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Recommendation for Space Duta System Standards Telem,etiy
Channel Coding, by the Consultative Cornnzittee for Space
Data Systems (see the Appendix).
No. 76
Annex 10 - Aeronautical Telecommunications Volume 111
6.4.3.1.2.2 Block lengths. The six RS-check octets shall. 6.4.3,1.4 Bit scrumbling. To aid clock recovety and to
be calculated on blocks of 249 octets. Longer transmissions stabilize the shape of the transmitted spectrum, bit scrambling
shall be split into blocks of 249 octets, per 6.4.3.1.3. Blocks of shall be applied. The pseudo noise (PN) sequence shall be a
shorter length shall be extended to 249 octets by a virtual fill 15-stage generator (see Figure 6-2) with the characteristic
of trailing zeros. The virtual fill shall not be transmitted. polynomial:
Blocks shall be coded according to 6.4.3.1.2.3 through
6.4.3.1.2.3.3. x '+~ X + 1
6.4.3.1.2.3 No error correction. For blocks with 2 or
fewer non-till octets, no error correction shall be used.
6.4.3.1.2.3.1 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 oclets shall be treated as erasures at the
decoder.
6.4.3.1.2.3.2 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.
The PN-sequence shall start after the frame synchronization
pattern with the initial value 1101 0010 101 1 001 with the
left-most bit in the first stage of the register as per Figure 6-2.
After processing each bit, the register shall be shifted one bit
to the right. For possible encryption in the future this initial
value shall be programrncd. Thc sequence shall be added
(modulo 2) to the data at the transmit side (scrambling) and to
the scrambled data at the receive side (descrambling) pcr
Table 6-3.
Note.- The concept of a PN scrambler is explained in
IXU-R Recommendation S.446-4, Annex I, Section 4.3.1,
Method I (see the Appendix to this Chapter).
6.4.3.1.2.3.3 Three-byte error correction. For blocks with
68 or more non-fill octets, all six RS-check octets shall be
generated and transrnittcd. 6.4.3.2 MODE 2 CHANNELSE NSING
6.4.3.1.3 jnterleaving. To improve the performance of the 6.4.3.2.1 Channel busy to idle detection. When a station
FEC, an octet-based table-driven interleaver shall be used. The receives on-channel power of at least -87 d]3m for at least 5
interleaver shall create a table having 255 octets per row and milliseconds, then:
c rows, where
transmission length (bits)
C =
1992 (bits)
a) with a likeljhood of 0.9, it shall continue to consider the
channel occupied if the signal level is attenuated to
below -92 dBm for less than 1 millisecond: and
where: b) with a likelihood of 0.9, it shall consider the
channel unoccupied if the signal level is attenuated to
a) t h t~ra nsmission length is as defined in 6.4.3.1.1.5; and below -92 dBm for at least 1.5 milliseconds.
b) c = the smallest integer greater than or equal to the value
of the fraction.
After extending the data to an even multiple of 1992 bits, the
interleaver shall wdte the transmission stream into the first
249 octets of each row by taking each consecutive group of
eight bits and storing them from the first column to the 24gth.
The first bit in each group of eight bits shall be stored in thc
eighth bit position; the first group of 1992 bits shall be stored
in the first row, the second group of 1992 bits in the second
row, etc. After thc FEC is computed on each row, the FEC data
(or erasures) shall be stored in columns 250 through 255. The
interleaver shall then pass the data to the scrambler by reading
out column by column, skipping any octet which contains
Note.- The m,mimum link throughput avuilable to all users
is highly sensitive to the RF channel sense delay Cfronz rhe time
when the channel actually changes state until a station detects
and acts on thut chunge) and RF channel seizure delay (from
the time when a station decides to transmit until the
transmitter is suficiently ramped up to lock out other stations).
Accordingly, it is imperutive that all efSorts are made to reduce
those times as the stute-of-the-art advances.
6.4.3.2.2 Channel idle to busy detection. With a
likelihood of at least 0.9, a station shall consider the channel
occupied within 1 millisecond after on-channel power rises to
at least -90 dBm.
crasures or all fill bits. All of the bits in an octet shall be
transmitted from bit 8 to bit 1. 6.4.3.2.3 Recommendation.- The detection of an
　
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