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tolerance are given in Tables 6-2 and 6-3.
6.4.3.1.2 Modulation mte. The Mode 2 symbol rate shall
be 10 500 symbolslsecond * 0.005 per cent, resulting in a
nominal bit rate of 31 SO0 bitsts.
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;
6.4.3.1 To transmit a sequence of frames, a station shall b) synchronization and ambiguity resolution;
insert the bit numbers and flags (per 6.5.3.3.1), compute the
FEC (per 6.4.3.1.4), interleave (per 6.4.3.1.5), prepend the cj reserved symbol;
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
6.4.3.1.1). * A11 tables are locatcd at the end of this chapter.
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No. 75
Annex I0 - AeronauticuE Telecommunications Volume III
d) transmission length; and R is the reserved symbol;
e) header EC. TLis the transmission Length symbol;
Note.- Immediately after these segments, an AVLC frame is the matrix transpose function; and
with the format specifled in 6.5.3.3.1 follows.
H is the parity matrix defined below:
6.4.3.1.3.1 Transmitter power stabilization and receiver
AGC setting. The first segment of the training sequence is the
transmitter power stabilization and receiver AGC setting,
which shall consist of four symbols each representing 000. The
transmitter shall be within 90 per cent of the steady state
power level by the end of the transmitter power stabilization
segment.
6.4.3.1.3.1.1 Recommendation.- Although it is 6.4.3.1.3.6 Bit tranimission order The five parity bits of
necessary to trunsmit all symbols, it is preferable for the resultant vector product shall be transmitted from the left
transmitters to rump-up to 90 per cent of steady state in no bit first.
more than two symbols.
6.4.3.1.4 Forward error correction. In order to improve
6.4.3.1.3.2 Synchronization arid ambiguity resolution. The the effective channel throughput by reducing the number of
second segment of the training sequence shall consist of the required retransmissions, FEC shall be applied after the
unique word: training sequence, regardless of frame boundaries.
000 010 011 110 000001 101 110 001 100011 6.4.3.1.4.1 FEC calculation. The FEC coding shall be
111 101 111 100 010 accomplished by means of a systematic fixed-length Reed-
Solomon (RS)(255,249) 2'-ary code.
and shall be transmitted from left to right.
Note.- This code is capable of correcting up to three octets
6.4.3.1-3.3 Reserved symbol- The third segment of the for data blocks of 249 octets (1992 bits). Longer transmissions
mdning sequence shall consist of the single symbol must be divided up into 1992 bit transmissions and shorter
representing 000. transmissions must be extended by virtual fill with trailing
zeros. Six RS-check octets are appended for a total bbck of
Note.- This field is reserved for future defirrition. 255 octets.
6.4.3.1.3.4 Transmission length. To allow the receiver lo me field defiaing the primitive polynomial of the code shall
determine the length of the final Reed-Solomon block, the ,, follows:
transmitter shall send a 17-bit word, from least significant bit
(Isb) to most significant bit (msb), indicating the total number p(x) = (x8 + x7 +. 2 -t .x + 1)
of data bits that follow the header FEC.
The generator polynomial shall be as follows:
Note.- The length does not ifidude those bits transmitted
for: the Reed Solonlon FEC, extra bits padded to ensure that 125
the interltraver genemtes an integral number of 8-bit words, or
the extra birs padded to ensure that the data encoder generates n (x - ai)
an illtegral number of 3-bit symbols. i = 120
where:
6.4.3.1.3.5 Header FEC. To correct bit errors in the
header, a (25, 20) block code shall be computed over the a is a primitive element of GF(256);
reserved symbol and the transmission length segments. 'fie GF(256) is a Galois field (GF) of size 256.
block code shall bc transmitted as the fifth segment. The
encoder shall accept the header in the bit sequence that is Note.- The Reed-Solomon codes are described iri the
being transmitted. The five parity bits to be transmitted shall Recommendation for Space Data System Standards Telerncty
be generated using the following equation: Channel Coding, by the Consultutive Committee for, Space
Data Systems (see Apperldix A).
6.4.3. I .4.2 Block lengths. The six RS-check. octets shall
where: be calculated on blocks of 249 octets. Longer transmissions
shall be split into blocks of 249 octets, per 6.4.3.1.5 Blocks of
P is the parity symbol (PI shall be transmitted first); shorter length shall be extended to 249 octets by a virtual fill
　
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