CODE DIVISION MULTIPLE ACCESS

CODE DIVISION MULTIPLE ACCESS

ABSTRACT:

With the rise in market for mobile communications, service providers have to face many problems. Like that of security, clarity of voice and a fixed bandwidth, to cater to a rapidly increasing number of users. A solution to these problems comes in the form of CDMA technology.

Code Division Multiple Access (CDMA) is a multiple access technology, which allows multiple users to simultaneously transmit and receive information over the entire system bandwidth. These multiple users are distinguished using ‘codes’. These codes are unique to every user and are known only to the transmitter and its’ corresponding receiver. The coding process at the transmitting end spreads the information signal and converts it into a spread spectrum (SS) signal whereas, the coding at the receiving end despreads the SS signal and converts it into the original (information) signal. Spreading increases the bandwidth but decreases the power level of the signal. The transmitted SS signal is received by many receivers but, due its low power level, it is treated as noise by them. The intended receiver, however, knows the code that was used at the transmitting end and successfully applies it to the received SS signal, to despread it and recover the original (information) signal. Thus, communication takes place only between the transmitter and its’ corresponding receiver, and a secure communication channel is established.

This paper starts off with a brief introduction to Spread Spectrum Modulation (SSM)-the underlying principle of CDMA technology. After explaining the concept of SSM, the generalized model of a SS system has been presented. The various types of SS systems are followed by ‘the suitability of Direct Sequence’ type of SS systems for CDMA. The applications of SSM preceed a brief introduction to CDMA. The principle of CDMA has been discussed in detail along with the technology needed to implement it. The ‘various spreading sequences’ have also been described in addition to. The merits and demerits of CDMA technology. A comparison table between the different multiple access technologies concludes the paper.

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