TETRA (TErrestrial Trunked RAdio) is a professional mobile radio (PMR) system used by public (police, military, emergency services) as well as private companies (public transportation, harbour, industrial/chemical plants, power/electricity companies, etc.)
It is specified by the ETSI (European Telecommunications Standardization Institute).
The specifiations can be downloaded from http://pda.etsi.org/
The main specification document is
- ETSI EN 300 392-1 (Voice plus Data (V+D); Part 1: General network design)
- ETSI EN 300 392-2 (Voice plus Data (V+D); Part 2: Air Interface (AI))
- ETSI EN 300 395-2 (Speech codec for full-rate traffic channel; Part 2: TETRA codec)
For a good technical book on TETRA, check out Digital Mobile Communications and the TETRA System by John Dunlop and others. You can preview it at Google books