Proceedings of the International Scientific Conference Modern Safety Technologies in Transportation 2007
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CONDITIONS FOR OFFSET CARRIER SYSTEMS IN REAL
ENVIRONMENT OF AIR RADIO COMMUNICATION NETWORKS
Stanislav RYDLO1
Abstract:
The article describes implementation methods of offset carrier systems determinated for supporting
real air connection according to the valid aviation rules and standards. Principle of this system is
based on exploitation only one radio freq
uency, wh ich is used by more ground transmitters. There
are described the possibilities and limitations of offset carrier systems when used in real
environment.
Keywords: offset, air radio communication
1. INTRODUCTION
Increasing number of the aircraft and thereby also growing communication requirements of the flight
area users at any altitude and any time evokes pressure on search possibility as this communication for
a certainty ensure on limitation number assigned frequency.
Air Navigation Service demands reliable radio communication between air traffic control and military
planes on assign frequency in VHF and UHF band throughout FIR (Flight Information Region) at
given altitude. It is necessary operate several radio-stations in more ranges on common frequency to
these requirements solution. There is the offset carrier system to operate several transmitters on
common radio channel. It is shift carrier frequency particular transmitter on common assigning
channel. It is described in International standard and recommended practices ANNEX 10 to the
convention on international civil aviation, Aeronautical Telecommunications, Volume III –
Communications systems.
2. OFFSET CARRIER SYSTEMS
Offset carrier systems can be used in 25 kHz, 50 kHz and 100 kHz channel spaced environments. The
stability of individual carriers of an offset carrier system shall be such as to prevent first-order
heterodyne frequencies of less than 4 kHz and, additionally, the maximum frequency excursion of the
outer carrier frequencies from the assigned carrier frequency shall not exceed 8 kHz. Offset carrier
systems shall not be used on 8.33 kHz spaced channels. The receiving system shall ensure an effective
rejection of 60 dB or more at the next assignable channel.
International standard ANNEX 10 Volume III a V describes the requirements for the offset carrier
system utilizing in VHF frequency band. For example:
a) 2-carrier system. Carriers should be spaced at plus and minus 5 kHz. This requires a frequency
stability of plus or minus 2 kHz (15.3 parts per million at 130 MHz).
b) 3-carrier system. Carriers should be spaced at zero and plus and minus 7.3 kHz. This requires
a frequency stability of plus or minus 0.65 kHz (5 parts per million at 130 MHz).
c) 4-carrier system. Carriers should be spaced at plus and minus 2.5 kHz and plus and minus
7.5 kHz. This requires a frequency stability of plus or minus 0.5 kHz (3.8 parts per million at
130 MHz).
d) 5-carrier system. Carriers should be spaced at zero and plus and minus 4 kHz and plus and
minus 8 kHz. A frequency stability in the order of plus or minus 0.5 kHz (0.3 parts per million
at 130 MHz) is an achievable and practicable interpretation of the requirement in this case.
The carrier frequency spacings referred to above are with respect to the assigned frequency.
In aircraft receivers which employ a measurement of the received carrier-to-noise ratio to operate the
mute, the audio heterodynes caused by the reception of two or more off-set carriers can be interpreted
as noise and cause the audio output to be muted even when an adequate wanted signal is present. In
order that that the airborne receiving system can conform with the sensitivity recommendations the
design of the receivers may need to ensure that their sensitivity is maintained at a high level when
receiving off-set carrier transmissions. The use of a carrier level override is an unsatisfactory solution
1
- Ing., CSc.; UO, Kounicova 65, 612 00 Brno, e- mail: stanislav.rydlo@unob.cz
ISBN 978-80-969760-2-7
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246
Proceedings of the International Scientific Conference Modern Safety Technologies in Transportation 2007
to this requirement, but where it is employed, setting the override level as low as possible can
ameliorate the problem.
In the case of receivers used in areas where offset carrier systems are in force, the characteristics of the
receiver should be such that:
the audio frequency response precludes harmful levels of audio heterodynes resulting from the
reception of two or more offset carrier frequencies,
the receiver muting circuits, if provided, operate satisfactorily in the presence of audio
heterodynes resulting from the reception of two or more offset carrier frequencies.
