Communications between pilots of aircraft and air traffic control are based on a document from ICAO published in Annex 10 Volume 2 (Communications Procedures) to the Convention on International Civil Aviation and ICAO PANS-ATM (Procedures for Air Navigation Services - Air Traffic Management) Doc. 4444.
Frequencies are a limited resource in the aviation radio spectrum as voice communications are handled between 117.975 - 137 MHz. In the busy European airspace the current and expected demand are very high for communication channels. Therefore VHF 8.33 kHz channel spacing was introduced in 1999 for airspace above FL245.
Every radio device must comply with certain rules regarding its electromagnetic behavoir in the radio spectrum. This applies to ground-based VHF hand-held, mobile and fixed radio transmitters, receivers and transceivers for the VHF aeronautical mobile service.
Excerpt from the document:
The present document states the minimum performance requirements for ground based radio transmitters, transceivers and receivers for the aeronautical mobile service operating in the VHF band (118 MHz to 137 MHz), using Double Sideband Amplitude Modulation with 8.33 kHz or 25 kHz channel spacing.
To continue reading see the pdf below: