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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 relationships 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 I shall ~rovide 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 bitsls + 0.02 per cent.
6.4.2.1.3 Training sequence. Data transmission shall
bcgin with a demodulator training sequence consisting of three
segments:
a) transmitter power stabilization;
b) continuous wave tone; and
c) bit ambiguity resolution.
Note.- Immediately a@er these segments, an AVLCframe
with the format specified in 6.5.3.3.1 follows.
6.4.2.1.3.3 Bit ambiguity resolution. Thc third segment
of the training sequence shall consist of the unique word 1101
0101 0101 0100 and shall be transmitted from lcft to right.
6.4.2.1.4 Transmission distortion. The distortion at
amplitude modulation up to 90 per cent shall not exceed
10 per cent over the modulation 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
6.4.3.1 To transmit a sequcncc ofrrarnes, a station shall
insert he bit numbers and flags (per 6.5.3.3.1), compute the
FEC (per 6.4.3.1.4), interleave (per 6.4.3.13, prepend the
training sequence (per 6.4.3.1.3), carry out bit scrambling (per
6.4.3.1.6) and finally encode and modulate the RF signal (per
6.4.3.1 .I).
6.4.3.1.1 Modulation scheme. Mode 2 shall use differentially
encoded 8 phase shift keying (DXPSK), using a raised
cosine filter with a = 0.6 (nominal value). The information to
be transmitted shall be differentially encoded with 3 bits per
symbol (baud) transmitted as changes in phasc 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
saparate binary streams X, Y, and Z so that bits 3n form X,
bits 3n + I form Y, and bits 3n + 2 form Z. The triplet at lime
k (X,, Y,, &) shall be converted to a change in phase as
shown in Table 6-l', and the absolute phase is the
accumulated series of A@,, that is:
+k = 4.1 + A@k
6.4.3.1.1.2 Transmitted signal fonn. The phasemodulated
baseband signal as defined in 6.4.3.1.1.1 shall
excite the pulse shape filter.
6.4.2.1.3.1 Transmitter power stabilization. The first where:
segment of the training sequence is the transmitter power h is the complex impulse response of the pulse shape
stabilization, which shall consist of no more than 128 binary filter;
ones. The transmitter shall be within 90 per cent of the steady
state power level by the end of the transmitter power
stabilization segment. 1. All tables are located at the end of this chapter.
61111~ 282B
No. 72
Part I Annex 10 - Aer~nautical Telecomm~nications
k is defined in 6.4.3.1.1.1;
b, is defined by the equation in 6.4.3.1.1 .l;
t is time;
T, is time duration of each symbol.
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
Inask tolerance are given in Tables 6-2 and 6-3.
6.4.3.1.2 Modulation rate. The Mode 2 symbol rate shall
be 10 500 symbols/second k 0.005 per cent, resulting in a
nominal bit rate of 31 500 bitsls.
6.4.3.1.3 Training sequence. Data transmission shall
begin with a demodulator training sequence consisting of five
segments:
a) transmitter power stabilization and receiver automatic
gain control (AGC) setting;
b) synchronization and ambiguity resolution;
c) reserved symbol;
d) transmission length; and
6 . 4 3 3 4 Transmission length. To allow the receiver
to determine the length of the final Reed-Solomon block, the
transmitter shall send a 17-bit word, from least significant bit
(Isb) to most significant bit (msb), indicating the total number
of data bits that follow the header FEC.
Note.- The length does not include those bits transmitted
for: the Reed Solomon FEC, extra bits padded to ensure that
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