Transcript
CDMA vs GSM
Submitted in partial fulfillment of the requirements
for the award of the degree ofMaster of Computer Application
(2006-2009)
Guided By: Submitted by:
Shalini Bhartiya Varun TaliyanLecturer(IT) Roll No.: 0331594406
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RUKMINI DEVI INSTITUTE OF ADVANCED STUDIES
(Aff. to Guru Gobind Singh Indraprastha University)
CERTIFICATE
This is to certify that the dissertation (MCA-331) entitled GSM vs
CDMA done by Mr. Varun Taliyan, Roll No. 0331594406 is an authentic
work carried out by him at RDIAS under my guidance. The matter
embodied in this project work has not been submitted earlier for the
award of any degree or diploma to the best of my knowledge and
belief.
Date:
Signature of the Guide
Shalini Bhartiya
Lecturer(IT)
RDIAS
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ACKNOWLEDGEMENT
Study of process Models Project would not have successfully achieved without the
guidance of my esteemed teacher. The institute and teacher made learning very
interesting by monitoring and suggesting the right approach.
So I hereby thank my respected teacher Ms Shalini Bhartiya (Lecturer) who have given
their precious time to help me better comprehend various essential things related to my
project. I also thank all the support staff of the institute for rendering me with their
valuable help and knowledge for completing this project.
(Varun Taliyan)
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ABSTRACT
This project is about the Mobile Technologies. Mobile phones or Cellular phones are one
of the most widely used devices in today’s era. At present there are two technologies
which are in use- GSM and CDMA. GSM stands for Global System for Mobile
Communication and CDMA stands for Code Division Multiple Access.
In this project I have explained what GSM and CDMA Technologies are all about
and draw a comparison between the two technologies, explaining the similarities an
differences between them. I have also explained the technical details of the two
technologies, their history, current scenario and try to figure out that what will be the
future of mobile technologies.
For gathering the information about this project, I have used Internet as a main
source but I have also be searched books from our library, magazines, journals and
newspapers and taken relevant information from there.
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List of Figures:
Figure 1. Cellular Subscriber Growth Worldwide 12
Figure 2. GSM Network Architecture 27
Figure 3. SIM card 29
Figure 4. MSC/VLR Service Areas 37
Figure 5. PLMN Network Areas 38
Figure 6. FDMA 56
Figure 7. TDMA 57
Figure 8. Spread Spectrum 58
Figure 9. Processing Gain 59
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List of Tables:
Table 1.Worldwide development of mobile telephone systems. 13
Table 2. Frequencies of GSM 31
Table 3. Number of GSM connections 40
Table 4. Comparative Study of GSM and CDMA 71
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Table of Contents
Introduction……………………………………………………………………………9
Objective……………………………………………………………………………..11
1. Evolution of Mobile Systems…………………………………………………….12
Evolution of GSM……………………………………………………………14
2. Services Provided by GSM………………………………………………………20
2.1Teleservice……………………………………………………………………20
2.2Supplementry Service………………………………………………………...22
2.3Newer GSM Service………………………………………………………….23
3. Architecture of GSM network…………………………………………………….26
3.1Mobile Station……………………………………………………………..….28
3.2Base Station Subsystem……………………………………………….……...29
3.3Network Subsystem…………………………………………………….…….30
4. Radio Aspects of GSM…………………………………………………………....31
4.1Multiple Access channel structure………………………………………..…..32
5. The GSM Network…………………………………………………………….….34
5.1Switching System………………………………………………………….…34
5.2Base Station Subsystem…................................................................................35
6. Evolution of CDMA………………………………………………………………42
7. CDMA overview…………………………………………………………………..47
7.1What is CDMA………………………………………………………………..48
7.2CDMA concepts……………………………………………………………….49
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8.CDMA Technique………………………………………………………………….56
8.1Multiple Access……………………………………………………………….56
8.2Spread Spectrum…………………………………………………………...…58
9.Differece between CDMA and GSM………………………………………………67
10.Comparitive study………………………………………………………………...71
11.Advantages and Disadvantages of CDMA and GSM…………………………….72
12.Conclusion………………………………………………………………………..74
13.Future Scope……………………………………………………………………...75
14.Biblography……………………………………………………………………….76
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Introduction
There are two main Mobile Technologies which are used at present throughout the world.
They are GSM and CDMA. GSM stands for Global System for Mobile Communications
and CDMA stands for Code Division Multiple Access.
Today's GSM Platform
GSM (Global System for Mobile communications) is the technology that underpins most
of the world's mobile phone networks. The GSM platform is a hugely successful wireless
technology and an unprecedented story of global achievement and cooperation. GSM has
become the world's fastest growing communications technology of all time and the
leading global mobile standard, spanning 218 countries.
Today, GSM technology is in use by more than one in three of the world's
population - by June 2008 there were over 2 billion GSM subscribers, representing
approximately 86% of the world's cellular market. The growth of GSM continues
unabated with almost 400 million new customers in the last 12 months.
GSM uses a variation of time division multiple access (TDMA) and is the most
widely used of the three digital wireless telephony technologies (TDMA, GSM, and
CDMA). GSM digitizes and compresses data, then sends it down a channel with two
other streams of user data, each in its own time slot. It operates at either the 900 MHz or
1800 MHz frequency band.
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Code division multiple access
Code division multiple access (CDMA) is a channel access method utilized by various
radio communication technologies. It should not be confused with the mobile phone
standards called cdmaOne and CDMA2000 (which are often referred to as simply
("CDMA"), that use CDMA as their underlying channel access methods.
One of the basic concepts in data communication is the idea of allowing several
transmitters to send information simultaneously over a single communication channel.
This allows several users to share a bandwidth of frequencies. This concept is called
multiplexing. CDMA employs spread-spectrum technology and a special coding scheme
(where each transmitter is assigned a code) to allow multiple users to be multiplexed over
the same physical channel. By contrast, time division multiple access (TDMA) divides
access by time, while frequency-division multiple access (FDMA) divides it by
frequency. CDMA is a form of "spread-spectrum" signaling, since the modulated coded
signal has a much higher data bandwidth than the data being communicated.
An analogy to the problem of multiple access is a room (channel) in which people
wish to communicate with each other. To avoid confusion, people could take turns
speaking (time division), speak at different pitches (frequency division), or speak in
different directions (spatial division). In CDMA, they would speak different languages.
People speaking the same language can understand each other, but not other people.
Similarly, in radio CDMA, each group of users is given a shared code. Many codes
occupy the same channel, but only users associated with a particular code can understand
each other.
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Objective
The Aim of doing this project is to find out the answers to the following Questions:
What are the major Technologies used for Mobile Communication?
What is GSM and CDMA Technologies all about?
How and When are they started?
Comparison of GSM and CDMA Technologies i.e. Similarities and differences.
What is the current scenario?
What will be the future trends in these technologies?
Future of Mobile Industry?
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1.The Evolution of Mobile Telephone Systems
Cellular is one of the fastest growing and most demanding telecommunications
applications. Today, it represents a continuously increasing percentage of all new
telephone subscriptions around the world. Currently there are more than 45 million
cellular subscribers worldwide, and nearly 50 percent of those subscribers are located in
the United States. It is forecasted that cellular systems using a digital technology will
become the universal method of telecommunications. By the year 2005, forecasters
predict that there will be more than 100 million cellular subscribers worldwide. It has
even been estimated that some countries may have more mobile phones than fixed
phones by the year 2000 (see Figure 1).
Figure 1. Cellular Subscriber Growth Worldwide
The concept of cellular service is the use of low-power transmitters where frequencies
can be reused within a geographic area. The idea of cell-based mobile radio service was
formulated in the United States at Bell Labs in the early 1970s. However, the Nordic
countries were the first to introduce cellular services for commercial use with the
introduction of the Nordic Mobile Telephone (NMT) in 1981.
Cellular systems began in the United States with the release of the advanced mobile
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phone service (AMPS) system in 1983. The AMPS standard was adopted by Asia, Latin
America, and Oceanic countries, creating the largest potential market in the world for
cellular. In the early 1980s, most mobile telephone systems were analog rather than
digital, like today's newer systems. One challenge facing analog systems was the inability
to handle the growing capacity needs in a cost-efficient manner. As a result, digital
technology was welcomed. The advantages of digital systems over analog systems
include ease of signaling, lower levels of interference, integration of transmission and
switching, and increased ability to meet capacity demands. Table 1 charts the worldwide
development of mobile telephone systems.
Year Mobile System
1981 Nordic Mobile Telephone (NMT) 450
1983 American Mobile Phone System (AMPS)
1985 Total Access Communication System (TACS)
1986 Nordic Mobile Telephony (NMT) 900
1991 American Digital Cellular (ADC)
1991 Global System for Mobile Communication (GSM)
1992 Digital Cellular System (DCS) 1800
1994 Personal Digital Cellular (PDC)
1995 PCS 1900—Canada
1996 PCS—United States
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1.1Evolution of GSM
More than 700 GSM mobile networks have been established in Europe, the North
America, South America, Iceland, Asia, Africa and Australasia up untill now, woven
together by international roaming agreements and a common bond called the
"Memorandum of Understanding" (MoU which defines the GSM standards and the
different phases of its world-wide implementation.
