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Part l Annex 10 - Aeronautical Telecommurrications
of trailing zeros. The virtual fill shall not be transmitted. 6.4.3.1.6 Bit scrambling. To aid clock recovery and to
Blocks shall be coded according . to 6.4.3.1.4.3 through stabilize the shape of the transmitted spectrum, bit scrambling
6.4.3.1.4.3.3. shall be applied. The pseudo noise (PN) sequence shall be a
15-stage generator (see Figure 6-1') with the character,istic
6.4.3.1.4.3 No error correction. For blocks with 2 or polynomial:
fewer non-fill octets, no error comection shall be used.
xlS + X + 1
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 fmt 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 non-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
interleaver shall create a table having 255 octets per row and
c columns, where
transmission length (hits)
C =
1992 (bits)
where:
The PN-sequence shall start after the frame synchronization
pattern with the initial value 1101 0010 101 1 001 with the leftmost
bit in the first stage of the register as per Figure 6-1.
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 programmed. The sequence shall be added
(modulo 2) to the data at the transmit side (scrambling) and to
the scrambled data at the receive side (descrambling) per
Table 6-4.
Note.-- The concept of a PN scrambler is explained in the
International Radio Consultative Committee (CCIR) Report
384-3, Annex 111, Section 3, Method I (see Appendix A).
6.4.3.2 CHANNELSE NSING
6.4.3.2.1. Channel busy to idle detection. When a station
receives on-channel power of at least -87 dBm for at least
5 brnilliseconds, then:
a) with a likelihood of 0.9, it shall continue to consider the
channel occupied if the signal level is attenuated to
below -92 dEm for less than 1 millisecond; and
b) with a likelihood of 0.9, it shall consider the
channel unoccupied if the signal level is attenuated to
below -92 dBm for at least 1.5 milliseconds.
a) the transmission length is as defined in 6.4.3.1.3.5; and
Note.- The maximum link throughput available to all users
b) c = the smallest integer greater than or equal to the value is highly sensitive to the RF channel sense delay Cfmm the time
when the channel actually changes state until a station detects
of the fraction.
and acts on that change) and RF channel seizure delay lfrom
After extcnding the data to an even multiple of 1992 bits, the
interleaver shall write the transmission stream into the first
249 octets of each row by caking each consecutive group of
eight bits and storing then1 from the first column to the 249th.
The first bit in each group of eight bits shall be stored in the
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 the 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
erasures or all fill bits. All of the bits in an octet shall be
lransmitted from bit 8 to bit 1.
On reception, the de-interleaver shall calculate the number of
rows and size of the last (potentially partial) row from the
length field in the header. It shall only pass valid data bytes to
the higher layer.
the time when a station decides to transmit until the
transmitter is suficiently ramped up to lock our other stations).
Accordingly, it is imperative that all eflorts are made to reduce
those times as the state-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.
6.4.3.2.3 Recommendation.- The detection of an
occupied channel should occur within 0.5 milliseconds.
Note.- A higher probability of false alann is acceptable on
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