# ISAC towards 6G Satellite-Terrestrial Communications: Principles, Status, and Prospects
## 4 Development Trend and Key Technology of ISAC towards 6G ISTNs
### Critical Techniques in Integrated Design
#### 4.2.5 Beam Enhancement Technology  
The beams formed by antennas in terrestrial mobile communications typically have low power. However, due to the need for long-distance propagation and large coverage areas in satellite communications, the enhancement of these beams is a necessity. Usually based on the target position, the optimal beam direction and shape parameters are obtained, and the optimal channel environment is switched to enhance the beam strength of the antenna in this direction. For 6G ISTNs, a satellite will form single or multiple beams to cover the ground area. In [127], the authors studied the robust beamforming problem for phase errors of channel state information in multi-beam satellite communications, achieving multi-regional beam coverage through the multiple beams of the satellite. In ISTNs, for multi-target sensing, the sensing terminal also requires multi-beam technology to locate multiple targets. For the detection and tracking of long-range targets, the transmitter antenna is required to form a narrower and more energy-concentrated beam. Therefore, for the signal transmission unit in the satellite and air, a large-scale antenna array with a more accurate beam direction will be adopted, and the antenna array parameters and beamforming algorithm will be optimized. As long-range beams inevitably consume a large amount of power, energy efficiency needs to be taken into account as an optimization metric in order to save energy. ISTNs from 5G to 6G will further achieve integration, and the connection between devices in the large architecture will be closer. In order to further promote the integration trend, we need to rationally allocate the resources of the overall equipment and minimize the negative interference between the internal architecture of the system. Optimized beam enhancement technology is an important part of this.  
In [128], the authors introduced an integrated space-air-ground system, using UAVs as relays to assist in the effective transmission of satellite signals and applying beamforming technology to optimize the beams of satellites and UAVs for optimal power distribution. In [129], the authors proposed a multi-beam ground multicast control scheme, designing an algorithm for determining cluster centers by leveraging the user distribution. This algorithm ensures that each beam can cover all users in each group, achieving multi-user access and eliminating inter-cluster interference. Additionally, ISAC beams of satellites can also sense environmental factors such as weather and remotely sensed information on the ground and enhance the quality of life. In addition, beamforming technology involves beamforming, beam tracking, and other algorithms, which improves the computational complexity of the system. Therefore, some low-cost algorithms also need to be optimized on the basis of the existing terrestrial ISAC beamforming technology.  
Overall, applying beam enhancement techniques in ISTNs will improve the detection range of radars, enhance target detection capability, and optimize communication quality.  
Table 7 summarizes the key technologies in the aforementioned satellite-terrestrial ISAC systems and their pros and cons.