Concept of CDMA

Code Division Multiple Access (CDMA) is a kind of multiplexing and common access which dividing channel not based on time (such as TDMA) or frequency (such as FDMA), but it make codes for the data with spesific orthogonal codes which associated to each channel dan uses constructive interference characteristic from that spesific codes to multiplex. CDMA is also based on digital cellular phone which uses common access scheme. Qualcom is a vendor who blazed it.

CDMA is a military technology which the first use was at Global War II by England’s ally to foiled Germany’s effort to disturbed their transmission. Allies decided to transmit not only in one frequency but also in another frequancies (many frequencies), so it would be awkward effort to get the complete signal. Start from that moment, CDMA becomes popular in communication system, include Global Positioning System (GPS) and also in OmniTRACS satelite for transportation logistic. Latest system designed and builed by Qualcomm, and became an embrio which helped Qualcomm engineers to found soft handoff technology dan fast power control, the technology to make CDMA become handy dan efficient for terrestrial celluler communication.

The priciple of CDMA is an ammount of users has a unique orthogonal codes which different with others. The standart of IS-95 decided rate of data at the end of spreading is 1,2288Mcps and it needs bandwidth approximately 1,25 Mhz. The performance of CDMA is limited by the inteference, it means the capacity and the qualities limited by interference power which happened in used RF band. Capacity defined as summary of users who could be served by system simultanously. BER quality is required to served users.

In digital cummunication, to show the quality it ussualy use a term called Eb/N0, Eb represent energy of bit, N0 represent about density of noise power. M = (W/R)/(Eb/N0)

Proportional capacity use processing gain. To achieve acceptable BER, we need Eb/N0 value. The capacity could be increased if the interference factor has reduced. Sectorize is a solution to reduce interferences. Beside that the perfection of control power is really affect the capacity. 100% loading factor is only theoritic and it never reached.

About Pradini Puspitaningayu

a big, big, big dreamer

Posted on August 12, 2009, in Knowledge. Bookmark the permalink. Leave a comment.

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