1982 - The Beginning
Nordic Telecom and Netherlands PTT propose to CEPT (Conference of European
Post and Telecommunications) the development of a new digital cellular standard
that would cope with the ever a burgeoning demands on European mobile
networks.
The European Commission (EC) issues a directive which requires member states
to reserve frequencies in the 900 MHz band for GSM to allow for roaming.
1986
Main GSM radio transmission techniques are chosen
1987
September - 13 operators and administrators from 12 areas in the CEPT GSM
advisory group sign the charter GSM (Groupe Spéciale Mobile) MoU "Club"
agreement, with a launch date of 1 July 1991.
The original French name was later changed to Global System for Mobile
Communications, but the original GSM acronym stuck.
GSM spec drafted.
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1989
The European Telecommunications Standards Institute (ETSI) defined GSM as
the internationally accepted digital cellular telephony standard
GSM becomes an ETSI technical committee
1990
Phase 1 GSM 900 specifications are frozen
DCS adaptation starts
Validation systems implemented
First GSM World congress in Rome with 650 Participants
1991
First GSM spec demonstrated
DCS specifications are frozen
GSM World Congress Nice has 690 Participants
1992
January - First GSM network operator is Oy Radiolinja Ab in Finland
December 1992 - 13 networks on air in 7 areas
GSM World Congress Berlin - 630 Participants
1993
GSM demonstrated for the first time in Africa at Telkom '93 in Cape Town
Roaming agreements between several operators established
December 1993 - 32 networks on air in 18 areas
GSM World Congress Lisbon with 760 Participants
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Telkom '93 held in Cape Town. First GSM systems shown.
1994
First GSM networks in Africa launched in South Africa
Phase 2 data/fax bearer services launched
Vodacom becomes first GSM network in the world to implement data/fax
GSM World Congress Athens with 780 Participants
December 1994 - 69 networks on air in 43 areas
1995
GSM MoU is formally registered as an Association registered in Switzerland -
156 members from 86 areas.
GSM World Congress Madrid with 1400 Participants
December 1995 117 networks on air in 69 areas
Fax, data and SMS roaming started
GSM phase 2 standardization is completed, including adaptation for PCS 1900
(PCS)
First PCS 1900 network live 'on air' in the USA
Telecom '95 Geneva - Nokia shows 33.6 kbps multimedia data via GSM
Namibia goes on-line
Ericsson 337 wins GSM phone of the year
US FCC auctions off PCS licenses
1996
GSM MoU is formally registered as an Association registered in Switzerland
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December 1996 120 networks on air in 84 areas
GSM World Congress in Cannes
GSM MoU Plenary held in Atlanta GA, USA
8K SIM launched
Pre-Paid GSM SIM Cards launched
Bundled billing introduced in South Africa
Libya goes on-line
Option International launches world's first GSM/Fixed-line modem
1997
Zimbabwe goes live
GSM World Congress Cannes 21/2/97
Mozambique goes live
Iridium birds launched
First dual-band GSM 900-1900 phone launched by Bosch
1998
Botswana GSM goes live
GSM World Congress Cannes (2/98)
Vodacom Introduces Free VoiceMail
MTN Gets Uganda Tender
GSM SIM Cracked in USA
Over 2m GSM 1900 users
MTN Gets Rwanda Tender
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MTN follows with free voicemail
Rwanda GSM Live
First HSCSD trials in Singapore
Vodacom launches Yebo!Net 10/98
Iridium Live 11/98
First GSM Africa Conference (11/98)
125m GSM 900/1800/1900 users worldwide (12/98)
Option International launches FirstFone
MTN launches CarryOver minutes
1999
GSM Conference in Cannes 2/99
165m GSM 900/1800/1900 users worldwide
GPRS trials begin and USA and Scandanavia 1/99
WAP trials in France and Italy 1/99
CellExpo Africa 5/99
Eight Bidders for Third SA Cell License
GSM MoU Joins 3GPP
MTN SA Head of GSM MoU
First GPRS networks go live
Bluetooth specification v1.0 released
2000
GSM Conference in Cannes 3/2000
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By 12/2000 480m GSM 900/1800/1900 users worldwide
First GPRS networks roll out
Mobey Forum Launched
MeT Forum Launched
Location Interoperability Forum Launched
First GPRS terminals seen
Nokia releases SmartMessaging spec
SyncML spec released
2001
GSM Conference in Cannes 2/2001
By 5/2001 500m GSM 900/1800/1900 users worldwide
16 billion SMS message sent in April 2001
500 million people are GSM users (4/01)
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2. Services Provided by GSM:
Telecommunication services can be divided into Bearer Services, Teleservices,
and Supplementary Services. Call diversion, caller identification, encrypted speech, fax
and error protected data are a few examples of current and new services provided by the
GSM.
Supplementary services are provided on top of teleservices or bearer services, and
include features such as caller identification, call forwarding, call waiting, multiparty
conversations, and barring of outgoing (international) calls, among others.
2.1 Teleservices:
A Teleservice utilises the capabilities of a Bearer Service to transport data, defining
which capabilities are required and how they should be set up.
The most basic Teleservice supported by GSM is telephony. There is an
emergency service, where the nearest emergency service provider is notified by dialing
three digits (similar to 911). The Telephony Teleservice and Emergency Teleservice
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cover normal speech calls. These are both the fundamental services for making ordinary
telephone calls, but they are separated because of a special need for Emergency calls.
When a call is made from a GSM Mobile Station, the type of service requested is
indicated in the set-up message. This means that the GSM operator has the option to treat
emergency calls differently by allowing mobile equipment without a SIM card to make
them.
The ISDN, on which GSM is based, has a great deal of potential for other
information and data services. These are the videotext, teletex, and electronic mail
services. The Videotex, Teletex and Advanced Message Handling Teleservices provide
these for in GSM. The last of these covers the electronic mail requirements.
This Advanced Message Handling Teleservice (or the Electronic Mail
Teleservice) is designed to allow quite long messages. GSM has one more Teleservice
that is designed for short, paging type messages. This Teleservice, called Short Message
Service (SMS), is by far the most widely used and flexible. The SMS Teleservice was
originally defined to utilise some spare signalling capacity in GSM. However, it soon
became apparent that SMS would become a key service in differentiating GSM from any
other cellular service. SMS is effectively an international paging service, overlaid on top
of the GSM network, with the capability to send, as well as receive, messages.
SMS is a bidirectional service for sending short alphanumeric (up to 160 bytes)
messages in a store and forward fashion. For point to point SMS, a message can be sent
to another subscriber to the service, and an acknowledgement of receipt is provided to the
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sender. SMS can also be used in a cell broadcast mode, for sending messages such as
traffic updates or news updates. Messages can be stored in the SIM card for later
retrieval.
2.2 Supplementary Services:
The supplementary services basically consist of call forwarding and call barring.
Call Forwarding:
The Call Forwarding Supplementary Service is used to divert calls from the
original recipient to another number, and is normally set up by the subscriber himself. It
can be used by the subscriber to divert calls from the Mobile Station when the subscriber
is not available, and so to ensure that calls are not lost. A typical scenario would be a
salesperson turns off his mobile phone during a meeting with customers, but does not
with to lose potential sales leads while he is unavailable.
Call Barring:
The concept of barring certain types of calls might seem to be a supplementary
disservice rather than service. However, there are times when the subscriber is not the
actual user of the Mobile Station, and as a consequence may wish to limit its
functionality, so as to limit the charges incurred. Alternatively, if the subscriber and user
are one and the same, the Call Barring may be useful to stop calls being routed to
international destinations when they are routed. The reason for this is because it is
expected that the roaming subscriber will pay the charges incurred for international re-
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routing of calls. So, GSM devised some flexible services that enable the subscriber to
conditionally bar calls.
2.3 Newer GSM Services:
The newer GSM services were not all generally available by the GSM operators at the
time of writing and comprise:
Number Identification:
Calling Line Identification Presentation: This service deals with the
presentation of the calling party's telephone number. The concept is for this
number to be presented, at the start of the phone ringing, so that the called
person can determine who is ringing prior to answering. The person
subscribing to the service receives the telephone number of the calling party.
Calling Line Identification Restriction: A person not wishing their number
to be presented to others subscribes to this service. In the normal course of
event, the restriction service overrides the presentation service.
Connected Line Identification Presentation: This service is provided to
give the calling party the telephone number of the person to whom they are
connected. This may seem strange since the person making the call should
know the number they dialled, but there are situations (such as forwardings)
where the number connected is not the number dialled. The person
subscribing to the service is the calling party.
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Connected Line Identification Restriction: There are times when the person
called does not wish to have their number presented and so they would
subscribe to this person. Normally, this overrides the presentation service.
