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264 CHAPTER NINE
Where code is the user’s unique code that selects the characteristics of the waveform
i
U(code ). These waveforms are typically a series of pulses that have the following char-
i
acteristics. Whereas represents a bit by multiplication (correlation):
U1code 2 U1code 2 Z V 1
i
k
unless k i and the two waveforms are synchronized, in which case
U1code 2 U1code 2 1
i i
In addition, U(code ) B is very small for uncorrelated signals (like radio trans-
i
missions) that may already exist in the channel. This means that SS signals can coexist
(overlay) in the channel with existing communication users.
In some SS protocols, the data is first modulated by the spreading waveform prior to
transmission. Consider the case where the channel is filled with the waveforms of two
users. We can extract a single user in the following way:

Channel D U1code 2 D U1code 2
i
i
k
k
where i and k are different and D is the data from user i.
i
Channel U1code i 2 U1code i 2 1D i U1code i 2 D k U1code k 22

Channel U1code i 2 D i U1code i 2 U1code i 2 D k U1code k 2 U1code i 2

Channel U1code i 2 D i Z D i
Similarly, the waveform for user k can be cleanly extracted as well.
On the plus side, SS communications can coexist with existing, uncorrelated com-
munication signals in the channel. This basically allows the channel spectrum to be
reused.
On the minus side, the different codes are not completely orthogonal. The previous
small signal Z is multiplied by the number of other users and can interfere with recep-
tion. This can limit the number of users.
Here are a few PDFs discussing shared access communication links:

http://courses.cs.vt.edu/ cs5516/spring02/Phy_mac_6.pdf
www.cse.sc.edu/ srihari/csce516/lecnotes/shared.6.pdf
http://web.mit.edu/course/16/16 .682/www/lec18.pdf