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training sequence (as per 6.4.2.1.3) and finally encode and
modulate the RF signal (as per 6.4.2.1.1, 6.4.2.1.1.1 and
6.4.2.1.4).
The aircraft and ground stations shall access the physical
medium operating in simplex mode. 6.4.2.1.1 Modulation scheme. Mode 1 shall use an
amplitude-modulated minimum shift keying (AM-MSK)
modulatioll scheme, which is a constant-phase, frequency shift
keying technique using two tones. The presence of the lower
6.4.1 Functions tone of a tone pair shall indicate a bit change from the previous
bit. The presence of the higher tone shall indicate that there is
6.4.1.1 The physical layer shall provide the following no bit change. The phases of the two tones shall be chosen so
functions: that the minimum phase discontinuity occurs at the interface
with rhe preceding bit and so that the amplitude of each tone
a) transmitter and receiver frequency control; is zero at the bit transition. The slope of the waveforms at the
end of a bit cell shall be positive for a binary one and negative
b) data reception by the receiver; for a binary zem.
--Z JlllOO
No. 75
Part I Annex 10 - Aeromutical Telecommunicalions
6.4.2.1.1.1 Phase continuity Phase coherence shall be
maintained through the transmission media for successful
decoding at the receiving end. The establishment of correct
phase relalionships shall be performed by the ground and
aircraft decoding hardware during the continuous wave (CW)
tone segment of the training sequence.
6.4.2.1.2 Modulation rate. Mode 1 shall provide for
binary ones and binary zeros to be modulated by using
1200-Hz and 2400-Hz tones, producing data transmission at a
bit rate of 2 400 bits/s + 0.02 per cent.
6.4.2.1.3 Training sequence. Data transmission shall
begin with a demodulator training sequence consisting of three
segments:
a) transmitter power stabilization;
6.4.3.1.1 Modulation scheme. Mode 2 shall use
differentially encoded 8 phase shift keying (DSPSK), using a
raised cosine filter with or = 0.6 (nominal value). The
information to be transmitted shall be differentially encoded
with 3 bits per symbol (baud) transmitted as changes in phase
rather than absolute phase. The data stream to be transmitted
shall be divided into groups of 3 consecutive data bits, least
significant bit first, Zeros shall be padded to the end of the
transmissions if needed for the final channel symbol.
6.4.3.1.1.1 Data encoding. A binary data stream entering
a differential data encoder shall be converted into three
separate binary streams X, Y, and Z so that bits 3n foml X, bits
3n + 1 form Y, and bits 3n + 2 form 2. The triplet at time k
(Xkr Yk, Zk) shall be converted to a change in phase as shown
in Table 6-l*, and the absolute phase h is the accumulated
series of A h , that is:
b) continuous wave tone: and h= h-1 + A h
c) bit ambiguity resolution. 6.4.3.1.1.2 Transmitted signal form. The phase-modulated
baseband signal as defined in 6.4.3.1.1.1 shall excite the pulse
Note.- Immediately after these segments, an AVLC frame shape filter.
with the format specijied br 6.5.3.3.1 follows.
6.4.2.1.3.1 Transmitter power stabilization. The first
segment of the training sequence is the transmitter power
stabilization, which shall consist of no more than 128 binary where:
ones. The transmitter shall be within 90 per cent of the steady
state power level by the end of the transmitter power h is the complex impulse response of the pulse shape
stabilization segment. filter;
k isdefrned in 6.4.3.1.1.1;
Note.- It is not necessary to transmit all 128 ones. The + is defined by the equation in 6.4.3.1.1.1;
next phase of the training sequence should begin as soon as t is time;
the zransmitter has reached 90 per cent power ouiput. Ts is time duration of each symbol.
6.4.2.1.3.2 Continuous wave tone. After the transmitter
has stabilized, a CW tone consisting of 88 binary ones, shall
be transmitted.
6.4.2.1.3.3 Bit ambigui~re solution. The third segment of
the training sequence shall consist of the unique word 1101
0101 0101 0100 and shall be transmitted from left to right.
6.4.2.1.4 Transmission distortion. The distortion at
amplitude modulation up to 90 per cent shall not exceed 10 pet
cent over the n~odulation frequency range of 600 Hz to
6.6 kHz. Transmitted noise shall be at least 45 dB below the
level of the carrier modulated 90 per cent at 1 kHz.
6.4.3 Mode 2 frames
The output (function of time) of the pulse shape filter (S(t))
shall modulate the carrier frequency. The pulse shape filter
shall have a nominal complex frequency response of a raisedcosine
filter with a=0.6. The spectral mask and the phase mask
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