AERONAUTICAL COMMUNICATIONS PANEL (ACP)
ththBangkok, 20?28 November 2003
Agenda Item 5?Maintenance of SARPs/Manuals for current COM systems
(HFDL, VDL Modes 2,3 and 4, AMSS)
Development and Evaluation of VDL Mode 3 System in Japan
Presented by Satoshi Kato, ENRI
Summary The Electronic Navigation Research Institute (ENRI) has been working on research & development, test and evaluation of VDL Mode 3 system based on 5-year program in the time frame 2000-2004. As the development of the test system was almost finalized, the testing to evaluate
and validate various VDL Mode 3 performances is presently in progress in a
laboratory and by flight tests using our Beechcraft B-99 aircraft. The
provisional test results on both data and voice communication, including
transfer delay, prioritized message processing, quality of voice, etc. have been successfully obtained so far. Whereas, large number of unintended link disconnections due to the collisions of M downlink bursts (for Reservation
Request) were observed in the course of the test. To cope with this issue, we introduced some measure that can ensure the integrity of transfer for M downlink Reservation Request message.
Future implementation of VDL Mode 3 system is being considered in Japan as a next generation A/G mobile communication system for air traffic services. With request from Japan Civil Aviation Bureau (JCAB), the Electronic Navigation Research Institute (ENRI) is promoting the development, test and evaluation of VDL Mode 3 System.
2. Activities of ENRI on VDL Mode 3 System
The ENRI had participated in the evaluation on vocoders from 1997 to 1998 to choose most suitable vocoder to be incorporated in ICAO SARPs. Currently we have developed the VDL Mode 3 test system, which can be a basis for the future operational system, and is conducting the evaluation of the data link and voice communication performances.
2.1 Overview of VDL Mode 3 development & Evaluation Program
Table 1 shows a program milestone for the development and evaluation on VDL Mode 3 system. We have completed developing the VDL Mode 3 test system except for its ground center station. With this system, the preliminary communication test was conducted in last February with the objective of evaluating basic functions and performances of the system. In April 2003, we carried out the first flight test, with being followed by the successive ones. Prior to those testing, the associated radio interference had been evaluated to investigate the impact of interferences that VDL Mode 3 system possibly provides to or is received from DSB-AM and other VDL systems operated in the same VHF band. The results of them were presented at the past ACP WG-B meetings. The entire testing schedule presently planned for VDL Mode 3 system is as illustrated in Table 2.
Meanwhile, the analysis with computer simulation on various communication performances such as channel throughput, transfer delay, etc. in various traffic models is also in progress using simulation software written with OPNET (Optimized Network Engineering Tool).
Table 1 Milestone of VDL Mode 3 Development and Evaluation
Summary and Report VDL Mode 3 System Development
System Radio Interferences Performance Test and Analysis Test & Evaluation
Table 2 Time Scales for Testing
? Vocoder Sept-Nov Radio ? ?
Interference Oct Sept Data Com ? ? Comprehensive Test &Evaluation (in Laboratory) Feb Dec
Voice Com ? ? (in Laboratory) Feb Sept
? ? ? Flight Test Apr Sept Mar ? ATN Connection May
2.2 Outline of VDL Mode 3 Test System
The configuration of VDL Mode 3 test system currently under development is as
given in Figure 1. It comprises two sets of Aircraft Stations (ASs), two sets of Ground
Stations (GSs) and the Ground Center Station (GCS). The development of most equipment excluding the GCS has been completed. Table 3 shows major specifications
of the system.
