Term paper on CDMA

8/10/2019 Term paper on CDMA

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Abstract Code division multiple access (CDMA) is

a channel access method used by various radio

communication technologies. CDMA is the

digitalcellular technology that uses spread

spectrum techniques. Unlike competing systems, such

asGSM, that useTDMA, CDMA does not assign a

specific frequency to each user. Instead, everychannel

uses the full available spectrum. Individual conversations

are encoded with a pseudo-randomised digital sequence.

CDMA consistently provides better capacity for voice

and data communications than other commercial mobile

technologies, allowing more subscribers to connect at any

given time, and it is the common platform on which 3G

technologies are built.

Introduction

CDMA is achannel access methodused by thevariousradiocommunication technologies.

CDMA is an example ofmultiple access, which is where

several transmitters can send information simultaneously

over a single communication channel. This allows several

users to share a band of frequencies (seebandwidth). To

permit this without undue interference between the users,

CDMA employsspread-spectrumtechnology and a

special coding scheme (where each is assigned a code).

CDMA is utilized as one of access methods in

manymobile phone utilization standardssuch

ascdmaOne,CDMA2000(the3Gevolution of

cdmaOne), andWCDMA(the 3G standard usedbyGSMcarriers), which are often referred to as

simplyCDMA.

History

CDMA is a military technology first used during World

War II by English allies to foil German attempts at

jamming transmissions. The allies decided to transmit

over several frequencies, instead of one, making it

difficult for the Germans to pick up the complete signal.

Because Qualcomm created communications chips for

CDMA technology, it was privy to the classified

information. Once the information became public,

Qualcomm claimed patents on the technology and became

the first to commercialize it.

The Cellular ChallengeThe world's first cellular networks were introduced in theearly 1980s, using analog radio transmission technologies

such as AMPS (Advanced Mobile Phone System).

Within a few years, cellular systems began to hit a

capacity ceiling as millions of new subscribers signed up

for service, demanding more and more airtime. Dropped

calls and network busy signals became common in many

areas. To accommodate more traffic within a limited

amount of radio spectrum, the industry developed a new

set of digital wireless technologies called TDMA (Time

Division Multiple Access) and GSM (Global System for

Mobile). TDMA and GSM used a time-sharing protocolto provide three to four times more capacity than analog

systems. But just as TDMA was being standardized, an

even better solution was found in CDMA.

Commercial Development

The founders of QUALCOMM realized that CDMA

technology could be used in commercial cellular

communications to make even better use of the radio

spectrum than other technologies. The first CDMA

networks were commercially launched in 1995, andprovided roughly 10 times more capacity than analog

networks - far more than TDMA or GSM. Since then,

CDMA has become the fastest-growing of all wireless

technologies, with over 100 million subscribers

worldwide. In addition to supporting more traffic, CDMA

brings many other benefits to carriers and consumers,

including better voice quality, broader coverage and

stronger security.

Taufeeq malik

Lovely Professional University

Code Division Multiple Access
http://www.webopedia.com/TERM/C/cellular.htmhttp://www.webopedia.com/TERM/C/GSM.htmhttp://www.webopedia.com/TERM/T/TDMA.htmlhttp://www.webopedia.com/TERM/C/channel.htmhttp://www.webopedia.com/TERM/C/channel.htmhttp://www.webopedia.com/TERM/T/TDMA.htmlhttp://www.webopedia.com/TERM/C/GSM.htmhttp://www.webopedia.com/TERM/C/cellular.htm


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.

Add in exciting Third-Generation (3G) wireless data

services and applications - such as wireless email, web,

digital picture taking/sending and assisted-GPS position

location applications- and wireless networks are asked to

do much more than just a few years ago. And thesenetworks will be asked to do more tomorrow. This is

where CDMA technology fits in. CDMA consistently

provides better capacity for voice and data

communications than other commercial mobile

technologies, allowing more subscribers to connect at any

given time, and it is the common platform on which 3G

technologies are built. CDMA is a "spread spectrum"

technology, allowing many users to occupy the same time

and frequency allocations in a given band/space. As its

name implies, CDMA assigns unique codes to each

communication to differentiate it from others in the same

spectrum.

