AMCP WGC4/IP-3
AERONAUTICAL MOBILE COMMUNICATIONS PANEL (AMCP)
Working Group C
Forth Meeting
Montreal, Canada
27 – 30 May 2002
Agenda Item 1 : Report of WGC/ 3
VDL Mode 2 Physical Layer Validation Test
(Prepared and Presented by S.Kato, ENRI)
INFORMATION PAPER
SUMMARY
This paper is related to Action Item WGC/2-1. It outlines the result of flight test on VDL Mode 2
physical layer. The observation of basic parameters including received power, bit error rate (BER) and
message error rate (MER) was made using VDL Mode 2 test equipment developed by ENRI. Then the
range and the relation between MER and BER were evaluated based on the result of measurements.
Major performance has been confirmed on VDL Mode 2 physical layer through the test and evaluation.
1. Introduction
ENRI has developed the test equipment of VDL Mode 2 until 1999 and the flight test
has been recently conducted to validate physical layer of VDL Mode 2. The characteristics for received
power, bit and message error rate were measured and evaluated for VDL Mode 2 transmission.
Consequently, the expected performance was almost confirmed on VDL Mode 2 physical layer under
near-real circumstances through the flight test extending beyond a radio horizon, passing above the
ground station, and also the ground operation test on airport surface.
2. Test Configuration
The bit error rate (BER) and message error rate (MER) were measured for uplink transmission with
the test configuration shown in Fig.1. With this configuration, the fixed data consisting of one Reed
Solomon Block (block length=238bytes) was transmitted from the ground station at the rate of 4 Hz.
The airborne station calculated and recorded the number of bit and message errors by comparing the
demodulated bits train with the fixed data. The recorded data included received power per a symbol,
phase deviation after demodulation, as well as bits train before/after error correction. The aircraft
position tracked by GPS was also recorded. To reduce the interferences from co-site VHF DSB-AM
transmission a crystal band path filter was installed at the input of airborne receiver.
The major parameters for VDL Mode 2 test equipment are shown in Tab.1
VDL2
RX
Uplink
Fixed Data
4HZ
Generator
Airborne Rx (Test Rx)
VDL2
TX
Ground Tx (Test Tx)
Fig.1
PC
Test Equipment Configuration
1
Parameters
Value
Comment
Emission Frequency
136.900 MHz
Emission Power
15W
Tx Cable Loss
1.75 dB
Tx Antenna Gain
2.15 dBi
Height
of
Tx
Antenna
Measured value
Value on catalog
λ/2 dipole
(No
pattern
considered)
23m
(Above
Level)
Rx Antenna Gain
0 dBi
Rx Cable Loss
1.2dB
Receiver sensitivity
-104dBm
Sea
loss
21m above ground
From MOPS
(No
pattern
considered)
loss
Measured value
At BER=10-3
(before error correction)
Tab. 1 Major Parameters
For the flight test, the ground transmitter antenna was installed near the runway at the Sendai Airport.
The test receiver equipment was installed on Beechcraft B99 aircraft. As shown in Fig.2, a couple of
VDL antennas were equipped at top and bottom on aircraft, among which bottom one was used for the
test except for some part of ground operation test.
Fig.2
Airborne VDL antenna (on Beechcraft B99 aircraft)
3. Transmitted data format
2
The transmitted data format complies with VDL 2 format and adopts Reed-Solomon (255,249) code
for error correction as defined in ICAO SARPs. However, due to the restriction on the hardware of test
equipment, total bytes available as user data for the test were limited to 238 bytes.
RS Block (238bytes)
TRNG*1 Address(9)*2
*1
*2
TRNG: Training sequence
Fig.3
FCS(3)*2
DATA(220)
FEC(6)
including Flag
Message Format for Flight Test
To estimate the effect by the restriction, we initially calculated the difference of message error rate
(MER) between a message encapsulating 238 bytes and 255 bytes (= maximum size of RS block)
according to the following formula.
