Digital trunking: air interface issues

Security, improved speech quality and high speed data are major objectives for a digital private mobile radio (DPMR) system, in addition to improved spectrum efficiency. As well as providing these features, there is the need to provide traditional PMR services such as group call, open channel calls and emergency calls. Unless sufficient new spectrum can be made available, there will also be a requirement to co-exist with existing analogue channels. Some of the issues which relate to the design and specification of the lower layers of the air interface for a trunked DPMR system are discussed. Emphasis is given to the aspects which distinguish DPMR from other mobile digital communication systems. Also, particular attention is given to the aspects of system design which affect the cost of implementation. The key topics addressed are spectrum efficiency, access method, speech codec, modulation, and power amplifiers