Data transmission over VHF and UHF land mobile radio channels

A microprocessor-controlled data transmission system is described for use over VHF and UHF land mobile radio channels. The system is based on ARQ operation of error control, additional information regarding the validity of the received data being provided by a signal strength detector. Direct modulation of an FM carrier is employed, although the system may also be used with subcarrier modulation in order to allow the use of existing voice communication equipment. The terminal equipment is predominantly digital in operation, thereby allowing full microprocessor control. As far as it is able the processor adapts the system to the current channel conditions by varying the coding technique employed and the data block size.