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place only when permitted by the MAC.
6.9.5.1.4.2 Order of transmission. The transmission shall
consist of the following stages in the following order:
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Part 1 Annex 10 - Aeronautical Telecommunications
a) transmitter power stabilization; 6.9.5.2.3 STAGEOSF TRANSMISSION
b) bit synchronization; 6.9.5.2.3.1 Transmitter power stabilization. The first
segment of the training sequence is the transmitter power
C) ambiguity resolution and data transmission; and stabilization, which shall have a duration of 16 symbol
periods. The transmitter power level shall be no less than 90
d) transmitter decay. per cent of the steady state power level at the end of the
transmitter power stabilization segment.
Note.- The definitions of the stages are given in Sections
6.9.5.2.3.1 to 6.9.5.2.3.4.
6.9.5.1.4.3 Automatic transmitter shutdown. A VDL
Mode 4 station shall automatically shut-down power to any
final stage amplifier in the event that output power from that
amplifier exceeds -30 dBm for more than 1 second. Reset to
an operational mode for the affected amplifier shall require a
manual operation.
Note.- This is intended to protect the shared channel
resource against so-called "stuck transmitters".
6.9.5.1.5 NOTIFICATION SERVICES
6.9.5.2.3.2 Bit synchronization. The second segment of
the training sequence shall be the 24-bit binary sequence 0101
0101 0101 0101 0101 0101, transmitted from left to right
immediately before the start of the data segment.
6.9.5.2.3.3 Ambiguity resolution and data transmission.
The transmission of the first bit of data shall start 40 bit
intervals (approximately 2083.3 microseconds) -t I microsecond
after the nominal start of transmission.
Note 1.- This is referenced to emissions at the output of
the antenna.
Note 2.- Ambiguity resolution is performed by the link
layel:
6.9.5.1.5.1 Signal quality. The operational parameters of 6.9.5.2.3.4 Transmitter decay. The transmitted power
the be monitored at the physical layer' level decay at least by 20 dB within 300 microseconds
quality be performed in the process after a transmission. The transmitter power level
and in the receive process. shall be less than -90 dBm within 832 microseconds after
completing a transmission.
Note.- Processes that may be evaluated in the
demodulator include bir error rate (BER), signal to noise ratio
(SNR), and timing jitte,: Processes that may he evaluated in 6.9.5.3 CHANNESLE NSING
the receiver include received signal level and group delay.
6.9.5.3.1 Estimation of noisejloo,: A VDL Mode 4 station
6.9.5.1 S.2 Arrival time. The amval time of each received shall estimate the noise floor based on power measurements of
transmission shall be measured with a two-sigma error of 5 the channel whenever a valid training sequence has not been
microseconds. detected.
6.9.5.1.5.3 Recommendation.- The receiver should be 6.9.5.3.2 The algorithm used to estimate the noise floor
of measuring the time within a two-sigma shall be such that the estimated noise floor shall be lower than
error of I microsecond. the maximum power value measured on the channel over the
last minute when the channel is regarded as idle.
Note.- The VDL Mode 4 receiver uses an energy sensing
6.9.5.2 PROTOCODLE FINITION FOR GFSK algorithm as one of the means to determine the state of the
channel (idle or busy). One algorithm that can be used to
6.9.5.2.1 Modulation scheme. The modulation scheme estimate the noise floor is described in the Manual on VDL
shall be GFSK. The first bit transmitted (in the training Mode 4 Technical Specifications.
sequence) shall be a high tone and the transmitted tone shall
be toggled before transmitting a 0 (i.e. non-return to zero 6.9.5.3.3 Channel idle to busy detection. A VDL Mode 4
inverted encoding). station shall employ the following means to determine the
channel idle to busy transition at the physical layer.
6.9.5.2.2 Modulation rate. Binary ones and binary zeros
shall be generated with a modulation index of 0.25 0.03 and 6.9.5.3.3.1 Detection of a training sequence. The channel
a BT product of 0.28 + 0.03, producing data transmission at a shall be declared busy if a VDL Mode 4 station detects a valid
bit rate of 19 200 bitsls * 50 ppm. training sequence followed by a frame flag.
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No. 76
Annex 20 - Aeronautical Telecommunications Volume ZZZ
6.9.5.3.3.2 Measurement of channel power Regardless of 6.9.5.5.2 Parameter P2 (nominal co-channel interference
the ability of the demodulator to detect a valid training performance)
sequence, a VDL Mode 4 station shall consider the channel
busy with at least a 95 per cent probability within 1 ms after 6.9.5.5.2.1 The parameter P2 shall be the nominal
　
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