Dual-Beam Dual-Frequency Secure Transmission for Downlink Satellite Communication Systems

In the multi-antenna satellite communication system, beamforming and artificial noise are usually adopted to enhance physical layer security. However, due to the long distance between the satellite and the terrestrial terminal, the coverage of the main lobe in the satellite downlink is so large that the eavesdropper can get into the coverage easily despite of using spot beams. When the eavesdropper is close to the legitimate user, the difference between the main channel and the wiretap channel is small, which makes the secrecy capacity decrease rapidly. This paper proposes a novel scheme to solve this problem by using two different beams which work on different frequencies to transmit the same signal to the legitimate user. This dual-beam dual- frequency scheme can enlarge the difference between the desired channel and the wiretap channel. Then, if we add some artificial noise set the proper beamforming vectors, the secrecy capacity will remain high when the eavesdropper is in the small zone near the legitimate user. In order to derive the artificial noise and the beamforming vectors, we transform the problem into a semidefinite problem and use CVX tools to solve the problem. Simulation results prove that this scheme can achieve high secrecy capacity even when the eavesdropper is very close to the legitimate user.