# ISAC towards 6G Satellite-Terrestrial Communications: Principles, Status, and Prospects
## 3 Development Status and Key Technologies of ISTNs
### Key Technologies of ISTNs  
#### 3.2.1 FSO of ISTNs  
Free-space optical (FSO) communication has attracted widespread attention in both academia and industry [69]. FSO communication has numerous advantages, including strong anti-interference capabilities, much higher communication rates than traditional radio frequency methods, large information capacity, compact and lightweight equipment, and low power consumption [70]. It has been proven that FSO communication is a promising technology. As early as 1995, the Japan Aerospace Exploration Agency successfully achieved 1 megabit per second data download speeds from its engineering test satellite KIKU-6 to a ground station in Tokyo [71]. In 2016, Europe proposed the European Data Relay System, aiming to use innovative laser communication technology to transmit data between LEO satellites and geostationary earth orbit satellites [72]. The National Aeronautics and Space Administration recently conducted a milestone experiment on the international space station, successfully establishing a bidirectional laser communication link between the laser communications relay demonstration equipment and the LEO user terminal at a speed of 1.2 gigabits per second [73].  
In recent years, academic research has also focused on optimizing FSO communication in inter-satellite and satellite-to-ground links. Studies on FSO communication have primarily involved modeling and analyzing atmospheric channels, satellite-to-ground and inter-satellite network architectures, energy-efficient collaboration [74], and satellite network routing strategies. In [75], dual-hop mixed radio frequency/free-space optical networks, where backhaul-to-relay and relay-to-user communications employ FSO and RF links, are investigated. For the backhaul-to-relay link, FSO communication is considered to meet the high throughput requirements of high-altitude platform satellites. By optimizing system performance through solving non-convex problems, simulations demonstrate achievable rates for FSO links under atmospheric conditions while proving that the proposed joint optimization algorithm enhances system throughput. In [76], the authors integrate high-altitude platforms into an integrated air-ground communication system and propose a novel FSO transmission network. Ground stations impose restrictions on the selection of high-altitude platforms to mitigate the effects of atmospheric turbulence. Experimental results demonstrate that the proposed architecture exhibits superior transmission rates and reliability. Additionally, in [77], the authors propose a multi-objective reinforcement learning-based routing strategy in a multi-layer network architecture. The satellite network considers large-scale, ultra-high-density, and highly dynamic LEO satellite networks and avoids conflicts by introducing cooperative mechanisms. Simulations demonstrate that the proposed algorithm outperforms existing routing algorithms in terms of packet loss rate and other performance metrics.  
Despite existing progress, there are still limitations, including a lack of research on inter-satellite routing protocols, insufficient consideration of the impact of spatial obstacles, and the absence of adaptive topological control mechanisms for existing satellite architectures.