Chaos-Based Dual-Function OTFS: Joint Security and PAPR Reduction for LEO Satellite Communications

Orthogonal time frequency space (OTFS) modulation is emerging as a strong candidate for Low Earth Orbit (LEO) satellite communication systems due to its ability to counter Doppler and delay impairments, enabling reliable performance in high-mobility environments. However, OTFS-based LEO Satcom still faces critical challenges in physical layer security (PLS) and peak-to-average power ratio (PAPR) reduction, particularly in Frequency Division Duplex (FDD) systems where channel reciprocity is only partial. To address these issues, this paper proposes a chaos-based secure OTFS scheme that jointly enhances secrecy and reduces PAPR. The scheme exploits the slow-varying delay-Doppler channel characteristics of the legitimate link to derive a common chaos sequence at both the satellite and ground receiver. This sequence is used to generate secure phase rotations, eliminating the need for side-information exchange and simultaneously enabling PAPR reduction through a selective mapping (SLM) process. A closed-form approximation of the ergodic secrecy rate is derived, and performance is validated through simulation. Results show that the proposed method significantly improves secrecy rate while maintaining a bit error rate (BER) close to 0.5 for the eavesdropper, and achieves notable PAPR reduction without additional signaling overhead. These findings confirm that the proposed scheme effectively addresses both the security and energy efficiency requirements of LEO Satcom.