AMCP /WG-B/14 WP-5

AERONAUTICAL MOBILE COMMUNICATIONS PANEL(AMCP)

Working Group B, 14th meeting13-17 January 2003Montreal, Canada

Impacts of Radio Interference on VDL Mode 3

Presented by Yasuyoshi NAKATANIPrepared by Jun KITAORI

Electronic Navigation Research Institute (ENRI)JAPAN

SUMMARYWe evaluated the impacts of radio interferences from undesired VHF DSB-AM or VDL

signal sources into VDL Mode 3. The characteristics of Bit Error Rate (BER), Adjacent Channel

Rejection (ACR) and Block Failure Rate (BFR) have been measured to evaluate the

interferences. We then calculated the minimum distances of isolation associated with channel

separation required for proper VDL Mode 3 operation. Consequently, it was identified for

example that at least 2 channels was needed for the channel separation between victim and

interferer at the minimum distance of 2000ft.

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1. Introduction

Radio wave interference on aeronautical VHF band deteriorates communication qualities of DSB-

AM analog radio or VDL. We investigated the impacts of VDL Mode 2/3 interferer on DSB-AM radio

and reported the required channel separation to avoid the interference on DSB-AM operation in the

past working paper (AMCP WG/B-12 WP-8). This time, we examined the impacts of DSB-AM or

VDL Mode 2/3 interference on VDL Mode 3 system and identified the channel separation for VDL

Mode 3 frequency assignment.

2. Fundamental performance of radios

2.1. Radios tested

As shown in Table 1, VDL Mode 3 trial system was used as a victim and VHF DSB-AM radios

and a VDL transmitter (a part of VDL Mode 3 trial system) were used as the interferers. When the

VDL Mode 3 trial equipment was used as an interferer, we regarded its continuous transmission mode

(in other words, pseudo random noise mode) as VDL Mode 2 signal and the TDMA burst transmission

mode as VDL Mode 3 signal. We labeled four DSB-AM radios in the following table as A, B, C and D

randomly to identify the test results.

Table 1 List of radios used

Radio type Manufacturer Major specifications

Victim (Mode 3) and

interferer (Mode 2

and Mode 3)

VDL Mode3

trial

equipment

NEC

Channel spacing: 25kHzBoth continuous transmission mode and

TDMA burst transmission mode are

equipped

Interferer (DSB-AM)

(Labeled A,B,C&D)

VHF-900Rockwell

Collins

Channel spacing: 25kHz

For airliners

Provision of VDL Mode 2 is provided

VHF-700Rockwell

Collins

Channel spacing: 25kHz

For airliners

VHF-700BRockwell

Collins

Channel spacings: 25kHz and 8.33kHz

For airliners

VHF-22ARockwell

Collins

Channel spacing: 25kHz

For general aviations

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2.2. Interferer spectrum characteristics (Adjacent Channel Power)

Adjacent channel power (ACP) characteristics of the interferers were measured.(Table 1) We

measured the ACP on the channel at the center frequency of 136.000MHz and on the 1st, 2nd, 3rd, 4th,

5th, 10th, 20th and 40th adjacent channels with increments/decrements of 25kHz. Since the upper limit of

aeronautical VHF band is 136.975MHz, ACP on the 39 th upper adjacent channel was measured instead

of the 40th. Notch filter was inserted to improve accuracy of ACP measurement on any channels except

for the 1st adjacent channel. Channel bandwidth (BW) over which ACP was measured was set to

25kHz and 16kHz as referred in VDL SARPs. The modulation parameters for each radio are presented

in Table 2. The VDL equipment was set to continuous transmission mode (i.e. Mode 2). Since VDL

Mode 3 has the same physical layer as VDL Mode 2, the measured results obtained on VDL Mode 2

are applicable to VDL Mode 3. Transmission power with modulation was measured.

Table 2 Setting of transmitters at ACP measurement

Radio IDs Modulation patternMeasured transmission

power with modulation

VDL (Mode 2)PN15 (15-stage pseudo

random noise sequence)15.6W (=41.9dBm)

A 1kHz tone, 85% modulation 21.8W (=43.4dBm)

B 1kHz tone, 85% modulation 38.6W (=45.9dBm)

C 1kHz tone, 85% modulation 37.6W (=45.8dBm)

D 1kHz tone, 85% modulation 41.8W (=46.2dBm)

Figure 1 System diagram for ACP measurement

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The measured results of ACP values are given in Figure 2 (BW=25kHz) and Figure 3

(BW=16kHz). The ACP on four DSB-AM radios (A, B, C and D) showed the difference of about

maximum 5dB between the values for both bandwidths. On the other hand, VDL Mode 2 had larger

difference than DSB-AM radios on the 1st adjacent channel, which reached to about 10 to 15 dB. This

can be explained as follows; because the spectrum of D8PSK modulated wave on VDL is broader than

that of DSB-AM, the emission power of VDL leaks out of the co-channel. This leaked power will

generate the uneven radiated spectrum in the 1st adjacent channel and produce the larger difference of

ACP.

The significant ACP degradation at the 4th adjacent channel for the radios A and D was

possibly brought about by higher spurious emission with 100kHz spacing contained in the spectrum of

these radios.

Figure 2 Result of ACP measurement at BW=25kHz

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Figure 3 Result of ACP measurement at BW=16kHz

2.3. VDL receiver performance

We measured the received power versus Bit Error Rate (BER) before FEC on VDL Mode 3

receiver and defined the minimum receiver sensitivity as fundamental characteristics of victim

receiver. In VDL Mode 3 SARPs, the minimum receiver sensitivity is specified as 20 V/m (equals to

–94dBm at 136.000MHz) when BER value before FEC is 10 -3. The VDL receiver satisfied the

required BER at the received power of –101.5dBm (Figure 4).

