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Service Manual
Digital Cellular
Multiplexing
Using a single frequency to carry two or more
communication links (e.g., conversations) is
called multiplexing. There are two types of
multiplexing that are feasible for cellular:
code division multiplexing and time division
multiplexing. Both code division multiplex-
ing and time division multiplexing digitize
voice before transmitting the signal. Another
type of multiplexing, frequency division mul-
tiplexing, was briefly considered, then aban-
doned. We will deal with each type of multi-
plexing separately.
FDMA (Frequency Division Multiple
Access)
Frequency Division Multiple Access
Figure 11. Digitizing Voice
(FDMA) uses two or more modulated sub-
carriers to modu-
late a third true carrier simultaneously.
While as many as six communication links
can be accommodated on a single frequency
with FDMA, the bandwidth requirements are
enormous. Given the relatively small 30 kHz
bandwidth of cellular, FDMA was never a
contender for improving the load carrying
capacity of cellular systems. Also, it should
be pointed out that FDMA is not necessarily
digital.
Digitizing Voice
If a person speaks into a microphone, a trans-
ducer in the microphone converts the me-
chanical air movements produced by the
person' s vocal cords into varying voltages. If
an oscilloscope probe is connected to the out-
put from a microphone, a varying voltage
Time
© 2000 Motorola, Inc.
Cellular Overview
Digital Cellular
17
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