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2.6 Interference
2.6.1 Intrasystem interjerence. 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 tequired C/No for a
given performance.
lllU0l
No. 76
Annex 10 - Aeronautical Telecontmunkatdons Volume ZXZ
2.6.2 Intersystem integemnce. Intetsystem 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 single-entry
interference resulting in a ATIT 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. RF'CHANNEL
CHARACTERISTICS
3.1 Modulation characteristics
3.1.1 Modulation types. n o modulation types are used in
aeronautical mobile-satellite service (AMSS), each providing a
system advantage. A form of binary phase shift keying (BPSK)
is used for channel rates up to 2.4 kbitsts. 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 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 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 +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 strategy is illustrated conceptually in
Figures A-3 and A-4 of this guidance material. Consequently,
A-BPSK is almost identical to minimum shift keying (MSK),
except that the pulse shaping has a 40 per cent root raised
cosine spectral shape, as opposed to sinusoidal weighting. The
amplitude and phase masks which this pulse-shaping filter
must satisfy are illustrated in Figures A-5 and A-7 of this
guidance material. These correspond to the transmit filter
requirements given in the definition of A-BPSK. Those
requirements apply to the transmitted signal before it
undergoes any non-linear amplification; their purpose is to
limit and control the distortion and corresponding degradation
in performance caused by nonlinear amplification. A-BPSK is
a linear modulation with nearly constant envelope.
Consequently, it may be transmitted through a "Class C"
amplifier with little spectral spreading and performance
degradation.
3.1.3 Aviation QPSK. Aviation QPSK is a form of offset
QPSK modulation that is used for data rates above 2.4 kbitsts
and is illustrated conceptually in Figures A-3 and A-4 of this
guidance material. The A-QPSK data encoder is driven by a
binary data sequence (ai) at the bit rate 2/T. The "even" bits are
switched onto the I line and the "odd" bits onto the Q line,
generating two data streams at rate 1TT. The synchronous
samplers S operate at rate 1lT and generate ideal positive and
negative impulses depending on whether the data bits are "1"
or "0". The pulse shaping filters in each channel have a
100 per cent root raised cosine spectral shape, except for the
8 400 bit& C channel, which has a 60 per cent root raised
cosine spectral shape. The outputs of the I and Q pulse shaping
filters modulate the same carrier in quadrature and are
　
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