Radiotelephone
Encyclopedia
A radiotelephone is a communications
system for transmission of speech over radio. Radiotelephone systems are not necessarily interconnected with the public "land line" telephone network. "Radiotelephone" is often used to describe the usage of radio spectrum where it is important to distinguish the type of emission from, for example, radiotelegraph or video signals. Where a two-way radio
system is arranged for speaking and listening at a mobile station, and where it can be interconnected to the public switched telephone system, the system can provide mobile telephone service.
has issued various commercial "radiotelephone operator" licenses and permits to qualified applicants. These allow them to install, service, and maintain voice-only radio transmitter systems for use on ships and aircraft. (Until deregulation in the 1990s they were also required for commercial domestic radio and television broadcast systems. Because of treaty obligations they are still required for engineers of international shortwave
broadcast stations.) The certificate currently issued is the General radiotelephone operator license
.
The word phone has a long precedent beginning with early US wireless voice systems. The term means voice as opposed to telegraph or Morse Code
. This would include systems fitting into the category of two-way radio or one-way voice broadcasts such as coastal maritime weather. The term is still popular in the amateur radio
community and in US Federal Communications Commission
regulations.
and Improved Mobile Telephone Service
used to place telephone calls.
Mobile radio telephone systems allowed a mobile unit to have a telephone number allowing access from the general telephone network, although some systems required mobile operators to set up calls to mobile stations. Mobile radio telephone systems before the introduction of cellular services suffered from few usable channels, heavy congestion, and very high operating costs.
allows both users to talk and listen simultaneously; effectively there are two open channels
between the two end-to-end users of the system. In a radiotelephone system, this form of working, known as full-duplex
, require a radio system to simultaneously transmit and receive on two separate channels, which both wastes bandwidth and presents some technical challenges. It is, however, the most comfortable method of voice communication for users, and it is currently used in cell phones and was used in the former IMTS
.
The most common method of working for radiotelephones is simplex
operation, which allows one person to talk and the other to listen alternately. If a single channel is used, both ends take turns to transmit on it. An eavesdropper would hear both sides of the conversation. Dual-frequency working splits the communication into two separate channels, but only one is used to transmit at a time. The end users have the same experience as single frequency simplex but an eavesdropper with one receiver would only hear one side of the conversation.
The user presses a special switch
on the transmitter
when they wish to talk—this is called the "press-to-talk" switch or PTT (colloquially, sometimes called "the tit"). It is usually fitted on the side of the microphone
or other obvious position. Users may use a special code-word such as "over" to signal that they have finished transmitting, or it may follow from the conversation.
where they are licensed to do so, though typically they are used in the various bands between 60 and 900 MHz. They may use simple modulation
schemes such as AM
or FM
, or more complex techniques such as digital coding, spread spectrum
, and so on. Licensing terms for a given band will usually specify the type of modulation to be used. For example, airband
radiotelephones used for air to ground communication between pilots and controllers operates in the VHF
band from 118.0 to 136.975 MHz, using amplitude modulation.
Radiotelephone receiver
s are usually designed to a very high standard, and are usually of the double-conversion superhet
design. Likewise, transmitters are carefully designed to avoid unwanted interference and feature power outputs from a few tens of milliwatts to perhaps 50 watt
s for a mobile unit, up to a couple of hundred watts for a base station
. Multiple channels are often provided using a frequency synthesizer
.
Receivers usually features a squelch
circuit
to cut off the audio
output from the receiver when there is no transmission
to listen to. This is in contrast to broadcast
receivers, which often dispense with this.
The crudest and oldest of these is called CTCSS
, or Continuous Tone-Controlled Squelch System. This consists of superimposing a precise very low frequency tone on the audio signal. Only the receiver tuned to this specific tone is able to receive the signal: this receiver shuts off the audio when the tone is not present or is a different frequency. By assigning a unique frequency to each mobile, private channels can be imposed on a public network. However this is only a convenience feature—it does not guarantee privacy.
