Page 266 - Satellite Communications, Fourth Edition

246 Chapter Eight

8.5 Transmit-Receive Earth Stations
In the previous sections, receive-only TV stations are described.
Obviously, somewhere a transmit station must complete the uplink to
the satellite. In some situations, a transmit-only station is required, for
example, in relaying TV signals to the remote TVRO stations already
described. Transmit-receive stations provide both functions and are
required for telecommunications traffic generally, including network TV.
The uplink facilities for digital TV are highly specialized and are covered
in Chap. 16.
The basic elements for a redundant earth station are shown in Fig. 8.5.
As mentioned in connection with transponders in Sec. 7.7.1, redundancy
means that certain units are duplicated. A duplicate, or redundant,
unit is automatically switched into a circuit to replace a correspon-
ding unit that has failed. Redundant units are shown by dashed lines
in Fig. 8.5.
The block diagram is shown in more detail in Fig. 8.6, where, for clar-
ity, redundant units are not shown. Starting at the bottom of the diagram,
the first block shows the interconnection equipment required between
satellite station and the terrestrial network. For the purpose of explana-
tion, telephone traffic will be assumed. This may consist of a number of
telephone channels in a multiplexed format. Multiplexing is a method of
grouping telephone channels together, usually in basic groups of 12, with-
out mutual interference. It is described in detail in Chaps. 9 and 10.
It may be that groupings different from those used in the terrestrial
network are required for satellite transmission, and the next block shows
the multiplexing equipment in which the reformatting is carried out.
Following along the transmit chain, the multiplexed signal is modulated
onto a carrier wave at an intermediate frequency, usually 70 MHz.
Parallel IF stages are required, one for each microwave carrier to be
transmitted. After amplification at the 70-MHz IF, the modulated signal
is then upconverted to the required microwave carrier frequency. A
number of carriers may be transmitted simultaneously, and although
these are at different frequencies they are generally specified by their
nominal frequency, for example, as 6-GHz or 14-GHz carriers.
It should be noted that the individual carriers may be multidestina-
tion carriers. This means that they carry traffic destined for different
stations. For example, as part of its load, a microwave carrier may have
telephone traffic for Boston and New York. The same carrier is received
at both places, and the designated traffic sorted out by filters at the
receiving earth station.
Referring again to the block diagram of Fig. 8.6, after passing through
the upconverters, the carriers are combined, and the resulting wideband
signal is amplified. The wideband power signal is fed to the antenna