RF : RF Unit MODEM: Mod/Demod Unit Aircraft Aircraft TDMA : TDMA Control Unit Aircraft Ground RF RF Station Station COM : Com Management Unit Station Station (AS)(AS) VOC : Voice Processing Unit (AS)(GS) MODEM MODEM GCS : Ground Center Station
VOC TDMA TDMA VOC GCS COM COM
ATN ATN Router Router
Figure 1 Configuration of VDL Mode 3 Test Equipment
Table 3 Major Specifications of VDL Mode 3 Test System
General ?Compliant with relevant ICAO SARPs and Manuals
Tx Power ?15W (=42dBm)
?Operational in any frequency between 118.0 and 136.975MHz Frequency (in 25kHz steps)
?Approved only at 136.900 and 136.925MHz for field testing
System Configurations ?2V2D and 3V1D implemented
Minimum Receiver -3 ?-103.2dBm (measured at BER=10 [beforeFEC] ) Sensitivity
Number of ?Ground Station (GS) - 2 sets, Aircraft Station (AS) - 2 sets
Equipment and Ground Center Station - 1set
3. Results of Test and Evaluation
3.1 Preliminary Laboratory Test
In February 2003, preliminary test for voice and data communications has been conducted in our laboratory.
3.1.1 Data Communication
The test setup used to evaluate data communication is as shown in Figure 2. The test included the measurement and evaluation of received power, transfer delay and channel throughput against BER (before FEC).
Variable Uplink Downlink ATT ATT ATT
RF RF Transmitted MSG Transmitted MSG (Uplink) (Downlink) MODEM MODEM Data length : 15seg Data length : 1 or 15seg. (Broadcast) ACK : Yes TDMA TDMA ACK : No Priority : 0 (low) Priority : 0 (low) Number of MSG: 100 COM COM Waiting time for next Next data transmission data transmission request request immediately upon AS :1.5sec GS receipt of ACK
Figure 2 Test Setup
Transfer delay referred in this test is defined as the period elapsed between the time when aircraft DLS (Data Link Service) sublayer delivers user data and that when ground DLS receives it. Therefore the transfer delay is the sum of the following elements of period;
a) The aircraft DLS sublayer delivers the information frame(s) containing user
data to aircraft MAC.
b) The aircraft MAC sublayer transmits a reservation request message towards
an associated ground station with a M downlink burst.
c) The ground station allocates TDMA time slot(s) in response to the reservation
request and replies a reservation response with a M uplink burst.
d) Finally the aircraft station sends the user data with V/D downlink burst(s)
and the ground DLS receives it (them).
(a) Transfer Delay and Channel Throughput vs. BER
Some examples of test results are given in Figure 3 (input received power vs. BER), Figure 4 (transfer delay vs. BER) and Figure 5 (channel throughput vs. BER). The
-3Minimum receiver sensitivity, which corresponds to BER equivalent to 10 appeared to
be -103.2dBm (Figure 3). The transfer delay was about 2 seconds for one segment data
(i.e. 62 byte-length data transferable within single TDMA slot) and about 3.75 seconds for 15 segment data (i.e. 930 byte-length data equivalent to maximum DLS frame size),
-3but each one rapidly increased from the vicinity where the BER exceeds 5×10 and 1
-3×10 respectively (Figure 4).
15seg data 151seg data BER 1.00E-0310 Minimum 5.87Received Power 3.7553.73 =-103.2dBm Transfer Delay (Cummulative 95%)2.042.032.032.031.00E-040
-110-108-106-104-102-1001.00E-041.00E-031.00E-021.00E-01 Input Reception Level (dBm)BERNote) a) Channel loading on this test was higher than 94% b) Transfer delay is as defined before (not including transfer ACK)
Figure 4 BER vs. Downlink Transfer Delay (95%) Figure 3 Input Received Power vs. BER
Likewise, the channel throughput quickly decreased from the point where BER
-3-3came up to 10. It is assumed that when BER exceeds around 10it will create rapid
increase of retransmissions of V/D (data) bursts as well as M bursts (used for reservation request/response). The analysis for the observed data explained that the degree of BER generating additional transfer delay due to retransmissions of burst
3-2messages is nearly 1×10-for V/D (data) burst, and 1×10 (ten times larger than that
for V/D burst) for M burst. It implies the retransmissions of V/D (data) bursts, as they contain longer message and different error correction code than that of M bursts, may be a primary factor for the deterioration of transfer delay and channel throughput.
