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Sec. 8–5 Satellite Communication Systems 591<br />

36 MHz bandwidth<br />

Common signaling<br />

channel (CSC)<br />

1<br />

3<br />

QPSK signals<br />

4 5 6 7<br />

Pilot carrier<br />

400 403 800<br />

160 kHz 38 kHz<br />

bandwidth<br />

7 kHz<br />

guardband<br />

kHz<br />

(a) Transponder Frequency Allocation<br />

To satellite<br />

Telephone VF signal<br />

(0-4 kHz, analog)<br />

PCM<br />

encoding<br />

PCM<br />

64 kB/sec<br />

QPSK<br />

transmitter<br />

QPSK<br />

signal<br />

<br />

Telephone VF signal<br />

(0-4 kHz, analog)<br />

PCM<br />

encoding<br />

PCM<br />

64 kB/sec<br />

QPSK<br />

transmitter<br />

QPSK<br />

signal<br />

(b) Possible QPSK SCPC Transmitter Configuration<br />

PA A B<br />

C D E PA A<br />

1 msec<br />

t<br />

One frame = 50 msec<br />

(c) TDMA CSC Signaling Format<br />

Figure 8–12 SPADE satellite communication system for telephone VF message transmission.<br />

the 36-MHz bandwidth of the transponder, as shown in Fig. 8–12a. Thus 800 VF messages<br />

may be transmitted simultaneously through one satellite transponder, where each QPSK signal<br />

is modulated by a 64-kbs PCM (digitized) voice signal (studied earlier in Example 3–1 and<br />

used as inputs to the North American digital hierarchy shown in Fig. 3–40). This SCPC-DAMA<br />

technique, illustrated in Fig. 8–12, is called the SPADE system, which is an acronym for single<br />

channel per carrier, pulse code modulation, multiple access, demand assignment equipment<br />

[Edelson and Werth, 1972]!<br />

The demand assignment of the carrier frequency for the QPSK signal to a particular Earth<br />

station uplink is carried out by TDM signaling in the common signaling channel (CSC) (see<br />

Fig. 8–12a). The CSC consists of a 128-kbs PSK signal that is time shared among the Earth

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