Specialized Mobile Radio (SMR) is a mobile communications concept for terrestrial mobile communications introduced as early as 1979. It is a very simple concept for two-way voice communications consisting of one or more stationary or mobile base stations, several antennas and the mobile terminals. The mobile terminals and base stations are linked via a single repeater.
As a terrestrial system, SMR radio operates in the VHF and UHF bands at frequencies between 30 MHz and 3 GHz, primarily at 800 MHz or 900 MHz, and covers a much wider geographic area from cellular networks. However, because it uses only one repeater and one transmission channel, and there is no linking among repeaters, its functionality is limited compared with cellular networks.
For half- duplex operation, SMR radio provides a channel pair with one transmit channel and one receive channel. Both channel frequencies with a channel width of 12.5 kHz are relatively close to each other and are identical for all mobile transceivers. In addition to half-duplex operation, in which only one can transmit at a time, SMR also offers a group call with push-to-talk operation.
With interconnect operation, SMR supports connection to fixed networks, enabling cross- network communication as with a cell phone. The improved SMR variant is Enhanced Specialized Mobile Radio( ESMR).