Robust Quinn-based hierarchical compensation for linewidth-induced impairment mitigation in RoF-OTFS ISAC system

Orthogonal time-frequency space (OTFS) is a promising 6G integrated sensing and communication (ISAC) waveform, with radio-over-fiber (RoF) serving as its essential distribution backbone. However, laser linewidth-induced phase noise hinders its integration by shifting signals off the discrete delay-Doppler grid. We propose a hierarchical compensation framework using Quinn's estimator to precisely rectify these fractional offsets across multiple domains. Simulations verify that the proposed scheme significantly enhances the laser linewidth tolerance of the OTFS waveform; specifically, it maintains the BER below the HD-FEC threshold for linewidths up to 140 kHz using QPSK and 10 kHz using 16-QAM, outperforming uncompensated systems. Experimental results over a 20 km fiber and 1 m wireless link with a 2 GHz bandwidth achieve a 4 Gbps peak rate, successfully surpassing the HD-FEC threshold. This approach enables the use of cost-effective lasers, providing a robust solution for high-capacity, photonic-assisted 6G ISAC networks.