Telephony Radiotelephone Essay, Research Paper
RADIOTELEPHONE
In addition to the wireline telephones described in The telephone instrument , there exist a number of wireless instruments that are connected to the public switched telephone network (PSTN). At the present time, these wireless telephones generally fall into one of three categories: cordless telephones, cellular radio systems, or personal communication systems. Eventually these systems will be expanded to include global satellite-based telephony. Cordless telephones.
Cordless telephones are devices that take the place of a telephone instrument within a home or office and permit very limited mobility (up to a hundred metres). Because they are plugged directly into an existing telephone jack, they essentially serve as a wireless extension to the existing home or office wiring. Cordless transceivers communicate with the plugged-in base unit over a pair of frequencies in the 46- and 48-megahertz bands or over a single frequency in the 902-928-megahertz band.
Cellular radio.
Cellular telephones are transportable by vehicle or personally portable devices that may be used in motor vehicles or by pedestrians. Communicating by radiowave in the 800-900-megahertz band, they permit a significant degree of mobility within a defined serving region that may be hundreds of square kilometres in area. In this section, the concept of cellular radio and the development of cellular systems are discussed. cellular telecommunication.
All cellular radio systems exhibit several fundamental characteristics, as summarized in the following:
1. The geographic area served by a cellular radio system is broken up into smaller geographic areas, or cells. Uniform hexagons most frequently are employed to represent these cells on maps and diagrams; in practice, though, radiowaves do not confine themselves to hexagonal areas, so that the actual cells have irregular shapes.
2. All communication with a mobile or portable instrument within a given cell is made to the base station that serves the cell.
3. Because of the low transmitting power of battery-operated portable instruments, specific sending and receiving frequencies assigned to a cell may be reused in other cells within the larger geographic area. Thus, the spectral efficiency of a cellular system (that is, the uses to which it can put its portion of the radio spectrum) is increased by a factor equal to the number of times a frequency may be reused within its service area.
4. As a mobile instrument proceeds from one cell to another during the course of a call, a central controller automatically reroutes the call from the old cell to the new cell without a noticeable interruption in the signal reception. This process is known as handoff. The central controller, or mobile telephone switching office (MTSO), thus acts as an intelligent central office switch that keeps track of the movement of the mobile subscriber.
5. As demand for the radio channels within a given cell increases beyond the capacity of that cell (as measured by the number of calls that may be supported simultaneously), the overloaded cell is split into smaller cells, each with its own base station and central controller. The radio-frequency allocations of the original cellular system are then rearranged to account for the greater number of smaller cells.
Frequency reuse between discontiguous cells and the splitting of cells as demand increases are the concepts that distinguish cellular systems from other radiotelephone systems. They allow cellular providers to serve large metropolitan areas that may contain hundreds of thousands of customers. The first mobile and portable subscriber units for cellular systems were large and heavy. With significant advances in component technology, though, the weight and size of portable transceivers have been significantly reduced. For example, lightweight portables in 1990 may have weighed 310 grams (10 ounces); by 1994 they weighed as little as 120 grams.
Development of cellular systems.
In the United States, interconnection of mobile radio transmitters and receivers (transceivers) with the PSTN began in 1946, with the introduction of mobile telephone service (MTS) by AT&T. The MTS system employed frequencies in either the 35-megahertz band or the 150-megahertz band. A mobile user who wished to place a call from a radiotelephone had to search manually for an unused channel before placing the call. The user then spoke with a mobile operator, who actually dialed the call over the PSTN. The radio connection was simplex i.e., only one party could speak at a time, the call direction being controlled by a push-to-talk switch in the mobile handset.
In 1964 AT&T introduced a second generation of mobile telephony, known as improved mobile telephone service (IMTS). This provided full-duplex operation, automatic dialing, and automatic channel searching. Initially 11 channels were provided in the 152-158-megahertz band, but in 1969 an additional 12 channels were added in the 454-459-megahertz band. Since only 11 (or 12) channels were available for all users of the system within a given geographic area (such as the metropolitan area around a large city) and since each frequency was used only once in that geographic area, the IMTS system faced a high demand for a very limited channel resource. For example, in New York City during 1976, the IMTS system served 545 customers with another 3,700 customers placed on a waiting list for service. Moreover, each base-station antenna was located on a tall structure and transmitted at high power in an attempt to provide coverage throughout the entire service area. Because of these high power requirements, all subscriber units in the IMTS system were mobile-based instruments that carried larger storage batteries.
During this time the American cellular radio system, known as the advanced mobile phone system, or AMPS, was developed primarily by AT&T and Motorola, Inc. AMPS was based on 666 paired voice channels, spaced every 30 kilohertz in the 800-megahertz region. The system employed an analog-modulation approach frequency modulation, or FM and was designed from the outset to support both mobile and portable subscriber units. It was publicly introduced in Chicago in 1983 and was a success from the beginning. At the end of the first year of service, there were a total of 200,000 AMPS subscribers throughout the United States; five years later there were more than 2,000,000. In response to this growth, an additional 166 voice channels were allocated to cellular carriers in each market. Still, the cellular system was expected to experience capacity shortages. (See BTW: The FCC and cellular telephony.) The American cellular industry responded with several proposals for increasing capacity without requiring additional spectrum allocations. One analog FM approach, proposed by Motorola in 1991, was known as narrowband AMPS, or NAMPS. In NAMPS systems each existing 30-kilohertz voice channel is split into three 10-kilohertz channels. Thus, in place of the 832 channels available in AMPS systems, the NAMPS system offered 2,496 channels. A second approach, developed by a committee of the Telecommunications Industry Association (TIA) in 1988, employed digital modulation and digital voice compression in conjunction with a time-division multiple access (TDMA) method; this also permitted three new voice channels in place of one AMPS channel. Finally, in 1994 there surfaced a third approach, developed originally by Qualcomm, Inc., but also adopted as a standard by the TIA. This third approach used a form of spread spectrum multiple access known as code-division multiple access (CDMA)–a technique that, like the original TIA approach, combined digital voice compression with digital modulation. The CDMA system offered 10 to 20
Airborne systems.
