mca cdma vs gsm

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

1

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

2

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)

3

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

5

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

7

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.

9

http://searchTelecom.techtarget.com/sDefinition/0,,sid103_gci213842,00.html

http://searchMobileComputing.techtarget.com/sDefinition/0,,sid40_gci213380,00.html

http://searchNetworking.techtarget.com/sDefinition/0,,sid7_gci214175,00.html

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.

10

http://en.wikipedia.org/wiki/Bandwidth_(computing)

http://en.wikipedia.org/wiki/Spread_spectrum

http://en.wikipedia.org/wiki/Frequency

http://en.wikipedia.org/wiki/Frequency-division_multiple_access

http://en.wikipedia.org/wiki/Time

http://en.wikipedia.org/wiki/Time_division_multiple_access

http://en.wikipedia.org/wiki/Spread-spectrum

http://en.wikipedia.org/wiki/Bandwidth_(signal_processing)

http://en.wikipedia.org/wiki/Channel_access_method

http://en.wikipedia.org/wiki/CDMA2000

http://en.wikipedia.org/wiki/IS-95

http://en.wikipedia.org/wiki/Channel_access_method

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


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


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


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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


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-

22

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

29

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

30

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


E-TACS

872-905 MHz 917-950 MHz


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.

31

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

32

http://www.privateline.com/Cellbasics/Cellbasics08.html

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

33

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).

37

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-

50

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

51

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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http://www.cdg.org/

http://www.educypedia.be/

http://www.comlab.hut/

http://www.wisegeek.com/

http://www.cdma.com/

http://www.cdg.org/

http://www.gsm.com/

http://www.gsmareana.com/

mca cdma vs gsm

Documents

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