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due to the poor discrimination characteristics of mobile
station antennas.
Guidelines that should be considered when co-ordinating
frequency plans to minimize infra and interservice interference
include:
a) compliance with the relevant lTU Radio Regulations;
b) each provider should provide monitoring facilities to
identify the actual usage of AMS(R)S and non-
AMS(R)S communications;
C) in those AMSS systems with global and spot beams,
operational measures to minimize the amount of global
bandwidth used and to maximize the use of spot
beams:
d) using the International Radio Consultative Committee
(CCIR) three-phase technical co-ordination method,
wherever possible (see CCIR Repon 11 85);
e) efficient spectmm use including the following:
I) using other system providers' satellite transponder
guard bands;
2) using frequency assignment by aircraft location;
3) taking advantage of improvements in aircraft
earth station antenna sidelobe discrimination;
4) using offset and interleaved carriers;
5) using satellite spotlshaped beams;
6) reducing spacecraft antenna sidelobe levels;
7) increasing the resistance of systems to interference;
8) using earth station power control:
9) using satellite transponder adjustable gain setting;
10) using knowledge of operational schdules to take
advantage of the difference in time zones;
1 1 ) appropriately grouping carriers;
Annex I0 - Aeronautical TeZecommunications Volume ZIZ
12) repositioning satellites; and
13) taking advantage of high-gain AES antennas and
the resulting ability to use lower carrier powers.
2.5.6 Transmitted phase noise. The phase noise mask
that the AES transmitter must meet is illustrated in Figure A-2
of this guidance material. The purpose of this mask is to
minimize the contribution of the AES transmitter phase noise
to the degradation of GES performance.
2.6 Interference
2.6.1 Intrasystem inte$erence. Intrasystem interFerence
refers to ,interference among AMS(R)S services. Some
examples would be co-channel, adjacent channel interference
and intermodulation noise. Due to disparate satellite system
designs, there is no single specification for intrasystem
interference. Each satellite system operator must be able to
show that intrasystem interference to AMS(R)S services, when
combined with other noise sources in the link, does not
degrade the achieved link C/N, below the required CfN,, for a
given performance.
2.6.2 Intersystem interference. Intersystem interference
refers to interference to an AMS(R)S service from any other
system, whether it is providing AMS(R)S services or otherwise.
Required performance should be maintained at whatever
level of interference is adopted as operable through coordination
among the particular satellite system operators. As
a minimum, the AMSS satellite system should provide adequate
performance in the presence of single-entry interference
resulting in a ATR of 6 per cent, as adopted by WARC-ORB-
88 as the threshold requiring co-ordination between satellite
systems. A suggested criterion for aggregate interference due
to all sources, including intrasystem interference, is a aT/T of
20 per cent.
3. RF CHANNEL
CHARACTERISTICS
3.1 Modulation characteristics
3.1.1 Modulation types. Two modulation types are used
in aeronautical mobile-sateIlite service (AMSS), each
providing a system advantage. A form of binary phase shift
keying (BPSK) is used for channel rates up to 2.4 kbitsls.
providing more robustness against phase noise generated in
frequency conversion processes in the aircraft earth station
(AES), satellite, and ground earth station (GES). Above 2.4
kbits/s, phase noise effects on the demodulation process are
diminished, and conservation of bandwidth at these higher
channel rates becomes important. Therefore, a more
bandwidth-efficient modulation type, quaternary phase shift
keying (QPSK), is used.
3.1.2 Aviafbn BPSK. Aviation BPSK is a form of
phase shift keyed modulation with shaped fllters especially
adapted to perform in an RF environment subject to fading.
It has four possible phase states of which only two are
permissible during any symbol period. The modulation
technique maps binary "0"s into a phase shift of -90" and
binary "1"s into +90°. This results in differential encoding
of the transmitted data, and implies that during any symbol
period two decisions separated by 180' are possible, and that
these two decisions are rotated by 90' from the possible
decisions in the previous symbol period. This modulation
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