Random Orthogonalization for Federated Learning in Massive MIMO Systems

TL;DR

This paper introduces random orthogonalization, a new communication scheme for federated learning in massive MIMO systems, leveraging channel properties to enable efficient over-the-air model aggregation with reduced channel estimation needs.

Contribution

It presents a novel random orthogonalization method that exploits massive MIMO characteristics for efficient FL communication, including uplink and downlink phases, with theoretical and experimental validation.

Findings

Achieves over-the-air model aggregation without transmitter CSI.

Reduces channel estimation overhead significantly.

Establishes a relationship between convergence rate, clients, and antennas.

Abstract

We propose a novel communication design, termed random orthogonalization, for federated learning (FL) in a massive multiple-input and multiple-output (MIMO) wireless system. The key novelty of random orthogonalization comes from the tight coupling of FL and two unique characteristics of massive MIMO -- channel hardening and favorable propagation. As a result, random orthogonalization can achieve natural over-the-air model aggregation without requiring transmitter side channel state information (CSI) for the uplink phase of FL, while significantly reducing the channel estimation overhead at the receiver. We extend this principle to the downlink communication phase and develop a simple but highly effective model broadcast method for FL. We also relax the massive MIMO assumption by proposing an enhanced random orthogonalization design for both uplink and downlink FL communications, that does not rely on channel hardening or favorable propagation. Theoretical analyses with respect to both communication and machine learning performance are carried out. In particular, an explicit relationship among the convergence rate, the number of clients, and the number of antennas is established. Experimental results validate the effectiveness and efficiency of random orthogonalization for FL in massive MIMO.