In addition to the terrestrial cellular radiotelephone systems, there also exist several systems that permit the placement of telephone calls to the PSTN by passengers on commercial aircraft. These in-flight radiotelephones, known by the generic name aeronautical public correspondence (APC) systems, are of two types: terrestrial-based, in which telephone calls are placed directly from an aircraft to an en route ground station; and satellite-based, in which telephone calls are relayed via a geostationary satellite to a ground station. In the United States the North American terrestrial system (NATS) was introduced by GTE Corporation in 1984. Within a decade the system was installed in more than 1,700 aircraft, with ground stations in the United States providing coverage over most of the United States and southern Canada. A second-generation system, GTE GenStar, employs digital modulation. In Europe the European Telecommunications Standards Institute (ETSI) adopted a terrestrial APC system known as the terrestrial flight telephone system (TFTS) in 1992. This system employs digital modulation methods and operates in the 1,670-1,675 and 1,800-1,805-megahertz bands. In order to cover most of Europe, the ground stations must be spaced every 50 to 700 kilometres.
The second type of APC system, based on satellite transmission, is available through the use of Inmarsat geostationary-orbit satellites. Because they do not depend on ground stations, satellite-based systems may be employed anywhere in the world.
Personal communication systems.
Although cellular radio systems provide a high degree of mobility within a given service area, they do so at the expense of providing voice-only service usually at a significant monthly fee. In recognition of this shortcoming, in a number of countries throughout the world a new radiotelephone service has been introduced that has been almost universally called the personal communication system (PCS). In the broadest sense, PCS includes all forms of radiotelephone communication that are interconnected to the PSTN, including cellular radio and aeronautical public correspondence, but the basic concept includes the following attributes: ubiquitous service to roving users, low subscriber terminal costs and service fees, and compact, lightweight, and unobtrusive personal portable units. The first PCS to be implemented was the second-generation cordless telephony (CT-2) system, which entered service in the United Kingdom in 1991. The CT-2 system was designed at the outset to serve as a telepoint system. In telepoint systems, a user of a portable unit may originate telephone calls (but not receive them) by dialing a base station located within several hundred metres. The base unit is connected to the PSTN and operates as a public (pay) telephone, charging calls to the subscriber. The CT-2 system transmits a digital signal at low power (10 megawatts) in the 864-868-megahertz band. Modifications that permit two-way call placement have been incorporated into the system. In 1988 the European Conference on Posts and Telecommunications (CEPT) began work on another personal communication system, which became known as the digital European cordless telephone (DECT) system. The DECT system was designed initially to provide cordless telephone service for office environments, but its scope soon broadened to include campuswide communications and telepoint services. DECT has been deployed in the United Kingdom and France as well as other countries. In Japan a PCS based loosely on the DECT concepts, the personal handy phone (PHP) system, was introduced to the public in 1994. The PHP system operates in the 1,895-1,907-megahertz band and is intended for home, office, and telepoint applications.
In the United States in 1994-95 the Federal Communications Commission (FCC) sold a number of licenses in the 1.85-1.99-gigahertz region for use in PCS applications. PCS operators in the United States will likely use many of the same technologies and systems that are employed in digital cellular systems at 800 megahertz. (See BTW: The FCC and personal communication systems.) Satellite-based radiotelephone communication.
In order to augment the terrestrial and aircraft-based mobile radiotelephone systems discussed in Cellular radio and Personal communication systems , several satellite-based systems are planned for operation. The goal of these new systems is to permit ready connection to the PSTN anywhere on the Earth s surface, especially in areas not presently covered by cellular radio. A form of satellite-based mobile communication is already available in airborne cellular systems that utilize the Inmarsat satellites. However, the Inmarsat satellites are geostationary, remaining fixed above a single point approximately 35,000 kilometres (22,000 miles) above the Earth. Because of this high-altitude orbit, Earth-based communication transceivers require high transmitting power, large communication antennas, or both in order to communicate with the satellite. In addition, such a long communication path introduces a noticeable delay, on the order of a quarter-second, in two-way voice conversations. One viable alternative to geostationary satellites would be a larger system of satellites in low earth orbit (LEO). Orbiting less than 1,600 kilometres above the Earth, LEO satellites are not geosynchronous and therefore cannot provide constant coverage of specific areas on the Earth. Nevertheless, by allowing radio communications with a mobile instrument to be handed off between satellites, an entire constellation of satellites can assure that no call will be dropped simply because a single satellite has moved out of range. (see also Index: communications satellite, satellite communication, Earth satellite) The first LEO system scheduled for commercial service was the Iridium system, designed by Motorola, Inc., and owned by Iridium, Inc., a consortium made up of corporations and governments from around the world. The Iridium concept employs a constellation of 66 satellites orbiting in six planes around the Earth. Each satellite, orbiting at an altitude of 778 kilometres, would have the capability to transmit 48 spot beams to the Earth. Meanwhile, all the satellites would be in communication with one another via 23-gigahertz radio crosslinks, thus permitting ready handoff between satellites when communicating with a fixed or mobile user on the Earth. The crosslinks would provide an uninterrupted communication path between the satellite serving a user at any particular instant and the satellite connecting the entire constellation with the gateway ground station to the PSTN. In this way the 66 satellites would provide continuous radiotelephone communication service for mobile and portable subscriber units around the globe.
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