Page 316 - Satellite Communications, Fourth Edition

296 Chapter Ten

A

1.0

0.5

f
f 1 R sym B
2
Figure 10.10 The raised cosine response.

and

1
f R sym (10.12)
1
2
For binary transmission, the symbol rate simply becomes the bit rate.
Thus, for the T1 signal, the required baseband bandwidth is

1
B 1.544 10 6
2

0.772(1 ) MHz (10.13)
For a rolloff factor of unity, the bandwidth for the T1 system becomes
1.544 MHz.
Although a satellite link requires the use of a modulated carrier wave,
the same overall baseband response is needed for the avoidance of ISI.
Fortunately, the channel for a satellite link does not introduce frequency
distortion, so the pulse shaping can take place in the transmit and
receive filters. The modulation of the baseband signal onto a carrier is
discussed in the following section.

10.6 Digital Carrier Systems
For transmission to and from a satellite, the baseband digital signal
must be modulated onto a microwave carrier. In general, the digital
baseband signals may be multilevel (M-ary), requiring multilevel mod-
ulation methods. The main binary modulation methods are illustrated
in Fig. 10.11. They are defined as follows:
On-off keying (OOK), also known as amplitude-shift keying (ASK).The
binary signal in this case is unipolar and is used to switch the carrier
on and off.