In recent years, the capacity of satellite has been improved by multi-beam technology. For the next-generation high throughput satellite (HTS), further improvements of capacity, flexibilization and digitization are required. In addition, cooperation with terrestrial 5G/Beyond 5G are also required to expand terrestrial network area or to utilize satellites as backup links in case of disaster or to realize completely new applications. We started research and development (R&D) on next-generation HTS which is scheduled for five years from fiscal year 2020 to fiscal year 2024. The development results will be verified by the on-orbit demonstration experiment using Engineering Test Satellite 9. In this paper, we introduce the outline of our R&D. In addition, we focus on one of the study items “satellite-terrestrial cooperation system” and present the concept of a satellite-terrestrial cooperation network simulator which will be used to extract potential issues or to verify some algorithms associated with the satellite-terrestrial cooperation system.