A more commonly used system is called Selective Calling or Selcall
. This also uses audio tones, but these are not restricted to sub-audio tones and are sent as a short burst in sequence. The receiver will be programmed to respond only to a unique set of tones in a precise sequence, and only then will it open the audio circuits for open-channel conversation with the base station. This system is much more versatile than CTCSS, as relatively few tones yield a far greater number of "addresses". In addition, special features (such as broadcast modes and emergency overrides) can be designed in, using special addresses set aside for the purpose. A mobile unit can also broadcast a Selcall sequence with its unique address to the base, so the user can know before the call is picked up which unit is calling. In practice many selcall systems also have automatic transponding
built in, which allows the base station to "interrogate" a mobile even if the operator is not present. Such transponding systems usually have a status code that the user can set to indicate what they are doing. Features like this, while very simple, are one reason why they are very popular with organisations that need to manage a large number of remote mobile units. Selcall is widely used, though is becoming superseded by much more sophisticated digital systems.
, an optical telephone invented jointly by Alexander Graham Bell
and his then-assistant and future associate Charles Sumner Tainter
on February 19, 1880. The device allowed normal conversations to be transmitted
on a beam of light. On April 1, 1880, Tainter transmitted the world's first wireless telephone
message to Bell, on the newly invented form of telecommunication
, the far advanced precursor to fiber-optic communication
s.
Although the Photophone was an important invention, it was many years before the significance of Bell and Tainter's work was fully recognized. While Bell had hoped his new Photophone could be used by ships at sea and to also displace the plethora of telephone lines that were blooming along busy city boulevards, his design failed to protect its transmissions from outdoor interferences
such as clouds, fog, rain, snow and such, that could easily disrupted the transmission of light.
Until the development of modern laser
and fiber-optic technologies for the secure transport of light, factors such as the weather or a lack of light inhibited the use of Bell and Tainter's invention.
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
system for transmission of speech over radio. Radiotelephone systems are not necessarily interconnected with the public "land line" telephone network. "Radiotelephone" is often used to describe the usage of radio spectrum where it is important to distinguish the type of emission from, for example, radiotelegraph or video signals. Where a two-way radio
Two-way radio
A two-way radio is a radio that can both transmit and receive , unlike a broadcast receiver which only receives content. The term refers to a personal radio transceiver that allows the operator to have a two-way conversation with other similar radios operating on the same radio frequency...
system is arranged for speaking and listening at a mobile station, and where it can be interconnected to the public switched telephone system, the system can provide mobile telephone service.
Mode of emission
Since the 1930s the Federal Communications CommissionFederal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
has issued various commercial "radiotelephone operator" licenses and permits to qualified applicants. These allow them to install, service, and maintain voice-only radio transmitter systems for use on ships and aircraft. (Until deregulation in the 1990s they were also required for commercial domestic radio and television broadcast systems. Because of treaty obligations they are still required for engineers of international shortwave
Shortwave
Shortwave radio refers to the upper MF and all of the HF portion of the radio spectrum, between 1,800–30,000 kHz. Shortwave radio received its name because the wavelengths in this band are shorter than 200 m which marked the original upper limit of the medium frequency band first used...
broadcast stations.) The certificate currently issued is the General radiotelephone operator license
General radiotelephone operator license
The General Radiotelephone Operator License is a United States commercial license, as opposed to an amateur radio license. It allows the holder to operate, maintain or install certain classes of United States licensed radio and television transmitters under the authority of the Federal...
.
The word phone has a long precedent beginning with early US wireless voice systems. The term means voice as opposed to telegraph or Morse Code
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...
. This would include systems fitting into the category of two-way radio or one-way voice broadcasts such as coastal maritime weather. The term is still popular in the amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
community and in US Federal Communications Commission
Federal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
regulations.
Mobile telephone
In the US, mobile telephone was more commonly used to refer to radio systems such as Mobile Telephone ServiceMobile Telephone Service
The Mobile Telephone Service is a pre-cellular VHF radio system that links to the Public Switched Telephone Network . MTS was the radiotelephone equivalent of land dial phone service. , only rural and wilderness areas were still using the system....
and Improved Mobile Telephone Service
Improved Mobile Telephone Service
The Improved Mobile Telephone Service is a "0G" pre-cellular VHF/UHF radio system that links to the PSTN. IMTS was the radiotelephone equivalent of land dial phone service...
used to place telephone calls.
Mobile radio telephone systems allowed a mobile unit to have a telephone number allowing access from the general telephone network, although some systems required mobile operators to set up calls to mobile stations. Mobile radio telephone systems before the introduction of cellular services suffered from few usable channels, heavy congestion, and very high operating costs.