Malicious Call Identification: The malicious call identification service was
provided to combat the spread of obscene or annoying calls. The victim
should subscribe to this service, and then they could cause known malicious
calls to be identified in the GSM network, using a simple command. This
identified number could then be passed to the appropriate authority for action.
The definition for this service is not stable.
Multi-Party:
Multi-Party Service: This service is similar to a conference type service, in
that several calls may be connected with all parties talking to each other.
However, there are enough differences, caused by its application in the mobile
environment, for it to be known by a different name.
Communication of Interest:
Closer User Group: This service is provided on GSM to enable groups of
subscribers to only call each other. In this way, intrusions can be limited only to
those members who wish to talk to each other.
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Charging :
Advice of Charge: This service was designed to give the subscriber an indication of the
cost of the services as they are used. Furthermore, those Service Providers who wish to
offer rental services to subscribers without their own Subscriber Identity Module (SIM)
can also utilize this service in a slightly different form.
Additional Information Transfer:
User-to-User Signalling: This service allows the subscriber to send and
receive information to and from the person with whom they have an active
call. The amount of information is limited, but may include text (such as
names and addresses), and numbers (such as telephone numbers).
Call Offering:
Call Transfer: The call transfer service allows the subscriber to transfer or
forward a call to another party. This party can be either another GSM Mobile
Station or indeed, a person on a different network. One of the difficulties with
this service is the billing ramifications. If A calls B, and B asks to be
transferred to C, then it is not clear who should be charged for the rest of the
call (A, who initiated the call but is no longer a participant, or B, who asked
for the call transfer. To charge B is technically difficult.)
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3. Architecture of the GSM Network:
The GSM mobile telephony service is based on a series of contiguous radio cells
which provide complete coverage of the service area and allow the subscriber operation
anywhere within it. Prior to this cellular concept, radiophones were limited to just the one
transmitter covering the whole service area. Cellular telephony differs from the
radiophone service because instead of one large transmitter, many small ones are used to
cover the same area. The basic problem is to handle the situation where a person using
the phone in one cell moves out of range of that cell. In the radiophone service there was
no solution and the call was lost, which is why the service area was so large. In cellular
telephony, handing the call over to the next cell solves the problem. This process is
totally automatic and requires no special intervention by the user, but it is a complex
technical function requiring significant processing power to achieve a quick reaction.
The functional architecture of a GSM system can be broadly divided into the
Mobile Station, the Base Station Subsystem, and the Network Subsystem. Each
subsystem is comprised of functional entities that communicate through the various
interfaces using specified protocols. The subscriber carries the mobile station; the base
station subsystem controls the radio link with the Mobile Station. The network
subsystem, which is the main part of which is the Mobile services Switching Center,
performs the switching of calls between the mobile and other fixed or mobile network
users, as well as management of mobile services, such as authentication.
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Fig 2: GSM Network Architecture.
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o Mobile Station:
The Mobile Station (MS) represents the only equipment the GSM user ever sees
from the whole system. It actually consists of two distinct entities. The actual hardware is
the Mobile Equipment (ME), which is anonymous and consists of the physical
equipment, such as the radio transceiver, display and digital signal processors. The
subscriber information is stored in the Subscriber Identity Module (SIM), implemented as
a Smart Card. The mobile equipment is uniquely identified by the International Mobile
Equipment Identity (IMEI). The SIM card contains the International Mobile Subscriber
Identity (IMSI), identifying the subscriber, a secret key for authentication, and other user
information. The IMEI and the IMSI are independent, thereby providing personal
mobility.
Thus the SIM provides personal mobility, so that the user can have access to all
subscribed services irrespective of both the location of the terminal and the use of a
specific terminal. By inserting the SIM card into another GSM cellular phone, the user is
able to receive calls at that phone, make calls from that phone, or receive other
subscribed services. The SIM card may be protected against unauthorized use by a
password or personal identity number.
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Figure 3. SIM card
SIM.
The ME provides generic radio and processing functions to
access the network through the radio interface as well as an
interface to the user (microphone loudspeaker, display and
keyboard) together with an interface to some other terminal
equipment (fax machine, PC).
The SIM contain all the subscriber-related information stored on
the user's side of the radio interface.
The MS is operational only when a valid SIM is placed in a ME.
o Base Station Subsystem:
The Base Station Subsystem is composed of two parts, the Base Transceiver
Station (BTS) and the Base Station Controller (BSC). The BTS houses the radio
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transceivers that define a cell and transmits and receives signals on the cells' allocated
frequencies with the mobile station.
A BSC operates with a group of BTSs and manages the radio resources for one or
more of them. The BSC is the connection between the MS and the Network Subsystem. It
manages the radio channel (setup, tear down, frequency hopping, etc.) as well as
handovers and the transmission power levels and frequency translations of the voice
channel used over the radio link to the standard channel used by the Public Switched
Telephone Network or ISDN.
o Network Subsystem:
The central component of the Network Subsystem is the Mobile services
Switching Center (MSC). It acts like a normal switching node of the normal telephones of
the land lines and in addition provides all the functionality needed to handle a mobile
subscriber, including registration, authentication, location updating and inter-MSC
handovers. These services are provided in conjunction with several functional entities,
which together form the Network Subsystem. The MSC provides the connection to the
public fixed network (PSTN or ISDN) and is the interface between the GSM and the
PSTN networks for both telephony and data.
Thus the MSC is primarily responsible for:
Traffic management
Call set-up
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Call Routing to a roaming subscriber
Termination
Charging and accounting information
4. Radio Aspects of GSM
The International Telecommunication Union (ITU), which manages the international
allocation of radio spectrum (among many other functions), allocated the bands 890-915
MHz for the uplink (mobile station to base station) and 935-960 MHz for the downlink
(base station to mobile station) for mobile networks in Europe. Since this range was
already being used in the early 1980s by the analog systems of the day, the CEPT had the
foresight to reserve the top 10 MHz of each band for the GSM network that was still
being developed. Eventually, GSM will be allocated the entire 2x25 MHz bandwidth.
American Cellular
AMPS, N-AMPS, D-AMPS (IS-136) CDMA
824-849 MHz 869-894 MHz
Mobile to base Base to mobile
American PCS/GSM
Narrowband 901-941 MHz
Broadband1850-1910MHz 1930-1990 MHz
Mobile to base Base to mobile
E-TACS
872-905 MHz 917-950 MHz
Mobile to base Base to mobile
GSM
GSM has three main frequency bands around the world: 900 MHz, 1800 MHz, and 1900 MHz. It all depends on
935-960MHz 890-915MHz
1800MHz
1900 MHz.
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the country. Other bands may be used in the
Table 2. Frequencies of GSM
Multiple access and channel structure.
Since radio spectrum is a limited resource shared by all users, a method must be devised
to divide up the bandwidth among as many users as possible. The method chosen by
GSM is a combination of Time- and Frequency-Division Multiple Access
(TDMA/FDMA). The FDMA part involves the division by frequency of the (maximum)
25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart. One or more
carrier frequencies are assigned to each base station. Each of these carrier frequencies is
then divided in time, using a TDMA scheme. The fundamental unit of time in this TDMA
scheme is called a burst period and it lasts 15/26 ms (or approx. 0.577 ms). Eight burst
periods are grouped into a TDMA frame (120/26 ms, or approx. 4.615 ms), which forms
the basic unit for the definition of logical channels. One physical channel is one burst
period per TDMA frame.
This is the correct, complete view of GSM. It's not enough to say, as I have too many
times, that GSM and conventional cellular (IS-136) are TDMA based. While that it is
true, it is more true to say such systems are TDMA and FDM based. First, we have a
number of radio frequencies, each separated by 200khz. This is the frequency division
multiplexing part. (Or the FDMA part, a minor semantic difference.) Secondly, we have
the transmission technology, TDMA, by which we put several calls on a single
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frequency. These calls are broken into many pieces, each piece of each call sent one after
another. Each call separated by slight differences in time. GSM is a TDMA/FDMA
system.
Weick calls a burst "a sequence of signals counted as a unit in accordance with some
specific criterion or measure." Bits are single pulses of electrical energy. Much like the
single dash of a Morse Code key. With Morse code we use long and short pulses of
energy to stand for letters. Although of uniform length, the pulses we use in digital radio
do the same thing. Bits grouped in patterns represent voice and data. We also use bits, as
shown in the diagram below, for signaling. In the channel depicted a burst of bits is a
marker, an indicator, a signal within a signal. It's what the mobile first looks for in the
digital stream flowing from the base station. More on this on the next page.
Channels are defined by the number and position of their corresponding burst periods. All
these definitions are cyclic, and the entire pattern repeats approximately every 3 hours.
Channels can be divided into dedicated channels, which are allocated to a mobile station,
and common channels, which are used by mobile stations in idle mode.
