563489578934

Sec. 8–5 Satellite Communication Systems 587
Down converter
6 GHz 4 GHz
Uplink
signals
BPF
Bandpass
filter
LNA BPF TWT
Low-noise
amplifier
Bandpass
filter
Traveling wave
tube amplifier
Downlink
signals
LO
Local
oscillator
Figure 8–9
Simplified block diagram of a communications satellite transponder.
Newer satellites operate in higher frequency bands, because there are so few vacant
spectral assignments in the 64-GHz band (C band). The Ku band satellites use 14 GHz on
the uplink and 12 GHz on the downlink, with an orbital spacing of 3°. Some new Ku-band
satellites have high-power amplifiers that feed 120 to 240 W into their transmitting antenna,
as compared with 20 to 40 W for low- or medium-power satellites. High-power satellites—
called direct-broadcast satellites (DBS)—provide TV service directly to the homeowner, who
has a small receiving antenna (2 ft or less in diameter). The resulting system is called the
digital satellite system (DSS) by the FCC.
Each satellite has a number of transponders (receiver-to-transmitter) aboard to amplify the
received signal from the uplink and to down-convert the signal for transmission on the downlink.
(See Fig. 8–9.) Figure 8–9 shows a “bent-pipe transponder” that does not demodulate the received
signal and perform signal processing but acts as a high-power-gain down converter. Most transponders
are designed for a bandwidth of 36, 54, or 72 MHz, with 36 MHz being the standard used for
C-band (64-GHz) television relay service. As technology permits, processing transponders have
come into use since an improvement in error performance (for digital signaling) is realized.
Each satellite is assigned a synchronous orbit position and a frequency band in which to
operate. In the 64-GHz band, each satellite is permitted to use a 500-MHz-wide spectral
assignment, and a typical satellite has 24 transponders aboard, with each transponder using
36 MHz of the 500-MHz bandwidth assignment. The satellites reuse the same frequency band
by having 12 transponders operating with vertically polarized radiated signals and 12
transponders with horizontally polarized signals. † A typical 64-GHz frequency assignment
for satellites is shown in Fig. 8–10. The transponders are denoted by C1 for channel 1, C2 for
channel 2, and so on. These satellites are used mainly to relay signals for CATV systems.
Digital and Analog Television Transmission
TV may be relayed via satellite using either digital or analog transmission techniques.
For digital transmission, the baseband video signal is sampled and digitized. The data
are usually compressed to conserve the bandwidth of the modulated satellite signal. The data
are compressed by removing redundant video samples within each frame of the picture and
removing redundant samples that occur frame to frame.
† A vertically polarized satellite signal has an E field that is oriented vertically (parallel with the axis of
rotation of the Earth). In horizontal polarization, the E field is horizontal.