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262 CHAPTER NINE
FREQUENCY DIVISION SYSTEMS
It is certainly possible to put different communication entities on different frequencies
within the allowable communication channel frequencies. Several issues arise, such as
frequency allocation and frequency separation.
Frequency Allocation

All of the same reservation issues of reserving bandwidth are present in frequency divi-
sion systems. If a frequency goes unused, then the bandwidth is wasted. If reservations
are required, then overhead exists for making the reservations.

Frequency Separation
Communication channels on adjacent frequencies must not interfere with one another.
Filters are used to remove adjacent frequencies from a communication band.
Since perfect filters are impossible to make, we must leave extra bandwidth between
frequency bands. It is impossible to pack different frequency bands too close together.
Both the transmitter and receiver run into trouble if they are too close.
A few other problems can crop up when frequency bands are packed close together.
Distortion The transmitter may have trouble with intermodulation distortion.
Consider the case where two frequencies, f1 and f2, are amplified and up-converted
together. The result is unwanted distortion signals at frequencies (f2 f1) and (f1
f2). Here is a PDF file and a few URLs speaking about such distortion:

www.sinctech.com/pdfs/Intermod.pdf
www.audiovideo101.com/dictionary/im-distortion.asp
www.atis.org/tg2k/_intermodulation.html

ISI If frequencies are too close together, the electronics handling each fre-
quency may have trouble filtering out the adjacent signals. Although frequency
division systems are viable and work fine, time division and code division sys-
tems have stolen the thunder of this technology.

CODE DIVISION SYSTEMS
Code division systems use a form of encryption where each user’s data is invisible to
the other users.