# A Survey on Satellite Communication System Security
## 5 Defense in Satellite Communication Systems
### Defense Methods for Confidentiality  
Anti-Eavesdropping--Physical methods  
**Beamforming:** Beamforming is a signal processing technique used in antenna arrays to direct the transmission or reception of signals in specific directions. Beamforming for anti-eavesdropping refers to the use of beamforming techniques in wireless communications to enhance the security against eavesdropping attempts. In most cases, beamforming for anti-eavesdropping in satellites prevents eavesdropping attacks by utilizing the size and directivity of the beam. The overall scheme is shown in Figure 12.  
The authors of [59] address the challenge of coexistence between a fixed satellite service (FSS) and cellular networks, specifically in the shared usage of the Ka-band. They present a system model involving satellites and base stations with multiple antennas and FSS terminals and BS users with single antennas, forming a multiple-input single-output (MISO) channel in the millimeter-wave frequency band. To enhance the PLS, they propose two beamforming schemes, non-cooperation-based and cooperation-based schemes, designed to optimize transmission beamforming and protect FSS terminals from eavesdropping. Similarly, ref. [60] presents a model of a MISO wiretap channel, specific to multibeam SCSs, where each ground terminal is associated with a distinct satellite beam, treating interference from other beams as cochannel interference. They concentrate on PHY design for secure SCS, focusing on power allocation and beamforming that is designed to eliminate cochannel interference and nullify eavesdropper signals, with an emphasis on understanding the impact of the eavesdropper's channel condition on security system design. [PERSON] et al. [61] introduce a novel framework for PLS in broadband multibeam satellite systems, emphasizing new beamforming requirements. It considers a general eavesdropping scenario, where each legitimate user could be targeted by multiple eavesdroppers, either unique to them or shared across users. Also, they propose a partial zero-forcing approach, designed to make the signal intended for legitimate users orthogonal  
Figure 11: Defense scheme in a satellite communication system. to the channels of eavesdroppers. To optimize transmit beamforming, an iterative algorithm based on semi-definite programming relaxation is developed. **Adding Artificial Noise:** Artificial noise (AN) for satellite anti-eavesdropping is a strategy used in SCSs to enhance security against unauthorized interception. The noise is designed to overlap with the eavesdropper's signal space, making it difficult for them to distinguish the actual communication from the noise. This process is depicted in Figure 12.  
For example, ref. [62] introduces a novel deep neural network (DNN)-based secure precoding scheme, named the deep AN scheme, for use in MISO wiretap channels. This scheme utilizes a DNN for the joint design and optimization of precoders for both the information signal and the AN signal. It inputs the estimated channel of the receiver and outputs predicted precoding vectors for both the information and AN signals. [PERSON] et al. [63] propose the artificial-noise-aided two-way opportunistic relay selection scheme, a novel approach to enhance security in wireless relay networks. In this scheme, two sources, S1 and S2, transmit signals to relays in separate time slots, with each source simultaneously emitting AN during the other's transmission to protect against eavesdropping. This scheme also incorporates a defense against side lobe eavesdropping, where the AN is tailored to interfere with potential eavesdroppers by introducing perturbations in their received signal. However, the effectiveness of this perturbation can be impacted by errors in channel state information estimation. **Optical Communication:** Optical wireless communication in satellites represents a technology that uses light, typically in the form of lasers, to transmit data wirelessly. This offers unique advantages for anti-eavesdropping measures compared to traditional RF.