25/09/07 International Civil Aviation Organization
AERONAUTICAL COMMUNICATIONS PANEL (ACP)
WORKING GROUP T
Montreal, Canada 2 – 5 October 2007
Agenda Item 4: FCS Candidate Technology assessment
B-AMC: Final Achievements
Presented by Christoph RIHACEK
Prepared by Christoph RIHACEK, Miodrag SAJATOVIC (Frequentis AG), and
Michael SCHNELL (German Aerospace Center, DLR)
In this working paper, the results obtained in the course of the B-AMC
study are reported. Especially, the following topics are covered by this
paper. The specific context of the B-AMC study, which was initiated and
funded by EUROCONTROL, is presented together with the high level
system design and the usage of the L-band spectrum. Finally, the
architectures for an airborne and a ground B-AMC system are
Detailed results on the B-AMC protocol and physical layer performance
as well as the results form the co-existence investigations are presented in
a separate working paper.
The ACP WG-T is invited to review and note this material.
The VHF COM frequency band (117.975 – 137.000 MHz) currently used for aeronautical air - ground
communications is becoming more and more congested. In some parts of Europe it is already extremely
difficult to make a new channel assignment. With the predicted increase in the number of flights, within
the next decade this situation will get progressively worse. In addition, future Air Traffic Management
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(ATM) concepts will require an air - ground data link with more capacity and better performance than today.
The International Civil Aviation Organisation (ICAO), through its Aeronautical Communications Panel (ACP), is seeking to define a Future Communication System (FCS). The FCS must support the new operational concepts, as well as the emerging requirements for communications of all types (both voice and data) with a minimum set of globally deployed technologies. The FCS will be the key enabler for new ATM services and applications that will bring operational benefits in terms of capacity, efficiency and safety. It will support both data and voice communications with an emphasis on data communications in the shorter term, by incorporating both legacy systems as well as the new technologies. In response, the Federal Aviation Administration (FAA) and EUROCONTROL initiated a joint study, with support from the National Aeronautics and Space Administration (NASA) and the United States (U.S.) and European contractors, to investigate suitable technologies and provide recommendations to the ICAO ACP Working Group T (WG-T). The first stage of the study was to conduct pre-screening of more than 50 candidate technologies and produce short lists for further investigations. In the next step, short listed technologies have been subject to an in-depth analysis to produce a further short list and recommendations for implementation.
2. B-AMC STUDY SPECIFIC CONTEXT The B-VHF project was a research project co-funded by the European Commission’s Sixth Framework
Programme. The project investigated the feasibility of a new multi-carrier-based wideband aeronautical communications system operating in the VHF communication band. The B-VHF project has completed a substantial amount of work in developing and designing the B-VHF system. As a result, B-VHF has been recognized as a very promising technology within the FCS and incorporated into the candidate shortlist. However, the current FCS approach is that the VHF COM band should continue to be used for voice services while the new data link system should preferably be deployed outside the VHF COM band. The candidate bands are:
— VHF navigation band: [112 or 116] – 118 MHz
— L band: 960 – [1024 or 1164] MHz
— C band: [5030 or 5091] – 5150 MHz
The B-AMC study is a specific EUROCONTROL project that has contributed to the ongoing work of FCS investigations by providing an in-depth feasibility study of implementing a system similar to B-VHF that would operate in the L-band. The generic name given to the system is Broadband - Aeronautical Multi-Carrier Communication (B-AMC).
The B-AMC study was kicked-off in February 2007 and was concluded by end of August 2007. The study team was led FREQUENTIS, comprising selected members of the former B-VHF project consortium: Deutsches Zentrum für Luft und Raumfahrt (DLR), University of Salzburg and being supported by the UK consulting company Mileridge Ltd.
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The results of the B-AMC project have been captured in seven deliverables:
— D1 – DME Spectrum Characterization and L-band System Availability for an OFDM-like
— D2.1 – B-AMC System High Level Description
— D2.2 – Technology Operating Concept and Deployment Scenarios
— D3 – System Specification and Standardization and Certification Considerations
— D4 – Interference Analysis and Spectrum Requirements
— D5 – Expected System Performance
— D6 – Aircraft Integration and Ground Infrastructure Considerations Combined together, these deliverables provide the information necessary for understanding operating
principles of the system, modes of operation, spectral requirements, provided communications services
and expected performance when the B-AMC system is operating under full loading and experiencing the
specific L-band interference. The B-AMC system’s functional architecture, deployment concept, details
of airborne and ground integration as well as the estimated interference impact of the B-AMC system
towards other L-band systems are provided as well.
3. HIGH LEVEL B-AMC SYSTEM DESIGN
The B-AMC system high-level design was conducted with the goal to keep as much as possible from the
original B-VHF design. The B-AMC system provides two modes of operation, namely Air-Ground (A/G)
communications and Air-Air (A/A) communications.
