Secure AFDM Waveform Design for High-Mobility Satellite–Air-Integrated Communications

The affine frequency division multiplexing (AFDM) waveform, with its superior capability in separating delay and Doppler shifts, emerges as a promising solution to achieve reliable high-mobility satellite-air integrated communications. However, the secure AFDM waveform design remains a critical challenge when deployed in satellite-air integrated communications. To tackle this issue, in this article, we first analyze the roles of the key parameters in the AFDM waveform, revealing the admissible ranges of these parameters. Afterwards, we propose a time-varying parameter-hopping (PH) AFDM scheme, where the parameters are dynamically adjusted during transmission, which enhances the flexibility of the AFDM waveform. In addition, the scheme can support both basic encryption strategy and advanced encryption strategy for different security requirements in high-mobility satellite-air integrated communications. Specifically, the basic strategy employs a single dynamically hopping parameter, while the advanced encryption strategy, trading the spectral efficiency for a larger parameter space, employs two dynamically hopping parameters. Besides, the anti-eavesdropping performance, quantified by waveform entropy, can be significantly improved through the proposed PH mechanism. Numerical simulations demonstrate the validity of the analysis and the effectiveness of the proposed scheme.