Textbook in PDF format This book provides solutions to some of the issues present in the physical layer of current high-frequency wireless communication systems. It reviews the development history of high-frequency wireless communication systems, pinpoints certain existing problems in contemporary high-frequency communication systems, and proposes solutions. The 6th Generation Mobile Networks (6G) is based on terrestrial cellular networks and integrates satellite communication, drone communication, and marine communication to form an integrated air-ground-sea network, providing comprehensive coverage, high speed, high security, and multifunctional communication solutions. High-frequency wireless communication, represented by millimeter-wave and terahertz communications, offers a wide available spectrum and high transmission rates, making it a highly promising broadband wireless access technology in 6G