Constant-Envelope ISAC via FM-OFDM: Analytical Framework and Receiver Design

TL;DR

This paper proposes FM-OFDM as a constant-envelope waveform for ISAC systems, providing an analytical framework and receiver design that enable high power efficiency and robust sensing in hardware-constrained, doubly dispersive channels.

Contribution

It introduces a comprehensive analytical model for FM-OFDM in challenging channels and designs a tailored receiver, demonstrating its advantages over traditional waveforms under hardware constraints.

Findings

FM-OFDM achieves low BER in saturated PA conditions.

Superior detection accuracy under high Doppler shifts.

Effective velocity estimation with the proposed receiver.

Abstract

Integrated Sensing and Communication (ISAC) systems face stringent hardware constraints, particularly regarding the high Peak-to-Average Power Ratio (PAPR) of standard OFDM, which necessitates power amplifier (PA) back-off and reduces sensing range. This paper investigates Frequency Modulated OFDM (FM-OFDM) as a constant-envelope solution capable of operating in the PA saturation region, thereby maximizing output power without the non-linear distortion penalties typical of conventional waveforms. We derive a comprehensive analytical framework for FM-OFDM in doubly dispersive channels, explicitly quantifying the inter-carrier interference (ICI) dynamics and effective channel gains in the discriminator domain. To address the unique phase structure of the waveform, we propose a tailored sensing receiver architecture utilizing slow time phase differencing for robust velocity estimation. Unlike prior works, we evaluate performance under a strictly normalized bandwidth constraint (B99), ensuring a fair comparison against CP-OFDM and Constant-Envelope OFDM (CE-OFDM). Simulation results demonstrate that FM-OFDM maintains superior detection accuracy and low BER even under fully saturated PA conditions and high Doppler shifts, validating its suitability for hardware-constrained ISAC transceivers.