## 5 High-Speed Scenarios Based on LEO Satellites for Future Autonomous Vehicles  
An autonomous vehicle, or a driver-less vehicle, is a vehicle that operates itself and performs necessary functions with minimal (or no) human intervention through its ability to sense its surroundings. There are six levels of automation according to [66]; level 0 refers to the no automation case where the vehicle is completely dependent on a human driver and level 5 refers to the full-automation case, where the vehicle is completely independent in all cases and necessitates no human intervention. Most of the realistic autonomous vehicles nowadays are between levels 2 and 4. For autonomous vehicles, the notion of high-speed scenarios include any case that deals with high speeds regardless of their domain; a high-speed case can be as simple as providing cellular connectivity to mobile devices inside a fast-moving train, going over 250 km/h, or it can be as complex as the need of instant wireless communication between different high-speed drones and flying taxis to avoid collision. For our discussions in this paper, we will consider LEO-satellite system scenarios that either require high speeds in parameters, as in the drone example, or scenarios that occur in high-speed vehicle motions, as in the train example. If we aim to achieve full automation in high-speed cases, future networks of autonomous vehicles will have stringent requirements in terms of communications, positioning, and sensing characteristics, which are not yet fully met by current cellular and IoT technologies. A summary of these stringent requirements is given in Table 4, together with example studies from the literature that have addressed these challenges to some extent and offered various solutions to them. It is also straightforward to see that the requirements listed in Table 4 can be perceived as boundaries for the optimization problems that have the overlapping metrics from Table 2.  
However, it is also important to discuss existing limitations of high-speed scenarios for autonomous vehicles. One limitation in the current terrestrial technologies in terms of autonomous vehicle services is the requirement for ubiquitous and seamless coverage, which should be as close to 100% as possible. In order to address this challenging limit, satellite-based networks, such as LEO, as well as their integration with terrestrial networks, have begun being investigated in the literature, e.g., in [78]. Another very important aspect, which becomes even more significant in high-speed scenarios due to time/delays and computation constraints, is the security aspect. Good and up-to-date surveys on the security aspects in automotive transportation and unmanned vehicles can be found in the literature, for example, in [79] (blockchain solutions for UAV), ref. [80] (physical layer security for UAV), ref. [81] (quantum cryptography for UAV), ref. [82] (integrated network security for terrestrial and aerial transportation), ref. [83] (a systematic literature review on AI in UAV safety), etc. Security aspects are very broad and are not typically a part of the design optimization of the space or ground segments, as there are many security solutions that can be devised in the post-design stage, such as using multi-system multi-frequency receivers, using various encryption methods or authentication signals, etc. While we highly recognize the importance of ensuring high security mechanisms for communication, sensing, and positioning purposes in autonomous transportation, it is our opinion that we cannot deal with the security aspects as normal optimization parameters, and therefore, the security part is seen as outside the scope of the current survey. That being said, we would like to point out that some of the optimization methods that we cover in this paper are also utilized in security-related aspects in autonomous vehicle implementations.