ACP-WGF29/WP12
International Civil Aviation Organization
2013-09-04
WORKING PAPER
AERONAUTICAL COMMUNICATIONS PANEL (ACP)
TWENTY NINETH MEETING OF WORKING GROUP F
Nairobi, Kenya 5 – 12 September 2013
Agenda Item 7: Development of material for ITU-R meetings
Use of Directional Antennas for Outside WAIC Systems
– Discussion of Concept –
(Presented by Uwe Schwark)
SUMMARY
Initial results on the compatibility between WAIC systems and radio altimeters in the
frequency band 4 200 – 4 400 MHz were presented at the most recent ITU-R Working Party
5B meeting in May/June 2013. These studies address the possibility of WAIC systems
installed on board one or multiple aircraft to interfere with a radio altimeter onboard another
aircraft. Meanwhile further studies have been undertaken addressing also the potential impact
radio altimeters might have onto WAIC systems. All results indicate that both, low and high
data rate WAIC systems located within the aircraft structure (LI and HI WAIC systems), are
compatible with all types of radio altimeters embraced within Draft New Recommendation
ITU-R M.[RadAlt].
Regarding outside WAIC systems, these studies indicate, that the “receiver desensitization”
and “false altitude report” protection criteria cannot be met, with the initially assumed set of
outside WAIC system parameters, in particular their transmission power and antenna
characteristics. This Working Paper discusses a technical concept for a significant reduction
of interference power radiated into the direction of the victim radio altimeter receiver, which
has the potential to prove, that also WAIC systems employing transmit antennas outside the
aircraft structure (i.e., LO and HO WAIC systems) can be implemented such that they are
compatible with the incumbent radio altimeters in the band 4 200 – 4 400 MHz.
ACTION
To provide feedback on the proposed technical concept and its acceptability for consideration
in further study activities on WRC-15 agenda item 1.17 in preparation of the next ITU-R
Working Party 5B meeting in November 2013.
(7 pages)
533568953
ACP WG-F/29 WP 12
1.
-2-
INTRODUCTION
Studies on the compatibility between WAIC systems and radio altimeters undertaken so far in the course
of finding solutions for WRC-15 agenda item 1.17 indicate that both, high data rate and low data rate
WAIC systems inside the aircraft structure (LI and HI WAIC systems) are compatible with all types of
radio altimeters embraced within Draft New Recommendation ITU-R M.[RadAlt]. Regarding both, low
and high data rate outside WAIC systems, these studies indicate that the “receiver desensitization” and
“false altitude report” protection criteria cannot be met with the initially assumed set of outside WAIC
system parameters, in particular the WAIC systems transmit power and antenna characteristics. This
Working Paper discusses a technical concept for a significant reduction of WAIC signal power radiated
into the direction of the victim radio altimeter receiver, which has the potential to prove, that also WAIC
systems employing transmit antennas outside the aircraft structure (i.e. LO and HO WAIC systems) can
be implemented such that they are compatible with the incumbent radio altimeter application in the band
4 200 – 4 400 MHz.
2.
DISCUSSION OF CONCEPT
It can be assumed, that in relevant cases the radio altimeter mainbeam points into nadir direction.
Consequently, significant amounts of potential interference into the radio altimeter receiver will only be
observed from aircraft equipped with LO or HO WAIC systems, if these aircraft are located below the
victim radio altimeter receiver. Likewise, it is also true that WAIC systems onboard an aircraft located
below a radio altimeter transmitter will experience the largest potential interference impact. This is the
underlying assumption for all interference geometries analyzed in the compatibility studies quoted above
(see Figures 1 and 2).
Hence, in order to avoid potential harmful interference into the radio altimeter receiver, it is proposed to
limit emissions of LO and HO WAIC systems into upwards direction by utilizing directive transmit
antennas. Likewise, in order to limit the potential interference impact of a radio altimeter transmitter into
LO or HO WAIC systems’ receivers, it is proposed to limit the amount of received power from sources
above the aircraft through employing directional receive antennas.
