Cellular
Radio Telephone
Cellular Telephone
Portable cellular telephones have become
an invaluable tool for people who need to stay in touch while on the move.
Cellular telephone systems combine radio and television technology with
computer systems. As a caller moves from one geographical cell (the name given
to a specific part of the area being covered by the system) to another,
computers in switching offices transfer calls among variously located antenna
transmitters without interrupting service.
Cellular Radio Telephone, also called cellular
telephone or cell phone, low-powered, lightweight radio transceiver
(combination transmitter-receiver) that provides voice telephone and other
services to mobile users. Cellular telephones primarily operate like portable
or cordless telephones. However, unlike conventional wire-based cordless
phones, cellular telephones are completely portable and do not require
proximity to a jack to access the wire-based networks operated by local
telephone companies. A new generation of services for cell phones includes
videoconferencing and Internet access with the ability to send e-mail. Cellular
telephones have become very popular with professionals and consumers as a way
to communicate while away from their regular, wire-based phones—for example,
while traveling or when in remote locations lacking regular phone service. As
cellular radio service proliferates and achieves greater market penetration,
some users have begun to consider it an alternative to conventional wire-based
services.
Cellular Radio Telephone
Students use a cellular radio telephone,
also known as a cell phone. As cell phones have grown in popularity, they have
also decreased in size.
Cellular telephones work by
transmitting radio signals to cellular towers. These towers vary in their
capability to receive cellular telephone signals. Some towers can receive
signals from distances of only 1.5 to 2.4 km (1.0 to 1.5 mi), while others can
receive signals from distances as far as 48 to 56 km (30 to 35 mi). The area a
tower can cover is referred to as a cell. However, more than one tower
may exist in a given cell area. The cells overlap so that the system can handle
increased telephone traffic volume. The towers within these cells are
networked to a central switching station, usually by wire, fiber-optic cable,
or microwave. The central switching station handling cellular calls in a given
area is directly connected to the wire-based telephone system. Cellular calls
are picked up by the towers and relayed to another cell telephone user or to a
user of the conventional wire-based telephone network. Since the cells overlap,
as a mobile caller moves from one cell into another, the towers “hand off” the
call so communication is uninterrupted.
Cellular phone networks exist in
nearly every metropolitan area throughout the world, and cellular coverage is
expanding in rural areas. Due to the convenience and mobility of cellular
telephones, users typically pay a higher fee than they would for normal
telephone use. A newer generation of cellular radio technology, called Personal
Communications Services (PCS), operates much like earlier cellular services,
but at higher frequencies, the number of times a radio wave oscillates
or completes a cycle, which is measured in a unit known as a hertz (Hz). (The
higher frequencies of PCS operate at around 1900 megahertz [MHz] in the United
States.) PCS also utilizes completely digital transmissions, rather than both
the analog and digital transmissions that many current cellular telephones use.
Digital transmissions convert sound into digital form, which can be transmitted
more efficiently than analog signals. Digital technologies can also generate
more channel capacity over the same amount of the radio spectrum.
II
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MECHANICS OF CELLULAR RADIO TELEPHONES
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Cell Phone
Tower
Cell phone towers are part of the
wireless communications network that makes cellular radio telephone calls
possible. Antennae on each tower receive high-frequency radio waves from cell
phones. Their range varies from distances as short as 1.5 to 2.4 km (1.0 to 1.5
mi) to distances as long as 48 to 56 km (30 to 35 mi). Cell phone towers are
now commonly seen along roadways as cell phone communication has become
increasingly popular.
Both cellular radio and PCS
use high-frequency radio waves to transmit calls. High-frequency waves have
short wavelengths that pass by a given point at a very high rate.
High-frequency waves can provide better sound quality and more reliable
short-distance transmission than lower-frequency waves (such as AM radio) as
they are less susceptible to sound degradation caused by the noise generated by
weather, such as lightning which causes static, and other noise generators such
as motors. However, high-frequency signals cannot effectively travel as far as
low-frequency signals can.
For cellular networks, the
limited range of high-frequency waves is actually advantageous because it means
the same frequencies can be reused at nearby locations. Cell phone calls
connect with short-range antennas known as towers. If there were only one tower
for a large area, more customers would be trying to use the same high-frequency
waves, and these waves would tend to overlap and cause interference. But
because cell phone networks establish many towers covering small areas, a
smaller number of customers access a given tower, and frequencies can be reused
when a cell phone call is handed off from one tower to another as a mobile cell
phone user travels. This ability to reuse frequencies is helpful because there
are a limited number of radio frequencies available to cell phone companies. It
also allows cellular network providers to accommodate a larger number of users.