MER  1  1  HER   1  BFR 
n  Number_of _ RS _ Block
n
1


k
( 25 k )
HER  1   ( BER   1  BER 
 Comb(k ,25)) 
 k 0

3


8 k
8  N  k 
BFR  1   1  1  BER   1  BER 
 Comb(k , N ) 
 k 0

Max : 255bytes
N  Data _ Length(bytes)



where, HER: Header Error Rate - Error Rate to Header part of VDL 2 message
BFR: Block Failure Rate - Error Rate to one RS Block
*The formula above is detailed on page 67 of “VDL Mode 2 physical layer validation report” developed by Eurocontrol.
The result of calculation provided that MER for a 255 bytes-message would exceed by maximum 30%
over that for a 238 bytes-message (Fig.4).
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Fig.4 MER Vs BER for 255 bytes and 238 bytes message
4. Flight Trajectory
The level flights having different courses and flight levels (FL100, FL150 and 180) were carried out
based at Sendai Airport (located about 300 km north of Tokyo). All the flights extended radially from
Sendai Airport to beyond radio horizon in order to obtain the data up to the distance limitation for signal
reception. The flight test passing over a ground station was also conducted at FL90 to evaluate the radio
coverage right over the station. The flight trajectory and estimated radio horizon for each flight are
given in Tab.2 and Fig.5.
Flight No.
Course
Flight Level
Estimated Radio
Horizon*(NM)
FLT 1
Sendai  R100  Sendai
(including Arc flight)
FL100
133
FLT 2
Sendai  Misawa  Sendai
Outbound FL150
Inbound FL100
144
118
FLT 3
Sendai  Oomiya  Sendai
FL180
123
FLT 4
Right over ground station
FL90
---
*The radio horizon is calculated taking account of rugged terrain.
Tab.2 Flight Trajectories and respective Radio Horizons
4
ARC Flight
(Constant distance flight)
(Ground Tx)
Fig.5 Flight Trajectories and Radio Horizons
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5. Flight Test Results
5.1 Level Flight (FLT 1~3)
The BER and MER before/after correction was observed for FLT1, FLT2 and FLT3 and
charted as the average value in every 5NM (for constant distance flight) or 10NM range (for other
flights) from the ground station.
FLT 1 is an eastbound flight from Sendai. This flight route was also chosen for a Arc flight (a
constant distance flight) test, as this route being almost located over sea, may have no such effect as
radio shielding from rugged terrain. In the Arc flight, aircraft maintained a constant distance more than
5 minutes for every 5NM different distance between 140NM and 100NM in order to obtain at least
1000 samples at the same distance.
Fig.6a gives the received power and the BER measured before and after FEC against a
distance. A degradation of around 4dB from outbound flight is observed in received power for Arc
flight. It may cause from the affects by a pattern loss of airborne antenna and a multipath from aircraft
wing. However, it was confirmed BER reaches below 10-4 if received power exceeding -90dBm.
The measured MER and BER in the same flight are given in Fig.6b. When BER after FEC is
around 10-4, MER (after FEC) is almost around 1%, which means messages will successfully arrive at
99% probability in one time transmission. The associated range at the time is 105NM, which
corresponds to 79% of radio horizon (133NM).
Range
Range
Rcvd Power
1-MER
Radio Horizon(RH)
Radio Horizon(RH)
.
BER
BER
Fig.6a BER and Rcvd Power Vs Distance
Fig.6b (1－MER) and BER Vs Distance
FLT 1 (SendaiR100 & Arc)
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The results from all the level flight tests are summarized in Tab.3. The MER reaches below
1% when BER after FEC being less than 10-4, and in every case except for the Arc flight the range
amounts to more than 80% of radio horizon. The measured received power, BER and MER against
distance for each flight are provided in Fig.7 ~ Fig.10.
