This book first surveys the relevant research progress of major institutions, organizations, projects and companies engaged in 6G research around the world, then comprehensively defines 6G from the perspectives of driving forces, vision, requirements, spectrum and technical characteristics, illustrating a complete picture of an end-to-end 6G mobile network with integration of communication, sensing, AI, data and security. The most promising technologies are presented, including distributed multiple-input and multiple-output (MIMO), Reflective Intelligent Surface (RIS), AI-enabled air interface, advanced full-duplex, OTFS, integration of communication and sensing, back-scatter communication, space-air-ground integration and ICDT converged network architecture.