-3-
ACP WG-F/29 WP 12
d Ground
dTaxi
yRA
dTaxi
y
Runway
d Hold
Runway
y
Taxiway
x
RA aircraft
RA aircraft

Holding Bay
x

d Ground
aRA
aRA
WAIC aircraft
WAIC aircraft
Runway
Taxiway
Runway
(a)
d Hold
(b)
Figure 1: Interference geometries assumed in compatibility studies between WAIC systems and
radio altimeters, (a) airport taxiway scenario (b) airport holding bay scenario1
RA aircraft
300m
Radio Altimeter
Main Beam
WAIC aircraft
Figure 2: Interference geometries assumed in compatibility studies between WAIC systems and
radio altimeters (in-flight scenario)
1
For more information on detailed scenario geometries (e.g. distances) please refer to document ITU-R
5B/TEMP/121-E
-4-
ACP WG-F/29 WP 12
2.1
Use of Directional Antennas for Outside WAIC Systems
To limit the emissions of WAIC systems into the direction of the radio altimeter antenna mainbeam,
directive antennas can be used for reception and transmission at the WAIC Gateway and End Nodes. The
End Node antennas are oriented such that their mainbeams point towards the aircraft body. In
corresponding areas Gateway Nodes are located. Their antenna mainbeams point into the direction of the
End Nodes. For this example End Nodes are always located below or approximately on the same
horizontal plane as Gateway Nodes, consequently antenna mainbeam elevation angles can be kept small
and interference into the direction of a possible victim radio altimeter receiver onboard another aircraft
located above the WAIC aircraft is reduced (see Figure 3).
WAIC
End
Node
WAIC
Gateway
Node
WAIC Gateway Node
antenna main beam
WAIC
Gateway
Node
WAIC
End
Node
WAIC End Node
antenna main beam
Figure 3: Exampe for use of directional antenneas at Gateway and End Node for providing WAIG
signal coverage aroud the wings
This antenna configuration provides two essential benefits which help avoid potential interference of
WAIC outside systems into radio altimeter receivers as well as potential interference of radio altimeters
into WAIC system receivers:
1.) The gain of the directive antennas used on both the WAIC Gateway and End Nodes, respectively,
improves the link budget (increases the SNR) of the corresponding communication links. This
allows a reduction of WAIC transmit power (while maintaining link quality) which in addition to
the antenna directivity will cause a significant decrease of the WAIC signal power received by
the radio altimeter’s antenna.
2.) Outside WAIC applications experience a better protection from the radio altimeter transmitter
through the additional isolation provided by antenna directivity.
2.2
Example Antenna Design
Resonant rectangular series-fed patch array antennas are one example of antennas which are capable of
generating a suitable beam pattern for outside WAIC system antenna installations (see Figures 4 and 5).
The antenna exemplarily shown in Figure 5, achieves a maximum gain of 17 dBi with a main-to-sidelobe
ratio of ~30 dB (see Figures 5 and 6). Antennas with gains of up to 20 dBi and a 35 dB main-to-sidelobe
ratio can be achieved using this antenna design.
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ACP WG-F/29 WP 12
Figure 4: Example of an resonant rectangular
series-fed patch array antenna
Figure 5: 3D plot of the radiation
characteristics of resonant rectangular seriesfed patch array antenna
0° 20dBi
45°
0dBi
-15dB
90°
Figure 6: Resonant rectangular series-fed patch array antenna used as WAIC Gateway Node
receive/transmit antenna installed laterally at center fuselage (arcraft front-view with WAIC
Gateway Node antenna radiation pattern in vertical plane) 2.
Figure 6 depicts an example application of a resonant rectangular series-fed patch array antenna as WAIC
Gateway node antenna installed at the lateral center fuselage. In this example the antenna is used to
provide radio coverage for WAIC End Nodes installed at the wings, nacelles or pylons. This example
assumes a 20 dBi antenna gain in mainbeam direction and a 35 dB mainbeam-to-sidelobe ratio.
2 For the sake of a simplified representation the radiation patterns of the gateway node antennas are centred in the
geometric center of the fuselage cross section. In reality gateway note antennas would be installed at the lateral
center fuselage. This simplification, however, has negligible impact on the results.
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ACP WG-F/29 WP 12
Exemplarily utilizing this kind of antenna for WAIC outside applications on both, the the Gateway and
End Nodes will provide up to 40 dB additional WAIC communications link budget (twice the maximum
antenna gain) as well as up to 15 dB of additional isolation between the radio altimeter and outside WAIC
systems. The additional link budget can be used to reduce the transmit power of outside WAIC
applications. Assuming a reduction of HO WAIC system transmit power of 22 dB (-5 dBm) will lead to
up to 37 dB of additional isolation (22 dB power reduction + 15 dB sidelobe suppression) into the
relevant direction (see Figures 1 and 2). This value is sufficient to protect the radio altimeter from harmful
interference.