Terminology alert! Cellular radio uses the word channel in many ways. It is a pair of
radio frequencies. And channels are part of the digital stream that flows back and forth
from the mobile to the base station. Channels, therefore, can be carried on a channel.
Confusing, isn't it? The discussion below focuses on data channels, not radio
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5. The GSM Network Aspects
GSM provides recommendations, not requirements. The GSM specifications define the
functions and interface requirements in detail but do not address the hardware. The
reason for this is to limit the designers as little as possible but still to make it possible for
the operators to buy equipment from different suppliers. The GSM network is divided
into three major systems: the switching system (SS), the base station system (BSS), and
the operation and support system (OSS).
5.1The Switching System
The switching system (SS) is responsible for performing call processing and subscriber-
related functions. The switching system includes the following functional units.
Home location register (HLR)—The HLR is a database used for storage
and management of subscriptions. The HLR is considered the most important
database, as it stores permanent data about subscribers, including a subscriber's
service profile, location information, and activity status. When an individual buys
a subscription from one of the PCS operators, he or she is registered in the HLR
of that operator.
Mobile services switching center (MSC)—The MSC performs the
telephony switching functions of the system. It controls calls to and from other
telephone and data systems. It also performs such functions as toll ticketing,
network interfacing, common channel signaling, and others.
34
Visitor location register (VLR)—The VLR is a database that contains
temporary information about subscribers that is needed by the MSC in order to
service visiting subscribers. The VLR is always integrated with the MSC. When a
mobile station roams into a new MSC area, the VLR connected to that MSC will
request data about the mobile station from the HLR. Later, if the mobile station
makes a call, the VLR will have the information needed for call setup without
having to interrogate the HLR each time.
Authentication center (AUC)—A unit called the AUC provides
authentication and encryption parameters that verify the user's identity and ensure
the confidentiality of each call. The AUC protects network operators from
different types of fraud found in today's cellular world.
equipment identity register (EIR)—The EIR is a database that contains
information about the identity of mobile equipment that prevents calls from
stolen, unauthorized, or defective mobile stations. The AUC and EIR are
implemented as stand-alone nodes or as a combined AUC/EIR node.
5.2 The Base Station System (BSS)
All radio-related functions are performed in the BSS, which consists of base station
controllers (BSCs) and the base transceiver stations (BTSs).
BSC—The BSC provides all the control functions and physical links between the
MSC and BTS. It is a high-capacity switch that provides functions such as
handover, cell configuration data, and control of radio frequency (RF) power
levels in base transceiver stations. A number of BSCs are served by an MSC.
35
BTS—The BTS handles the radio interface to the mobile station. The BTS is the
radio equipment (transceivers and antennas) needed to service each cell in the
network. A group of BTSs are controlled by a BSC.
5.3 The Operation and Support System
The operations and maintenance center (OMC) is connected to all equipment in the
switching system and to the BSC. The implementation of OMC is called the operation
and support system (OSS). The OSS is the functional entity from which the network
operator monitors and controls the system. The purpose of OSS is to offer the customer
cost-effective support for centralized, regional, and local operational and maintenance
activities that are required for a GSM network. An important function of OSS is to
provide a network overview and support the maintenance activities of different operation
and maintenance organizations.
Additional Functional Elements
Other functional elements are as follows:
message center (MXE)—The MXE is a node that provides integrated voice, fax,
and data messaging. Specifically, the MXE handles short message service, cell
broadcast, voice mail, fax mail, e-mail, and notification.
mobile service node (MSN)—The MSN is the node that handles the mobile
intelligent network (IN) services.
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gateway mobile services switching center (GMSC)—A gateway is a node used
to interconnect two networks. The gateway is often implemented in an MSC. The
MSC is then referred to as the GMSC.
GSM interworking unit (GIWU)—The GIWU consists of both hardware and
software that provides an interface to various networks for data communications.
Through the GIWU, users can alternate between speech and data during the same
call. The GIWU hardware equipment is physically located at the MSC/VLR.
5.4 GSM Network Areas
The GSM network is made up of geographic areas. As shown in Figure 3, these areas
include cells, location areas (LAs), MSC/VLR service areas, and public land mobile
network (PLMN) areas.
The cell is the area given radio coverage by one base transceiver station. The GSM
network identifies each cell via the cell global identity (CGI) number assigned to each
cell. The location area is a group of cells. It is the area in which the subscriber is paged.
Each LA is served by one or more base station controllers, yet only by a single MSC.
Each LA is assigned a location area identity (LAI) number.
An MSC/VLR service area represents the part of the GSM network that is covered by one
MSC and which is reachable, as it is registered in the VLR of the MSC (see Figure 4).
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Figure 4. MSC/VLR Service Areas
The PLMN service area is an area served by one network operator (see Figure 6).
Figure 5. PLMN Network Areas
GSM Specifications
Before looking at the GSM specifications, it is important to understand the following
basic terms:
bandwidth—the range of a channel's limits; the broader the bandwidth, the faster
data can be sent
bits per second (bps)—a single on-off pulse of data; eight bits are equivalent to
one byte
frequency—the number of cycles per unit of time; frequency is measured in hertz
(Hz)
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kilo (k)—kilo is the designation for 1,000; the abbreviation kbps represents 1,000
bits per second
megahertz (MHz)—1,000,000 hertz (cycles per second)
milliseconds (ms)—one-thousandth of a second
watt (W)—a measure of power of a transmitter
Specifications for different personal communication services (PCS) systems vary among
the different PCS networks. Listed below is a description of the specifications and
characteristics for GSM.
frequency band—The frequency range specified for GSM is 1,850 to 1,990 MHz
(mobile station to base station).
duplex distance—The duplex distance is 80 MHz. Duplex distance is the
distance between the uplink and downlink frequencies. A channel has two
frequencies, 80 MHz apart.
channel separation—The separation between adjacent carrier frequencies. In
GSM, this is 200 kHz.
modulation—Modulation is the process of sending a signal by changing the
characteristics of a carrier frequency. This is done in GSM via Gaussian minimum
shift keying (GMSK).
transmission rate—GSM is a digital system with an over-the-air bit rate of 270
kbps.
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access method—GSM utilizes the time division multiple access (TDMA)
concept. TDMA is a technique in which several different calls may share the same
carrier. Each call is assigned a particular time slot.
speech coder—GSM uses linear predictive coding (LPC). The purpose of LPC is to
reduce the bit rate. The LPC provides parameters for a filter that mimics the vocal tract.
The signal passes through this filter, leaving behind a residual signal. Speech is encoded
at 13 kbps
Number of Connections, GSM
Market Q4 2006 Q2 2007 Q4 2007 Q2 2008
World 2,190,084,047 2,432,990,168 2,709,900,985 2,925,454,308
Africa 195,832,145 232,061,178 273,079,330 306,485,511
Americas 218,384,266 255,639,490 302,471,377 338,342,270
Asia Pacific 825,958,067 949,496,716 1,082,653,571 1,219,674,193
Europe: Eastern
339,735,325 361,706,937 395,030,491 401,945,699
Europe: Western
390,738,824 390,666,845 389,712,986 370,819,907
Middle East 128,538,868 148,180,842 170,277,699 190,634,697
USA/Canada 90,896,552 95,238,160 96,720,693 97,552,031
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5.5 GSM security
GSM was designed with a moderate level of security. The system was designed to
authenticate the subscriber using a pre-shared key and challenge-response.
Communications between the subscriber and the base station can be encrypted. The
development of UMTS introduces an optional USIM, that uses a longer authentication
key to give greater security, as well as mutually authenticating the network and the user
whereas GSM only authenticated the user to the network (and not vice versa). The
security model therefore offers confidentiality and authentication, but limited
authorization capabilities, and no non-repudiation. GSM uses several cryptographic
algorithms for security. The A5/1 and A5/2 stream ciphers are used for ensuring over-the-
air voice privacy. A5/1 was developed first and is a stronger algorithm used within
Europe and the United States; A5/2 is weaker and used in other countries. Serious
weaknesses have been found in both algorithms: it is possible to break A5/2 in real-time
with a ciphertext-only attack, and in February 2008, Pico Computing, Inc revealed its
ability and plans to commercialize FPGAs that allow A5/1 to be broken with a rainbow
table attack. The system supports multiple algorithms so operators may replace that
cipher with a stronger one.
