Centralized Microwave-Photonic OCDM-IM-FMCW for Free-Space Optical ISAC Systems

Abstract

Free-space optical (FSO) integrated sensing and communication (ISAC) has emerged as a promising approach for high-capacity optical wireless links while enabling simultaneous communication and sensing. Unmanned aerial vehicles (UAVs) can further enhance network flexibility by mitigating blockage and providing dynamic links in FSO networks. However, most existing UAV-assisted FSO-ISAC schemes either treat sensing as an auxiliary function or require additional optical hardware, while UAV platforms are subject to strict size, weight, and power (SWaP) constraints. In this paper, we propose a centralized microwave-photonic FSO-ISAC framework for reduced-complexity UAV platforms based on orthogonal chirp division multiplexing with index modulation (OCDM-IM). In the proposed architecture, waveform generation and digital signal processing are centralized at the ground control station, allowing the UAV terminal to employ simplified optical components with reduced onboard processing. Moreover, carrier-suppressed double-sideband (CS-DSB) modulation enables simultaneous ranging and velocity estimation using a single Mach–Zehnder modulator (MZM). Building on our previous OCDM-FMCW design, the proposed OCDM-IM-FMCW scheme extends the framework by embedding information in subcarrier-index patterns and employing a single-MZM carrier-suppressed double-sideband optical implementation, thereby improving spectral efficiency while enabling joint range and velocity estimation with simplified UAV-side optical relaying. Simulation results show centimeter-level ranging accuracy and reliable velocity estimation, while improving communication spectral efficiency through index modulation. These results indicate the potential of the proposed architecture for UAV-assisted FSO-ISAC applications.