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").
Private land mobile radio systems are available for use by public safety organizations such as police, fire, ambulance, and other governmental entities.
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.
Low band has longer range capability, but requires mobile antennas as long as nine feet (2.7 m) tall.
Commercial and public safety users are required to obtain U.S. Federal Communications Commission licenses in the United States.
Mobile and portable stations have a fairly limited range, usually three to twenty miles (~5 to 32 km) depending on terrain.
The controller activates the repeater when it detects a carrier on one of its incoming channels, representing a user talking.
In November 2005, many automatic garage doors in Ottawa, Ontario, Canada, had suddenly[1] stopped working, due to a powerful radio signal that appeared to be interfering with the remote controls that open them.
The strong radio signals on the 390 MHz band overpowered the garage door openers.
Nevertheless, because garage door openers operate as unlicensed devices, they must accept any interference from authorized spectrum users.
Congress requested that GAO review the potential spectrum interference caused by DOD's recent deployment of land mobile radios.
Specifically, Congress asked us to (1) determine the extent of the problem of spectrum interference associated with the recent testing and use of mobile radios at military facilities in the United States, (2) review the efforts made by DOD during the development of its land mobile radio system to identify and avoid spectrum interference, and (3) identify efforts to address the problem.