On Signal Peak Power Constraint of Over-the-Air Federated Learning

TL;DR

This paper investigates the impact of peak-power constraints on over-the-air federated learning, revealing that practical power limitations can cause distortions and degrade learning performance, especially in multi-carrier systems.

Contribution

It is the first to analyze the effects of instantaneous peak-power constraints in AirComp-FL, highlighting the importance of power amplifier linearity in practical wireless FL systems.

Findings

Peak power often exceeds amplifier limits in AirComp-FL.

Clipping and filtering can degrade federated learning performance.

Multi-carrier OFDM systems are more affected by in-band distortions.

Abstract

Federated learning (FL) has been considered a promising privacy preserving distributed edge learning framework. Over-the-air computation (AirComp) leveraging analog transmission enables the aggregation of local updates directly over-the-air by exploiting the superposition properties of wireless multiple-access channels, thereby alleviating the communication bottleneck issues of FL compared with digital transmission schemes. This work points out that existing AirComp-FL overlooks a key practical constraint, the instantaneous peak-power constraints due to the non-linearity of radio-frequency power amplifiers. Operating directly in non-linear region causes in-band and out-of-band distortions. We present and analyze the effect of the default method that limits the signal's peak power and out-of-band distortions, iterative amplitude clipping combined with filtering. We investigate the effect of imposing instantaneous peak-power constraints in AirComp-FL for both single-carrier and multi-carrier orthogonal frequency-division multiplexing (OFDM) systems. Simulation results demonstrate that, in practical settings, the instantaneous transmit power in AirComp-FL regularly exceeds the power-amplifier linearity limit. As the first work of this line of research, it is essential to evaluate if this is an actual problem that has an impact on FL performance. We therefore apply the classic method of iterative clipping and filtering, and show that the FL performance degrades more or less depending on the scenarios. The degradation becomes pronounced especially in multi-carrier OFDM systems due to the in-band distortions caused by clipping and filtering.