An Ambiguity-Function-Assisted Active Sensing Scheme for OFDM-Based ISAC Systems Toward Low-Altitude Airspace

The drastic growth of the low-altitude airspace (LAA) applications, such as over the horizon control, low-altitude logistic, and all-time supervision, produces strong demands for communication and sensing services toward LAA. Under the idea of integrated sensing and communication (ISAC), this work investigates the low-complexity/high-accuracy active sensing problem toward LAA using a communication information-bearing orthogonal frequency division multiplexing (OFDM) waveform. A general and structured fast/slow-time representation for the OFDM input-output relation is first derived, which can clearly show the effects of delay and Doppler frequency shift (or equivalently, range and radial velocity of target) on the received echo. Based on the fast/slow-time representation, an ambiguity function (AF)-assisted active sensing scheme for the OFDM-based ISAC system is proposed. The scheme consists of three modules, i.e., a coarse search and triangular AF matching module, a gradient-based fine tune module and a modified cell-averaging constant false alarm rate (CA-CFAR) target detection module, which work coordinately to realize joint target detection and high-accuracy target range/velocity estimation. Simulation results show that the proposed scheme can realize better detection reliability and range/velocity estimation accuracy with much less computations than the reference schemes.