, the channel hardening degree at each user, and the mobility conditions for the users. These elements, properly combined together, are used to define a user pilot utility metric, which measures the user's real need of a downlink pilot for efficient data decoding. The proposed strategy consists in assigning orthogonal downlink pilots only to the users having a pilot utility metric exceeding a predetermined threshold. Instead, users that are not assigned with an orthogonal downlink pilot decode the data by using the statistical channel state information. The utility-based approach guarantees higher downlink net sum throughput, better support both for high-speed users and shorter coherent intervals than prior art approaches.