To obtain a visa to attend IET International Radar Conference 2023, many embassies will require an invitation letter as part of the visa application process.   The entry requirements for China can be found at the webpage of Foreign Affairs.
Persons who complete a registration and submit payment will be able to request an official invitation letter on Beijing Institute of Technology Letterhead.   The letter will be a PDF file delivered by email upon request.   There is no additional cost for the emailed invitation letter.