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6. Evolution of CDMA
November 1988
CDMA cellular concept
November 1989
QUALCOMM proposes CDMA as a more efficient, higher-quality wireless technology
CDMA open demonstration conducted in San Diego
February 1990
NYNEX and QUALCOMM successfully demonstrate CDMA in New York City
1991
QUALCOMM successfully performs large-scale capacity tests in San Diego
1992
US West orders the first CDMA network equipment
CDMA soft handoff patent granted
1993
CDMA IS-95A standard complete
CDMA is adopted by the Telecommunications Industry Association (TIA) as a North
American digital cellular standard
First commercial CDMA market trial
South Korea adopts CDMA
1994
Sprint PCS adopts CDMA
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1995
CDMA standardized for U.S. PCS
First commercial launch of cdmaOne (Hutchison Telecom, Hong Kong)
QUALCOMM launches first commercial cdmaOne handset
1996
cdmaOne is commercially launched in South Korea
PrimeCo launches cdmaOne in 14 U.S. cities (now Verizon Wireless)
CDMA Development Group (CDG) announces more than one million cdmaOne
subscribers
1997
IS-95B standard completed (including 64 kbps data transmission capability)
Commercial service available in 100 U.S. cities
CDMA chosen in Japan
1998
TIA endorses CDMA2000 to be 3G solution for International Telecommunication Union
(ITU)
LG Telecom launches first CDMA data services
CDMA2000 submitted to ITU as part of the IMT-2000 process for global 3G standards
More than 12.5 million cdmaOne subscribers in 30 countries
First 1xEV-DO demonstration
1999
China Unicom joins CDG and announces plans for commercial services
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83 CDMA operators in 35 countries
CDG announces CDMA is fastest growing mobile technology with nearly 42 million
subscribers
CDMA2000 1X and WCDMA are selected as standards for 3G wireless by ITU
2000
Japan's IDO and DDI start nationwide 64 kbps CDMA packet data service
DDI announces they will use CDMA2000 for 3G wireless service
IUSACELL becomes first Latin American operator to offer wireless Internet services
QUALCOMM, Samsung and Sprint PCS make first 3G CDMA2000 voice call
Lucent and QUALCOMM complete the first 153 kbps 3G CDMA2000 data call
QUALCOMM and Sprint commence U.S. trials for 3G CDMA2000 solution
SK Telecom launches world's first 3G CDMA2000 commercial service
2001
More than 100 million CDMA subscribers globally
More than 22 million cdmaOne Internet and data users
CDMA2000 surpasses three million subscribers
QCT and Nortel Networks conduct industry's first mobile IP call
QCT, SchlumbergerSema and Samsung demonstrate CDMA/GSM roaming using R-
UIM-enabled CDMA handsets
KDDI announces successful completion of CDMA2000 1xEV-DO trial with
QUALCOMM, Hitachi, Sony and Kyocera
Telesp Cellular in Brazil is first Latin American operator to deploy 3G CDMA2000
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Romania launches world's first CDMA2000 network at 450 MHz (CDMA450)
CDMA2000 1xEV-DO is recognized as a 3G standard by the ITU
2002
3G CDMA subscribers surpass 27 million
China Unicom launches nationwide cdmaOne network in China
SK Telecom launches CDMA2000 1xEV-DO in South Korea
14 countries launch commercial CDMA2000 services (Australia, Canada, Chile,
Columbia, Ecuador, India, Israel, Japan, Moldova, New Zealand, Panama, Russia, United
States and Venezuela)
First chipset is shipped enabling use of CDMA and GSM networks during travel
2003
3G CDMA subscribers surpass 73 million
Cumulative shipment of CDMA chips surpass the one billion mark
Reliance begins deployment of a nationwide CDMA2000 network in India
China Unicom launches its nationwide CDMA2000 network
18 countries launch commercial CDMA2000 services (Argentina, Belarus, Bermuda,
Brazil, Canada, China, Dominican Republic, Guatemala, Indonesia, Kazakhstan, Mexico,
Nicaragua, Nigeria, Peru, Puerto Rico, Taiwan, Thailand, Vietnam)
Verizon Wireless begins nationwide CDMA2000 1xEV-DO deployment in the United
States
KDDI launches CDMA2000 1xEV-DO in Japan
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2004
240.2 million CDMA subscribers worldwide
146.8 million CDMA2000 subscribers
CDMA2000 1xEV-DO Revision A approved by Third Generation Partnership Project 2
(3GPP2)
Eurotel Praha (Czech Republic) launches world's first CDMA2000 1xEV-DO network at
450 MHz (CDMA450)
2005
More than 200 million commercial CDMA2000 subscribers worldwide
143 CDMA2000 operators commercially deployed in 67 countries on 6 continents
950 CDMA2000 devices offered commercially since 2000
64 CDMA2000 device manufacturers
Number of commercial CDMA2000 1xEV-DO operators doubled from 16 to 29
Number of commercial CDMA450 operators increased to 31 in 22 countries
Average growth rate of nearly 4.9 CDMA2000 subscribers per month
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7. CDMA Overview
Code Division Multiple Access (CDMA) is a radically new concept in wireless
communications. It has gained widespread international acceptance by cellular radio system
operators as an upgrade that will dramatically increase both their system capacity and the service
quality. It has likewise been chosen for deployment by the majority of the winners of the United
States Personal Communications System spectrum auctions. It may seem, however, mysterious
for those who aren't familiar with it. This site is provided in an effort to dispel some of the
mystery and to disseminate at least a basic level of knowledge about the technology.
CDMA is a form of spread-spectrum , a family of digital communication techniques that have
been used in military applications for many years. The core principle of spread spectrum is the
use of noise-like carrier waves, and, as the name implies, bandwidths much wider than that
required for simple point-to-point communication at the same data rate. Originally there were two
motivations: either to resist enemy efforts to jam the communications (anti-jam, or AJ), or to hide
the fact that communication was even taking place, sometimes called low probability of intercept
(LPI). It has a history that goes back to the early days of World War II.
The use of CDMA for civilian mobile radio applications is novel. It was proposed theoretically in
the late 1940's, but the practical application in the civilian marketplace did not take place until 40
years later. Commercial applications became possible because of two evolutionary developments.
One was the availability of very low cost, high density digital integrated circuits, which reduce
the size, weight, and cost of the subscriber stations to an acceptably low level. The other was the
realization that optimal multiple access communication requires that all user stations regulate
47
their transmitter powers to the lowest that will achieve adequate signal quality.
CDMA changes the nature of the subscriber station from a predominately analog device to a
predominately digital device. Old-fashioned radio receivers separate stations or channels by
filtering in the frequency domain. CDMA receivers do not eliminate analog processing entirely,
but they separate communication channels by means of a pseudo-random modulation that is
applied and removed in the digital domain, not on the basis of frequency. Multiple users occupy
the same frequency band. This universal frequency reuse is not fortuitous. On the contrary, it is
crucial to the very high spectral efficiency that is the hallmark of CDMA. Other discussions in
these pages show why this is true.
7.1 What is CDMA?
CDMA (Code-Division Multiple Access) refers to any of several protocols used in so-
called second-generation (2G) and third-generation (3G) wireless communications. As
the term implies, CDMA is a form of multiplexing, which allows numerous signals to
occupy a single transmission channel, optimizing the use of available bandwidth. The
technology is used in ultra-high-frequency (UHF) cellular telephone systems in the
800-MHz and 1.9-GHz bands.
CDMA employs analog-to-digital conversion (ADC) in combination with spread
spectrum technology. Audio input is first digitized into binary elements. The frequency
of the transmitted signal is then made to vary according to a defined pattern (code), so it
can be intercepted only by a receiver whose frequency response is programmed with the
same code, so it follows exactly along with the transmitter frequency. There are trillions
of possible frequency-sequencing codes, which enhances privacy and makes cloning
difficult.
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The CDMA channel is nominally 1.23 MHz wide. CDMA networks use a scheme called
soft handoff, which minimizes signal breakup as a handset passes from one cell to
another. The combination of digital and spread-spectrum modes supports several times
as many signals per unit bandwidth as analog modes. CDMA is compatible with other
cellular technologies; this allows for nationwide roaming.
The original CDMA standard, also known as CDMA One and still common in cellular
telephones in the U.S., offers a transmission speed of only up to 14.4 Kbps in its single
channel form and up to 115 Kbps in an eight-channel form. CDMA2000 and wideband
CDMA deliver data many times faster.
CDMA Concepts In CDMA each user is assigned a unique code sequence it uses to encode its information-
bearing signal. The receiver, knowing the code sequences of the user, decodes a received
signal after reception and recovers the original data. This is possible since the
crosscorrelations between the code of the desired user and the codes of the other users are
small. Since the bandwidth of the code signal is chosen to be much larger than the
bandwidth of the information-bearing signal, the encoding process enlarges (spreads) the
spectrum of the signal and is therefore also known as spread-spectrum modulation. The
resulting signal is also called a spread-spectrum signal, and CDMA is often denoted as
spread-spectrum multiple access (SSMA) [13, 1112].
The spectral spreading of the transmitted signal gives to CDMA its multiple access
capability. It is therefore important to know the techniques necessary to generate spread-
spectrum signals and the properties of these signals. A spread-spectrum modulation
technique must be fulfill two criteria: The transmission bandwidth must be much larger
49
than the information bandwidth. The resulting radio-frequency bandwidth is determined
by a function other than the information being sent (so the bandwidth is statistically
independent of the information signal). This excludes modulation techniques like
frequency modulation (FM) and phase modulation (PM).
