FREQUENCY DIVISION MULTIPLE ACCESS

Jun 25,2007 00:00 by admin
FREQUENCY DIIVIISIION MULTIIPLE ACCESS
Three multiple access schemes are in use today, providing the
foundation for mobile communications systems (Figure F-5): Frequency Division Multiple Access (FDMA), which
serves the calls with different frequency channels
Time Division Multiple Access (TDMA), which serves the
calls with different time slots
Code Division Multiple Access (CDMA), which serves the
calls with different code sequences
All three technologies are widely used in cellular networks.
FDMAis still used on some first-generation cellular
analog networks, such as Advanced Mobile Phone Service
(AMPS)1 and TACS (Total Access Communications System).1
TDMAis used on second-generation digital cellular networks,
such as North American Digital Cellular and Global
System for Mobile (GSM) communications. CDMAis also
used on second-generation digital cellular networks, such as
PCS 1900. Both TDMAand CDMAhave been enhanced to
support emerging third-generation networks.
Of the three, FDMAis the simplest and still the most
widespread technology in use today for mobile communications.
For example, FDMAis used in the CT2 system for
Power Power
Power
Frequency Frequency Frequency
Time
Time
Time
FDMA TDMA CDMA
Figure F-5 Simple comparison of Frequency Division Multiple Access
(FDMA), Time Division Multiple Access (TDMA), and Code Division Multiple
Access (CDMA).
1There is a digital version of AMPS called D-AMPS (Digital-Advanced Mobile Phone
Service). D-AMPS adds Time Division Multiple Access (TDMA) to AMPS to get three
subchannels for each AMPS channel, tripling the number of calls that can be handled
on a channel.
FREQUENCY DIVISION MULTIPLE ACCESS 121
cordless telecommunications. The familiar cordless phone
used in the home is representative of this type of system. It
creates capacity by splitting bandwidth into radio channels
in the frequency domain. In the initial call setup, the handset
scans the available channels and locks onto an unoccupied
channel for the duration of the call.
The traditional analog cellular systems, such as those
based on AMPS, also use FDMAto derive the channel. In the
case of AMPS, the channel is a 30-kHz “slice” of spectrum.
Only one subscriber at a time is assigned to the channel. No
other conversations can access the channel until the subscriber’s
call is finished, or until the call is handed off to a
different channel in an adjacent cell.
The analog operating environment poses several problems.
One is that the wireless devices are often in motion.
Current analog technology does not deal with call handoffs
very well, as evidenced by the high incidence of dropped calls.
This environment is particularly harsh for data, which is less
tolerant of transmission problems than voice. Whereas
momentary signal fade, for instance, is a nuisance in voice
communications, it may cause a data connection to drop.
Another problem with analog systems is their limited
capacity. To increase the capacity of analog cellular systems,
the 30-kHz channel can be divided into three narrower channels
of 10 kHz each. This is the basis of the narrowband
AMPS (N-AMPS) standard. However, this band-splitting
technique incurs significant base station costs, and its limited
growth potential makes it suitable only as a short-term
solution.
While cell subdivision often is used to increase capacity,
this solution has its limits. Since adjacent cells cannot use
the same frequencies without risking interference, a limited
number of frequencies are being reused at closer distances,
which makes it increasingly difficult to maintain the quality
of communications. Subdividing cells also increases the
amount of overhead signaling that must be used to set up
and manage the calls, which can overburden switch resources. In addition, property or rights of way for cell sites
are difficult to obtain in metropolitan areas where traffic volume
is highest and future substantial growth is anticipated.
These and other limitations of analog FM radio technology
have led to the development of second-generation cellular
systems based on digital radio technology and advanced
networking principles. Providing reliable service in this
dynamic environment requires digital radio systems that
employ advanced signal processing technologies for modulation,
error correction, and diversity. These capabilities are
provided by TDMAand CDMA.
Summary
FM systems have supported cellular service for nearly 20
years, during which demand has finally caught up with the
available capacity. Now first-generation cellular systems
based on analog FM radio technology are rapidly being
phased out in favor of digital systems that offer higher
capacity, better voice quality, and advanced call handling
features. TDMA- and CDMA-based systems are contending
for acceptance among analog cellular carriers worldwide