The major mobile wireless technologies can be classified according to the
method by which the medium is shared. The mobile wireless options are varied
and often confusing, partly because there are so many and partly because the
same technologies are known by different names in different parts of the world.
FDMA
In the 1970s, Bell Telephone Labs developed the first
practical wireless mobile phone system. It was an analog system known as
Advanced Mobile Phone System (AMPS). The spectrum was shared among users
through frequency division multiple access (FDMA). With FDMA, a cell is
subdivided by frequency into distinct channels, allowing multiple users to
access the cells.
TDMA
Second generation (2G) mobile wireless
used time division multiple access (TDMA), to share the bands more efficiently.
TDMA divides radio channels into time slots. Each slot consists of a fraction
of a second. TDMA uses the same frequency and band allocation as AMPS FDMA.
However, TDMA provides three to six channels in the same bandwidth as a single
AMPS channel.
CDMA
Code Division Multiple Access (CDMA) is a
form of multiplexing that allows numerous signals to occupy a single
transmission channel, optimizing the use of available bandwidth. It is a spread
spectrum technology that allows multiple users to share radio frequencies at
the same time without interfering with each other. The new 3G services are
almost all based on CDMA.
GSM
GSM is an open, nonproprietary
system that is the dominant cellular technology throughout much of the world
today. GSM uses a variation of the Time Division Multiple Access (TDMA)
protocol. Data is digitized and compressed and then sent down a channel with
two other streams of user data, which are each in their own time slot.
GPRS
General packet radio services (GPRS), based on GSM
technologies, are already extremely popular in Europe and are beginning to
emerge in the United States. With GPRS, data streams are broken up into packets
of data rather than the continuous stream of GSM circuit-switched networks.
Packetized GPRS offers always-on connectivity as opposed to GSM.
EDGE
The next stage in GSM evolution is Enhanced Data rates for
Global Evolution (EDGE), with potential data rates of up to 384 Kbps. Developed
specifically to meet the bandwidth needs of 3G, EDGE is a new modulation scheme
for the air interface that retains the basic frame structure of GSM and uses
GPRS packet data protocols.
WCDMA
The GSM 3G equivalent,
Wideband Code Division Multiple Access (WCDMA), can support mobile voice,
images, data, and video communications at much higher speeds. The input signals
are digitized and transmitted in a coded, spread-spectrum mode over a range of
frequencies.
CDMA2000
Code division multiple access 2000, 1x
Evolution-Data Optimized (CDMA2000 1xEV-DO) is a 3G wireless technology that
lets service providers deliver video and audio streaming, as well as other
multimedia services. Unlike WCDMA, it is completely separate from the legacy
circuit-switched wireless voice network.
Some of the more important
wireless technologies in the United States are summarized in Figure
. Figure
lists
the different technologies according to whether they are considered first,
second, or third generation.