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CDMA

Decades ago, the military organizations of the world adopted the Code Division Multiple Access (CDMA) scheme to meet the need for highly secure and easily concealed communication. CDMA uses a spreading code to scramble the data and spread the bandwidth. In the past 15 years CDMA has become popular in commercial wireless and satellite communications.

Another term for CDMA is Spread Spectrum, which indicates that the bandwidth of the information is actually multiplied by the spreading code. Spreading factors range from 4 to 1000, where the improvement gained is proportional to this factor (converted to dB, it is called the spreading gain).

How CDMA works

In CDMA, multiple stations can transmit on top of each other - on the same or overlapping frequencies and at the same time.

This seeming contradiction is possible because the signals are separately encoded and appear to each other to be background noise.

Techniques

There are two basic CDMA techniques, Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FSSS). FSSS is the dominant CDMA technique in satellite communications, illustrated below for a single channel of communication. Qualcomm successfully adapted it to cellular telephone.

The coding comes about through a spreading process within the top circle with the cross through it (a multiplier). This takes the information bits, at the top left, and multiplies it by a high-speed spreading code called the chip.

Once transmitted, the CDMA signal can be added to other CDMA signals as well as noise and interference.

In the receiver, the incoming signal is again multiplied by the same chip code, which is carefully synchronized. The output at the bottom left is the original data.

Because the receiver has its own spreading multiplier and identical copy of the chip code, any other CDMA signals and interference after this second multiplication will get even more spread out and hence become less troublesome.

While the principle would appear to make communication perfect, in actuality the unwanted signals accumulate and can overwhelm the desired signal to the point of non-reception.

 
A Direct Sequence Spread Spectrum communication system, composed of a transmitter (top), satellite link (right), and receiver (bottom).



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