A land mobile radio system (LMRS) is a person-to-person voice communication system consisting of
two-way radio
A two-way radio is a radio that can both transmit and receive radio waves (a transceiver), unlike a broadcast receiver which only receives content. It is an audio (sound) transceiver, a transmitter and receiver in one unit, used for bidirection ...
transceiver
In radio communication, a transceiver is an electronic device which is a combination of a radio ''trans''mitter and a re''ceiver'', hence the name. It can both transmit and receive radio waves using an antenna, for communication purposes. Thes ...
s (an audio
transmitter
In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
and
receiver in one unit) which can be stationary (
base station units), mobile (installed in vehicles), or portable (handheld
walkie-talkie
A walkie-talkie, more formally known as a handheld transceiver (HT), is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald Hings, radio engineer Alfred J. Gross, ...
s). Public land mobile radio systems are made for use exclusively by
public safety
Public security or public safety is the prevention of and protection from events that could endanger the safety and security of the public from significant danger, injury, or property damage. It is often conducted by a state government to ensur ...
organizations such as police, fire, and ambulance services, and other governmental organizations, and use special frequencies reserved for these services. Private land mobile radio systems are designed for private commercial use, by firms such as
taxi
A taxi, also known as a taxicab or simply a cab, is a type of vehicle for hire with a driver, used by a single passenger or small group of passengers, often for a non-shared ride. A taxicab conveys passengers between locations of their choic ...
s or delivery services. Most systems are
half-duplex
A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
, with multiple radios sharing a single radio channel, so only one radio can transmit at a time. The transceiver is normally in receiving mode so the user can hear other radios on the channel; to talk, the user presses a
push to talk
Push-to-talk (PTT), also known as press-to-transmit, is a method of having conversations or talking on half-duplex communication lines, including two-way radio, using a momentary button to switch from voice reception mode to transmit mode.
H ...
button that turns on the
transmitter
In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
mode of the transceiver. Land mobile radio systems use channels in the
VHF or
UHF bands, since the antennas used at these short
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...
s are small enough to mount on vehicles or handheld transcievers. Transmitter power is usually limited to a few watts, to provide a reliable working range on the order of depending on terrain.
Repeater
In telecommunications, a repeater is an electronic device that receives a signal and retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side of an obstruction. Som ...
s installed on tall buildings, hills or mountain peaks can be used to increase the coverage area. Older systems use
AM or
FM modulation, while some recent systems use
digital modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
allowing them to transmit data as well as sound.
Military use
Land mobile radio systems are widely used by the military. Separate bands in the radio spectrum are reserved for military use.
Commercial use
Many businesses and industries throughout the world use LMR as their primary means of communication, especially from a fixed location to mobile users (i.e. from a base site to a fleet of mobiles). Commercial LMR Radios are typically available in the
VHF and
UHF frequency bands. 30−50 MHz (sometimes called "Low VHF Band" or "Low Band"), 150−172 MHz (sometimes called "High VHF Band" or "High Band"), 450−470 MHz "UHF". Many larger populated areas have additional UHF frequencies from 470 to 490 MHz, and 490−512 MHz. Low band has longer range capability, but requires mobile antennas as long as tall. VHF bands works well in outdoor environments, over bodies of water, and many other applications. UHF bands typically perform better in urban environments and with penetrating obstacles such as buildings. There are also frequencies in the 800 and 900 MHz range available. Commercial, public safety and government users are required to obtain
U.S. Federal Communications Commission
The Federal Communications Commission (FCC) is an independent agency of the United States federal government that regulates communications by radio, television, wire, satellite, and cable across the United States. The FCC maintains jurisdictio ...
licensing in the United States and must follow Government law.
Interference in the spectrum
In November 2005, many automatic garage doors in
Ottawa, Ontario
Ottawa (, ; Canadian French: ) is the capital city of Canada. It is located at the confluence of the Ottawa River and the Rideau River in the southern portion of the province of Ontario. Ottawa borders Gatineau, Quebec, and forms the core ...
, Canada, had suddenly stopped working, due to a powerful radio signal that appeared to be interfering with the remote controls that open them.
In the summer of 2004, garage door operators noticed similar phenomena around
U.S. military bases. The strong radio signals on the 390-megahertz band overpowered the garage door openers. One technician likened it to a whisper competing with a yell.
Repeaters
Mobile and portable stations have a fairly limited range, usually three to twenty miles (~5 to 32 km) depending on terrain. Repeaters can be used to increase the range of these stations. They are usually placed upon hills and buildings to increase range.
Repeaters have one or more receivers and a transmitter, with a controller. The controller activates the repeater when it detects a carrier on one of its incoming channels, representing a user talking. The repeater receives the radio signal,
demodulates it to an
audio signal which is filtered to remove noise, and retransmits it on a second channel to avoid interference with the first signal. This is received by a second two way radio in the repeater's expanded listening area. When the second user replies on the second channel, representing the other half of the
half-duplex
A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
conversation, his signal is received by the repeater and similarly translated and retransmitted on the first channel back to the first user. Most controllers also decode
Continuous Tone-Coded Squelch System
In telecommunications, Continuous Tone-Coded Squelch System or CTCSS is one type of in-band signaling that is used to reduce the annoyance of listening to other users on a shared two-way radio communication channel. (See squelch.) It is sometimes ...
tones, which allows the repeater to activate only if the station is sending a particular pre-programmed code, preventing unauthorized stations from using the repeater. Additionally, as repeaters are placed on high locations it also prevents distant stations on the same frequency from interfering. A Morse code or a synthesized voice module may produce station ID to comply with station identification regulations; this is most common on
amateur radio repeaters, some LMR stations must also identify to be legal in some areas.
Along with the repeater units, most stations utilize duplexers. These are notch filters usually in an array of six, eight, and sometimes four units. They separate the transmitter and receiver signals from each other so one antenna and coaxial line can be utilized. While this solution is very efficient and easy to install, factors such as humidity and temperature can affect the performance of duplexers, so in most configurations a dryer is installed to keep humidity out of the duplexers and coaxial cable, along with heated buildings in which they are installed in. Excellent quality coaxial cable, connectors, and antennas must also be used, as a single-antenna is not as forgiving as a dual antenna system since any RF leakage or poor connection can greatly decay the reliability and performance of the repeater. In some applications, cables going from the repeater and duplexers must be tuned to mitigate these issues.
In dual antenna systems, there are two antennas and two lengths of coaxial cable running from the transmitter and receiver. Usually, triple shield coax and or low loss Heliax are used to keep the two systems isolated. Two antenna systems are usually used if tower space is not limited, or space to build an array is available. The only issues with dual antenna systems is isolating the antennas so the receiver is not receiving what the transmitter is putting out. If this happens, it creates a loop, much like the feedback heard when a microphone is placed near a speaker. When this happens the repeater amplifies its own signal until it is either powered off or a TOT (time out timer) is expired.
To solve this antennas must be placed several wavelengths from each other in opposite vertical planes. For example, the receiver antenna is vertically polarized, while the transmitter antenna is placed one wavelength (or more) below the receiver antenna, but rotated 180° as to maintain vertical polarization. Antennas that have a null spot directly above and below them are excellent choices since another antenna can be placed in the null zone and isn't affected as much. Antennas must also be polarized the same as the stations trying to access the repeater—usually vertical polarization.
See also
*
Land mobile service
Land mobile service (short: LMS) is – in line to ITU Radio Regulations – a mobile service between base stations and land mobile stations, or between land mobile stations.
In accordance with ''ITU Radio Regulations'' (article 1) variations of ...
*
Astro (Motorola)
References
{{Two-way radio
Military radio systems