Like B-VHF, the B-AMC A/G sub-system is a multi- application cellular broadband system capable of
simultaneously supporting various kinds of Air Traffic Services (ATS) and Airline Operational
Communications (AOC) data link services. The A/G mode assumes a star-topology where B-AMC
aircraft within a certain volume of space (the B-AMC cell) are connected to the B-AMC controlling
Ground Station (GS). The B-AMC A/G system design includes propagation guard times sufficient for
operation at a maximum distance of 200 nm from the GS. Each cell operates in the Frequency-division
Duplex (FDD) mode, using its dedicated forward/reverse link (FL/RL) channel pair. As the aircraft moves,
services are handed-over between the involved cells transparently to the air and ground users. Therefore,
the physical cell coverage is de-coupled from the operational service coverage.
The B-AMC A/G data link sub-network can be integrated within the existing (ATN/OSI) Aeronautical
Telecommunication Network and is prepared for the integration within the future Internet Protocol-based
(ATN/IP) network solution. Additionally, B-AMC-specific (non-ATN) data links and voice A/G channels
can be optionally configured.
Fig. 1 shows the B-AMC framing structure and different kinds of system (green) and user data OFDM
frames (red/blue) used when the system is operated in the A/G mode.
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Fig. 1: B-AMC Framing Structure in the A/G Mode
Like B-VHF, the B-AMC A/G mode uses an Orthogonal Frequency Division Multiplex (OFDM)-based
physical layer. However, some modifications had to be made with respect to the original B-VHF design
due to the specific L-band conditions:
— Frequency-Division Duplex (FDD) is used instead of Time-Division Duplex (TDD),
— OFDM/OFDMA is the proposed access scheme for the FL instead of MC-CDMA,
— OFDM sub-carrier spacing is increased from 2 kHz to 10.4 kHz,
— OFDM FL/RL frame duration and position of pilot symbols have been adjusted. In addition to the A/G mode, the B-AMC system offers an A/A mode with direct air-air connectivity,
supporting broadcast A/A surveillance link and addressed (point-to-point) A/A data link. As the A/A
mode was not part of the B-VHF system design, it had to be designed within the B-AMC study.
The physical layer for the A/A mode is also based on OFDM, but with completely different parameters
than those used for the A/G mode:
— OFDM sub-carrier spacing is set to 25 kHz, with OFDM symbol duration of 50 µs.
— Self-organised Time Division Multiple Access (TDMA) scheme is used. All B-AMC A/A communications between aircraft take place in a decentralized, self-organized way,
without any need for ground support. Involved aircraft transmit and receive on a single radio channel, the
Common Communication Channel (CCC). There are several physical layer design options for the frame
structures and CCC bandwidths (2.6/1.3 MHz, respectively) dependent on what different operational
ranges (120/200 nm) are required. For synchronization purposes, the availability of a global time
reference is assumed. Any aircraft can receive transmissions from any other aircraft, which is within the
reception range. The receiver distinguishes broadcast-, multicast- and unicast messages based on the
message header. No A/A voice services are offered by B-AMC.
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The "lessons learned" retrieved from the B-VHF performance simulations and the B-VHF demonstrator as well as the addition of the A/A mode have lead to some adaptations/optimisation of the B-VHF communications protocols.
4. USAGE OF L-BAND SPECTRUM Currently, multiple systems are operating in the L-band. The L-band plan is shown in Fig. 2.
Fig. 2: L-band Usage And Allocations for B-AMC A/G and A/A Communications
The Distance Measuring Equipment (DME) and military Tactical Air Navigation (TACAN) channels are allocated on 1 MHz grid between 960 and 1213 MHz. Not all these channels are actually used for DME/TACAN operations.
One fixed channel (978 MHz) has been allocated for Universal Access Transceiver (UAT). Another fixed channel (1030 MHz) is being used for forward link transmissions of the ground Secondary Surveillance Radar (SSR) equipment as well as for air-air interrogations of the Airborne Collision Avoidance System (ACAS). A third channel (1090 MHz) is used for reverse link SSR transmissions and for Automatic Dependent Surveillance Broadcast (ADS-B) by using the 1090 Extended Squitter (1090 ES) technology. Several fixed allocations in the upper part of the L-band are used by GPS and GALILEO navigation systems.
Additionally, in some regions military Multi Functional Information Distribution System (MIDS) operates on three MHz channels placed in three separate blocks (969-1008 MHz, 1053-1065 MHz, and 1113-1206 MHz) of the L-band spectrum.
Fig. 3 shows that the DME system is the major user of the L-band. In spite of 1 MHz channel grid, most energy of DME/TACAN signal-in-space is contained within only about 500 kHz of the spectrum. Based on that fact, EUROCONTROL proposed three options to be considered for the B-AMC technology deployment in the L-band.
— Option 1 - utilising spectrum between successive Distance Measuring Equipment (DME)
channels for B-AMC without taking DME frequency planning into account. With that
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option, the B-AMC channels would be placed exactly between (at 500 kHz offset from)
adjacent DME allocations.
— Option 2 - alternatively, a relationship between potential B-AMC channel assignments and
existing DME assignments may be established by taking frequency planning constraints
into account. With that option, the B-AMC channels would be assigned in areas of the L-
band that are not locally used by DME.
