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(372 NM) without conflict.
23/11/06 APP B-78
Appendix B Annex 10 — Aeronautical Communications
Table B-60. Burst data content
Element Data content Number of bits
Beginning of burst all zeros 15
Power stabilization
Synchronization and ambiguity resolution 3.6.3.2.1 48
Scrambled data: 3.6.3.3
station slot identifier (SSID) 3.6.3.3.1 3
transmission length 3.6.3.3.2 17
training sequence FEC 3.6.3.3.3 5
application data 3.6.3.3.4 up to 1 776
application FEC 3.6.3.3.5 48
fill bits (Note) 3.6.2.2 0 to 2
Note.— Data scrambling of the fill bits is optional (3.6.3.3.6).
3.6.3.3 SCRAMBLED DATA CONTENT
3.6.3.3.1 Station slot identifier (SSID). The SSID shall be a numeric value corresponding to the letter designation A to
H of the first time slot assigned to the GBAS ground subsystem, where slot A is represented by 0, B by 1, C by 2, … and H
by 7. The identifier is transmitted LSB first.
the total number of bits in both application data
and
where
Pn = the nth bit of the training (P1 shall be transm first);
SSIDn = the nth bit of the station sl );
TLn = the nth bit in the transmis ; and
HT = the transpose of the parity efined below:
0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 T
3.6.3.3.2 Transmission length. The transmission length shall indicate
application FEC. The transmission length is transmitted LSB first.
3.6.3.3.3 Training sequence FEC. The training sequence FEC shall be computed over the SSID and transmission
length fields, using a (25, 20) block code, in accordance with the following equation:
[P1, ..., P5] = [SSID1, …, SSID3, TL1, …, TL17] HT
sequence FEC itted
ot identifier (SSID1 = LSB
sion length (TL1 = LSB)
matrix, d
0 0 1 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1
HT = 1 1 0 0 0 1 1 1 0 0 1 1 0 0 0 0 1 1 1 1
1 1 0 1 1 0 1 1 0 1 0 1 0 0 1 1 0 0 1 1
0 1 1 0 1 0 0 1 1 1 1 0 0 1 0 1 0 1 0 1
Note.— This code is capable of correc d detec 75 of 300 possible double bit errors.
3.6.3.3.4 Application data. The appli f one or re message blocks, as defined in 3.6.3.4. The
message blocks shall be mapped directly in data with no ad onal overhead of intervening layers.
3.6.3.3.5 Application FEC. The application FEC shall be calculated using the application data by means of a systematic,
xed-length, Reed-Solomon (R-S) (255, 249) code.
ting all single bit errors an ting
cation data shall consist o mo
to the application diti
fi
APP B-79 23/11/06
Annex 10 — Aeronautical Communications Volume I
3.6.3.3.5.1 The field-defining primitive, p(x), of the R-S code shall be:
p(x) = x8 + x7 + x2 + x + 1
=
where α is a root of p(x) used for construction of the Galois Field of size 28, GF(256), and αi is the ith primitive element in
GF(256).
d, m(x), shall be grouped into 8-bit R-S symbols.
ll l be ordered such as specified in Tables B-61 and
-62
he
a248 represents the message block identifier, with the rightmost bit defined as the LSB and the first bit of the application
h e block CRC, with the leftmost bit defined as the MSB and the last bit of
the ap ; and
3.6.3.3.5.4 The 6 R-S check symbols (bi) shall be defined as the coefficients of the remainder resulting from dividing
the message polynomial x6m(x) by the generator polynomial g(x):
i 5 4 3 2 6
8-bit R-S check symbol shall
0 5 bler shall be the MSB of b0
Note 1.— This R-S code is capable of correcting up to 3 symbol errors.
Note 2.— The order of the transmitted 8-bit R-S check symbols of the appended application FEC differs from the VHF
ata k symbol is transmitted LSB first.
are given in Attachment D, 7.15.
3.6.3.3.5.2 The generator polynomial of the R-S code, g(x), shall be:
125
g(x) = Π (x − αi ) = x6 + α176x5 + α186x4 + α244x3 + α176x2 + α156x + α225
i 120
3.6.3.3.5.3 In generating the application FEC, the data to be encode
A data fields in the message blocks that define the application data shal
B , and in the message tables in 3.6.6. However, since the R-S code is a block code, application data blocks shorter than
249 bytes (1 992 bits) shall be extended to 249 bytes by virtual fill bits set to zero and appended to the application data.
These virtual fill bits shall not be transferred to the bit scrambler. The data to be encoded, m(x), shall be defined by:
m(x) = a248x248 + a247x247 + .... + a248-length+1 x248-length+1 + a248-length x248-length + .... + a1x+a0
w re
length represents the number of 8-bit bytes in the application data block;
data sent to the bit scrambler;
a248-length+1 represents t e last byte of the messag
　
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