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Basic Concept of CDMA


Posted Date: 08-Aug-2010  Last Updated:   Category: Computer & Technology    
Author: Member Level: Silver    Points: 25


Hi Friends, Here I'm sharing the basic concept of CDMA technology with you all, hope you like it.



The CDMA system is a self-interfering system, in which all the mobile users occupy the same bandwidth and frequency.
This system allocates each user a unique code sequence (spread spectrum code), which is used to encode the signals of the bearer information.
The receiver that knows the code sequence decodes the received signals and restores them to the original data because the cross correlation between the code sequences of one user and another user is very slight.
Image the bandwidth as a large room. All the people will enter the only large room.
If speaking in completely different languages, they can clearly hear the voice of their fellows, and can only be interfered by the conversations of other people. The air in the room can be imaged as the carrier of the bandwidth, and different languages are regarded as the codes.
We can constantly add the users until we are blocked by the background noise.
If the user's signal intensity can be controlled, we can accommodate more users while keeping high-quality communication.
As the bandwidth of the code sequence is much wider than the bandwidth of signal bearing information, and the frequency spectrum of the signal has been spread in the encoding process, so this is called as spread spectrum modulation, and the generated signals are called as spread spectrum signals.
CDMA is often indicated by Spectrum Spreading Multiple Access (SSMA).
The spread spectrum of the transmitted signals is to provide the CDMA with multiple access capability. The spread spectrum modulation technique must cater to two basic requirements:
1. The bandwidth of the transmitted signals must be much greater than the information bandwidth.
2. The bandwidth of the generated RF signals is independent of the transmitted information.
In the spread spectrum system, the ratio of the bandwidth (Bt) of the transmitted signals to the information bandwidth (Bi) is called as the processing gain of the spread spectrum system, i.e., Gp = Bt/Bi. It can be seen that the processing gain of the CDMA system is very high.
The receiver adopts the same spread spectrum code and the received signals to conduct the correlation operation to restore the carried original information.
As the spread spectrum signal is a spectrum-spread signal, it has many advantages that are different from those of the narrowband signal.
a. Multiple access capability
b. Multi-path interference resistance capability
c. Privacy
d. Man-made interference resistance
e. Low interception probability
f. Narrowband interference resistance capability
According to different spread spectrum modulation modes, CDMA can be divided into Direct Sequence Spread Spectrum (DS-SS), Frequency Hopping Spread Spectrum (FH-SS), Time Hopping Spread Spectrum (TH-SS) and hybrid spread spectrum.


Regards,
Arijit Mukherjee


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