3. OFFSET CARRIER SYSTEM UTILIZING IN THE AIR-GROUND
COMMUNICATION SYSTEM
It is supposed the of the air-ground radio stations on spot-height utilization, from where is warranted
the reliable reception of signal in all the special-interest area, i.e. FIR ýR from AGL to FL 095. Air
traffic services are provided on one single radio channel and partial frequency separation in frames of
one channel. It is utilized partial space diversity to ground transceivers separate. This is utilized in the
systems with frequency shift carrier – so – called offset carrier system.
The main advantage of that system is a fact, that airborne radio station retuning is not demanded
through the flight. The communication relating is realized on one same radio channel. Disadvantages
of this solution are very strict requirements for ground and airborne technical equipments. If the
requirements are not followed, the situation is worse by three and more carrier systems, then it does
not be enough the decorrelation of sources and it will be troubles with voice intelligibility and
communication relating failure. Next problem is a fact, that a civilian and military standard does not
define offset carrier system in 220-400 MHz bandwidth.
4. OPERATION REQUIREMENTS FOR THE AIR-GROUND RADIOSTATIONS IN OFF-SET MODE
It is necessary to go from two postulates for determination requirements on frequency stability and
bandwidth:
1. Point 1.2 Attachment A to Parts II ANNEX 10 Volume III where are presentation Off-set carrier
system examples, which it satisfy requirements point 2.2.1.1.1Part II ANNEX 10 Volume III,
2. Determination of effective bandwidth in offset mode for one carrier frequency transmission. The
bandwidth contains 2500 Hz acoustic bandwidth, necessary transmitter frequency instability
bandwidth and 140 Hz Doppler shift.
Bandwidth of one carrier frequency transmission in offset mode is given by equation:
'f On
where:
'f O
'f Z
'f S
'f D
n
2 * ('f Z 'f Sn 'f D )
(1)
- one transmission carrier frequency in offset mode bandwidth
- audio frequencies bandwidth
- transmitter instability bandwidth
- Doppler shift
- carriers in offset mode number
Following Table 1 shows computed values of transmitting band. Value N obtains from 2 to n rate.
Value 'f Nn determines frequency distance between carrier frequency in Offset - n mode and assigned
frequency channel.
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Proceedings of the International Scientific Conference Modern Safety Technologies in Transportation 2007
247
Offset-5
Offset-4
Offset-3
Offset-2
Table 1 Computed values according to the equation (1)
'f Nn
'f Nn ± 'f Z
[Hz]
[Hz]
-7 500
-2 500
2 500
7 500
-9 800
-4 800
-2 500
2 500
4 800
9 800
-10 000
-5 000
-5 000
0
0
5 000
5 000
10 000
-10 500
-5 500
-6 500
-1 500
-2 500
2 500
1 500
6 500
5 500
10 500
1st frequency
-5 000
2nd frequency
5 000
1st frequency
-7 300
2nd frequency
0
3rd frequency
7 300
1st frequency
-7 500
2nd frequency
-2 500
3rd frequency
2 500
4th frequency
7 500
1st frequency
-8 000
2nd frequency
-4 000
3rd.frequency
0
4th frequency
4 000
5th frequency
8 000
'f Nn ± 'f Z ± 'f S
'f Nn ± 'f Z ± 'f S ± 'f D
[Hz]
-9 500
-500
500
9 500
-10 450
-4 150
-3 150
3 150
4 150
10 450
-10 500
-4 500
-5 500
500
-500
5 500
4 500
10 500
-10 540
-5 460
-6 540
-1 460
-2 540
2 540
1 460
6 540
5 460
10 540
[Hz]
-9 640
-360
360
9 640
-10 590
-4 010
-3 290
3 290
4 010
10 590
-10 640
-4 360
-5 640
640
-640
5 640
4 360
10 640
-10 680
-5 320
-6 680
-1 320
-2 680
2 680
1 320
6 680
5 320
10 680
It is possible to find whole transmission bandwidth for particular offset mode from calculated values.