Modes of operation
A standard land line based telephoneTelephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
allows both users to talk and listen simultaneously; effectively there are two open channels
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
between the two end-to-end users of the system. In a radiotelephone system, this form of working, known as full-duplex
Duplex (telecommunications)
A duplex communication system is a system composed of two connected parties or devices that can communicate with one another in both directions. The term multiplexing is used when describing communication between more than two parties or devices....
, require a radio system to simultaneously transmit and receive on two separate channels, which both wastes bandwidth and presents some technical challenges. It is, however, the most comfortable method of voice communication for users, and it is currently used in cell phones and was used in the former IMTS
Improved Mobile Telephone Service
The Improved Mobile Telephone Service is a "0G" pre-cellular VHF/UHF radio system that links to the PSTN. IMTS was the radiotelephone equivalent of land dial phone service...
.
The most common method of working for radiotelephones is simplex
Simplex communication
Simplex communication refers to communication that occurs in one direction only. Two definitions have arisen over time: a common definition, which is used in ANSI standard and elsewhere, and an ITU-T definition...
operation, which allows one person to talk and the other to listen alternately. If a single channel is used, both ends take turns to transmit on it. An eavesdropper would hear both sides of the conversation. Dual-frequency working splits the communication into two separate channels, but only one is used to transmit at a time. The end users have the same experience as single frequency simplex but an eavesdropper with one receiver would only hear one side of the conversation.
The user presses a special switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....
on the transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
when they wish to talk—this is called the "press-to-talk" switch or PTT (colloquially, sometimes called "the tit"). It is usually fitted on the side of the microphone
Microphone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
or other obvious position. Users may use a special code-word such as "over" to signal that they have finished transmitting, or it may follow from the conversation.
Technology
Radiotelephones may operate at any frequencyFrequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
where they are licensed to do so, though typically they are used in the various bands between 60 and 900 MHz. They may use simple modulation
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
schemes such as AM
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
or FM
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
, or more complex techniques such as digital coding, spread spectrum
Spread spectrum
Spread-spectrum techniques are methods by which a signal generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth...
, and so on. Licensing terms for a given band will usually specify the type of modulation to be used. For example, airband
Airband
Airband or Aircraft band is the name for a group of frequencies in the VHF radio spectrum allocated to radio communication in civil aviation, sometimes also referred to as VHF, or phonetically as "Victor"...
radiotelephones used for air to ground communication between pilots and controllers operates in the VHF
Very high frequency
Very high frequency is the radio frequency range from 30 MHz to 300 MHz. Frequencies immediately below VHF are denoted High frequency , and the next higher frequencies are known as Ultra high frequency...
band from 118.0 to 136.975 MHz, using amplitude modulation.
Radiotelephone receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
s are usually designed to a very high standard, and are usually of the double-conversion superhet
Superheterodyne receiver
In electronics, a superheterodyne receiver uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency...
design. Likewise, transmitters are carefully designed to avoid unwanted interference and feature power outputs from a few tens of milliwatts to perhaps 50 watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
s for a mobile unit, up to a couple of hundred watts for a base station
Base station
The term base station can be used in the context of land surveying and wireless communications.- Land surveying :In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to derive correction information for nearby portable GPS...
. Multiple channels are often provided using a frequency synthesizer
Frequency synthesizer
A frequency synthesizer is an electronic system for generating any of a range of frequencies from a single fixed timebase or oscillator. They are found in many modern devices, including radio receivers, mobile telephones, radiotelephones, walkie-talkies, CB radios, satellite receivers, GPS systems,...
.
Receivers usually features a squelch
Squelch
In telecommunications, squelch is a circuit function that acts to suppress the audio output of a receiver in the absence of a sufficiently strong desired input signal.-Carrier squelch:...
circuit
Electronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...
to cut off the audio
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
output from the receiver when there is no transmission
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
to listen to. This is in contrast to broadcast
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
receivers, which often dispense with this.
Privacy and selective calling
Often, on a small network system, there are many mobile units and one main base station. This would be typical for police or taxi services for example. To help direct messages to the correct recipients and avoid irrelevant traffic on the network's being a distraction to other units, a variety of means have been devised to create addressing systems.The crudest and oldest of these is called CTCSS
CTCSS
In telecommunications, Continuous Tone-Coded Squelch System or CTCSS is a circuit that is used to reduce the annoyance of listening to other users on a shared two-way radio communications channel. It is sometimes called tone squelch...
