Page 429 - Satellite Communications, Fourth Edition

Interference 409

Calculation of [RTC] for various combinations of wanted and inter-
fering carriers is very complicated and has to be done by computer. As
an example, taken from Sharp (1983), when the wanted carrier is TV/FM
with a modulation index of 2.571 and the interfering carrier is TV/FM
with a modulating index of 2.560, the carrier frequency separation being
zero, the [RTC] is computed to be 31.70 dB. These computations are lim-
ited to low levels of interference (see Sec. 13.2.8).

13.2.7 Specified interference objectives
Although [C/I] pb or [S/I] gives a measure of interference, ultimately,
the effects of interference must be assessed in terms of what is toler-
able to the end user. Such assessment usually relies on some form of
subjective measurement. For TV, viewing tests are conducted, in which
a mixed audience of experienced and inexperienced viewers (experi-
enced from the point of view of assessing the effects of interference)
assess the effects of interference on picture quality. By gradually
increasing the interference level, a quality impairment factor can be
established which ranges from 1 to 5. The five grades are defined as
(Chouinard, 1984)

5. Imperceptible
4. Perceptible, but not annoying
3. Slightly annoying
2. Annoying
1. Very annoying

Acceptable picture quality requires a quality impairment factor of at
least 4.2. Typical values of interference levels which result in acceptable
picture quality are for broadcast TV, [S/I] 67 dB; and for cable TV,
[C/I] pb 20 dB.
For digital circuits, the [C/I] pb is related to the bit error rate (BER)
(see, e.g., CCIR Rec. 523, 1978). Values of the required [C/I] pb used
by Sharp (1983) for different types of digital circuits range from 20
to 32.2 dB.
To give some idea of the numerical values involved, a summary of the
objectives stated in the FCC single-entry interference program is pre-
sented in Table 13.1. In some cases the objective used differed from the
reference objective, and the values used are shown in parentheses. In
some entries in the table, noise is shown measured in units of pW0p.
Here, the pW stands for picowatts. The 0 means that the noise is meas-
ured at a “zero-level test point,” which is a point in the circuit where a
test-tone signal produces a level of 0 dBm. The final p stands for psopho-
metrically weighted noise, discussed in Sec. 9.6.6.