# Review of Physical Layer Security in Integrated Satellite-Terrestrial Networks
## 4 Review
### Beamforming  
Beamforming is a signal processing technique used in both terrestrial and satellite communications to focus the transmission or reception of radio waves in a specific direction [74]. Beamforming allows for the highly directional transmission of signals, focusing energy toward specific intended receivers. This can reduce the likelihood of signals being intercepted by unauthorized parties. By concentrating energy in desired directions, beamforming can help mitigate interference from other sources, both within and outside the network [75]. This can make it more difficult for attackers to eavesdrop on or disrupt communications. In conjunction with other physical layer security techniques, beamforming can enhance the overall security of the communication channel. For example, it can be used to create artificial noise or to exploit channel fading to make it difficult for attackers to decode transmitted signals [76]. Beamforming can be used to direct signals toward specific UE and minimize interference with other users or eavesdroppers as shown in Figure 3[77]. This can be particularly beneficial in densely populated areas where multiple users share the same spectrum. Beamforming on satellites can focus signals on designated areas on Earth, reducing signal leakage and improving security. Additionally, it can enhance uplink transmissions from ground stations to the satellite, making them more difficult to intercept. While PLS leverages the physical properties of the channel itself for security, beamforming can strengthen this security posture in several ways [68]:  
* Reduced Signal Leakage: By directing signals toward intended receivers, beamforming minimizes the signal power broadcast in unintended directions. This makes it more challenging for potential eavesdroppers to intercept data, even if they are not actively trying to jam the communication [78]. * Improved SNR: Beamforming concentrates the transmitted signal toward the receiver, leading to a stronger signal and a better SNR. This can make it more difficult for attackers to inject noise or manipulate the signal to compromise data integrity [79]. In Figure 4, we show the beamforming scheme to secure a satellite link. Assuming that the eavesdropper is located slightly far away from Bob, in our case, we have taken the difference in elevation angle to be 0.5\({}^{\circ}\). The main beam points toward Bob, while the gain in the direction of Eve will be taken as the gain away from the boresight with a difference of 0.5\({}^{\circ}\). The simulation parameters are shown in Table 2. The array antenna at the satellite is taken to be a uniform linear array with half-wavelength spacing. The antennas at Eve and Bob are parabolic reflectors with a diameter of 1 m. The transmitted power of the satellite is taken to be 1 W. We observe that as the number of elements increases, the beamwidth of the array antenna decreases, and therefore, the gain in the direction of Eve will be lower. We show the performance in terms of normalized secrecy capacity as the number of elements in the array is varied. It is observed that as the number of elements increases, the secrecy capacity improves, thus improving the PLS of the communication link. Without beamforming, the difference in capacity would be zero under uniform illumination by the satellite. Figure 4: The number of elements in the array antenna of a satellite is increased to improve the normalized secrecy capacity. Figure 3: By utilizing beamforming, [PERSON] can concentrate the signal power toward Bob, creating a highly focused beam that is difficult for [PERSON] to intercept. This is achieved by strategically adjusting the phase and amplitude of the signals emitted from multiple antennas within [PERSON]’s array. As a result, the signal strength at [PERSON]’s location is significantly amplified, while the signal energy dispersed in other directions is minimized.