Page 421 - Satellite Communications, Fourth Edition

Interference 401

of the effects that this has on the B 1 and B 2 levels of interference are
examined later in the chapter. It may be noted, however, that although
the larger authorized operators will in general be able to meet the
costs of technical improvements needed to offset the increased inter-
ference resulting from reduced orbital spacing, the same cannot be
said for individually owned television receive-only (TVRO) installations
(the “home satellite dish”), and these users will have no recourse to
regulatory control (Chouinard, 1984).
Interference with individually owned TVRO receivers also may occur
from terrestrial station transmissions in the 6/4-GHz band. Although
this may be thought of as an A 1 mode of interference, the fact that
these home stations are considered by many broadcasting companies
to be “pirates” means that regulatory controls to reduce interference are
not applicable. Some steps that can be taken to reduce this form of
interference are described in a publication by the Microwave Filter
Company (1984).
It has been mentioned that the Radio Regulations place limits on the
energy spectral density which may be emitted by an earth station.
Energy dispersal is one technique employed to redistribute the trans-
mitted energy more evenly over the transmitted bandwidth. This prin-
ciple is described in more detail later in this chapter.
Intermodulation interference, briefly mentioned in Sec. 7.7.3, is a
type of interference which can occur between two or more carriers using
a common transponder in a satellite or a common high-power amplifier
in an earth station. For all practical purposes, this type of interference
can be treated as noise, as described in Sec. 12.11.

13.2 Interference between Satellite Circuits
(B and B Modes)
2
1
A satellite circuit may suffer the B and B modes of interference shown
1
2
in Fig. 13.1 from a number of neighboring satellite circuits, the result-
ant effect being termed aggregate interference. Because of the difficul-
ties of taking into account the range of variations expected in any
practical aggregate, studies of aggregate interference have been quite
limited, with most of the study effort going into what is termed single-
entry interference studies (see Sharp, 1984a). As the name suggests,
single-entry interference refers to the interference produced by a single
interfering circuit on a neighboring circuit.
Interference may be considered as a form of noise, and as with noise,
system performance is determined by the ratio of wanted to interfering
powers, in this case the wanted carrier to the interfering carrier power
or C/I ratio. The single most important factor controlling interference
is the radiation pattern of the earth-station antenna. Comparatively