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shall not exceed onc part in lo6.
4.4.2.2 FRAME FORMAT
Note.- The dumnzy field is included to make each frame
0.5 seconds long. There is no dummy field required for data
rates of 2.4 kbits/s and below.
4.4.2.2.5 Information field. The information ficld shall
contain multiple signal units which are scrambled, coded and
interleaved, in that order. The number of bits in the
information field shall be as indicated in Table 4-10.
Note.- The number of bits in the information field is
depehdenr oh the data rate and the number of interleaver
blocks in t hfi~eld .
4.4.2.2.5.1 Scrambling. A scrambler with a 15-stage
generator register shall be used for data scrambling before FEC
coding. The polynomjal for the generator register of the
scrambler and the descrambler shall be 1 + X + XI5. The
scrambler and descrarnbler shall be clocked at the information
rate with the first scrambled bit output before the first shift. In
the absence of programming commands, the shift register shall
be initialized to 1101 0010 1011 001 (leftmost bit in shift
register stage 1) at the beginning of the information field of
each frame. The scrambler and descrambler functions shall be
as illustrated in Figure 4-3. The scrambler shall be re-initialized
at the beginning of the information field of each frame.
4.4.2.2.1 General characteristics. All P channel frames Note.- The concept of a scrambler is explained in ITU-R
shall be either 500 ms, or a multiple of 500 ms to provide Reconzmendation S.446-4, Annex I, Section 4.3.1 Method I .
simple derivation of an 8-second superframe which shall be
used for R channel and T channel slot allocation. Each P 4.4.2.2.5.2 Fom~ard error correction (FEC). The Standard
channel frame shall consist of five fields identified as: format in 4.4.5.3.3.5.2 shall apply.
identifier, superframe boundary marker, dummy field (for data
rates greater than 2.4 kbitsls), information field and unique Note.- There are no flush bits on the P chamlel.
word, as shown in Figure 4- 1 * far channel rates of 2.4 kbitsls
and less, and in Figure 4-2 for channel rates greater than
2.4 kbitsls. * All figures are located at the end of this chapter.
1/11/01
No. 76
Annex 10 - Aeronautical Telecommunications Volurne I11
4.4.2.2.5.3 Interleaving. All P channels shall employ
block interleaving. The column depth (number of rows) of the
interleaver shall be 64 transmission bits, while the number of
columns shall depend on the transmission rate as shown in
Table 4-1 1. At the transmitter, the output of the convolutional
encoder shall be written into the 64-bit columns until the
prescribed number of columns are full. The rows shall then be
permuted using the algorithm Rowj = (Rowi * 27) modulo 64.
The content of the interleaver shall then be transmitted row by
row as shown in Figure 4-4. At the receiver, the soft decision
data from the demodulator shall be written into the interleaver
row by row, and when it is full the interleaver rows shall be
permuted using the converse algorithm Rowj = (Rowi * 19)
modulo 64. The soft decision data shall then be read column
by column into the FEC decoder.
4.4.2.2.6 Unique word. Wtth A-BPSK each P channel
frame shall end with the 32-bit unique word 11 10 0001 0101
1010 1110 1000 ZOO1 0011, with the leftmost bit transmitted
first. With A-QPSK, the unique word shall be the A-BPSK
unique word repeated in each of the in-phase and quadrature
channels.
4.4.2.3 Performance. The over-all physical layer shall be
configured and operated such that the average bit error rate is
loL5 or less after descrambling.
4.4.3 R channel
4.4.3.1 Channel rate accuracy. The chann~l rate error
shall not exceed one part in 2R, where R is the channel rate,
and one part in lo4.
4.4.3.2 Burst timing. The beginning of each R channel
burst shall occur within 2300 ps of the beginning of an R
channel slot defined by the received P channel superframe. As
shown in Figure 4-5, each P channel superframe shall define
8, 16, 32 and 64 random access slots for R channel data rates
of 0.6, 1.2, 2.4 and 10.5 kbitsls, respectively.
4.4.3.3 BURST FORMAT
4.4.3.3.1 General characteristics. Each R channel burst
shall consist of three fields: the preamble, the unique word and
the information field as shown in Figure 4-6.
4.4.3.3.2 Preamble. The preamble for the R channel shall
consist of an unmodulated carrier portion followed by a
modulated portion. The length of these depend on the data rate
as shown in Table 4-12. The unmodulated portion of the
A-BPSK preamble shall be a signal of constant phase and the
modulated portion shall consist of alternating "0" and "1"
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