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a) AMSS includes safety services;
b) there is concern about the availability of adequate
AMSS spectrum, and adequate capacity for AMSS
safety services; and
c) the difficulty in co-ordinating mobile satellite networks
due to the poor discrimination characteristics of mobile
station antennas.
Guidelines that should be considered when co-ordinating
frequency plans to minimize intra 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 Report 1 185);
e) efficient spectrum use including the following:
1) using other system providers' satellite transponder
guard bands;
2) using frequency assignment by aircraft loc~on;
3) taking advantage of improvements in aircraft earth
station antenna sidelobe discrimination;
4) using offset and interleaved carriers;
5) using satellite spot/shaped 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 schedules to take
advantage of the difference in time zones;
1 1) appropriately gruuping carriers;
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 lntrasystem interfemnce. 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/No below the required C/No for a
given performance.
Zll UOO
No. 75
Annex 10 - Aeronautical Telecommunic~rrs ~olumeIX I
2.6.2 Iaterxystem inte$emnce. 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
co-ordination among the particular satellite system operators.
As a minimum, the AMSS satellite system should provide
adequate performance in the presence of singIe-entry
interference resulting in a aTfl 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 ATIT of 20 per cent.
3. FW CHANNEL
CHARACTERISTPCS
3.1 Modulation characterfstics
3.1.1 Modulation types. Rvo modulation types are used in
aeronautical mobile-satellite service (AMSS), each providing a
system advantage. A fonn of binary phase sbift keying (BPSK)
is used for channel rates up to 2.4 kbiWs, 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 kbitsls,
phase noise effects on the demodulation process are
diminished, and consemation of bandwidth at these higher
channel rates becomes important. Therefore, a morebandwidth-
efficient modulation type, quaternary phase shift
keying (QPSK), is used,
3.1.2 Aviation BPSK. Aviation BPSK is a form of phase
shift keyed modulation with shaped filters 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 +*9Q0. This
results in differential encoding of the transmitted data, and
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