Radio communication

Some open-source electronic projects involve radio communication.

Some amateur radio operators enjoy building custom radio communication systems. Some of them have built systems with features unavailable from any off-the-shelf system, such as software-defined radio.

Other people typically use a pair of off-the-shelf Integrated Circuits#RF ICs/Modules in the unlicensed spectrum as a kind of "virtual wire" between two systems. Such as, for example, Xbee wireless module, etc.

Modern walkie talkies (also known as HTs or Handie-Talkie or handheld tranceivers) ... Family Radio Service (FRS) uses audio frequency modulation in frequency channels around 462 and 467 MHz; all FRS channels are shared with GMRS radios ... GMRS requires a license in the US; GMRS uses audio frequency modulation in frequency channels near 462 MHz and 467 MHz, sharing frequency bands with FRS ... citizens band radio (CB radio) in frequency channels around 72 MHz (11 meters) ...

The ESP32 and ESP8266 chips are popular for connecting to the internet over standard 2.5 GHz WiFi, and also can be used for direct-connection walkie talkies over the 2.5 GHz WiFi frequencies<ref> "Hackaday: articles tagged walkie talkie". </ref> and mesh communication using the ESP-NOW protocol. (Is the "ESP32-A1S Audio Module" open source?)

The Open Mobile Gadgets project(s) use(s) the cell-phone-licensed spectrum to build an open-hardware that talks to cell phone towers like a cell phone, but does things that no closed-source cell phone can do.

Open-source RC flying craft (multi-rotor, etc.) typically use FM modulated servo control the aircraft remote control reserved frequency bands (72 MHz ... and others ...) or use frequency-hopping (FHSS) in the unlicensed spectrum.

Open-source RC surface craft (RC cars; RC boats; RC submarines towing a floating antenna; etc.) typically use (FM modulated???) servo control the surface vehicle remote control reserved frequency bands (73 MHz ... and others ...) or use frequency-hopping (FHSS) in the unlicensed spectrum.

The Serval Project is developing open-source technology to connect cell phones into a mesh directly to each other through their WiFi interfaces (without using some intermediate cell phone tower or WiFi access point). When the mesh can find a stable route between participants, it supports live voice calls. Even without a stable route, Serval supports text messages between participants using store-and-forward.

(FIXME: mention other "wireless" technologies, such as acoustic, ultrasonic, infrared remote control, free-space optical, etc.)