Land mobile radio system

A land mobile radio system (LMRS) is a person-to-person voice communication system consisting of two-way radio transceivers (an audio transmitter and receiver in one unit) which can be stationary (base station units), mobile (installed in vehicles), or portable (handheld transceivers e.g. "walkie-talkies").

Public land mobile radio systems are available for use by commercial businesses offering communications service to the public for a fee. This would include mobile telephone and paging service, as examples.

Private land mobile radio systems are available for use by public safety organizations such as police, fire, ambulance, and other governmental entities. They are allocated frequencies exclusively for their use. Commercial Private Land Mobile systems are available for businesses in the Business, Industrial, and Land Transportation sectors. Most frequencies are shared with other users. Land mobile radio systems use channels in the VHF or UHF bands, since the antennas used at these short wavelengths are small enough to mount on vehicles or handheld transceivers. Transmitter power is usually limited to a few watts, to provide a reliable working range on the order of 3 to 20 miles (4.8 to 32 km) depending on terrain. Repeaters installed on tall buildings, hills or mountain peaks can be used to increase the coverage area. Older systems use AM or, usually, FM modulation, while some recent systems use digital modulation allowing them to transmit data as well as voice. Most (30-174 MHz) systems operate simplex, with multiple radios sharing a single radio channel. Only one radio can transmit at a time. The transceiver is otherwise normally in receiving mode so the user can hear other radios on the channel. To talk, the user presses a push to talk button that turns on the transmitter of the transceiver.