, or Continuous Tone-Controlled Squelch System. This consists of superimposing a precise very low frequency tone on the audio signal. Only the receiver tuned to this specific tone is able to receive the signal: this receiver shuts off the audio when the tone is not present or is a different frequency. By assigning a unique frequency to each mobile, private channels can be imposed on a public network. However this is only a convenience feature—it does not guarantee privacy.
A more commonly used system is called Selective Calling or Selcall
Selcall
Selcall is a type of squelch protocol used in radio communications systems, in which transmissions include a brief burst of sequential audio tones...
. This also uses audio tones, but these are not restricted to sub-audio tones and are sent as a short burst in sequence. The receiver will be programmed to respond only to a unique set of tones in a precise sequence, and only then will it open the audio circuits for open-channel conversation with the base station. This system is much more versatile than CTCSS, as relatively few tones yield a far greater number of "addresses". In addition, special features (such as broadcast modes and emergency overrides) can be designed in, using special addresses set aside for the purpose. A mobile unit can also broadcast a Selcall sequence with its unique address to the base, so the user can know before the call is picked up which unit is calling. In practice many selcall systems also have automatic transponding
Transponder
In telecommunication, the term transponder has the following meanings:...
built in, which allows the base station to "interrogate" a mobile even if the operator is not present. Such transponding systems usually have a status code that the user can set to indicate what they are doing. Features like this, while very simple, are one reason why they are very popular with organisations that need to manage a large number of remote mobile units. Selcall is widely used, though is becoming superseded by much more sophisticated digital systems.
Archaic name usage
In the 19th century radiophone was an alternate name for the PhotophonePhotophone
The photophone, also known as a radiophone, was invented jointly by Alexander Graham Bell and his then-assistant Charles Sumner Tainter on February 19, 1880, at Bell's 1325 'L' Street laboratory in Washington, D.C...
, an optical telephone invented jointly by Alexander Graham Bell
Alexander Graham Bell
Alexander Graham Bell was an eminent scientist, inventor, engineer and innovator who is credited with inventing the first practical telephone....
and his then-assistant and future associate Charles Sumner Tainter
Charles Sumner Tainter
Charles Sumner Tainter was an American scientific instrument maker, engineer and inventor, best known for his collaborations with Alexander Graham Bell, Chichester Bell, Alexander's father-in-law Gardiner Hubbard, and for his significant improvements to Thomas Edison's phonograph, resulting in the...
on February 19, 1880. The device allowed normal conversations to be transmitted
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
on a beam of light. On April 1, 1880, Tainter transmitted the world's first wireless telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
message to Bell, on the newly invented form of telecommunication
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
, the far advanced precursor to fiber-optic communication
Fiber-optic communication
Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information...
s.
Although the Photophone was an important invention, it was many years before the significance of Bell and Tainter's work was fully recognized. While Bell had hoped his new Photophone could be used by ships at sea and to also displace the plethora of telephone lines that were blooming along busy city boulevards, his design failed to protect its transmissions from outdoor interferences
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
such as clouds, fog, rain, snow and such, that could easily disrupted the transmission of light.
Until the development of modern laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
and fiber-optic technologies for the secure transport of light, factors such as the weather or a lack of light inhibited the use of Bell and Tainter's invention.
See also
- Car phoneCar phoneA car phone is a mobile phone device specifically designed for and fitted into an automobile.In the late 1970s and 1980s, the car phone was more popular than the regular mobile phone...
- Improved Mobile Telephone ServiceImproved Mobile Telephone ServiceThe Improved Mobile Telephone Service is a "0G" pre-cellular VHF/UHF radio system that links to the PSTN. IMTS was the radiotelephone equivalent of land dial phone service...
, the technology which replaced the earliest 'car telephone' service (Mobile Telephone Service) - Mobile radio telephone
- Mobile Telephone ServiceMobile Telephone ServiceThe Mobile Telephone Service is a pre-cellular VHF radio system that links to the Public Switched Telephone Network . MTS was the radiotelephone equivalent of land dial phone service. , only rural and wilderness areas were still using the system....
, the earliest 'car telephone' service provided by the Bell System, later replaced by IMTS technology - Two-way radioTwo-way radioA two-way radio is a radio that can both transmit and receive , unlike a broadcast receiver which only receives content. The term refers to a personal radio transceiver that allows the operator to have a two-way conversation with other similar radios operating on the same radio frequency...