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input to a standard A-BPSK modulator. The first bit of the
modulated portion shall be a "0" and shall give a -90 degree
phase change relative to thc phase of the unmodulated signal.
The unmodulated portion of the A-QPSK preamble shall be a
signal of constant phase corresponding to Ihe output of an
ideal. A-QPSK modulator with all "0"s at its input. The
modulated portion shall consist of alternating "0" and "1"
(commencing with a "0" in the first bit) on the 1 channel and
continuous "0"s on the Q channel.
Note.- It is intended that the umodulated portion of the
preamble be used for carrier acquisition and the modulated
portion for clock acquisition.
4.4.3.3.3 Unique word. The Standard in 4.4.2.2.6 shall
apply.
4.4.3.3.4 Information field. The information field of each
R channel burst shall consist of 160 bits and shall contain an
extended signal unit plus Kflush bits prior to convolutional
encoding, where an extended SU shall have 152 bits and a
Flush field shall be 0000 0000. The information field shall be
scrambled, coded and interleaved, in that order.
4.4.3.3.4.1 Scrambling. The Standard in 4.4.2.2.5.1 shall
apply, except that the flush bits are not scrambled.
4.4.3.3.4.2 Forward error correction (FEC) coding. The
Standard in 4.4.5.3.3.5.2 shall apply, except that the encoder is
re-initialized to the all 0s state at the beginning of the
information field of each burst.
4.4.3.3.4.3 Interleaving. All R channels shall employ
block interleaving. The number of rows in the interleaver shall
be 64 transmission bits, while the number of columns shall be
5. Row interchanging in the interleaver shall be performed in
accordance with 4.4.2.2.5.3.
4.4.3.4 Performnce. The Standard in 4.4.2.3 shall apply.
4.4.4 T channel
4.4.4.1 Channel rate accuracy. The channel rate error
shall not exceed one part in 2R, where R is the channel rate,
and one part in lo4.
4.4.4.2 Timing relative to P channel. The beginning of
each T channel burst shall occur within k300ps of the
beginning of the assigned T channel slot defined by the
received P channel superframe. As shown in Figure 4-7, each
P channel superframe shall be divided into 16 nominal frames
with 64 T channel slots in each frame. The shortest guard time
between the T channel bursts of two different aircraft is under
control of the GES and shall be 5 slots.
4.4.4.3 BURST STRUCTURE
4.4.4.3.1 General chamcterisrics. Each T channel burst
shall consist of three fields: the preamble, the unique word and
the infomation field as shown in Figure 4-8.
14
CHAPTER 6. VHF AIR-GROUND DIGITAL LINK (VDL)
6.1 DEFINITIONS AND
SYSTEM CAPABILITIES
Note I.- The very high frequency (VHF) digital link (VDL)
Mode 2 and the VDL Mode 4 provide data service capabilities.
Tile VLX Mode 3 provides both voice and data service
capabilities. The data capability is a constituent nzobile
subnetwork of the aeronautical telecommunication network
(ATN). In addition, the VDL may provide non-ATN ficnctions.
Standards and Recommended Practices (SARPs) for tlze VDL
are defined and referenced below.
Note 2.- Additional information on VDL is contnined in
the Manuals on VDL Mode 2, VDL Mode 3 and VDL Mode 4
Technical Specij5cations.
Note 3.- Sections 6.1.2 to 6.8.2 contain Standards and
Recommended Practices for VDL Modes 2 and 3. Section 6.9
contuirzs Stundards and Recommended Practices for VDL
Mode 4.
Note 4.- VDL Mode 4 SARPs apply to surveillance
applications (e.g. ADS and ADS-C).
Automatic dependent surveillance-broadcast (ADS-B). A
surveillance technique in which aircraft automatically
provide, via a broadcast mode data link, data derived from
on-board navigation and position-fixing systems, including
aircraft identification, four-dimensional position, and
additional data as appropriate.
Broadcast. A transmission of information relating to air navigation
that is not addressed to a specific station or stations.
Burst. A time-defined, contiguous set of one or more related
signal units which may convey user information and
prolocols, signalling, and any necessary preamble.
Current slot. The slot in which a received transmission begins.
Data circuit-terminating equipment (DCE). A DCE is a
network provider equipment used to facilitate
communications between DTEs.
Data link entity (DLEj. A protocol state machine capable of
setting up and managing a single data link connection.
Data link service (DLSj sublayer: The sublayer that resides
above the MAC sublayer. For VDL Modc 4, the DLS
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