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Code Division Multiple Access

Anas Bin Numan

on 6 November 2013

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Transcript of CDMA

Stands for Code Division Multiple Access
A channel access method,
used by various communication technologies
Each user in a CDMA system uses a different code to modulate their signal.

The separation of the signals is made by correlating the received signal with the locally generated code of the desired user. If the signal matches the desired user's code then the correlation function will be high and the system can extract that signal. If the desired user's code has nothing in common with the signal the correlation should be as close to zero as possible (thus eliminating the signal); this is referred to as cross correlation.
=> Synchronous
=> Asynchronous
The mobile phone standards cdmaOne,
cdma2000 and WCDMA uses CDMA
as theire underlying channel access method.
A spread spectrum multiple access technique
CDMA in mobile
Each user is assigned a unique code are orthogonal to each other called chips code

Orthogonal codes have a cross-correlation nearly equal to zero

Walsh code/table are used to assign users a unique chips code
Uses a binary code to encode /decode stations signal.

The binary sequence /code is random but it can be
reproduced by a deterministic manner by intended receiver

Additional techniques to control the signal power
caused by Multiple Access Interference (MAI)
which one is better ?
Depends on the user's requirements
(cc) image by anemoneprojectors on Flickr
CDMA is prominent in North America, but GSM reaches a lot more users worldwide (about 1 billion vs 270 million).

In the CDMA vs. GSM debate, GSM wins if you plan to travel to foreign countries but CDMA might have a better coverage in your area.
Data transfer
The best data transfer technology it has to offer is the EVDO technology, allowing for a maximum download speed of about 2mb/s (about 700kbps in practice), which is similar to what a DSL line has to offer.
=> Offers EDGE, allowing for a maximum 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.
Thank you
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