The rapid deteriorations in received power observed between 40-60NM in Fig.7a is not
brought about by the actual changes in radio propagation, but is caused by the operation of attenuator
installed at the input of airborne VDL receiver which was triggered by the airborne DSB-AM
transmission. Whereas the steep decline in received power at around 30NM in Fig.9a is assumed due to
multipath by ground reflection. But it will not be critical because the related value of BER still remains
in a degree of 10-4.
Flight
FLT 1
FLT 1
(Arc)
FLT 2
(Outbou
nd)
FLT 2
(Inbound
)
FLT 3
Estimated
Radio Horizon
(A)
Flight
Level
Range
(BER after FEC < 10-4)
B/A
(B)
100
133NM
110NM
83%
100
133NM
105NM
79%
150
144NM
120NM
83%
100
118NM
100NM
85%
180
123NM
120NM
98%
MER at
(B)
Less
than
1%
Tab.3 Range Vs Radio Horizon
1-MER
Range
RH
Rcvd Power
Range
RH
RH
BER
BER
* The transmitting station is located in the origin of coordinates.
Fig.7a BER and Rcvd Power Vs Distance
Fig.7b (1－MER) and BER Vs Distance
FLT1 (Sendai  R100)
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1-MER
Rcvd Power
Range
Range
BER
BER
Fig.8a BER and Rcvd Power Vs Distance
Fig.8b
(1－MER) and BER Vs Distance
FLT2 (Sendai  Misawa)
Range
1-MER
RH
Rcvd Power
Range
RH
BER
BER
Fig.9a BER and Rcvd Power Vs Distance
Fig.9b
(1－MER) and BER Vs Distance
FLT2 (Sendai  Misawa)
nce
RH
Range
1-MER
Rcvd Power
RH
Range
BER
BER
Fig.10a BER and Rcvd Power Vs Distance
Fig.10b
FLT3 (Sendai  Oomiya)
8
(1－MER) and BER Vs Distance
5.2 Flight over ground station (FLT 4)
Some message errors were observed in the airspace right over the ground transmitter antenna,
probably causing from the pattern loss of ground and onboard antenna, as well as from the ground
reflection. Fig.11 shows the location where message errors occur on the horizontal and vertical flight
trajectories. The flight level was FL90 for all the flights. From the figure, it was found that the
occurrence of message errors concentrated within 1NM range (above 56deg in elevation angle) from the
ground antenna and the relevant BER (after FEC) in this range exceeds 10-4.
EL Angle=56deg
* The transmitting station is located in the origin of coordinates.
Fig.11 Flight over Station (at FL90)
Distance(NM)
0.0--1.0
1.0--2.0
2.0--3.0
3.0--4.0
4.0--5.0
Number of
Messages
463
473
466
471
481
BER before FEC
BER after FEC
2.52E-3
1.56E-5
2.56E-5
2.05E-5
4.73E-6
2.54E-3
0
0
1.57E-5
0
Tab.4 Flight over Station (at FL90)
9
MER
6.26E-2
0
0
2.12E-3
0
5.3 Ground operation test
The ground operation test was performed to evaluate the continuity of communication service on the
airport surface. The test comprises two sets of round trip trial at the Sendai Airport with the use of bottom
and top antenna onboard. The use of bottom antenna caused several message errors probably due to lower
antenna height and shielding by aircraft body, in contrast, top antenna gave no message error while
accompanying with some correctable bit errors.
In general, no major problem was observed in VDL Mode 2 capability to communicate with
aircraft on ground, although it may largely depend on the siting condition of ground and airborne
antenna. Actually the siting condition for the ground antenna was not optimal in the trial.
Fig.12
Ground operation test using bottom antenna
Fig.13 Ground operation test using top antenna
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6. Evaluation of the results
6.1
BER Vs Received Power Level
The measured BER data was accumulated and then plotted for each 5dB range in average
received power of burst signal (Fig.14). The figure shows that BER after FEC becomes less than the
value of 10-4, which is stipulated in ICAO SARPs, when received power level exceeds -90dBm.
