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Principles of CDMA

THE GOAL OF SPREAD SPECTRUM is a substantial increase in bandwidth of an information-bearing signal, far beyond that needed for basic communication. The bandwidth increase, while not necessary for communication, can mitigate the harmful effects of interference, either deliberate, like a military jammer, or inadvertent, like co-channel users. The interference mitigation is a well-known property of all spread spectrum systems. However the cooperative use of these techniques in a commercial, non-military, environment, to optimize spectral efficiency was a major conceptual advance.

SPREAD SPECTRUM systems generally fall into one of two categories: frequency hopping (FH) or direct sequence (DS). In both cases synchronization of transmitter and receiver is required. Both forms can be regarded as using a pseudo-random carrier, but they create that carrier in different ways.

FREQUENCY HOPPING is typically accomplished by rapid switching of fast-settling frequency synthesizers in a pseudo-random pattern. The references can be consulted for further discussions of FH, which is not a part of commercial CDMA.

CDMA uses a form of direct sequence. Direct sequence is, in essence, multiplication of a more conventional communication waveform by a pseudonoise (PN) ±1 binary sequence in the transmitter.

We are taking some liberties with the details. In reality spreading takes place prior to any modulation, entirely in the binary domain, and the transmitted signals are carefully bandlimited.

A second multiplication by a replica of the same ±1 sequence in the receiver recovers the original signal.
The noise and interference, being uncorrelated with the PN sequence, become noise-like and increase in bandwidth when they reach the detector. The signal-to-noise ratio can be enhanced by narrowband filtering that rejects most of the interference power. It is often said, with some poetic license, that the SNR is enhanced by the so-called processing gain W/R, where W is the spread bandwidth and R is the data rate. This is a partial truth. A careful analysis is needed to accurately determine the performance. In IS-95A CDMA W/R = 10 log(1.2288 MHz/9600Hz) = 21 dB for the 9600 bps rate set.Show me the math!To get this right, you have to bite the bullet, and go do some math! We've tried to present it in as simple a fashion as possible.


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