AMCP WGC4/IP-3

AERONAUTICAL MOBILE COMMUNICATIONS PANEL (AMCP)

Working Group CForth Meeting

Montreal, Canada27 – 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

SUMMARYThis 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.

PC

1. IntroductionENRI 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 ConfigurationThe 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

Ground Tx (Test Tx)

Fig.1 Test Equipment Configuration

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VDL2TX

Fixed DataGenerator

VDL2RX

4HZ

Uplink Airborne Rx (Test Rx)

Parameters Value CommentEmission Frequency 136.900 MHz

Emission Power 15W Measured valueTx Cable Loss 1.75 dB Value on catalog

Tx Antenna Gain 2.15 dBiλ/2 dipole(No pattern loss

considered)

Height of Tx Antenna

23m (Above Sea

Level)21m above ground

Rx Antenna Gain 0 dBi From MOPS(No pattern loss

considered)Rx Cable Loss 1.2dB Measured value

Receiver sensitivity -104dBm 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)

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3. Transmitted data formatThe 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.

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.

where, HER: Header Error Rate - Error Rate to Header part of VDL 2 messageBFR: 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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RS Block (238bytes)

TRNG*1 Address(9)*2 DATA(220) FEC(6)FCS(3)*2

Fig.3 Message Format for Flight Test

*1 TRNG: Training sequence *2 including Flag

4. Flight TrajectoryThe 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 LevelEstimated Radio Horizon*(NM)

FLT 1Sendai R100 Sendai

(including Arc flight) FL100 133

FLT 2 Sendai Misawa SendaiOutbound FL150Inbound FL100

144118

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

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Fig.4 MER Vs BER for 255 bytes and 238 bytes message

5

ARC Flight(Constant distance flight)

Fig.5 Flight Trajectories and Radio Horizons

(Ground Tx)

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).

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Fig.6a BER and Rcvd Power Vs Distance Fig.6b (1 － MER) and BER Vs Distance

FLT 1 (SendaiR100 & Arc)

Range

Radio Horizon(RH)Rcvd Power

BER

Range

Radio Horizon(RH)

BER

1-MER

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 Flight Level

Estimated Radio Horizon

(A)

Range(BER after FEC < 10-4)

(B)B/A MER at

(B)

FLT 1 100 133NM 110NM 83%

Less than

1%

FLT 1(Arc) 100 133NM 105NM 79%

FLT 2(Outbou

nd)150 144NM 120NM 83%

FLT 2(Inbound

)100 118NM 100NM 85%

FLT 3 180 123NM 120NM 98%

Tab.3 Range Vs Radio Horizon

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RH

* 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

1-MER

BER

Range RH

RH

BER

Range

Rcvd Power

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FLT1 (Sendai R100)

Fig.8a BER and Rcvd Power Vs Distance Fig.8b (1 － MER) and BER Vs Distance

FLT2 (Sendai Misawa)

Rcvd Power

BER

Range

1-MER

BER

Range

Fig.9a BER and Rcvd Power Vs Distance Fig.9b (1 － MER) and BER Vs Distance

FLT2 (Sendai Misawa)nce

Rcvd Power

BER

Range RH

BER

1-MER Range

RH

Rcvd Power

BER

Range

RH

BER

1-MER RangeRH

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.

* The transmitting station is located in the origin of coordinates. Fig.11 Flight over Station (at FL90)

Distance(NM)Number of Messages

BER before FEC BER after FEC MER

0.0--1.0 463 2.52E-3 2.54E-3 6.26E-21.0--2.0 473 1.56E-5 0 0

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EL Angle=56deg

Fig.10a BER and Rcvd Power Vs Distance Fig.10b (1 － MER) and BER Vs Distance FLT3 (Sendai Oomiya)

2.0--3.0 466 2.56E-5 0 03.0--4.0 471 2.05E-5 1.57E-5 2.12E-34.0--5.0 481 4.73E-6 0 0

Tab.4 Flight over Station (at FL90)

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5.3 Ground operation testThe 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 results6.1 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 BER Vs Received Power

6.2 Estimation of error rate for longer message

6.2.1 Theoretical FER Vs BER for different frame lengthThe 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.

Fig.15 VDL Mode 2 Message Format

Fig.16 FER Vs BER (Theoretical value)

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OR

Flag Address FCSUser Data(128~2048)

FlagLink Control

TRNG Data Field(249) FEC

Data Field(249) FECData Field(249) FEC

N block

IPI ACARS Block(Max238)

ISO 8208 Packet

AVLC Frame

If limited toone ACARS 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%.

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Note: One symbol (*,▽)represents the average value of many data.

Fig.17 Measured MER (Log Scale) Vs BER7. 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 WorkThe 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. ConclusionThe meeting is invited to note the information provided in this paper.

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