Abstract: Satellite downlinks face severe eavesdropping threats due to their openness and wide-area coverage. Traditional satellite downlink anti-eavesdropping schemes centered on encryption technology have dual bottlenecks in computational complexity and anti-quantum attack capabilities. Moreover, the application scenarios of existing physical-layer security anti-eavesdropping schemes for satellite downlinks have limitations. To address these issues, an anti-eavesdropping scheme for satellite downlinks based on Channel State Information (CSI) and cooperative relaying was proposed through the cascade design of scrambling codes and coding based on dynamic expansion factors. First, a satellite downlink communication model based on cooperative relaying was established by deploying ground relay base stations to expand the CSI randomness difference between the legitimate link and the eavesdropping link. Second, the expansion factor of the quasi-cyclic low-density parity-check code was dynamically regulated through the CSI of the legitimate link to increase coding randomness and thus increase the decoding difficulty for eavesdroppers. Finally, a scrambling and descrambling mechanism was constructed at the satellite end and the user end using the dynamic expansion factor and the CSI of the legitimate link, so that eavesdroppers could not descramble the confidential information due to the lack of the CSI of the legitimate link. Simulation results show that when the bit error rate at the user end is as low as 10-6, by leveraging the scrambling code's dependence on CSI to construct a descrambling barrier for eavesdroppers, the bit error rate of eavesdroppers can be made to approach 0.5. The proposed scheme, relying on the cooperative design of CSI and ground cooperative relaying, has the potential to resist quantum computing attacks and meets the requirements of satellite communication network engineering deployment for high efficiency and low consumption. It can effectively balance the contradiction between the reliability and security of information transmission in satellite downlinks and can provide a technical reference path with engineering practice value for information security transmission in the future 6G space-air-ground integration scenario.

Key words: satellite downlink, anti-eavesdropping, cooperative relay, CSI, QC-LDPC codes