Chirp Parameters Hopping over Time for Affine Frequency Division Multiplexing with Physical Layer Security

Affine frequency division multiplexing (AFDM) is a promising solution recently proposed to resist Doppler effects and enable reliable communications. Due to the openness of the channel, ensuring the security of communications is crucial, especially in scenarios like integrated sensing and communications (ISAC) in 6G networks. In this paper, we demonstrate that AFDM inherently offers security advantage, which is provided by its two adjustable parameters in the underlying discrete affine Fourier transform (DAFT). To further enhance physical layer security of AFDM-based system, we propose a novel scheme of chirp parameters hopping (CPH), where the parameters randomly change for each symbol over time. We derive the Cramér-Rao lower bound (CRLB) for the chirp parameters, considering the scenario where the eavesdropper knows the channel between legitimate transmitter and receiver, in order to determine the theoretical estimation accuracy achievable by the eavesdropper. Simulation results demonstrate that the proposed scheme effectively prevents information leakage and achieves excellent security performance.