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CDMA

Code Division Multiple Access

Compiled By:Muhummad Sherjeel

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Multiple Access

Multiple access is a radio transmissionscheme that allows several earth stations totransmit in the same time span.

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Types of Multiple Access

There are three types of Multiple AccessMethods

Frequency Division Multiple Access (FDMA)

Time Division Multiple Access (TDMA)

Code Division Multiple Access (CDMA SpreadSpectrum)

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Introduction of CDMA

Code division multiple access (CDMA) isa channel access method used by variousradio communication technologies.

CDMA employs spread-spectrum technologyand a special coding scheme (where eachtransmitter is assigned a code) to allowmultiple users to be multiplexed over thesame physical channel.

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Spread spectrum

Spread-spectrum techniques are methods by

which a signal (e.g. an electrical,

electromagnetic, or acoustic signal) generated in

a particular bandwidth is deliberately spread inthe frequency domain, resulting in a signal with a

wider bandwidth. These techniques are used fora variety of reasons, including the establishment

of secure communications, increasing resistance

to natural interference, noise and jamming, to

prevent detection.

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Generation standards of CDMA

2G

CdmaOne

3G

Cdma 2000

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Types of CDMA

Frequency hop CDMA

Direct Sequence CDMA

Multi Carrier CDMA

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DS-CDMA (Direct Sequence)

Most commonly used CDMA

Data Signal is multiplied by PN code.

Each user is asssigned a unique code.

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PN Codes

Pseudorandom noise (PN) is a signal similarto noise which satisfies one or more of thestandard tests for statistical randomness.

Although it seems to lack anydefinite pattern, pseudorandom noiseconsists of adeterministic sequence of pulses that willrepeat itself after its period.

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Block diagram of modulator

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Waveform of CDMA modulation

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Working

Each user in a CDMA system uses a different code tomodulate their signal.

Choosing the codes used to modulate the signal is

very important in the performance of CDMAsystems. The best performance will occur whenthere is good separation between the signal of adesired user and the signals of other users.

The separation of the signals is made by correlatingthe received signal with the locally generated codeof the desired user. If the signal matches the desireduser's code then the correlation function will be highand the system can extract that signal.

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