Digital wireless and cellular date back to the 1940s when commercial mobile
telephony began. The wireless revolution began after low cost microprocessors
and digital switching became available, and the regulatory climate changed to
allow less control of radio transmission equipment
.
Cellular Radio
Cellular radio provides mobile telephone service
by employing a network of cell sites distributed over a wide area. A cell site
contains a radio transceiver and a base station controller. The base station
manages, transmits, and receives traffic from the mobile radios in its
geographical area. A cell site also has a tower and antennas, as well as a link
to a distant switch, which is called a mobile telecommunications switching
office (MTSO). The MTSO connects calls from land based telephones to wireless
customers, switches calls between cells as mobiles travel across cell
boundaries, and authenticates wireless customers before they make calls.
Cellular networks use a principle called frequency reuse to greatly increase
the number of customers served. Frequency reuse is shown in Figure
. Low
powered mobile radios and the radio equipment at each cell site permit the same
radio frequencies to be reused in different, noncontiguous cells, thus
multiplying calling capacity without creating interference. This
spectrum-efficient method sharply contrasts with earlier mobile systems that
used a high-powered, centrally located transmitter to communicate with
high-powered car mounted mobiles on a small number of frequencies. Channels
were used and could not be re-used over a wide area.
Complex signaling routines handle call placements, call requests, and call
transfers from one cell to another. These routines are referred to as handovers
and roaming, and occur when a user moves from one carrier area to another
carrier area. Different cellular radio systems use frequency division, multiple
access (FDMA) analog, time division multiple access (TDMA), and spread spectrum
code division multiple access (CDMA) techniques.
WLAN design is similar
to that of cellular technologies. Instead of having one large, centralized,
high-powered access point or bridge, WLANs favor the cellular model of using
multiple low-powered base stations to maximize coverage, redundancy, and
bandwidth capabilities.
Third Generation (3G)
Wireless 3G
systems provide access to a wide range of telecommunication services supported
by the fixed telecommunication networks, and to other services that are
specific to mobile users. A range of wireless terminal types is encompassed,
linking users to terrestrial or satellite-based networks. The terminals may be
designed for mobile or fixed use.
3G systems have several key design
features:
- A high degree of commonality of design worldwide
- Compatibility of services worldwide
- Use of small pocket terminals with worldwide roaming capability
- Access to the Internet and other multimedia applications
- A wide range of services and terminals
According to the International Telecommunication Union (ITU)
International Mobile Telecommunications 2000 Initiative (IMT-2000), 3G system
services were scheduled to be initiated around the year 2000, depending on
market considerations. It was later predicted that 3G services would be offered
in 2001. However, 3G services did not begin to appear until 2002 and most
industry observers say that these services are not true 3G services because
they do not achieve 3G data rates.
Figure
describes some
of the key service attributes and capabilities expected of 3G systems.