IF
-60
Receiver desensitizations criterion for HO systems
10
A1
A2
0
A3
A4
A5
-10
A6
D1
-20
D2
D3
D4
-30
200
400
600
Altitude [m]
800
1000
Figure 7: Results of radio altimeter receiver
desensitaization for the “airport taxiway”
scenario
0
200
Altitude
400 [m]600
Altitude [m]
800
Receiver desensitizations criterion for LO systems
I/S criterion for HO systems - WAIC channel model:F
10
A1
30
A2A1
0
A2
A3
A3
20
A4
-10
A5A4
A6A5
-20
10
D1A6
-30
D2D1
D3D2
0
-40
D4D3
D4
-10 0
200
400
600
800
200
400
600
800
1000
Altitude [m]
Altitude [m]
Figure 8: Results for I/S protection criteria for
WAIC HO systems the for the “airport taxiway”
scenario
-20
-30
-40
-50
-60
1000
IIF/N [dB]
I/S [dB]
IF
I /N [dB]
Altitude [m]
-10
I/S [dB]
-20
A6
A4
Figure
8 depicts the resulting interference-to-signal
power
(I/S) ratio at the HO WAIC system’sA6
receiver
-20
-20
D1
D1A5
onboard the taxiing aircraft vs. the altitude over ground -30
of the flying-by aircraft again for the “airport
D2
D2A6
-25 scenario”. Also in this case, it is assumed that the
-25 directive antenna concept described above
taxiway
is
D3
D3D1
-40
applied
to
both,
outside
WAIC
high
data
rate
Gateway
and
End
Nodes.
The
protection
criterion
of
the
D4
D4
D2
-30
-30
interference-to-signal power ratio I/S of -14 dB for high data
for any
-50 rate WAIC systems is not exceeded D3
-35radio altimeter types embraced in Draft New Recommendation
-35
of the
ITU-R
M.[RadAlt].
D4
200
400
600
800
1000
200
400
600
800
1000
I/S criteri
0
0
I/S criterio
30
20
I/S [dB]
IIF/N [dB]
I/S [dB]
I /N [dB]
Figure 7 depicts the resulting interference-to-receiver noise power (I/N) ratio at the radio altimeter
receiver on board the aircraft which flies-by the taxiing aircraft vs. this aircraft’s altitude over ground
(“Airport taxiway scenario” see Figure 1a). For the depicted case, it is assumed that the directive antenna
conceptReceiver
described
above is applied to both, outside WAIC high
data rate Gateway and End Nodes. This
desensitizations criterion for HI systems
Receiver desensitizations criterion for LI systems
combination
of WAIC system category and scenario is 0discussed here, since it represents the most
0
A1 lowest isolation
A1
stringent case, due to highest power level,
and most
stringent
protection
criterion.
I/S criterion
for HI systems
- WAIC
channel model:A
-5
-50
A2 desensitization
A2 noise
Figure 7 shows that the radio altimeter receiver
criterion of interference-to-receiver
A3
A3A1
-10ratio I/N of -6dB (the most stringent of all radio altimeter
-10
power
protection criteria) is not exceeded
for
-10
A4
A4A2
any of
the
radio
altimeter
types
embraced
in
Draft
New
Recommendation
ITU-R
M.[RadAlt].
A5
A5A3
-15
-15
10
0
-10
-20
-30
0
-7-
3.
ACP WG-F/29 WP 12
CONSLUSION
Employing directive antennas is deemed a suitable concept for mitigating harmful interference which
could potentially be caused by outside WAIC systems when they would utilize omni-directional antennas.
A specific exemplary antenna type (Resonant rectangular series-fed patch array antennas) which provides
sufficient directivity and sidelobe suppression has been identified and analyzed. Furthermore, this antenna
type is specifically suitable for conformal integration into the aircraft’s surface due to its planar structure,
which is a great advantage over many other antenna designs.
For resonant rectangular series-fed patch array antennas, when designed properly, the reduction of
radiated power into the relevant directions, e.g. the elevation angle under which the radio altimeter
receive antenna of a landing aircraft is seen by a WAIC aircraft on ground (see e.g. Figures 1 and 2) is
sufficiently large to ensure enough isolation between radio altimeters and outside WAIC systems.
It can be concluded that outside WAIC applications can share the frequency band 4 200 – 4 400 MHz
with all radio altimeter types embraced in Draft New Recommendation ITU-R M.[RadAlt], if antennas
with sufficiently large directivity, as discussed in this Working Paper, are utilized.
4.
ACTION BY WG-F
To provide feedback on the proposed technical concept and its acceptability for consideration in further
study activities on WRC-15 Agenda Item 1.17 in preparation of the next ITU-R Working Party 5B
meeting in November 2013.