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COMMUNICATIONS 263
Code Division Multiple Access (CDMA)
CDMA systems, also known as spread spectrum (SS) systems, generally use a wide
frequency bandwidth. The data for each user is spread across the entire frequency band
using a spreading code. Every user’s communication is broadcast in the band at the
same time, but they do not interfere with one another. Each user gets a unique spread-
ing code that is used to separate users. Because of the nature of the codes, little or no
interference exists between users. Furthermore, no synchronization is required between
the users.
In 1940, Hollywood actress turned inventor Hedy Lamarr copatented a frequency-
hopping device for military use. It’s kind of nice to have her picture in here among all
the men in powdered white wigs (see Figure 9-15). One interesting quote is attributed
to her: “Any girl can be glamorous ...all she has to do is stand still and look stupid.”
More information on her work can be found at www.inventions.org/culture/female/
lamarr.html and at www.edu-cyberpg.com/Teachers/womenmonth.html#ahedy.
Here’s how the most popular SS systems work. Each user is assigned a coded spread-
ing waveform:

U1code 2
i

FIGURE 9-15 Hedy Lamarr, pioneer in spread spectrum communications
and actress