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Guizani WL040/Bidgolio-Vol I WL040-Sample.cls July 16, 2003 10:7 Char Count= 0
822 WIRELESSCOMMUNICATIONSAPPLICATIONSD-AMPS (Digital Advanced Mobile
Phone System)
D-AMPS (digital advanced mobile phone system) is de-
signed to be compatible with older analog AMPS tech-
nology. It uses the 30-MHz frequency channel as AMPS,
but divides each one up into three TDMA slots. D-AMPS
uses the same-paired spectrum and structure as AMPS.
The difference is that instead of sending a single FM ra-
dio transmission over a 30-MHz channel, it allows each
one to be used by three simultaneous conversions. Oper-
ators of AMPS networks can selectively allocate channels
to be either digital or analog, allowing the two systems
to coexist. The modulation scheme is called differential
quadrature phase shift keying (DQPSK).PDC/JDC (Personal Digital Cellular/Japanese
Digital Cellular)
Japan has deployed a system based on D-AMPS, but de-
signed for backward compatibility with its own J-TACS
analog system. Despite being used in only one country,
it is the world’s second most popular mobile standard
(behind that of GSM).
J-TACS used channels of only 25 MHz, which means
that some changes were needed to the D-AMPS system
designed for 30 MHz. The same size time slots and modu-
lation are used, resulting in a lower overall bit rate. How-
ever, the D-AMPS voice codecs and data rate of 9.6 Kbps
can still be achieved, by missing out some of the error cor-
rection and protocol overhead. This makes the system less
reliable, but this problem can be overcome by keeping the
cells small so that every user has a relatively clear link to
the base stations.D-AMPS+
D-AMPS+ is a scaled down version of GPRS that is im-
plemented in the United States. When ETSI designed
GPRS, they hoped that it could be applied to every TDMA-
based system, including D-AMPS and PDC. The principle
is simple—use more slots in the multiplex to increase the
capacity until a single user has the entire channel all the
time. GSM has eight time slots, while D-AMPS has only
three, and each of these already offers a lower data rate
of 9.6 Kbps compared to GSM’s 14.4 Kbps.
One problem is that to achieve a faster rate, terminals
need to be able to transmit and receive at the same time.
A standard D-AMPS phone transmits for one third of the
time, receives for another third, and is idle for the remain-
ing third. Without simultaneous transmission and receiv-
ing, there is only one spare time slot (compared to six in
GSM), limiting speed to only 19.2 Kbps in one direction
and 9.6 Kbps in the other.CdmaOne
CdmaOne is the only 20th century system to use CDMA.
It was developed by Qualcomm, and it has been standard-
ized by the Telecommunications Industry Association as
IS-95a. CDMA systems seem superficially simpler than
those based on TDMA. They involve no slot of frame struc-
ture. Every phone just transmits and receives all the time,sending many duplicates of the same information to en-
sure that at least one gets through. The very high transmis-
sion rate is achieved by using two different phase modu-
lation techniques, quadrature phase shift keying (QPSK)
on the downlink and offset quadrature phase shift keying
(OQPSK) on the uplink. The second type required more
forward error correction, because individual phones can-
not coordinate their transmission in the same way that
base stations can.
One large disadvantage of CDMA systems is their
power consumption. By transmitting everything 64 times,
a CdmaOne phone would seem to drain its battery 64
times faster than necessary and cover its user with
64 times as much microware radiation. CdmaOne gets
around this problem by carefully controlling the transmis-
sion power. The aim is to ensure that the signal strength
at the base station is the same for every user, ensuring that
all can be heard equally.CdmaTwo
Phones based on CdmaOne already transmit and receive
simultaneously and at very high data rates. Most of these
data are redundant, but it doesn’t have to be. A phone
could use more than one of the available Walsh codes,
multiplying its 16 Kbps capacity by any factor up to the
number of calls per channel.THIRD-GENERATION STANDARDS
Third generation systems are critical to the wireless Inter-
net services often touted as the future of mobile commu-
nications. They will offer permanent access to the Web,
interactive video, and voice quality that sounds more like
a CD player than cellular phone. The term 3G was orig-
inally defined as any standard that provide mobile users
with the performance of ISDN or better, at least 144 Kbps.
Technologically, the increased capacity is found in part by
using extra spectrum and in part by new modulation tech-
niques that squeeze higher data rates from a given wave-
band. Vendors, operator, and regulators all accept that
the move toward higher data rates and better services will
be evolutionary. Standards must be backward compati-
ble with their predecessors so that phones can maintain a
connection while moving between cells based on the old
and the new.IMT-2000
The third generation system was first planned in 1992,
when the ITU realized that mobile communications was
playing an increasingly important role. An international
study group predicted that mobile phones would rival
fixed lines within 10 years. It began work on a project
called FPLMTS (Future Public Land Mobile Telecommu-
nications System), aiming to unite the world under a sin-
gle standard. The acronym was awkward, so ITU adopted
the name IMT-2000. The number 2000 represents the year
2000, which the ITU wanted to make globally available for
the new technology. The number 2000 also means the data
rate of 2000 Kbps and the frequencies in the 2000-MHz
region.
The ITU’s original definition of IMT-2000 concerned
only the data rate. Three different rates were suggested,