The whole bandwidth is given by the lowest value of the 1st frequency shift and the highest value of
the last frequency shift. You can see whole bandwidth for offset mode from 2 to 5 carrier frequencies
according to the examples in point 1.2 of the Attachment A to Parts II ANNEX 10 Volume III. It is
presented in the Table 2. Figure 1 shows computed values graphical representation from the Table 1.
Table 2 Ground transmitters’bandwidth in offset mode with 2 – 5 carriers.
Offset mode
Offset-2
Offset-3
Offset-4
Offset-5
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Bandwidth [Hz]
19 280
21 180
21 280
21 360
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Proceedings of the International Scientific Conference Modern Safety Technologies in Transportation 2007
Offset - 2
[Hz]
-12000
-8000
0
-4000
4000
8000
12000
Offset - 3
[Hz]
-12000
-8000
0
-4000
4000
8000
12000
0
4000
8000
12000
0
4000
8000
12000
Offset - 4
[Hz]
-12000
-8000
-4000
Offset - 5
[Hz]
-12000
-8000
-4000
Figure 1 Frequency spectrum of the ground transmitting par t with existent tolerations for frequency
instability and Doppler shift
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Proceedings of the International Scientific Conference Modern Safety Technologies in Transportation 2007
249
Next important requirement on realization offset mode is a transmitter carrier frequency relative
stability. Table 3 presents minimal required relative stability values for selected carrier frequency from
VHF, UHF bandwidths for aeronautical services. The carrier frequencies are chosen for objectivity in
the middle and in the limits in VHF, UHF bandwidths.
Table 3 M
inimal required relative stability va
lues of the carrier frequency
VHF
Offset-5 Offset-4 Offset-3 Offset-2
Absolute frequency
stability [kHz]
2.00
0.65
0.50
0.04
Carrier
frequency
[MHz]
118
130
137
118
130
137
118
130
137
118
130
137
UHF
Relative
frequency stability
[ppm]
17.0
15.4
14.6
5.5
5.0
4.7
4.2
3.8
3.6
0.34
0.31
0.29
Carrier
frequency
[MHz]
220
260
400
220
260
400
220
260
400
220
260
400
Relative
frequency stability
[ppm]
9.1
7.7
5.0
2.9
2.5
1.6
2.3
1.9
1.3
0.18
0.15
0.1
5. CONCLUSION
Offset mode can be realized for two or three carriers frequencies, because AM neighbouring carrier
frequencies spectres do not touch, according to 2.5 kHz maximum audio frequency bandwidth and
±2 kHz frequency instability for two carrier frequencies system and ±0.65 kHz for three carrier
frequencies system. 720 Hz is distance between spectres several carrier frequencies in these examples.
Distance between carrier frequencies with audio bands can be larger with increment frequency
stability in these two examples. It is impossible reduce the audio-band; it is defined precisely. The
result is a fact, that it is necessarily need the frequency stability keep.
There is a touch between AM side bands in four frequencies offset carrier system and AM side bands
are overlaid with the frequency instability and Doppler shift. See in the Figure 1.
There is a AM side bands overlaying in five frequencies offset carrier system without frequency
instability and Doppler shift yet.
The result of the analysis is, that four and five offset carrier systems make worse the voice
intelligibility and they can be used only in additional services (for example Volmet).
REFERENCES
1. ETSI EN 300 676: Electromagnetic compatibility and Radio spectrum matters; Ground- based
V
HF hand-held, mobile and fixed radio transmitte rs, receivers and transmitters, receivers and
transmitters for aeronautical mobile service using amplitude modulation; Technical
characteristics and methods measurement.
2. ICAO: ANNEX 10, Aeronautical Telecommunications. Volume III – Communications systems.
Opponent: Doc. Ing. Drahoslav JUěÍK, CSc., SLI, Rampová 7, 040 01 Košice
ISBN 978-80-969760-2-7
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