Mobile radio or mobiles refer to wireless communications systems and devices which are based on radio frequencies, and where the path of communications is movable on either end. There are a variety of views about what constitutes mobile equipment. For US licensing purposes, mobiles may include hand-carried, (sometimes called portable), equipment. An obsolete term is radiophone.Some mobile radios are mounted in aircraft, (aeronautical mobile), shipboard, (maritime mobile), on motorcycles, or railroad locomotives. Power may vary with each platform. For example, a mobile radio installed in a locomotive would run off of 72- or 30-volt DC power. A large ship with 117V AC power might have a base station mounted on the ship's bridge.
2.Disambiguation: Two-way versus telephone
The distinction between radiotelephones and two-way radio is becoming blurred as the two technologies merge. The backbone or infrastructure supporting the system defines which category or taxonomy applies. A parallel to this concept is the convergence of computing and telephones.
Radiotelephones are full-duplex (simultaneous talk and listen), circuit switched, and primarily communicate with telephones connected to the public switched telephone network. The connection sets up based on the user dialing. The connection is taken down when the end button is pressed. They run on telephony-based infrastructure such as AMPS or GSM.
Two-way radio is primarily a dispatch tool intended to communicate in simplex or half-duplex modes using push-to-talk, and primarily intended to communicate with other radios rather than telephones. These systems run on push-to-talk-based infrastructure such as Nextel's iDEN, Specialized Mobile Radio (SMR), MPT-1327, Enhanced Specialized Mobile Radio (ESMR) or conventional two-way systems. Certain modern two-way radio systems may have full-duplex telephone capability.
2.1 Mobile Radio
The mobile radio is a two-way communication gadget that operates through radio frequencies. As such, the channel of information and messages in a mobile radio is variable. Used to be known as radiophone, the earlier versions of the mobile radio, were one-way communication systems used for broadcast.
Contemporary mobile radio systems can have as much as a hundred channels and may be controlled by microprocessors. These types require the use of software to encode channels and operate their integrated functions. The mobile radio, also known as a two-way radio system, allows the exchange of messages only with other mobile radios through push-to-talk (PTT) functions. A mobile radio also features wireless transceivers, making mobile radios portable. Mobile radio systems may be used for communications in aircraft, ships, automobiles, and other vehicles. The power supply on which mobile radios run depend on the type of vehicle these are mounted on.
A mobile radio system is composed of a transceiver and microphone with a push-to-talk key. It has an antenna that links to the transceiver. Since most types of mobile radio are used in moving vehicles, where the surrounding noise can be loud, some mobile radio types come with an external speaker. Other models have headsets and microphones with noise-reduction capabilities.
2.2. How does a mobile radio work?
Most mobile radios operate on a single band of frequency. The radio transceiver contains transmit and receive frequencies. Very high frequency (VHF) and ultra high frequency (UHF) allow a mobile radio to operate on maximum coverage. This means that its average operating range is from 150 to 470 MHz.
To transmit a message, the PTT key must be pressed during talk-time to allow the voice message to be dispatched by the sending party. During this period, the sending party cannot hear or receive any incoming messages from the mobile radio. Once the PTT button is released, the sender may hear the response of the receiving party.
2.3. Why do we need a mobile radio
The use of mobile radio in transportation, security, and general operations makes communication fast, efficient and safe. It allows control centers to monitor location of vehicles and dispatch announcements to several receivers simultaneously. Additionally, the range of its area coverage is very high and is not dependent on a cellular network, which may fluctuate during emergency situations.
Different types of mobile radio are portable and capable of withstanding shock and severe weather conditions. Most countries impose certain requirements on the manufacture, sale and use of two-radio systems. This helps ensure that the communication gadget functions according to standards and that its use does not interfere with other communication systems.
3. History of mobile radio
Early users of mobile radio equipment included transportation and government. These systems used one-way broadcasting instead of two-way conversations. Railroads used medium frequency range (MF) communications (similar to the AM broadcast band) to improve safety. Instead of hanging out of a locomotive cab and grabbing train orders while rolling past a station, voice communications with rolling trains became possible. Radios linked the caboose with the locomotive cab. Early police radio systems were initially one way using MF frequencies above the AM broadcast band, (1.7 MHz). Some early systems talked back to dispatch on a 30-50 MHz link, (called crossband).