The ratio of transmitted bandwidth to information bandwidth is called the processing
gain, Gp, of the spread-spectrum system, where Bt is the transmission bandwidth and Bi is
the bandwidth of the information-bearing signal.
The receiver correlates the received signal with a synchronously generated replica of the
spreading code to recover the original information-bearing signal. This implies that the
receiver must know the code used to modulate the data.
Because of the coding and the resulting enlarged bandwidth, SS signals have a number of
properties that differ from the properties of narrowband signals. The most interesting
ones, from the communication systems point of view, are discussed below. To have a
clear understanding, each property has been briefly explained with the help of
illustrations, if necessary, by applying direct sequence spread-spectrum techniques.
Multiple Access Capability -- If multiple users transmit a spread-spectrum signal at the
same time, the receiver will still be able to distinguish between the users provided each
user has a unique code that has a sufficiently low cross-correlation with the other codes.
Correlating the received signal with a code signal from a certain user will then only
despread the signal of this user, while the other spread-spectrum signals will remain
spread over a large bandwidth. Thus, within the information bandwidth the power of the
desired user will be larger than the interfering power provided there are not too many
interferers, and the desired signal can be extracted. If two users generate a spread-
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spectrum signal from their narrowband data signals. If both users transmit their spread-
spectrum signals at the same time. At the receiver 1 only the signal of user 1 is
"despread" and the data recovered.
Protection Against Multipath Interference -- In a radio channel there is not just one
path between a transmitter and receiver. Due to reflections (and refractions) a signal will
be received from a number of different paths. The signals of the different paths are all
copies of the same transmitted signal but with different amplitudes, phases, delays, and
arrival angles. Adding these signals at the receiver will be constructive at some of the
frequencies and destructive at others. In the time domain, this results in a dispersed
signal. Spread-spectrum modulation can combat this multipath interference; however, the
way in which this is achieved depends very much on the type of modulation used. In the
next section, where CDMA schemes based on different modulation methods are
discussed, we show for each scheme how multipath interference rejection is obtained.
Privacy -- The transmitted signal can only be despread and the data recovered if the code
is known to the receiver.
Interference Rejection -- Cross-correlating the code signal with a narrowband signal
will spread the power of the narrowband signal thereby reducing the interfering power in
the information bandwidth. This is illustrated in Fig. 3. The spread-spectrum signal (s)
receives a narrowband interference (i). At the receiver the SS signal is "despread" while
the interference signal is spread, making it appear as background noise compared to the
despread signal.
Anti-Jamming Capability, Especially Narrowband Jamming -- This is more or less
the same as interference rejection except the interference is now willfully inflicted on the
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system. It is this property, together with the next one, that makes spread-spectrum
modulation attractive for military applications.
Low Probability of Interception (LPI) -- Because of its low power density, the spread-
spectrum signal is difficult to detect and intercept by a hostile listener.
A general classification of CDMA is given in Fig. 4. There are a number of modulation
techniques that generate spread-spectrum signals. We briefly discuss the most important
ones. Direct sequence spread-spectrum -- The information-bearing signal is multiplied
directly by a high chip rate code signal. Frequency hopping spread-spectrum -- The
carrier frequency at which the information-bearing signal is transmitted is rapidly
changed according to the code signal Time hopping spread-spectrum -- The information-
bearing signal is not transmitted continuously. Instead the signal is transmitted in short
bursts where the times of the bursts are decided by the code signal. Hybrid modulation --
Two or more of the above-mentioned SS modulation techniques can be used together to
combine the advantages and, it is hoped, to combat their disadvantages. Furthermore, it is
possible to combine CDMA with other multiple access methods: TDMA, multicarrier
(MC), or multitone (MT) modulation. In the case of MC-CDMA, spreading is done along
the frequency axis, while for MT-CDMA spreading is done along the time axis. Note that
MC-CDMA and MT-CDMA are based on orthogonal frequency division multiplexing
(OFDM).
In the next section the above-mentioned pure CDMA modulation techniques are used to
show the multiple access capability of CDMA. However, the remainder of the sections
will mainly concentrate on direct sequence (DS)-CDMA and its related subjects.
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CDMA is altering the face of cellular and PCS communication by:
Dramatically improving the telephone traffic capacity
Dramatically improving the voice quality and eliminating the audible effects of multipath fading
Reducing the incidence of dropped calls due to handoff failures
Providing reliable transport mechanism for data communications, such as facsimile and internet traffic
Reducing the number of sites needed to support any given amount of traffic
Simplifying site selection
Reducing deployment and operating costs because fewer cell sites are needed
Reducing average transmitted power
Reducing interference to other electronic devices
Reducing potential health risks
Commercially introduced in 1995, CDMA quickly became one of the world's fastest-growing
wireless technologies. In 1999, the International Telecommunications Union selected CDMA as
the industry standard for new "third-generation" (3G) wireless systems. Many leading wireless
carriers are now building or upgrading to 3G CDMA networks in order to provide more capacity
for voice traffic, along with high-speed data capabilities.
CDMA is a form of Direct Sequence Spread Spectrum communications. In general, Spread
Spectrum communications is distinguished by three key elements:
1. The signal occupies a bandwidth much greater than that which is necessary to send the
information. This results in many benefits, such as immunity to interference and jamming and
multi-user access, which we'll discuss later on.
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2. The bandwidth is spread by means of a code which is independent of the data. The
independence of the code distinguishes this from standard modulation schemes in which the data
modulation will always spread the spectrum somewhat.
3. The receiver synchronizes to the code to recover the data. The use of an independent code and
synchronous reception allows multiple users to access the same frequency band at the same time.
In order to protect the signal, the code used is pseudo-random. It appears random, but is actually
deterministic, so that the receiver can reconstruct the code for synchronous detection. This
pseudo-random code is also called pseudo-noise (PN).
There are three ways to spread the bandwidth of the signal:
Frequency hopping. The signal is rapidly switched between different frequencies within
the hopping bandwidth pseudo-randomly, and the receiver knows before hand where to
find the signal at any given time.
Time hopping. The signal is transmitted in short bursts pseudo-randomly, and the
receiver knows beforehand when to expect the burst.
Direct sequence. The digital data is directly coded at a much higher frequency. The code
is generated pseudo-randomly, the receiver knows how to generate the same code, and
correlates the received signal with that code to extract the data.
How spread spectrum works:
Spread Spectrum uses wide band, noise-like signals. Because Spread Spectrum signals are noise-
like, they are hard to detect. Spread Spectrum signals are also hard to Intercept or demodulate.
Further, Spread Spectrum signals are harder to jam (interfere with) than narrowband signals.
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These Low Probability of Intercept (LPI) and anti-jam (AJ) features are why the military has used
Spread Spectrum for so many years. Spread signals are intentionally made to be much wider band
than the information they are carrying to make them more noise-like.
Spread Spectrum signals use fast codes that run many times the information bandwidth or data
rate. These special "Spreading" codes are called "Pseudo Random" or "Pseudo Noise" codes.
They are called "Pseudo" because they are not real gaussian noise.
Spread Spectrum transmitters use similar transmit power levels to narrow band transmitters.
Because Spread Spectrum signals are so wide, they transmit at a much lower spectral power
density, measured in Watts per Hertz, than narrowband transmitters. This lower transmitted
power density characteristic gives spread signals a big plus. Spread and narrow band signals can
occupy the same band, with little or no interference. This capability is the main reason for all the
interest in Spread Spectrum today.
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8. CDMA technique
8.1 Multiple Access
The concept behind multiple access is to permit a number of users to share a common
channel. The two traditional ways of multiple access are Frequency Division Multiple
Access (FDMA) and Time Division Multiple Access (TDMA).
FDMA
In Frequency Division Multiple Access, the frequency band is divided in slots. Each user
gets one frequency slot assigned that is used at will. It could be compared to AM or FM
broadcasting radio where each station has a frequency assigned. FDMA demands good
filtering.
Figure6 .FDMA
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TDMA
In Time Division Multiple Access, the frequency band is not partitioned but users are
allowed to use it only in predefined intervals of time, one at a time. Thus, TDMA
demands synchronization among the users.
Figure7 TDMA
CDMA
CDMA, for Code Division Multiple Access, is different than those traditional ways in
that it does not allocate frequency or time in user slots but gives the right to use both to
all users simultaneously. To do this, it uses a technique known as Spread Spectrum. In
effect, each user is assigned a code which spreads its signal bandwidth in such a way that
only the same code can recover it at the receiver end. This method has the property that
the unwanted signals with different codes get spread even more by the process, making
them like noise to the receiver.
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8.2 Spread Spectrum
Spread Spectrum is a mean of transmission where the data occupies a larger bandwidth
than necessary. Bandwidth spreading is accomplished before the transmission through the
use of a code which is independent of the transmitted data. The same code is used to
demodulate the data at the receiving end. The following figure illustrate the spreading
done on the data signal x(t) by the spreading signal c(t) resulting in the message signal to
be transmitted, m(t).
