[1]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.

   [2]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.

References

   1. https://phys.org/news/2021-06-quantum-cascade-lasers-private-free-space.html
   2. http://www.microwave-museum.org/exhibits/mwm0022.htm