Early mobile radios used amplitude modulation (AM) to convey intelligence through the communications channel. In time, problems with sources of electrical noise showed that frequency modulation (FM) was superior for its ability to cope with vehicle ignition and power line noise. The frequency range used by most early radio systems, 25 to 50 MHz (vhf "low band") is particularly susceptible to the problem of electrical noise. This plus the need for more channels led to the eventual expansion of two-way radio communications into the VHF "high band" (150 to 174 MHz) and UHF (450 to 470 MHz). The UHF band has since been expanded again.
One of the major challenges in early mobile radio technology was that of converting the six or twelve volt power supply of the vehicle to the high voltage needed to operate the vacuum tubes in the radio. Early tube-type radios used dynamotors - essentially a six or twelve volt motor that turned a generator to provide the high voltages required by the vacuum tubes. Some early mobile radios were the size of a suitcase or had separate boxes for the transmitter and receiver. As time went on, power supply technology evolved to use first electromechanical vibrators, then solid-state power supplies to provide high voltage for the vacuum tubes. These circuits, called "inverters", changed the 6V or 12V direct current (DC) to alternating current (AC) which could be passed through a transformer to make high voltage.
Examples of US 1950s-1960s tube-type mobile radios with no transistors:
- Motorola FMTRU-140D (dynamotor powered)
- Motorola Twin-V, named for its "universal" 6 or 12 Volt power supply
- General Electric Progress Line (Early models without "T-Power" power supply)
- Kaar Engineering Model 501
The requirement that unauthorized persons be prohibited from using the radio transmitter meant that many radios were wired so they could not transmit unless the vehicle ignition was on. Persons without a key to the vehicle could not transmit. Equipment had to be "type accepted", or technically approved, by the FCC before it could be offered for sale. In order to be type accepted, the radio set had to be equipped with an indicator light, usually green or yellow, that showed power was applied and the radio was ready to transmit. Radios were also required to have a lamp (usually red) indicating when the transmitter was on. These traits continue in the design of modern radios.
Early tube-type radios operated on 50 kHz channel spacing with plus-or-minus fifteen kilohertz modulation deviation. This meant that the number of radio channels that could be accommodated in the available radio frequency spectrum were limited to a certain number, dictated by the bandwidth of the signal on each channel.
In the early 1970s, the California Department of Forestry requirement for 6-frequency transmit, 3-frequency receive, scanning, and a 5-tone burst encoder was unusual, leading to this custom-built mobile radio.
Solid state equipment arrived in the 1960s, with more efficient circuitry and smaller size. Channel spacing narrowed to 20-30 kHz with modulation deviation dropping to plus-or-minus five kilohertz. This was done to allow more radio spectrum availability to accommodate the rapidly growing national group of two-way radio users. By the mid 1970s, tube-type transmitter power amplifiers had been replaced with high-power transistors. From the 1960s to the 1980s, large system users with specialized requirements often had custom built radios designed for their unique systems. Systems with multiple tone encoders and more than two channels are unusual. Examples of US hybrid partially solid state mobile radios:
- Motorola Motrac.
- Motorola MJ IMTS Car Telephone (1963)
- General Electric Transistorized Progress Line
- General Electric MASTR Professional and MASTR Executive
- RCA Super Carfone.
4. Mobile radio in everyday routine
Custom design for a particular customer is a thing of the past. Modern mobile radio equipment is "feature rich". A mobile radio may have 100 or more channels, be microprocessor controlled and have built-in options such as unit ID. A computer and software is typically required to program the features and channels of the mobile radio. Menus of options may be several levels deep and offer a complicated array of possibilities. Some mobile radios have alphanumeric displays that translate channel numbers (F1, F2) to a phrase more meaningful to the user, such as "Providence Base", "Boston Base", etc. Radios are now designed with a myriad of features to preclude the need for custom design.
Examples of US microprocessor-controlled mobile radios:
- Motorola Astro Digital Spectra W9
- Kenwood TK-690
As use of mobile radio equipment has virtually exploded, channel spacing has had to be narrowed again to 12.5-15 kHz with modulation deviation dropping to plus-or-minus 2.5 kilohertz. In order to fit into smaller, more economical vehicles, today's radios are trending toward radically smaller sizes than their tube-type ancestors.
