Ted Rappaport's paper, Millimeter wave mobile communications for 5G cellular: It will work! inspired 5G. The new Wireless Communications and Applications Above 100 GHz: Opportunities and Challenges for 6G and Beyond points the way forward in wireless above 100 GHz. Ted and colleagues believe, "The mobile industry will be able to work well up to 800 GHz in the future using the small-cell architectures envisioned for 5G."
The reach of millimetre wave 5G is a major issue, due to signal loss. The paper discusses several ways to compensate for the increased loss in even higher frequencies. The key is that the antenna size goes down as frequencies go up. At 28 GHz, a 256 antenna module is the size of a chip. The beam will be pencil sized and hard to detect.
"THz will enable new sensing applications such as miniaturized radars for gesture detection and touchless smartphones, spectrometers for explosive detection and gas sensing , THz security body scanning, air quality detection , personal health monitoring systems , precision time/frequency transfer, and wireless synchronization , , ."