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seconds. There is a single indicator for either of these
conditions and, consequently, the action by the AES
management for either event is the same.
363 w l m
No. 75
Annex 10 - Aeronautical Telecommunifatlons Volume III
4.2 P channel
4.2.1 General. At least one -P channel is transmitted
continuously by each GES that forms part of the AMSS
service. Each AES must continuously monitor the P channel
transmitted by the GES to which it is logged on. The P channel
implements a time division multiplexing strategy .to send small
packets of information to the AESs that are monitoring it. The
functional blocks at the GES end of each P channel are as
follows:
a) data scrambler;
b) forward error correction (FEC) encoder;
c) interleaver;
d) timing mark inserter (unique word); and
e) modulator.
The functional blocks at the receive end of each P channel are
complementary to those at the transmit .end. The complete
series of functional blocks from transmit end to receive end is
shown in Figure A- 11 of this guidance material.
4.2.2 m C r r O N ~BtL OCKS
4.2.2.1 Daru scradel: A data scrambler is a logical
device which multiplies the data sequence by a known pseudorandom
sequence. The data is unscrambled at the receiver by
multiplying by the same sequence of random bib. This has no
direct effect on he bit error rate.perform2mce on the link.
However, it prevents the possibility af transmitting long
sequences of 1's or O's over the link. The latter could be
detrimental to the performance of acquisition- and tracking
-circuitry.
4.2.2.2 Forwurd crmr correction coding. Satellite
communications are, in general, power-limited due to the
limited resources at the satellite. One can reduce the amount of
power required for communications by introducing forward
error correction coding, which adds redundancy to the
transmitted signal at the expense of requiring more bandwidth.
At the receiver, a &coder uses the redundancy to c o m t those
errors which normally occur when transmitting at a lower
power.
4.2.2.3 Interleaving. Mobile communications are
generally subject to fading due to reflections from nearby
objects. The fading is correlated with time and when it wurs,
can cause a sequence of correlated errors in the data detected
at the receiver. These errors can be corrected by the forward
error correction coding. However, most decoders work best
with uncamelated errors. The purpose of the (transmit)
interleaver and its inverse, the (receive) de-interleaver, is to
randomize the order of bits presented to the channel compared
to those presented to the decoder, thus reducing,any correlation
between the errors which may be caused by the channel.
4.2.2.4 i7ming mark. A unique word is inserted in each
data stream, at periodic intervals, to provide timing
information to the receiver. This timing information is needed
in order to properly synchronize the de-scrambler, the decoder,
and the de-interleavet with their counterparts on the transmit
side. It also allows recovery of this synchronization if the
signal happens to be lost momentarily.
4.2.3 P chamwl fmme dumtion. The frame duration is
either 500 milliseconds (channel rates of 2.4 kbiWs and
higher) or a multiple of 500 milliseconds to provide simple
derivation of a supframe u dfo r R channel slotting and the
reservation TDMA T channel. Scrambling and FEC coding of
rate 112 is used on all P channels. The FEC encoder is not reset
between frames, but starts a new frame in the state resulting
from the previous frame. With the exception of the 0.6 kbits/s
P channel, the duration of the interleaver is 500 milliseconds.
Fbr the 0.6 kbitsh, the interleaver is 384 bits corresponding to
213 second.
4.2.4 P channel frame f~mrA. ll P channel frame
formats include a 16-bit field (uncoded) as a fannat identifier
and for derivation of the superframe that has a duration of
8 seconds. The format identifier is a +bit field and is &ways
set to the value 0001 when the channel is a P channel. The
remaining 12 bits of this field are used as a superframe
counter. To achieve this, the field is subdivided into three 4-bit
fields, of which the first is used to indicate the start of a new
superframe, while the remaining two are used to indicate the
frame number. It is advisable for AES implementations to
make use of all 12 bits to achieve reliable superframe
synchronization in the presence of bit errors.
4.2.5 For P channels of 4.8 and 10.5 kbits/s, a small
number of "dummy" bits m included in the frame just after
the framing bits. This matches the number of bits of the
　
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