5. Description of Mobile radio comm’n
Commercial and professional mobile radios are often purchased from an equipment supplier or dealer whose staff will install the equipment into the user's vehicles. Large fleet users may buy radios directly from an equipment manufacturer and may even employ their own technical staff for installation and maintenance.
A modern mobile radio consists of a radio transceiver, housed in a single box, and a microphone with a push-to-talk button. Each installation would also have a vehicle-mounted antenna connected to the transceiver by a coaxial cable. Some models may have an external, separate speaker which can be positioned and oriented facing the driver to overcome ambient road noise present when driving. The installer would have to locate this equipment in a way that does not interfere with the vehicle's sun roof, electronic engine management system, vehicle stability computer, or air bags.
Mobile radios installed on motorcycles are subject to extreme vibration and weather. Professional equipment designed for use on motorcycles is weather and vibration resistant. Shock mounting systems are used to reduce the radio's exposure to vibration imparted by the motorcycle's modal, or resonant, shaking.
Mobile radio equipment is manufactured to specifications developed by the Electronics Industries Association/Telecommunications Industries Association (EIA/TIA). These specifications have been developed to help ensure that mobile radio equipment not only performs as expected by the user, but also to prevent the sale and distribution of inferior equipment which could degrade communications. All specifications should be reviewed by radio system engineers to ensure that appropriate equipment is used to meet the end user's needs.
6. Antenna
A mobile radio must have an associated antenna. The most common antennas are stainless steel wire or rod whips which protrude vertically from the vehicle. Physics defines the antenna length: length relates to frequency and cannot be arbitrarily lengthened or shortened (more likely) by the end user. The standard "quarter wave" antenna in the 25-50 MHz range can be over nine feet long. A 900 MHz antenna may be three inches long for a quarter wavelength. A transit bus may have a ruggedized antenna, which looks like a white plastic blade or fin, on its roof. Some vehicles with concealed radio installations have antennas designed to look like the original AM/FM antenna, a rearview mirror, or may be installed inside windows, or hidden on the floor pan or underside of a vehicle. Aircraft antennas look like blades or fins, the size and shape being determined by frequencies used. Microwave antennas may look like flat panels on the aircraft's skin. Temporary installations may have antennas which clip on to vehicle parts or are attached to steel body parts by a strong magnet.
Though initially relatively inexpensive mobile radio system components, frequently damaged antennas can be costly to replace since they are usually not included in maintenance contracts for mobile radio fleets. Some types of vehicles in 24-hour use, with stiff suspensions, tall heights, or rough diesel engine idle vibrations may damage antennas quickly. The location and type of antenna can affect system performance drastically. Large fleets usually test a few vehicles before making a commitment to a certain antenna location or type.
7. Multiple radio sets
Dispatch-reliant services, such as tow cars or ambulances, may have several radios in each vehicle. For example, tow cars may have one radio for towing company communications and a second for emergency road service communications. Ambulances may have a similar arrangement with one radio for government emergency medical services dispatch and one for company dispatch.
Mobile radio telephone
These early mobile telephone systems can be distinguished from earlier closed radiotelephone systems in that they were available as a commercial service that was part of the public switched telephone network, with their own telephone numbers, rather than part of a closed network such as a police radio or taxi dispatch system.
These mobile telephones were usually mounted in cars or trucks, though briefcase models were also made. Typically, the transceiver (transmitter-receiver) was mounted in the vehicle trunk and attached to the "head" (dial, display, and handset) mounted near the driver seat.
8. Applications
8.1 Multiple controls, microphones
A mobile radio in a US ambulance often has two sets of controls: one in the patient area and another near the driver.
US ambulances often have radios with dual controls and dual microphones allowing the radio to be used from the patient care area in the rear or from the vehicle's cab.
8.2 Data radio
Both tow cars and ambulances may have an additional radio which transmits and receives to support a mobile data terminal. A data terminal radio allows data communications to take place over the separate radio. In the same way that a facsimile machine has a separate phone line, this means data and voice communication can take place simultaneously over a separate radio. Early Federal Express (FedEx) radio systems used a single radio for data and voice. The radio had a request-to-speak button which, when acknowledged, allowed voice communication to the dispatch center.
Each radio works over a single band of frequencies. If a tow car company had a frequency on the same band as its auto club, a single radio with scanning might be employed for both systems. Since a mobile radio typically works on a single frequency band, multiple radios may be required in cases where communications take place over systems on more than one frequency band.
