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Cellular OverviewService Manual

17

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

(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

Amplitude (voltage)

Digital Cellular

Figure 11. Digitizing Voice

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