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the interleaver generates an integral number of 8-bit words,
or the extra bits padded to ensum that the data encoder
generates an integral number of 3-bit symbols.
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
reserved symbol and the transmission length segments. The
block code shall be transmitted as the fifth segment. The
encoder shall accept the header in the bit sequence that is
being transmitted. The five parity bits to be transmitted shall
be generated using the following equation:
where:
P is the parity symbol (P, shall be transmitted first);
e) header FEC. R is the reservcd symbol;
Note.- Immediately after these segments, an AVLC frame
with the fonnat specified in 6.5.3.3.1 follows.
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
necessary to transmit all symbols, it is preferable for
transmitters to ramp-up to 90 per cent of steady state in no
more than two symbols.
6.4.3.1.3.2 Synchronization and ambiguity resolution.
The second segment of the training sequence shall consist of
the unique word:
and shall be transmitted from left to right.
6.4.3.1.3.3 Reserved symbol. The third segment of the
training sequence shall consist of the single symbol
representing 000.
Note.- This $eld is mserved for future definition.
TL is the transmission Length symbol;
is the matrix transpose function; and
H is the parity matrix defined below:
6.4.3.1.3.6 Bit transmission ordel: The five parity bits
of the resultant vector product shall be transmitted from the
left bit first.
6.4.3.1.4 Forward error correction. In order to improvc
the effective channel throughput by reducing the number of
required rclransmissions, FEC shall be applied after the
training sequence, regardless of frame boundaries.
6.4.3.1.4.1 FEC calculation. The EEC coding shall be
accomplished by means of a systematic fixed-length Reed-
Solomon (RS)(255,249) 2'-ary code.
Note.- This code is capable of correcting up to three
octets for data blocks of 249 octets (1992 bits). Longer
-6/11/97
No. 72
Annex 10 - Aeronautical Telecommunications Volume ZZZ
transmissions must be divided up into 1992 bit transmissions
and shorter transmissions must be extended by virtual Jill with
trailing zeros. Six RS-check octets are appended for a total
block of 255 octets.
The field defining the primitive polynomial of the code shall
be as follows:
The generator polynomial shall be as follows:
where:
a is a primitive element of GF(256);
GF(256) is a Galois field (GF) of size 256.
Note.- The Reed-Solomon codes am described in the
Recommendation for Space Data System Standards Telemetry
Channel Coding, by the Consultative Committee for Space
Data Systems (see Appendix A).
6.4.3.1.4.2 Block lengths. The six RS-check octets shall
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
shorter length shall be extended to 249 octets by a virtual fill
of trailing zeros. Thc virtual fill shall not be transmitted.
Blocks shall be coded according to 6.4.3.1.4.3 through
6.4.3.1.4.3.3.
6.4.3.1.4.3 No error correction. For blocks with 2 or
fewer non-fill octets, no error correction shall be used.
6.4.3.1.4.3.1 Single-byte error correction. For blocks
with 3 to 30 non-fill octets, all six RS-check octets shall be
generated, but only the first two shall be transmitted. The last
four RS-check octets shall be treated as erasures at the
decoder.
6.4.3.1.4.3.2 Two-byte error correction. For blocks with
31 to 67 non-fill octets, all six RS-check octets shall be
generated, but only the first four shall be transmitted. The last
two RS-check octets shall be treated as erasures at the
decoder.
6.4.3.1.4.3.3 Three-byte error correction. For blocks
with 68 or more nno-fill octets, all six RS-check octets shall
be generated and transmitted.
6.4.3.1.5 Interleaving. To improve the performance of
the FEC, an octet-based table-driven interleaver shall be used.
The intetleaver shall create a table having 255 octets per row
and c columns, where
transmission length (bits)
C =
1992 (bits)
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