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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Copyright © 1996-1999 Arthur H. M. Ross, Ph.D., Limited