How Wireless Communications Work
Cellular radio telephones, also known as
cell phones, communicate by sending radio signals to a cell tower. Each cell
tower has a certain range within which it can receive the radio signals. The
range of each tower overlaps with that of another tower so as a mobile cell
phone user travels, communication is uninterrupted. To communicate with the
user of a wired telephone, the cell phone radio signals are routed from the
cell tower to a mobile switching center, which in turn routes the signals to
the telephone company. The signals then travel over telephone lines to reach a
wired telephone.
The transceiver inside a
cellular phone is a much more complex device than a conventional phone used
over the wire-based network. A cellular telephone has circuitry that creates a
unique identity code that is used to locate and track the telephone. This
identity code is necessary for coordinating calls to and from the telephone,
and for billing such calls. Because a cellular telephone user may move quite a
distance during the duration of a call, the cellular radio network must manage
calls from different tower sites as the telephone moves out of the range of one
tower and into the range of another tower.
Current cellular telephones
offer such features as a memory database for storing frequently called numbers
and a lock to deter theft. Most cell phones, whether old or new, also have a
small liquid crystal screen to display the telephone number being called or the
number from which an incoming call originated. Many newer cell phones can
display a short text message, much like a pager displays this information. Some
cellular phones can also access the Internet and display text from Web sites,
such as stock quotes and news stories. Internet-capable cell phones can also
send and receive e-mail. Because mobile telephones use radio waves to send and
receive calls, the device must include a power source. Rechargeable batteries
provide the usual source of power, but most cell phones can also be attached to
the cigarette lighter in a vehicle or to some other external power device.
III
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THE CELLULAR NETWORK
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The cells in a cellular
radio network refer to the coverage area of each tower that receives and
transmits calls from mobile telephones. The cells are arranged in a honeycomb
pattern, and they overlap so that the system can handle increases in
anticipated telephone traffic volume. Network management functions, performed
by computers at a central facility known as a Mobile Telephone Switching Office
(MTSO), include the ability to measure and compare the transmission quality
between a single handset and multiple towers. This function is important so
that the MTSO can select the best transmission link between mobile telephones
and towers. This optimal link is then used to pass transmissions from one tower
to another as the mobile telephone moves between cells.
All cell towers in a given
area connect with the MTSO, which in turn has links to the wire-based local
exchange carrier that handles normal telephone calls. The link between the MTSO
and the wire-based local telephone company is essential for connecting wireless
and wire-based calls. The vast majority of calls handled by a cellular radio
network either begin on the wire-based network or end there.
IV
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HISTORY
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The first commercial cellular
telephones were tested in the late 1970s by Illinois Bell in Chicago, Illinois,
and they were a great success. Cellular service carriers began nationwide
operations in the mid-1980s operating in the 800- to 900-MHz frequency band.
Before the availability of cellular radio service, mobile telephone service
consisted of bulky mobile telephone radio units. These two-way radio units
communicated with a single antenna in a city or area. The radio signals often
interfered with those of other commercial radios, and due to the technology of
the day the frequencies could not be reused as they can today. Therefore,
limited numbers of channels were available for callers, and the service was
unreliable and costly. Because of the consumer demand for cellular
telecommunications, the Federal Communications Commission (FCC) in the
mid-1990s authorized up to six additional mobile telephone service providers in
each service area.
There were more than 120
million wireless subscribers in the United States in 2001, according to the
Cellular Telecommunications and Internet Association, with the number of new
users increasing significantly each year. Forecasters and regulators did not
anticipate this growth, however. Network equipment and start-up costs were
substantial, and the cost to consumers was high. Providers had limited their
networks to a small group of high-volume business users. Eventually, the
providers recognized that their businesses would be more profitable if they
created innovative service packages and aggressively marketed their services to
the general public. As a result, prices for cellular telephones and network
access have dropped considerably.
Cellular radio telephone service
has achieved great commercial success because users recognize that mobile
telephone access can improve productivity and enhance safety. Delivery drivers,
repair technicians, lawyers, and other professionals were early adopters of
mobile telephone service. As more geographic areas are covered by cellular
networks and as rates drop, new subscribers are buying cellular services for
personal security, safety on the road, and general convenience. See also Telecommunications;
Wireless Communications.
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