8.3 Walkie talkie converters in place of mobile radios
Intended as a cost savings, some systems employ vehicular chargers instead of a mobile radio. Each radio user is issued a walkie talkie. Each vehicle is equipped with a charger system console. The walkie talkie inserted into a vehicular charger or converter while the user is in the vehicle. The charger or converter :-
1) connects the walkie talkie to the vehicle's two-way radio antenna,
2) connects an amplified speaker,
3) connects a mobile microphone, and
4) charges the walkie talkie's battery. The weak point of these systems has been connector technology which has been proven unreliable in some installations. Receiver performance is a problem in congested radio signal and urban areas. These installations are sometimes referred to as jerk-and-run systems.
9. Professional Mobile Radio
Motorola HT1000 hand-held two-way radio
Professional Mobile Radio (also known as Private Mobile Radio (PMR) in the UK and Land Mobile Radio (LMR) in North America) are field radio communications systems which use portable, mobile, base station, and dispatch console radios. Operation of PMR radio equipment is based on such standards as MPT-1327, TETRA and APCO 25 which are designed for dedicated use by specific organizations, or standards such as NXDN intended for general commercial use. Typical examples are the radio systems used by police forces and fire brigades. Key features of professional mobile radio systems can include:
- Point to multi-point communications (as opposed to cell phones which are point to point communications)
- Push-to-talk, release to listen — a single button press opens communication on a radio frequency channel
- Large coverage areas
- Closed user groups
- Use of VHF or UHF frequency bands.
9.1 Selective calling
Many systems operate with the remote or mobile stations being able to hear all the calls being made. This may not always be satisfactory and a system of selective calling may be required. There are several ways of achieving this, including Dual Tone Multiple Frequency (DTMF) signalling and Continuous Tone Coded Squelch System (CTCSS).
9.2 DTMF
DTMF is a system that is widely used for telephone signalling and is almost universally used for touch tone dialing for landline telephones today. It uses set pairs of tones that are used to carry the information. The eight frequencies used are 697, 770, 852, 941 Hz which are termed the "low tones" and 1209, 1336, 1477, and 1633 Hz which are "high tones". One high and one low tone is used together and the various combinations are used to represent different numbers and symbols.
The relevant code consisting of one or more digits is sent and the station programmed to respond to the number, typically one or two digits responds by opening the squelch on the receiver to let the audio through. The disadvantage of this system is that if the receiver does not pick up the code at the instant the DTMF signalling takes place then it will not respond to any of the message. This can be a significant disadvantage because mobile stations often lose the signal for short periods as they are on the move.
9.3 CTCSS
Another widely used system is CTCSS, also referred to as subaudible tones or PL tones (a Motorola trademark). As the name suggests it uses subaudible tones (below about 250 Hz) to carry the selection information. These are transmitted in addition to the normal voice channel, but as they appear below the audio range passed by most mobile radios (roughly 300-3000Hz), they are filtered out and therefore not heard.
Only when the correct tone for the required station is transmitted will the squelch for that receiver be opened and the transmitted audio will be heard. The advantage of this system is that the subaudible tones are transmitted for the whole period of the transmission so if the signal fades at the beginning of the transmission is lost but later then increases in strength, the continuously transmitted tones will enable the squelch to open and the audio to be heard. Systems typically are able to provide up to 37 different tones, the lowest frequency of which is 67 Hz and the highest 254.1 Hz. This enables a variety of different mobiles to be called selectively..
9.4 Modulation
In general narrow band frequency modulation is the chosen form of modulation, although airport services use amplitude modulation. Typically a deviation of 2.5 kHz is used for FM and this enables a channel spacing of 12.5 kHz to be implemented. As the demands for PMR are high, it is necessary to make effective use of the channels available. This is achieved by re-using the frequencies in different areas. Base stations must be located sufficiently far apart so that interference is not experienced, and also selective calling techniques such as CTCSS and DTMF are used to ensure that as many mobiles as possible can use a given channel.
10. Conclusion
This term paper reveals all the aspects of mobile radio. Though mentioned in the above term paper, mobile radio has evolved a lot but it is an ever growing field and has its technology being advanced in near future. It is ever growing in private sector and giving room to an blooming industry. Hence, in near future, it will increase to ever lasting heights
No comments:
Post a Comment
leave your opinion