— Option 3 - the lower part of the band (960 – 978 MHz) could be used for B-AMC,
considering potential interference to non-aeronautical systems operated in the lower
adjacent band (UMTS, GSM).
The B-AMC study has proven that Option 1 is not feasible. Instead, the B-AMC A/G sub-system is
proposed to be deployed using Option 2, by placing B-AMC channels between DME channels, as shown
in Fig. 3. The B-AMC A/A sub-system could be optimally deployed in the lower part of the L-band by
using Option 3.
Fig. 3: Channel Allocations for B-AMC A/G Sub-system
The required number of channels for the B-AMC A/G mode and the single CCC for the A/A mode should
be placed within the L-band in such a way that the mutual interference impact between the B-AMC
system and legacy systems would be minimised. The finally proposed allocations are shown in Fig. 2.
The guiding principle was to keep as much margin as possible to the fixed L-band allocations (UAT and
The airborne architecture for the B-AMC A/G sub-system is illustrated in Fig. 4 is regarded as generally
applicable to large transport aircraft.
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Fig. 4: B-AMC Airborne Architecture – A/G Mode In the A/G mode, the B-AMC airborne radio – comprising the TX/RX duplexer - is placed within the existing airborne communications architecture, together with legacy VHF voice radios (VHF1, VHF3)
and the airborne voice system (Audio Management Unit - AMU). The components related to the A/G data
link usage (Communications Management Unit – CMU, Flight Management System – FMS, Multi-
purpose Control and Display Unit – MCDU, Dedicated Control and Display Unit – DCDU) are shown as
well. The B-AMC radio is wired to the CMU and (optionally, if voice is required) to the AMU. It would
be attached to its dedicated L-band antenna that itself may be a part of a combined VHF/L-band antenna
The B-AMC A/G airborne architecture concept closely follows the approach adopted for other
aeronautical systems. The B-AMC L-band radio should be preferably installed as a separate box,
comprising both A/G and A/A RF front ends and common parts of the B-AMC protocol stack (physical
layer, MAC). Higher layers of the B-AMC sub-network stack would be integrated within the existing
CMU. Alternatively, the B-AMC A/G L-band radio could be integrated within existing VHF radio units,
possibly using combined VHF/L-band antennas. In order to relax airborne co-site problems, the airborne
B-AMC radio (TX) would be attached to the common Suppression Bus.
The B-AMC radio operating in the A/A mode interacts with external airborne ASAS/surveillance systems
(not shown in Fig. 4), but may also itself integrate ASAS/surveillance processing functions, becoming
more attractive to General Aviation (GA), but also requiring an increased number of external interfaces.
The proposed ground B-AMC A/G infrastructure (Fig. 5) applies to all environments considered for the
B-AMC deployment (APT/TMA/ENR). The required components are physical B-AMC radios (G_RX,
G_TX), the Ground Station Controller (GSC) and the Ground Network Interface (GNI), together with the
B-AMC management system. The same components, however with reduced functionality, may be
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sufficient in the APT environment where no wide area coverage requirements apply. The required GNI functionality and complexity may be significantly reduced if no voice option is required.
Fig. 5: B-AMC Ground Architecture – A/G Mode
The GNI is attached to an A/G ATN router that in turn is connected via ground networks to ATS and AOC data link systems. The B-AMC system would appear within the ATN/OSI concept as any other sub-network, but with improved capacity and performance. Similar integration concepts would apply with respect to the B-AMC integration within the IP-based end-to-end ATN framework (ATN/IP). 6. CONCLUSIONS AND KEY RECOMMENDATIONS
B-AMC interference investigations performed indicate that a system deployment without considering existing frequency planning (referred as "Option 1" in Chapter 4) is not feasible, due to the excessive interference impact at 500 kHz frequency spacing between the involved channels, which might occur if worst case conditions are assumed.
If frequency planning constraints are considered (referred as "Option 2" in Chapter 4), the B-AMC A/G system will perform well under assumed interference scenarios. With that option, the B-AMC physical layer design allows for cells with coverage up to 200 nm.
The interference from the military Joint Tactical Information Distribution System (JTIDS) has a strong impact on the B-AMC system, in particular when combined with the Distance Measuring Equipment (DME) interference. Hence, the impact of JTIDS is subject to further investigations. The B-AMC A/A sub-system designed within the scope of this study and deployed by using the third option (referred as "Option 3" in Chapter 4) should be able to support A/A data link services on the single CCC designed for 170 nm range fulfilling the QoS requirements.
Based on the B-AMC results and lessons learned within the B-AMC study, two recommendations can be made with respect to future FCS activities:
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Detailed commonly agreed interference scenarios based on the common inputs should be prepared and used in the next phases of the FCS investigations in order to enable a fair comparison of the candidate L-band technologies.
Detailed commonly agreed data traffic scenarios, based on the EUROCONTROL / FAA Communications Operating Concept and Requirements document, should be prepared for different flight phases in different domains (APT Surface, APT Zone, TMA, ENR, ORP, and AOA). Such detailed scenarios should be used as a common input in the next phases of FCS activities and would allow for detailed investigations and fair comparison of candidate FRS technologies.