Figure8. Spread Spectrum
Originally for military use to avoid jamming (interference created on purpose to make a
communication channel unusable), spread spectrum modulation is now used in personal
communication systems for its superior performance in an interference dominated
environment.
Processing Gain
In spread spectrum, the data is modulated by a spreading signal which uses more
bandwidth than the data signal. Since multiplication in the time domain corresponds to
convolution in the frequency domain, a narrow band signal multiplied by a wide band
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signal ends up being wide band. One way of doing this is to use a binary waveform as a
spreading function, at a higher rate than the data signal.
Figure9. Processing gain
Here the three signals corresponds to x(t), c(t) and m(t) discussed above. The first two
signals are multiplied together to give the third waveform.
Bits of the spreading signal are called chips. On the above figure, Tb represents the
period of one data bit and Tc represents the period of one chip. The chip rate, 1/Tc, is
often used to characterize a spread spectrum transmission system.
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The Processing Gain or sometimes called the Spreading Factor is defined as the ratio of
the information bit duration over the chip duration:
PG = SF = Tb / Tc
Hence, it represents the number of chips contained in one data bit. Higher Processing
Gain (PG) means more spreading. High PG also means that more codes can be allocated
on the same frequency channel (more on that later).
Pseudo-Noise Sequences
So far we haven't discussed what properties we would want the spreading signal to have.
This depends on the type of system we want to implement. Let's first consider a system
where we want to use spread spectrum to avoid jamming or narrow band interference.
If we want the signal to overcome narrow band interference, the spreading function needs
to behave like noise. Random binary sequences are such functions. They have the
following important properties:
Balanced: they have an equal number of 1's and 0's
Single Peak auto-correlation function
Multiple-Access in CDMA
The advantage of CDMA for personal communication services is its ability to
accommodate many user on the same frequency at the same time. As we mentioned
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earlier, a specific code is assigned to each user and only that code can demodulate the
transmitted signal.
There are two ways of separating users in CDMA:
Orthogonal Multiple Access
Non-orthogonal Multiple Access or Asynchronous CDMA
Spread-Spectrum Multiple Access
Direct Sequence -- In DS-CDMA the modulated information-bearing signal (the data
signal) is directly modulated by a digital, discrete-time, discrete-valued code signal. The
data signal can be either analog or digital; in most cases it is digital. In the case of a
digital signal the data modulation is often omitted and the data signal is directly
multiplied by the code signal and the resulting signal modulates the wideband carrier. It
is from this direct multiplication that the direct sequence CDMA gets its name.
In Fig. 5 a block diagram of a DS-CDMA transmitter is given. The binary data signal
modulates a RF carrier. The modulated carrier is then modulated by the code signal. This
code signal consists of a number of code bits called "chips" that can be either +1 or 1. To
obtain the desired spreading of the signal, the chip rate of the code signal must be much
higher than the chip rate of the information signal. For the code modulation various
modulation techniques can be used, but usually some form of phase shift keying (PSK)
like binary phase shift keying (BPSK), differential binary phase shift keying (D-BPSK),
quadrature phase shift keying (QPSK), or minimum shift keying (MSK) is employed.
If we omit the data modulation and use BPSK for the code modulation, we get the block
diagram given in Fig. 6. The DS-SS signal resulting from this transmitter is shown in Fig.
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7. The rate of the code signal is called the chip rate; one chip denotes one symbol when
referring to spreading code signals. In this figure, 10 code chips per information symbol
are transmitted (the code chip rate is 10 times the data rate) so the processing gain is
equal to 10.
After transmission of the signal, the receiver (shown in Fig. 8) uses coherent
demodulation to despread the SS signal, using a locally generated code sequence. To be
able to perform the despreading operation, the receiver must not only know the code
sequence used to spread the signal, but the codes of the received signal and the locally
generated code must also be synchronized. This synchronization must be accomplished at
the beginning of the reception and maintained until the whole signal has been received.
The code synchronization/tracking block performs this operation. After despreading a
data modulated signal results, and after demodulation the original data can be recovered.
In the previous section a number of advantageous properties of spread-spectrum signals
were mentioned. The most important of those properties from the viewpoint of CDMA is
the multiple access capability, the multipath interference rejection, the narrowband
interference rejection, and with respect to secure/private communication, the LPI. We
explain these four properties for the case of DS-CDMA.
Multiple access: If multiple users use the channel at the same time, there will be
multiple DS signals overlapping in time and frequency. At the receiver coherent
demodulation is used to remove the code modulation. This operation concentrates
the power of the desired user in the information bandwidth. If the
crosscorrelations between the code of the desired user and the codes of the
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interfering users are small, coherent detection will only put a small part of the
power of the interfering signals into the information bandwidth.
Multipath interference: If the code sequence has an ideal autocorrelation function,
then the correlation function is zero outside the interval [Tc,Tc], where Tc is the
chip duration. This means that if the desired signal and a version that is delayed
for more than 2Tc are received, coherent demodulation will treat the delayed
version as an interfering signal, putting only a small part of the power in the
information bandwidth.
Narrowband interference: The coherent detection at the receiver involves a
multiplication of the received signal by a locally generated code sequence.
However, as we saw at the transmitter, multiplying a narrowband signal with a
wideband code sequence spreads the spectrum of the narrowband signal so that its
power in the information bandwidth decreases by a factor equal to the processing
gain.
LPI: Because the direct sequence signal uses the whole signal spectrum all the
time, it will have a very low transmitted power per hertz. This makes it very
difficult to detect a DS signal.
Spread Spectrum Characteristics of CDMA
Most modulation schemes try to minimize the bandwidth of this signal since bandwidth is
a limited resource. However, spread spectrum techniques use a transmission bandwidth
that is several orders of magnitude greater then the minimum required signal bandwidth.
One of the initial reasons for doing this was military applications including guidance and
communication systems. These systems were designed using spread spectrum because of
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its security and resistance to jamming. Asynchronous CDMA has some level of privacy
built in because the signal is spread using a pseudorandom code; this code makes the
spread spectrum signals appear random or have noise-like properties. A receiver cannot
demodulate this transmission without knowledge of the pseudorandom sequence used to
encode the data. CDMA is also resistant to jamming. A jamming signal only has a finite
amount of power available to jam the signal. The jammer can either spread its energy
over the entire bandwidth of the signal or jam only part of the entire signal.
CDMA can also effectively reject narrowband interference. Since narrowband
interference affects only a small portion of the spread spectrum signal, it can easily be
removed through notch filtering without much loss of information. Convolution encoding
and interleaving can be used to assist in recovering this lost data. CDMA signals are also
resistant to multipath fading. Since the spread spectrum signal occupies a large
bandwidth only a small portion of this will undergo fading due to multipath at any given
time. Like the narrowband interference this will result in only a small loss of data and can
be overcome.
Another reason CDMA is resistant to multipath interference is because the delayed
versions of the transmitted pseudorandom codes will have poor correlation with the
original pseudorandom code, and will thus appear as another user, which is ignored at the
receiver. In other words, as long as the multipath channel induces at least one chip of
delay, the multipath signals will arrive at the receiver such that they are shifted in time by
at least one chip from the intended signal. The correlation properties of the
pseudorandom codes are such that this slight delay causes the multipath to appear
uncorrelated with the intended signal, and it is thus ignored.
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Some CDMA devices use a rake receiver, which exploits multipath delay components to
improve the performance of the system. A rake receiver combines the information from
several correlators, each one tuned to a different path delay, producing a stronger version
of the signal than a simple receiver with a single correlator tuned to the path delay of the
strongest signal.
Frequency reuse is the ability to reuse the same radio channel frequency at other cell sites
within a cellular system. In the FDMA and TDMA systems frequency planning is an
important consideration. The frequencies used in different cells need to be planned
carefully in order to ensure that the signals from different cells do not interfere with each
other. In a CDMA system the same frequency can be used in every cell because
channelization is done using the pseudorandom codes. Reusing the same frequency in
every cell eliminates the need for frequency planning in a CDMA system; however,
planning of the different pseudorandom sequences must be done to ensure that the
received signal from one cell does not correlate with the signal from a nearby cell.
Since adjacent cells use the same frequencies, CDMA systems have the ability to perform
soft handoffs. Soft handoffs allow the mobile telephone to communicate simultaneously
with two or more cells. The best signal quality is selected until the handoff is complete.
This is different than hard handoffs utilized in other cellular systems. In a hard handoff
situation, as the mobile telephone approaches a handoff, signal strength may vary
abruptly. In contrast, CDMA systems use the soft handoff, which is undetectable and
provides a more reliable and higher quality signal.
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8.3 Uses One of the early applications for code division multiplexing—predating, and
distinct from cdmaOne—is in GPS.
The Qualcomm standard IS-95, marketed as cdmaOne.
The Qualcomm standard IS-2000, known as CDMA2000. This standard is used
by several mobile phone companies, including the Globalstar satellite phone
network.
