Trade-off Frontier of the OCDM-Based ISAC System with Pilot Arrangements

The severe Doppler effects in high-mobility scenarios pose critical challenges for integrated sensing and communications (ISAC), motivating the development of next-evolution waveforms (NEW). Therefore, this paper investigates orthogonal chirp division multiplexing (OCDM) waveform for ISAC, leveraging its inherent robustness derived from the time-frequency duality of chirp modulation. First, a compact Fresnel-transform representation is adopted to establish an analytical signal and channel model for OCDM over multipath Doppler channels. Based on this model, we characterize the communication performance under Doppler-induced interference and develop a range-velocity estimation method via FFT-based processing. Furthermore, closed-form expressions for the achievable rate and the Cramér-Rao bounds (CRBs) are derived for adjustable pilot density and placement. These expressions quantify the OCDM-specific resource trade-off and reveal how pilot arrangement determines the performance boundary. Simulation results validate that the proposed OCDM scheme with optimized pilot arrangements achieves a lower bit error rate and higher sensing accuracy in high-mobility environments.