CDMA

 CDMA (IS-95):

CDMA


A US digital cellular system based on CDMA that has increased system capacity was standardized as Interim Standard 95 (IS-95) or CDMA. It is one of the basis for many commercial 2G cellular systems around the world.

It is used in many communication systems like Personal Communication System (PCS), fixed Wireless Local Loop (WLL), Global Positioning System (GPS) and Omni TRACS satellite system for the transportation logistics.

IS-95 allows each user within a cell to use the same radio channel and users in adjacent cell to use the same radio channel.

CDMA / IS-95 systems allow many data signals to be multiplexed and transmitted over a wireless channel at the same time and in the same frequency band without interfering with each other. It is done by spreading the spectrum occupied by user with high speed unique codeword to that user.

The spectrum spreading is done by multiplying the user data by identifying the code and modulating the carrier with spread spectrum. At the receiver end the original data is recovered by correlating the demodulated waveform by the spreading code.

The spread spectrum was a used for enhancing the security of the data transmitted. It is used in applications where high security is needed.

DS/SS system is generally used. The data signals that are to be transmitted are changed through the use of pseudo-noise code (PN code). Whereas in time division systems, the channels are separated by time slot e.g. GSM. The CDMA systems can be distinguished according to the PN code that the systems use.

The PN codes used must be orthogonal i.e. they must not correlate with the time shift versions of each other. Hence, the PN codes of each signal are unique.

The pseudo noise code can be generated by using one or more shift registers in feedback connections. The PN spreading code is called as chipping code.

The processing gain is,

 G =Rchip/Rdata

Thus, the processing gain is proportional to the ratio of spreading code rate to the data rate.

In CDMA systems three types of PN codes are used. They are:

1.

  • Walsh code: It is used as 64 bit different orthogonal codes on 
  • downlink: for different users 
  • uplink: for modulation

2.

  • Long PN code: It is a 42-bit code used on 
  • downlink: for data scrambling 
  • uplink: for identifying the mobile station (spreading)

3.

  • Short PN code: It is a 16 bit code used on
  • downlink: to identify the base stations. 
  • uplink: for extra signal robustness without offset.

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