CDMA has been used in the OmniTRACS satellite system for transportation
logistics.
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9. Difference Between GSM and CDMA
In cellular service there are two main competing network technologies: Global System
for Mobile Communications (GSM) and Code Division Multiple Access (CDMA).
Cellular carriers including Sprint PCS, Cingular Wireless, Verizon and T-Mobile use one
or the other. Understanding the difference between GSM and CDMA will allow you to
choose a carrier that uses the preferable network technology for your needs.
The Origin
CDMA: Code Division Multiple Access (CDMA) is a technology developed by
Qualcomm in the United States and it is currently the dominant network standard in
North America.
GSM: Global System for Mobile communication (GSM) was invented in 1987 by the
GSM Association, an international organization dedicated to developin the GSM standard
worldwide.
Coverage
CDMA: CDMA is mostly used in America and some part of Asia. It is currently making
progress in other parts of the world, but the coverage is still limited compared to the
GSM technology. Its support is currently non-existent in Europe because the European
Union mandates the sole use of GSM. In North America however, CDMA generally
offers a better coverage than GSM in some rural areas because it was deployed earlier.
The CDMA network reaches over 475 million users worldwide.
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GSM: GSM being an international standard, it is better suited for international roaming,
provided u own a quad-band cell phone (850/900/1800/1900 MHZ). The GSM network is
also well established in North America, but not as much as CDMA network yet. The
GSM reaches over2.5 billion users worldwide.
Data Transfer
CDMA: The best data transfer technology CDMA has to offer is the DVDO technology,
allowing for a maximum download speed of about 2mbps ( but only about 700kbps in
practice), which is similar to what a DSL line has to offer. EVDO is not available
everywhere yet aan requires a cell phone that is EVDO ready.
GSM: GSM on the other hand offers EDGE, allowing for a mximum download speed of
384kbps (around 140kbps in practice). More technologies are being developed on top of
EDGE such as HSDPA to boost the transfer rate to over 384kbps in practice. This
technology requires an EDGE-ready cell phone.
CDMA offers faster data download. GSM is catching up fast, but its EDGE technology is
subject to interferences. CDMA would therefore be the favored choice for data transfer.
Phone Identification (SIM cards)
GSM: On a GSM phone your account information along with your contact list and other
personal data are stored on a SIM card (Subscriber Identity Module) which is a small
chip you can freely remove from your phone. When you get a new mobile device, ypu
can simply insert your SIM card into it and it will work with your current account
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information and contact list. If you travel to another country, it might even be possible to
purchase a prepaid SIM card which you can use to avoid roaming fees.
CDMA: The CDMA equivalent, an R-UIM card, is only available in parts of Asia but
remain on the horizon for the US market. To upgrade a CDMA phone, the service
provider must deactivate the old phone then activate the new one. The old phone
becomes useless. If you want to change your phone, you have to contact your service
provider and have reprogram your new phone. You will also need to re-enter your contact
list and calendar information into your new phone.
GSM is clear winner here. The SIM card technology offers many advantages.
Talk Time on phone:
CDMA: has lower talk time than GSM because the transmitter(CDMA) is active all the
time.
GSM: has a higher talk time than CDMA based phones because the transmitter(TDMA)
does not require constant transmit. The transmitter can be idle when not actually
transmitting packets.
Range to antennae:
CDMA: allows greater range to antennae (110KM under ideal circumstances) and low
power transmitters (200mW) with greater than GSM.
GSM: Lower distances to antennae (75-105KM under ideal conditions) than CDMA.
Higher power transmitters(2W).
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For urban areas this is probably not an issue but in rural and less travel in areas cell tower
density is very low. CDMA may win this point.
International Roaming:
Most of the world only supports GSM. The phone you buy must support the frequency
used in the country you wish to visit. GSM the more internationally accepted and has
more international “roamability” than CDMA.
Patents and extra fees:
CDMA: is patented by Qualcom-So additional fees need to be paid to Qualcom for its
use. This limits its appeal in international markets.
GSM: has no such patents and is an international standard.
Limited number of active calls per cell:
CDMA: has no hard limit of number of calls per cell.The quality goes dowm the more
calls are active and the provider may cap the total number of active users.
GSM: Have a hard limited number of calls per cell.Once the limit is exceeded you cannot
use the cell.
Account and phone migration/upgrading.
GSM: phones have a SIM card that carries all account/phone data.They can be ‘locked’
By the carriers but typically this can be circumvented by secret codes that are entered into
the phone.
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10. Comparative Study between CDMA and GSM.
Basis GSM CDMA
Origin Invented in 1987 by the GSM Association
Developed by Qualcomm in US
Coverage Used in North America and Europe Mostly used in America and some parts of Asia
Data transfer GSM offers EDGE technology, easy to copy or hack the phone
CDMA offers EVDO technology, high speed data transfer
Phone Identification SIM card R-UIM card
Talk time on phone Higher talk time Lower talk time
International Roaming
More internationally accepted Less internationally accepted
Patents and extra fees
Has no such patents and is international standard
Is patented by Qualcomm, fees need to be paid to Qualcomm for its use
Limited no. of active calls per cell
Have a hard limited no of calls per cell
Has no hard limited no of calls per cell
Account and phone migration
Phones have a SIM card that carries all account/phone data.
Some migration services are offered by carrier for upgrade to higher version, phones are specific to carrier.
Voice quality Better voice quality Not very good voice quality.
Total market size Far larger in both number of subscribers and amount of total area covered.
Not large
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11. Advantages and Disadvantages of CDMA and GSM:
Advantages of CDMA include:
Increased cellular communications security.
Simultaneous conversations.
Increased efficiency, meaning that the carrier can serve more subscribers.
Smaller phones.
Low power requirements and little cell-to-cell coordination needed by
operators.
Extended reach - beneficial to rural users situated far from cells.
Disadvantages of CDMA include: Due to its proprietary nature, all of CDMA's flaws are not known to the
engineering community.
CDMA is relatively new, and the network is not as mature as GSM.
CDMA cannot offer international roaming, a large GSM advantage.
Advantages of GSM:
GSM is already used worldwide with over 450 million subscribers.
International roaming permits subscribers to use one phone throughout
Western Europe. CDMA will work in Asia, but not France, Germany, the
U.K. and other popular European destinations.
GSM is mature, having started in the mid-80s. This maturity means a more
stable network with robust features. CDMA is still building its network.
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GSM's maturity means engineers cut their teeth on the technology, creating an
unconscious preference.
The availability of Subscriber Identity Modules, which are smart cards that
provide secure data encryption give GSM m-commerce advantages.
In brief, GSM is a "more elegant way to upgrade to 3G," says Strategies Group senior
wireless analyst Adam Guy.
Disadvantages of GSM:
Lack of access to burgeoning American market.
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12.Conclusion
Today, the battle between CDMA and GSM is muddled. Where at one point Europe
clearly favored GSM and North America, CDMA, the distinct advantage of one over the
other has blurred as major carriers like AT&T Wireless begin to support GSM, and recent
trials even showed compatibility between the two technologies.
GSM still holds the upper hand however. There's the numerical advantage for one thing:
456 million GSM users versus CDMA's 82 million.
Other factors potentially tipping the scales in the GSM direction include :
AT&T Wireless' move to overlay GSM atop its TDMA network means the European
technology (GSM) gains instant access to North America's number two network.
Qualcomm's recently announced that Wideband-CDMA (WCDMA) won't be ready in
Europe until 2005. This comes amidst reports that GSM's successor, General Packet
Radio Services (GPRS) remains on target for deployment in 2001-2002.
For all of the historical and technological reasons outlined above, it appears that GSM, or
some combination of GSM and CDMA, will become the long sought after grail for a
global wireless standard. A universalization of wireless technologies can only stand to
benefit the compatibility and development costs and demands on all wireless commerce
participants.
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13 Future scope of the project
The mobile industry in India believes the market still offers loads of opportunities for
both GSM and CDM technologies. While the services providers have been launching
Various palns to lure customers, it’s still a big question as to which of the two
technologies will lead eventually. There’re twelve operators in India where the
population has crossed one billion.The Telecom tariff in India is just 2 cents/minute as
against 4 cents in china. This is something that is absolutely in favor of the Indian mobile
industry.
Boom in telecom has completely changed the teledensity scenario in India.With
companies continuously penetrating deeper in to interiors of this country,teledensity is
ecpected to grow further in the years to come. As per industry estimates GSM subscribe
base rose by 81% year-on-year in October 2006.However ,CDMA subscribe base has
more than doubled compared to the previous year. This technologies have been quite
successful in increasing the mobile and will likely continue to do so in the near future as
well.
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14.Biblography
Digital living Electronics for you ETRI Journal,volume19 Information Technology-Magazine www.gsmworld.com www.gsmareana.com www.gsm.com www.cdg.org www.cdma.com www.ITBusinessEdge.com www.wisegeek.com www.comlab.hut www.educypedia.be www.cdg.org
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