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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 nonlinear
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 (a,) at the bit rate 2/T.T he "even" bits
are switched onto the I line and the "odd" bits onto the Q line,
generating two data streams at rate In. The synchronous
samplers S operate at rate 1TT and generate ideal positive and
negative impulses depending on whether the data bits are "I"
or "0". The pulse shaping filters in each channel have a 100
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 combined linearly. The amplitude and
phase masks that the pulse shaping filter must satisfy are
shown in Figures A-6 and A-7 of this guidance material.
These correspond to the transmit filter requirements given in
the definition of A-QPSK. 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 non-linear amplification. There is no requirement
for actual modulators to be implemented in this way, as long
as the modulated RF signal is indistinguishable from one that
was generated by an ideal modulator.
Amchment A to PaH I Annex 10 - Aeronautical Telecommunicutions
33 %mds on radiated power
spectral density
3.2.1 Spectrum masks. These spectrum masks allow for
degradation from the ideal Nyquist model that could occur due
to non-ideal system characteristics, e.g. saturation in the
amplifier chain.
3.2.2 From-aitrraft. The spectral mask that must be
satisfied by any A-BPSK signal transmitted in the from-aircraft
direction is shown in Figure A-8 of this guidance material.
This was derived assuming a non-linear amplifier (Class C) is
used on board the AES, but it is applicable to Class A linear
amplifiers as well. The same spectral mask (Figure A-8)
applies to A-QPSK except that the frequency axis is scaled by
the symbol rate.
3.2.3 To-aircraft. The spctral mask that must be met in
the to-aircraft direction with A-BPSK is shown in Figure A-9
of this guidance material. This was derived assuming that all
amplifiers in the to-aircraft transmission path are operating
linearly. The corresponding spectral mask for A-QPSK is
shown in Figure A- 10 of this guidance material.
3.3 hmodulator performance
3.3.1 The performance specified in the Standards can be
attained using coherent detection and a Viterbi decoder with
3-bit soft decisions. The R and T channel demodulaton are
allowed more F&N, to achieve the bit error rate of 1u5
because of the shon bursty nature of communications over
these channels. The theoretical performance of A-QPSK in
additive white Gaussian noise is better than that of A-BPSK
because the bits are not differentially encoded. However, for
A-QPSK modulation, more margin is included (relative to
theoretical) because of its poorer perfomance over fading
channels.
3.3.2 The relative motion of the aircraft and the satellite
means that any signal reflections from aircraft wings or tail, or
the sea or ground below can mult in time-varying multipath.
This is in part due to the rather broad beamwidth of the AES
antenna. The characterisrics of this multipath depend on a
number of characteristics including the aircraft velocity, the
look-angle of the satellite with respect to the aircraft and the
slope of the reflecting surface. The rate of these variations
　
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