Working of CDMA

CDMA takes an entirely different approach from TDMA.

CDMA, after digitizing data, spreads it outover the entire

available bandwidth. Multiple calls are overlaidon each

other on the channel,with each assigned a unique

sequence code. CDMA is a formof spread spectrum,

which simply means that data is sent in small pieces over

a number of the discrete frequencies available for use at

any time in the specified range

All of the users transmit in the same wide-bandchunk of

spectrum. Each user's signal is spread over the entire

bandwidth by a unique spreading code. At the receiver,

that sameunique code is used to recover the signal.

Because CDMA systems need to put an accurate time-stamp on each piece of a signal, it references the

GPSsystem for this information. Between eight and 10

separate calls can be carried inthe same channel space as

one analog AMPS call.

Spread Spectrum Communications technology in

CDMA

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 well discuss

later on.

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 multipleusers 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).

Three Types of Spread Spectrum

Communications

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.


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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.

Direct Sequence Spread Spectrum

Fig.1

CDMA is a Direct Sequence Spread Spectrum system.

The CDMA system works directly on 64 kbit/sec digital

signals.These signals can be digitized voice, ISDN

channels, modem data, etc.

Signal transmission consists of the following

steps:

1. A pseudo-random code is generated, different for

each channel and each successive connection.

2. The Information data modulates the pseudo-

random code (the Information data is spread).

3. The resulting signal modulates a carrier.

4. The modulated carrier is amplified and broadcast.

Signal reception consists of the foll owing steps:

1. The carrier is received and amplified.

2. The received signal is mixed with a local carrier to

recover the spread digital signal.

3. A pseudo-random code is generated, matching the

anticipated signal.

4. The receiver acquires the received code and phase

locks its own code to it.

5. The received signal is correlated with thegenerated code, extracting the Information data.

Implementing CDMA Technology

The following sections describe how a system might

implement the steps illustrated in Figure 1.

I nput data

CDMA works on Information data from several

possible sources, such as digitized voice or ISDN

channels. Data rates can vary, here are some

examples:

The system works with 64 kBits/sec data, but can

accept input rates of 8, 16, 32, or 64 kBits/sec.

Inputs of less than 64 kBits/sec are padded with

extra bits to bring them up to 64 kBits/sec.


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For inputs of 8, 16, 32, or 64 kBits/sec, the system

applies Forward Error Correction (FEC) coding, whichdoubles the bit rate, up to 128 kbits/sec. The Complex

Modulation \scheme (which well discuss in more detail

later), transmits two bits at a time, in two bit symbols.

For inputs of less than 64 kbits/sec,each symbol is

repeated to bring the transmission rate up to 64

kilosymbols/sec. Each component of the complex signal

carries one bit of the two bit symbol, at 64 kBits/sec, as

shown below.

Transmitting Data

The resultant coded signal next modulates an RF carrier

for transmission using Quadrature Phase Shift Keying

(QPSK). QPSK uses four different states to encode each

symbol. The four states are phase shifts of the carrier

spaced 90_ apart. By convention, the phase shifts are 45,

135, 225, and 315 degrees. Since there are four possible

states used to encode binary information, each state

represents two bits. This two bit word is called a

symbol.

General Specification of CDMA

Rx: Rx 869-894MHz Tx: 824-849MHz

20 Channels spaced 1250kHz apart (798 users/channel)

QPSK/(Offset) OQPSK modulation scheme

1.2288Mbps bit rate

IS-95 standard

Operates at both800 and 1900 MHz frequency bands

Advantages of CDMA techniques:

1.Flexible allocation of resources.

2.

