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Unit 7: Wireless Networks

The system is scalable, even though it has a finite number of channels. If channel demand Notes
increases in a specific area (such as a metro area), the service provider divides cells into a
number of smaller cells. Transmitter power is turned down to fit the new smaller cell size and
channel frequencies are allocated so that no adjoining cells use the same channels. However,
channel reuse is possible in cells that are at least one cell apart. Thus, frequency reuse and
smaller cell size allow the system to scale. Metro areas may have many small cells while rural
area may have large cells. The cell size is designed to accommodate the number of people in the
area.
When a user turns a phone on, its phone number and serial number are broadcast within the
local cell. The base station picks up these signals and informs the switching office that the
particular device is located within its area. This information is recorded by the switching office
for future reference. An actual call takes place when the user enters a phone number and hits the
Send button. The cellular system selects a channel for the user to use during the duration of the
call.
As users travel, they may move from one cell to another, necessitating a handoff and the selection
of a new channel. While in the vicinity of a cell, mobile phone users are under the control of the
transmitter/receiver in that cell. A handoff takes place when the base station in one cell transfers
control for a user’s call to a base station in another cell. When a base station begins to lose a
user’s signal, it notifies base stations in all the surrounding cells that the user may be moving
into their cells. As the user moves into a new cell, the base station in that cell takes over the call.

Notes The frequency of the call is changed to a frequency used in the new cell during the
transition. This is because adjoining cells cannot use the same frequencies.

7.2.1 From Analog to Digital Systems

Mobile wireless analog communication systems have been around since the 1950s. The early
systems were single channel “over-and-out” systems. Instead of a cellular configuration, a
single radio tower serviced a metropolitan area, which severely limited the scalability of the
systems. Service quality varied depending on the location of the caller. Later systems added
multiple two-way channels but still had limited capacity.

Analog cellular services were introduced by AT&T in the 1970s and became widespread in the
1980s. The primary analog service in the United States is called AMPS (Advanced Mobile Phone
Service). There are similar systems around the world that go by different names. The equivalent
system in England is called TACS (Total Access Communications System).
The AMPS system is a circuit-oriented communication system that operates in the 824-MHz to
894-MHz frequency range. This range is divided into a pool of 832 full-duplex channel pairs (1
send, 1 receive). Any one of these channels may be assigned to a user. A channel is like physical
circuit, except that it occupies a specific radiofrequency range and has a bandwidth of 30 kHz.
The circuit remains dedicated to a subscriber call until it is disconnected, even if voice or data is
not being transmitted.
Cellular systems are described in multiple generations, with third- and fourth- generation (3G
and 4G) systems just emerging:
1G systems These are the analog systems such as AMPS that grew rapidly in the 1980s and
are still available today. Many metropolitan areas have a mix of 1G and 2G systems, as
well as emerging 3G systems. The systems use frequency division multiplexing to divide
the bandwidth into specific frequencies that are assigned to individual calls.

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