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Telecom Courses : CDMA

Tutorial by:      Date: 2016-08-06 04:09:59

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CDMA-   Code division multiple access


Multiple Access in CDMA:
Each user is assigned a unique PN code.
Each user transmits its information by spreading with unique code.
Direct Sequence spread spectrum is used.
Users are seperated by code not by time slot and freq slot.


Spread Spectrum and Multiple Access:
Spread Spectrum
In Spread spectrum data is transmitted with BW in excess to minimum BW  necessary to send it.
Spread spectrum is achieved by spreading with PN code at transmitter . Same code is used to despread the received signal at reciever
How do we get increased spectrum


Code division multiple access (CDMA) is a channel access method used by various radio communication technologies.

CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication channel. This allows several users to share a band of frequencies (see bandwidth). To permit this without undue interference between the users, CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code).


        Multiple Access Techniques
             Multiple access techniques are used to allow a large number of mobile users to share
                  the allocated spectrum in the most efficient manner. As the spectrum is limited, so
                   the sharing is required to increase the capacity of cell or over a geographical area
                   by allowing the available bandwidth to be used at the same time by different users.
                       And this must be done in a way such that the quality of service doesn’t degrade
                       within the existing users.
          8.1 Multiple Access Techniques for Wireless Communi-cation
                      In wireless communication systems it is often desirable to allow the subscriber to
                  send simultaneously information to the base station while receiving information from
                     the base station.
                 A cellular system divides any given area into cells where a mobile unit in each
                   cell communicates with a base station. The main aim in the cellular system design
                       is to be able to increase the capacity of the channel i.e. to handle as many calls
                       as possible in a given bandwidth with a sufficient level of quality of service. There
                    are several different ways to allow access to the channel. These includes mainly the
                   1) Frequency division multiple-access (FDMA)
                    2) Time division multiple-access (TDMA)
                   3) Code division multiple-access (CDMA)



Steps in CDMA modulation

CDMA is a spread-spectrum multiple access technique. A spread spectrum technique spreads the bandwidth of the data uniformly for the same transmitted power. A spreading code is a pseudo-random code that has a narrow ambiguity function, unlike other narrow pulse codes. In CDMA a locally generated code runs at a much higher rate than the data to be transmitted. Data for transmission is combined via bitwise XOR (exclusive OR) with the faster code. The figure shows how a spread spectrum signal is generated. The data signal with pulse duration of T_{b} (symbol period) is XOR’ed with the code signal with pulse duration of T_{c} (chip period). (Note: bandwidth is proportional to 1/T, where T = bit time.) Therefore, the bandwidth of the data signal is 1/T_{b} and the bandwidth of the spread spectrum signal is 1/T_{c}. Since T_{c} is much smaller than T_{b}, the bandwidth of the spread spectrum signal is much larger than the bandwidth of the original signal. The ratio T_{b}/T_{c} is called the spreading factor or processing gain and determines to a certain extent the upper limit of the total number of users supported simultaneously by a base station.





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