Page 439 - Satellite Communications, Fourth Edition

Interference 419

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quantity. Thus, 1 W/Hz is equivalent to 1 J/s ÷ s , which is simply J.
The units for power spectral density therefore can be stated as joules.
This is verified by Eq. (12.15) for noise power spectral density.

13.5 Problems and Exercises
13.1. Describe briefly the modes of interference that can occur in a satellite
communications system. Distinguish carefully between satellite and terrestrial
modes of interference.
13.2. Define and explain the difference between topocentric angles and
geocentric angles as applied to satellite communications. Two geostationary
satellites have an orbital spacing of 4°. Calculate the topocentric angle subtended
by the satellites, measured (a) from the midpoint between the subsatellite points
and (b) from either of the subsatellite points.

13.3. Westar IV is located at 98.5°W and Telstar at 96°W. The coordinates for
two earth stations are 104°W, 36°N and 90°W, 32°N. By using the look angle and
range formulas given in Sec. 3.2, calculate the topocentric angle subtended at
each earth station by these two satellites.

13.4. Explain what is meant by single-entry interference. Explain why it is the
radiation pattern of the earth-station antennas, not the satellite antennas,
which governs the level of interference.

13.5. A geostationary satellite employs a 3.5-m parabolic antenna at a
frequency of 12 GHz. Calculate the 3-dB beamwidth and the spot diameter on
the equator.
13.6. Calculate the 3-dB beamwidth for an earth-station antenna operating
at 14 GHz. The antenna utilizes a parabolic reflector of 3.5-m diameter. Compare
the distance separation of satellites at 2° spacing with the diameter of the beam
at the 3-dB points on the geostationary orbit.
13.7. Compare the increase in interference levels expected when satellite
orbital spacing is reduced from 4° to 2° for earth-station antenna sidelobe
patterns of (a) 32 25 log
dB and (b) 29 25 log
dB.

13.8. A satellite circuit operates with an uplink transmit power of 28.3 dBW
and an antenna gain of 62.5 dB. A potential interfering circuit operates with an
uplink power of 26.3 dBW. Assuming a 4-dB polarization discrimination figure
and earth-station sidelobe gain function of 32 25 log
dB, calculate the [C/I ]
ratio at the satellite for 2° satellite spacing.

13.9. The downlink of a satellite circuit operates at a satellite [EIRP] of 35 dBW
and a receiving earth-station antenna gain of 59.5 dB. Interference is produced
by a satellite spaced 3°, its [EIRP] also being 35 dBW. Calculate the [C/I ] ratio