Private free-space communication.

https://phys.org/news/2021-06-quantum-cascade-lasers-private-free-space.html Jim Bell's comment: I remember a device called a "gunnplexer" in the late 1970s.http://www.microwave-museum.org/exhibits/mwm0022.htm It allowed two-way communication over the Gunn-oscillator frequency, usually 10.25 GHz.------------------- I have long thought that designers of WiFi routers should have incorporated an ability to communicate between nearby units.  The vast majority of WiFi routers are within hearing distance of other routers.