300 1seg data
100A/G Total Throughput
Figure 5 BER vs. A/G Total Channel Throughput
(b) Transfer of Prioritized Message
Next, we evaluated the transfer delay characteristics for the data with different priorities under certain conditions. The prioritized message processing and transmission is one of the outstanding features offered by VDL Mode 3 system. The test setup for the evaluation is as presented in Figure 6. Each of three aircraft stations successively transmits 1 segment-length (=62bytes) data having different priority. The ground station broadcasts 15 segment-length (=930bytes) data at constant time spacing. With changing this time spacing, we varied the total channel loading on air-ground link.
Some results from this test are shown in Figure 7 and 8. Figure 7 indicates that the transfer delay for aircraft station with lowest priority (AS1) has significantly outdistanced those for other two aircraft stations with higher priorities and reached near 40 seconds in maximum. This is due to that the transmission request from AS1 continually contended with the uplink broadcast having the same priority, while the requests from AS2 and AS3 in this order preceded other requests. It was also recognized that the delay on AS1 was rapidly reducing with the decrease of channel load (See Figure 8). When the downlink data length was 15 segments, the maximum transfer delay went beyond 4 minutes.
Downlink Uplink Combiner
GS : Ground Station ATT ATT ATT ATT AS : Aircraft Station ATT : Attenuator
ACK : Acknowledged Frame GS AS1 AS2 AS3 (Yes or No) Xmt’d Data Transmitted Data Xmt’d Data Xmt’d Data a. 1seg a. Data length: 15seg a. 1seg a. 1seg b. Yes b. ACK : No b. Yes b. Yes c. 2 (high) c. Priority : 0 (low) c. 1 (middle) c. 0 (low) d. 100 max. d. Number of Data: NA d. 100 max. d. 100 max. e. the same as e. Waiting time for next e. next data e. the same as *The test was terminated when request upon data transmission AS1 the number of data transmitted AS1 receipt of ACK request : 1-3sec from any of aircraft stations reached 100.
Figure 6 Test Setup for Different Priority Message
40 35 30 25AS1 (Pri=low) 20AS2 (Pri=mid) AS3 (Pri=high)15
10Downlink Transfer Delay (sec) 5
Elapsed Time from Test Started (HH:MM:SS)
Figure 7 Transfer Delay for Message with Different Priority
40Delay for AS1 (Pri=0)
Delay for AS2 (Pri=1) 30Delay for AS3 (Pri=3)
10 Cummulative 95% Downlink
Transfer Delay (sec) 0
Channel Loading (%)
Figure 8 Relation between Transfer Delay and Channel Loading
3.1.2 Voice Communication
As for voice communication, we measured the voice processing delay within the vocoder, the end-to-end system transfer delay, and the degradation of voice quality and transfer delay according to the reduction of received signal.
Consequently, the internal delay for voice processing within the vocoder was 59~64 msec (total time required for encoding and decoding both). The end-to-end voice transfer delay was about 208msec (See Figure 9), which satisfied maximum time delay (=215ms) specified in the ICAO VDL Mode 3 Manual (section 8.4.2 in Part II). The observation on the voice quality and transfer delay against BER (before FEC) explained that the received voice can be clearly understood if the BER is lower than 1
-2×10 (equivalent to –106dBm of received power), and also that irrespective of the received power, the transfer delay is almost constant at 208ms as far as received voice is perceivable.
Input Voice VDL Mode 3
VDL Mode 3 Output Voice System
end-to-end delay Input Voice =208ms
Figure 9 Voice Com End-to-End Delay
3.2 Flight Test
The flight test was conducted using our Beechcraft B-99 aircraft in the airspace near Sendai Airport to investigate radio coverage and downlink transfer delay for the VDL Mode 3 system. When the radio coverage was defined as the airspace satisfying
-3BER being less than 10 (before FEC), it reached nearly 90% of estimated radio horizon (Table 4). It was almost comparable with the radio coverage for VDL Mode 2, which had been assessed before with similar flight profiles.
Figure 10 represents the flight trajectories, estimated radio horizon, as well as the location of V/D burst errors observed at the flight tests. The symbol, “A”, “B” and “C” in
the figure correspond to those in Table 4.
