FMCW Lidar Beyond Nyquist by Instantaneous Frequency Fitting

TL;DR

This paper introduces advanced signal processing techniques, matched filtering and instantaneous frequency fitting, to extend FMCW lidar range beyond Nyquist limits and improve robustness against phase noise.

Contribution

It proposes novel methods that recover larger distances and velocities in FMCW lidar by leveraging full waveform analysis and robustness to phase noise, surpassing traditional FFT-based approaches.

Findings

Methods recover larger distance and velocity ranges.

Instantaneous frequency fitting is more robust to phase noise.

Non-linear modulation can enhance sensitivity.

Abstract

Frequency-modulated continuous-wave (FMCW) lidar conventionally estimates distance and velocity from constant beat frequencies generated through interferometry. Existing FMCW implementations emphasize simple signal processing -- e.g., beat frequency estimation via a fast Fourier transform (FFT) algorithm plus peak-finding -- which results in hardware-focused solutions requiring linear swept-frequency laser sources or linearized resampling. However, the maximum achievable distance by this method is limited by the need to sample the interference signal without aliasing. In this work, we propose two signal processing methods: matched filtering and instantaneous frequency fitting. These two methods can recover larger ranges of distance and velocity by considering the full waveform despite aliasing in the frequency domain. Furthermore, the FMCW lidar signal is often corrupted by phase noise, and we show that the instantaneous frequency fitting approach is more robust than matched filtering by considering the deviation in the phase. We present comprehensive simulation studies along with theoretical analysis using the misspecified Cram\'er--Rao bound. As these methods are flexible to arbitrary frequency modulation, we also show results for non-linear modulations that could yield better sensitivity to distance and velocity compared to the popular triangular modulation.