Note: One symbol (◇,+)represents the average value of many data.
Fig.14
6.2
BER Vs Received Power
Estimation of error rate for longer message
6.2.1
Theoretical FER Vs BER for different frame length
The message structure of VDL Mode 2 is as shown in Fig.15. The size of user data in AVLC frame
varies from 128 to 2048 bytes. Accordingly the number of RS blocks included in a frame differs from 1
to 9. The frame error rate (FER) was calculated for various frames comprising different number of RS
blocks.
Fig.16 presents the calculated FER Vs BER for different frames having different number (1,3,5,9) of
RS blocks. It is known from the figure, for example, that BER (before FEC) must be suppressed under
5.4x10-4 for 9 RS block-frame if envisaging FER below 20%, which is near half of the value (1.1x10-3)
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for 1 RS block-frame. It is also known that FER will reach to the negligible level of below 0.1% when
BER being less than 10-4.
ISO 8208 Packet
OR
If limited to
ACARS Block(Max238)
IPI
one ACARS block
AVLC Frame
Flag
Address
TRNG
Link Control
User Data(128~2048)
Data Field(249)
Data Field(249)
Data Field(249)
Fig.15
Fig.16
FCS
FEC
FEC
FEC
VDL Mode 2 Message Format
FER Vs BER (Theoretical value)
12
Flag
N block
6.2.2 MER Vs BER derived from the results of flight test
Fig.17 presents the measured MER (depicted as “*”) for the message containing one 238bytes-RS
block in opposition to the value of BER. It also shows the MER for the messages with nine
255bytes-RS blocks (depicted as “▽”) that was theoretically calculated from the measured MER. The
figure further gives the theoretical curves of MER for messages having one 238 and 255bytes length RS
block.
The measured MER(“*”) is similar to theoretical one in its tendency but its value goes
significantly below them. It will come from the fact that bit errors possibly do not occur uniformly over
the entire messages but rather concentrate in some messages. However, the value of measured MER
stays at around 1% when BER (after FEC) is 10-4, which is assumed fairly low to perform transmission.
The calculated MER at the same BER is around 10% for the message containing 9 RS blocks. This
means that message transmission will succeed with more than 99% probability if 10% in all messages
are again retransmitted.
If judging from only the flight test of this time, it is considered that BER below 10-4 is required for
the transmission of longer messages (near up to 9 RS blocks), while the level of 10-3 required for shorter
messages (within 1 RS block) in order to obtain the MER around 10%.
Note: One symbol (*,▽)represents the average value of many data.
Fig.17
Measured MER (Log Scale) Vs BER
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7. Summary
The flight test for VDL Mode 2 physical layer provided the following results:
(1) VDL Mode 2 transmission demonstrated good performance up to beyond 80% of the
respective radio horizon with minor influence due to ground reflection.
(2) If average received power of bursts is approximately above -90dBm, BER after FEC
becomes better than 10-4, which is specified in ICAO SARPs.
(3) As the interference by VHF analogue radio operated in the same aircraft is significant
(so-called “co-site issue”), effective method for rejecting such interference should be
considered, including ensuring enough isolation between antennas.
(4) No message error was observed up to 56 degrees in the elevation angle at the flight right
over ground transmitter antenna.
(5) While it may depend on the location of airborne and ground antennas, good
transmission can be achieved for aircraft ground operation with using top antenna onboard.
(6) The BER after correction must be kept under 10-4 for the transmission of message
containing 9 Reed Solomon (RS) blocks (= maximum size of blocks assumed for AVLC) if
intending message error rate (MER) to be less than 10%. For the message containing only
one RS block, that value can be mitigated up to the level of 10-3.
8. Future Work
The flight tests and analysis to validate upper layers on VDL Mode 2 including data link and
sub-network layers is in progress at the ENRI. The results will be presented in near future at the
meeting of this Working Group.
9. Conclusion
The meeting is invited to note the information provided in this paper.
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