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Figure 4 VDL Mode 3 receiver performance

3. Laboratory test for interference

3.1. BER measurements

We conducted the measurements of BER under the following conditions.

Pseudo random pattern PN15 was generated as a desired signal source.

Received power of desired signal at the end of receiver input was set to 40 V/m (converts to –

88dBm). This value is defined at the paragraph 6.3.5.4 “Interference immunity performance” in

the VDL SARPs.

VDL Mode 3 radios were used as the desired signal generator and the victim receiver. Interferers were

those listed in Table 1. The BER was measured using the VDL equipment as Mode 2 and Mode 3

interferer. We set it to transmit V/D bursts in ‘A’ and ‘C’ slots of TDMA frame as the desired signal

source of Mode 3. However, for co-channel operation continuous transmission mode was used as

interference signal source because the interference by continuous mode could have higher adverse

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effect than that by burst mode. System diagram for BER measurement is shown in Figure 5. The level

of undesired signal source was varied with increments of 1 dB, and BER was measured within the

range between 10-5 and 10-2.

Figure 6 shows the results of BER measured for interference on co-channel. D/U ratios on

each interferer satisfying the same BER value were distributed in the range about 2 dB width, and no

specific difference of BER characteristics was observed between DSB-AM and VDL. The D/U ratio

was about 17dB at BER=10-3.

Figure 7 shows a graph of Adjacent Channel Rejection (ACR) performance when BER

before FEC satisfies 10-3. ACR is defined as a reciprocal value of D/U ratio, so ACR = –D/U when

represented in dB. The ACR curves up to the 4 th adjacent channel significantly differed from each

interfering radio. As described in 2.2, it can be explained that the VDL modulation spectrum extending

wider than DSB-AM produces the degradations of ACR on the 1st adjacent channel, and the 100kHz

spacing spurious on the radios ‘A’ and ‘D’ creates them on the 4th adjacent channel. The ACR values

were approximately constant (about 70dB) beyond the 4th adjacent channel, with no major difference

observed for each interferer. In other words, the impacts of interference on VDL Mode 3 system can

be described as follows; if the channel of interferer is within the 4 th adjacent channel of VDL Mode 3,

the degree of interference varies depending on the type of interferer radios. Beyond the 4 th adjacent

channel, that variation will decline.

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Figure 5 System diagram for BER measurement

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Figure 6 BER for interference on co-channel

Figure 7 ACR performance

3.2. Block Failure Rate (BFR) measurements

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We also evaluated the BFR characteristics as the performance including error correction function.

The BFR was evaluated to a V/D burst block that is used to transfer VDL Mode 3 user data. The V/D

burst consists of a V/D header block generated as a Golay(24,12) code word and a V/D data block

encoded as a RS(72,62) code word. For BFR measurement, the undesired signal was tuned to the same

frequency as the VDL Mode 3 receiver (co-channel), then the initial power of undesired signal was set

so as to produce BER before FEC of 10-5. We increased the power of undesired signal in steps of 1dB

and measured both BER and BFR until no desired signal could be received any longer. The undesired

signal sources were the DSB-AM ‘B’ and VDL Mode 2. The system block diagram is given in Figure

8. Theoretical curve in Figure 9 was calculated by following equations, where HDFR and VDFR are

block failure (or error) rate of V/D header block and V/D data block respectively. The results of

measurements almost agreed with the theoretical curve (Figure 9). BFR values were zeros in the

region of BER < 10-3.

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Figure 8 System diagram for BFR measurement

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Figure 9 BFR performance

4. Consideration

To determine the required channel separation for VDL Mode 3, we computed an isolation level

based on ACR performance. We assumed that the undesired signal source was an airborne station and

the victim was another airborne station. Desired signal power at the antenna input of VDL Mode 3

receiver (Pd) was assumed to be –82dBm. Transmission power of interferer was assumed to be 44dBm

(for VDL Mode 3 and DSB-AM) or 42dBm (for VDL Mode 2). So emission power into space (Pe) was

calculated to be 41dBm or 39dBm when feeder losses and antenna gains of airborne stations were

respectively –3dB and 0dBi. Interference power at the antenna input of the receiver (Pu) was derived

from the next equation in dB.

For DSB-AM interferers, Pu was averaged among four types of DSB-AM radios. Isolation in space (Is) was defined as . We regarded Is as a free space propagation loss and converted it into a

separation distance (Figure 10). If we take the vertical minimum separation in flight as 2000ft (=

600m) for example, so the required channel separation is as follows.

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Table 3 Required channel separation for VDL Mode 3

Required separation

distance

Interferer

Mode 2 Mode 3 DSB-AM

2000ft 2 channels 2 channels 2 channels

Figure 10 Isolation converted into separation distance

5. Conclusion

We carried out the radio interference tests using VDL Mode 3 trial system and obtained the

interference characteristics that DSB-AM and VDL Mode 2/3 transmitters gave to the VDL Mode 3

receiver. Four types of airborne DSB-AM radios were employed as the interferers and the VDL Mode

3 trial system was used as VDL Mode 3 and pseudo VDL Mode 2 signal source. After the test,

isolation of space and separation distance were calculated based on ACR performance. The result

showed the required BER specified in VDL Mode 3 SARPs was satisfied if frequency of the interferer

is allocated in more than the 2nd channels apart from VDL Mode 3 assuming that the interferers were

VDL Mode 2/3 or DSB-AM and the minimum distance between the interferer and the victim was

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2000ft (minimum vertical separation in same direction).

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