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implies that during any symbol period two decisions separated
by 180' are possible, and that .hse 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 roat 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 modulaticm that is used for data rates above 2.4 kbitds
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 1iT. The synchronous
samplers S operate at rate 1m 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 bits/s 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
combined linearly. The amplitude and phase masks that the
100 per cent raised cosine pulse shaping filter must satisfy are
shown in Figures A-6 and A-7 of this guidance material, while
those of the 60 per cent root raised cosine filter are shown in
Figures A-23 and A-24, 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 RP signal is indistinguishable
from one that was generated by an ideal modulator.
3.2 Bounds on radiated power
wtral density
3.2.1 Spectrum mush+ These spectrum mkks allow for
degradation from the ideal Nyquist model that could occur due
to non-ideal system chwacteristics, e-g. saturation in the
amplifier chain.
3.2.2 Fmm-aimraft. The spectra1 mask that must be
satisfied by any A-BPSK signal transmitted in the fromaircraft
direction is shown in Figure A-8 of this guidance
material. This mask is applicable for 100 per cent root r a i d
cosine filtering. 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, The spectral
mask (Figure A-22) applies to AQPSK with a 60 per cent mot
raised cosine filter.
3.2.3 To-aimmfr. The spectral 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.
A#achm#nt A & Pad I Annex 10 - Aerorrauticul Telecommunications
33 I)emodulator performance
3.3.1 The performance specified in the Standards can be
attained using coherent detection and a Mterbi decoder with
3-bit soft decisions. The R and T channel demodulators are
allowed more E@Jo to achieve the bit error rate of loq5
because of the short 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, mom margin is included (relative to
theoretical) because of its pomr performance over fading
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