Transmitting chips instead of symbols can lead to good multipath performance
through the use of a RAKE receiver. This performance can be achieved by using PN codes with excellent
correlation properties in a frequency selective fading (FSF) environment.
This technique, though, suffers from multiple access interference (MAI). The source of this interference
is the addition of DS users operating in the same frequency band. The best and most often
used analogy is the cocktail party effect. The party starts with two people having a conversation; as
more couples engage in conversation of their own, using a different language, the background noise
or sound begins to increase. This noise can cause the original two conversants to speak louder and, in
turn, cause the other attendees to speak louder themselves. Caution must be exercised here, since
instability (i.e., unintelligible speech) can be reached. The FH technique is less affected by the nearfar
problem, but requires a more complex frequency synthesizer so that the information signal can
???hop??? around the wide bandwidth available for transmission.
In an FSF channel, certain frequency bands experience spectral ???nulls??? while others don??™t.
Hopping through the entire bandwidth will reduce the possibility that your transmitted signal is in a
fade.
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