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mobile radio[′mō·bəl ′rād·ē·ō]
Radio communication in which one or both ends of the communication path are movable. The term mobile refers to movement of the radio rather than association with a vehicle (for example, hand-held portable radios are included by the definition). The Federal Communications Commission (FCC) licenses and regulates nonfederal government radio activity in the United States, while the National Telecommunications and Information Administration (NTIA) oversees federal government users. Other countries have similar agencies. International coordination is afforded through the International Telecommunication Union (ITU) and international treaty.
Users who lease or purchase radio equipment for personal communication fall into this category. Examples are public safety, special emergency, industrial, land transportation, and radiolocation radio services. Spectrum over a wide range of frequency bands is allocated; for example, low band (30–50 MHz), high band (150–174 MHz), ultrahigh frequency or UHF (450–512 MHz), the 800 band (806–824 MHz, paired with 851– 869 MHz), and the 900 band (896–901 MHz, paired with 935–940 MHz). Dispatch is the normal mode of operation; that is, all members of the group hear all communications. To accomplish this high-power, high-site base, repeaters are generally used so that the entire area of interest is covered by a single site. Coverage radius varies with frequency band, local terrain, and permissible power levels, but values on the order of 20 mi (32 km) are commonplace. Where areas to be covered are even larger (for example, statewide police systems) or where coverage reliability must be greater than that possible from a single site (for example, for ambulance communications), multiple sites can simulcast the communications. Current technology allows for data exchanges, vehicle location, and secure, digitized voice.
Specialized mobile radio (SMR) is a type of mobile radio service in which individual users with business interests are licensed to operate their mobiles, portables, and control stations on channel pairs repeated by specialized mobile radio base stations. Full interconnection to the public switched telephone network (PSTN) is possible. To boost the spectrum efficiency of specialized mobile radios relative to shared repeaters already in use, the FCC requires that channels be trunked. Trunking in the context of radio systems means not only sharing equipment but sharing frequencies as well. Trunking channels means that when a user wishes to place a call it can be served by any one of the channel pairs that is available.
Although paging is primarily a one-way radio system, two-way operation with such functions as page acknowledgment and short message reply are available. Some types of paging receivers display digits and letters (alphanumeric displays) that allow the calling party's number or a brief message to be displayed, and a message operator becomes unnecessary. Since display of paging messages involves little information, thousands of users can share each paging channel, thus making the service extremely spectrum efficient. Other types of paging receivers provide for brief voice messages following the alert (tone and voice).
Cellular technology allows hundreds of thousands of users to be handled in a single metropolitan area. Rather than link into the telephone system from a single high-power, high site that covers the entire metropolitan ares, users are linked via many low-power, low sites. A single low site, of course, can cover only a limited area, termed a cell, but many low sites taken together can cover the entire metropolitan area. Spectrum efficiency stems from reusing the same frequency at all sites that are sufficiently separated. To further limit interference caused by frequency reuse, each cell may be divided into sectors and directive antenna patterns may be used.
An attractive feature of cellular radio is the ability to vary the size of the cells in accordance with user density; hence, cell size can increase away from city centers. To sustain the reuse pattern with mixed cell sizes, power levels are tailored to produce comparable signal levels at all cell boundaries. Also, as more customers are added, radio channels can be created to serve them by constructing new base stations (hence, new cells) in geographical locations between existing cells. This concept is called cell splitting. Geographical coverage of the system can be expanded as well by constructing new base stations on the periphery of the existing system and assigning frequencies consistent with the original reuse pattern.
Automatic, continuous coverage as users move across cell boundaries is provided by the call handoff feature of cellular (also termed handover and automatic link transfer). Calls in need of handoff are recognized by monitoring call quality and comparing it to some required threshold. Handoff control procedures for first-generation analog frequency-modulation cellular systems are in operation.
The great demand for cellular phones and related wireless services has been addressed to some extent by the addition of new spectrum, by the introduction of narrow-band and digital cellular systems, and by cell splitting where practical. Acknowledging that these techniques for increasing capacity would quickly be exhausted, most countries allocated additional spectrum for mobile and portable communication. Since these frequency bands have much more spectrum than those previously allocated to cellular service, a greater variety of services are possible.
A spectrum of 120 MHz in the 1850–1910 and 1930–1990 MHz bands was allocated for licensed personal communication system (PCS) operation in the United States, and 20 MHz of spectrum in the 1910–1930 MHz band for unlicensed operation, split evenly between voice (isochronous) and data (asynchronous) applications. The spectrum allocated for licensed PCS operation is divided into six frequency blocks, three of which contain 30 MHz of spectrum and the other three, 10 MHz. It is thus possible in a given region to have as many as six competing service providers, in addition to the two 900-MHz cellular service providers. See Data communications, Telephone service