X X Misawa X X
SDE R100 10,500feet10,500feet ? XX XX X X X Sendai C FL180FL180 10,500ft 10,500feet10,500feet A 15,000ft FL150FL150 FL180FL180 18,000ft X X X X X X Omiya XX X X X
X : V/D burst error
Figure 10 Flight Trajectory and Occurrence of V/D Burst Error
Table 4 Radio Horizon and Coverage for Each Flight Trajectory
Radio Coverage Estimated Altitude -3Radio Horizon (BER?10) Flight Routes Y/X (feet) ?X? ?Y?
Sendai; Omiya 18,000 123NM 114NM 93% A Omiya ; Sendai 18,000 123NM 111NM 90%
Sendai ; Misawa 15,000 144NM 156NM 108% B Misawa ; Sendai 10,000 118NM 117NM 99%
C SDE R100 10,500 133NM 114NM 86%
The transfer delay for downlink was measured on the same flight trajectories. One of the results on the transfer delay, the definition of which is the same as specified in 3.1, is presented in Figure 11. It can be known from the figure that the range in which the transfer delay stays at lower level is nearly coincide with the radio coverage identified before.
The reason can be logically explained as follows;
As we adopted the cumulative 95% value as the transfer delay here, the occurrences of V/D burst error below 5%, which may of course produce message retransmission, but will not effect the value of transfer delay.
Figure 12 shows the interrelation of the error rate of V/D burst message and the BER (before FEC), which is calculated based on the formulas provided in the figure. As known from the theoretical curves in the figure, the error rate of V/D bursts at BER
-3equivalent to 10 is substantially below 5% for both 1 and 15 segment-length data.
-3Therefore, the V/D burst errors occurring when BER is lower than 10 will give no
effect on the cumulative 95% transfer delay. More precisely, the transmission errors on M bursts (used for reservation request and response) also relate to this transfer delay. However the probability of such M burst error over the same BER is much lower than that of V/D burst, hence, it will provide further less adverse effect on the transfer delay.
Radio Coverage=114NM Radio Horizon=123NM
7 Transfer Delay (1seg)6 Transfer Delay (15seg)
1 Cumulative 95% Transfer Delay0 (sec)
Distance from GS (NM)
Figure 11 Transfer Delay vs. Distance
(Flight: Sendai; Omiya)
a) Error rate of V/D burst for 1 segment-length data 1.00E-013 ，(5% k48;k；1;BER.(1;BER).Comb(48,k)！；？ ，，k；0 315seg data ，(m24;m1.00E-02 ？BER.(1;BER).Comb(24,m)！；？ ，，m；051seg data ，( 8n8.(72;n) ？(1;(1;BER)).(1;BER).Comb(72,n) ！；？---A n；0，，1.00E-03 -43.5x10 b) Error rate of V/D burst for 15 segment-length data
151.00E-04Error Rate of V/D Burst = 1;(1;A)
Figure 12 V/D Burst Error Rate vs. BER (Theoretical Calculation)
4. Issues Arising from the Test and its Resolution
4.1 Unintentional Link Disconnection
A lot of unintentional link disconnections were observed in a laboratory test at the test setup as shown in Figure 13. The ground station misinterpreted the M downlink (random access) Reservation Request messages for the Leaving Net messages. For example, in the scenario where the ground station broadcasts 15 segment-length (=930bytes) data with the time spacing of 3 seconds, at the same time each aircraft station continually transmits 1 segment-length (=62bytes) data with the same priority, the rate of such misinterpretation at the ground station amounted to as high as 8% for a total of 80 M downlink (Reservation Request) messages transmitted in random access.
Combiner GS : Ground Station AS : Aircraft Station ATT ATT ATT ATT ATT : Attenuator
ACK : Acknowledged Message GS AS1 AS2 AS3
Transmitted Data Transmitted Data from each AS
a. Data length: a.Data length: 1seg 15seg (broadcast) b.Priority : 0 (low) b.Priority: 0 (low) c.Number of messages:100 max. for each station c.Number of messages: NA d.Next data transmission request *Channel loading reached d.Waiting time for next data immediately upon receipt of ACK transmission request: 3sec approx. 85% during the test.
Figure 13 Test Setup for Data Link with Multiple Aircraft Stations