Many users of CDMA use the same frequency,

TDD or FDD may be used

3.

Multipath fading may be substantially reduced

because of large signal bandwidth

4. No absolute limit on the number of users, Easy

addition of more users.

5. Impossible for hackers to decipher the code sent

6. Better signal quality

7.No sense of handoff when changing cells

8. The CDMA channel is nominally 1.23 MHz wide.

9.CDMA networks use a scheme called soft handoff,

which minimizes signal breakup as a handset

passes from one cell to another.

10.

CDMA is compatible with other cellulartechnologies; this allows for nationwide roaming.

11. The combination of digital and spread-spectrum

modes supports several times as many signals per

unit bandwidth as analog modes.

12.Efficient practical utilization of fixed frequency

spectrum.


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Disadvantages to using CDMA:

1.As the number of users increases, the overall

quality of service decreases

2.Self-jamming

3.Near- Far- problem arises

CDMA Applications

One of the early applications for code division

multiplexing is in GPS. This predates and is distinct from

its use in mobile phones.

1.The Qualcomm standard IS-95, marketed as

cdmaOne.

2.

The Qualcomm standard IS-2000, known asCDMA2000. This standard is used by several

mobile phone companies, including the

Globalstar satellite phone network.

3. The UMTS 3G mobile phone standard, which

uses W-CDMA.

4. CDMA has been used in the OmniTRACS

satellite system for transportation logistics.

5.Navigation Purpose

6. Real time stock of different exchanges

Conclusion

The basic problem of cellular traffic is removed by the

use of CDMA. It provides about 10 times more capacity

then analog networks- far more then TDMA & GSM

systems. CDMA is a "spread spectrum" technology,

allowing many users to occupy the same time and

frequency allocations in a given band/space. CDMA

consistently provides better capacity for voice and data

communications.

Bibliography

[1] Lee JS and Miller LM, CDMA System Engineering

Handbook, Arttech Publishing House, 1998

[2] Viterbi A, CDMA-Spread Spectrum Communication,

Addison Wesley 1995.

[3] Ageev, D. V. (1935). "Bases of the Theory of Linear

Selection. Code Demultiplexing". Proceedings of the

Leningrad Experimental Institute of Communication: 3

35.

[4] http://www.webopedia.com

[5]http://www.thestudymaterial.com/presentation-

seminar/electronics-presentation/52-cdma.html?start=4

[6]http://wirelessresource.blogspot.in/2012/07/what-is-

cdma-advantage-disadvantage-and_4266.html
http://www.thestudymaterial.com/presentation-seminar/electronics-presentation/52-cdma.html?start=4http://www.thestudymaterial.com/presentation-seminar/electronics-presentation/52-cdma.html?start=4http://www.thestudymaterial.com/presentation-seminar/electronics-presentation/52-cdma.html?start=4http://www.thestudymaterial.com/presentation-seminar/electronics-presentation/52-cdma.html?start=4http://wirelessresource.blogspot.in/2012/07/what-is-cdma-advantage-disadvantage-and_4266.htmlhttp://wirelessresource.blogspot.in/2012/07/what-is-cdma-advantage-disadvantage-and_4266.htmlhttp://wirelessresource.blogspot.in/2012/07/what-is-cdma-advantage-disadvantage-and_4266.htmlhttp://wirelessresource.blogspot.in/2012/07/what-is-cdma-advantage-disadvantage-and_4266.htmlhttp://wirelessresource.blogspot.in/2012/07/what-is-cdma-advantage-disadvantage-and_4266.htmlhttp://wirelessresource.blogspot.in/2012/07/what-is-cdma-advantage-disadvantage-and_4266.htmlhttp://www.thestudymaterial.com/presentation-seminar/electronics-presentation/52-cdma.html?start=4http://www.thestudymaterial.com/presentation-seminar/electronics-presentation/52-cdma.html?start=4

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Term paper on CDMA