The IEEE defines electromagnetic compatibility (EMC) as "The requirements for
electromagnetic emission and susceptibility dictated by the physical environment and
regulatory governing bodies in whose jurisdiction a piece of equipment is operated."
We'll call electromagnetic emission (EMI) RFI. It just means the level of RF interfer-
ence caused by a certain piece of equipment such as a microwave terminal. Susceptibility
deals with how well a piece of equipment can operate in an RFI environment. EMC can
be a real headache for a microwave engineer.
Satellite communications is an extension of LOS microwave technology covered in
Section 9.2. The satellite must be within line-of-sight of each participating earth ter-
minal. We are more concerned about noise in satellite communication links than we were
with LOS microwave. In most cases, received signals will be of a much lower level.
On satellite systems operating below 10 GHz, very little link margin is required; there
is essentially no fading, as experienced with LOS microwave. The discussion here only
deals with geostationary orbit (GEO) communication satellites.
Satellite communications presents another method of extending the digital network
(Chapter 6). These digital trunks may be used as any other digital trunks for telephony,
data, the Internet, facsimile, and video. However, fiber optics has become a strong com-
petitor of satellite communications. Only very small aperture terminal (VSAT) systems
are showing any real growth in the GEO arena. A new type of communication satellite
is being fielded. This is the LEO class of satellites, which we discuss in Chapter 15.
Three-fourths of the satellite transponders over North America are used to provide
entertainment services such as direct broadcast television and cable system head-end
feeds, as well as for private broadcaster connectivity.
Most of the commercial communication satellites that are presently employed are RF
repeaters. A typical RF repeater used in a communication satellite is illustrated in
Figure 9.17. The tendency is to call these types of satellite bent pipe as opposed to
processing satellites. A processing satellite, as a minimum, demodulates and regenerates
the received digital signal. It may also decode and recode (FEC)
a digital bit stream.
It also may have some bulk switching capability, switching to crosslinks connecting to
other satellites. Theoretically, three GEO satellites placed correctly in equatorial orbit
could provide connectivity from one earth station to any other located anywhere on the
surface of the earth (see Figure 9.18). However, high latitude service is marginal and it
is nil north of 80
N and south of 80
Three Basic Technical Problems
As the reader can appreciate, satellite communication is nothing more than LOS micro-
wave using one (or two)
located at great distances from the terminal earth stations,
FEC, forward error correction. (See Reference 3, Chapter 4.)
For voice communications, connectivity is limited to only one GEO satellite link because of the delay