A Paradigm Shift in High-Resolution Depth Estimation Using SPAD-Based LiDAR Histograms: From Signal Filtering to Lightweight Similarity Learning

Accurate and efficient depth estimation from time-of-flight (ToF) LiDAR is essential for autonomous systems operating in real-world environments. However, traditional histogram-based depth estimation (HBDE) algorithms face fundamental limitations in balancing depth performance and computational cost, and they struggle under signal-induced pile-up distortion. While deep learning has shown promise, existing neural network-based methods rely on large models that are impractical for deployment on edge hardware. To bridge this critical gap, we propose a paradigm shift in histogram-based ToF estimation, reframing depth estimation from signal filtering to lightweight similarity learning. Instead of attempting to correct the distorted signal, our approach learns a specialized metric where the measure of similarity between the distorted histogram and a reference pulse is the temporal shift itself. The resulting 57.61 KB model, over 215.2 times smaller than state-of-the-art deep learning approaches, achieves real-time performance (106.27 fps) on an FPGA. It delivers superior accuracy across nearly all signal-noise conditions, including 2.21 cm RMSE at severe pile-up scenarios, significantly outperforming conventional methods while remaining practical for on-device deployment.