ACP-WGF15/WP23
International Civil Aviation
Organization
26/05/06
WORKING PAPER
AERONAUTICAL COMMUNICATIONS PANEL (ACP)
FOURTEENTH MEETING OF WORKING GROUP F
Cairo, Egypt 7 – 13 June 2006
Agenda Item 5: AM(R)S spectrum requirements (Future Communications System
(FCS))
Bandwidth Requirements and Channelization Approaches for Future Airport Surface
Communications in the 5091-5150 MHz Band.
(Presented by Robert Kerczewski, NASA Glenn Research Center)
SUMMARY
This paper extends the results of previous work in determining
bandwidth requirements for future airport wireless surface
communications. In particular, wake vortex sensing and information
security requirements are described, as well as considerations for
channelization of the 5091-5150 MHz band to accommodate future
such systems currently being tested.
ACTION
The meeting is invited to consider the airport surface communications
bandwidth requirements relative to AM(R)S allocations needed to
support them.
ABSTRACT
NASA, in cooperation with the FAA, industry, and academia, has been performing
research and testing for a new generation of airport surface communications to enable
advanced surface CNS systems and improved information distribution and provide lower
cost, safer and more efficient airport surface operations. The key element is a wireless
airport surface communications network, envisioned to operate in the 5091-5150 MHz
band. Previous work has included assessment of communications requirements,
characterization of channel performance in a dynamic airport environment, the
development of channel models and simulations, waveform analysis, communications
technology assessments, and airport surface network architecture definition. The next
step is the development of an airport surface communications testbed, which is underway
at the NASA Glenn Research Center and the adjacent Cleveland Hopkins International
Airport. This paper will review communications requirements and present possible 50915150 MHz channelization schemes that will be evaluated in a prototype airport surface
wireless communications testbed.
1 - INTRODUCTION
During the NASA Glenn Research Center (GRC) Space-Based Technologies (SBT)
Project, research and development for next-generation airport surface communications
networks was performed. The purpose was to develop and evaluate a wireless digital
communications network that would enable deployment of advanced air traffic
management systems required for safer and more efficient surface operations. Such
systems include vehicle surveillance and weather sensors, navigation and landing, ATC
voice and data, AOC voice and data, and surface management systems. Key reasons for
the need to develop such a system include the requirement for increasing digital data flow
on the airport surface to enable safe increases in traffic load; the need to communicate
with mobile airport surface assets (requiring a wireless network solution); the increasing
congestion on VHF communications channels; the high installation and maintenance
cost of hard-wiring many new sensors at various locations on the airport surface; the
vulnerability of underground communications links to cuts and service losses; and the
need to integrate various types of data into an integrated network architecture.
NASA GRC’s research program for airport surface wireless communications, after the
end of the SBT Project, is continuing through the NGATS-CNS National Testbed,
described in another working paper. Work elements completed to date include:
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5091-5150 MHz channel sounding campaign at CLE, BKL, MIA, FLL, and JFK
airports (by NASA GRC, FAA and Ohio University)
Development of 5091-5150 MHZ airport channel models for communications
link optimization (by Ohio University)
Analysis of technology alternatives for future ATC/AOC communications under
the Eurocontrol/FAA Future Communications Study (by NASA GRC, ITT
Industries, and QinetiQ)
Study of communications requirements for a large airport (DFW) (by Trios/SAIC)
Study of spectrum requirements for Airport Network and Location Equipment
(ANLE) system (by Mitre Corp.)
Analysis of interference compatibility of airport wireless network with satellite
feeder links in the 5091-5150 MHz band (by Mitre Corp.)
Development of NGATS-CNS National Testbed Design, including airport surface
wireless network prototype testbed (by Sensis Corp and NASA GRC)
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Laboratory and field testing (at SYR) of prototype airport surface wireless
network equipment (by Sensis Corp.)
Laboratory testing of mobile network components (by NASA GRC)
This paper provides additional results regarding the development of the next-generation
airport surface wireless communications system focusing on three areas: review and
extension of data and bandwidth requirements analysis (to include wake vortex sensing
and data security requirements); application of IEEE 802.16e standards to the airport
surface wireless network architecture; and considerations for channelization of the 50915150 MHz band for airport surface wireless network use.
2.0 - HIGH LEVEL CONCEPT OF USE
It is forecasted that aviation will continue to grow at an anticipated rate of 3% annually.
To meet the demand, modernization of surface operations and communications systems is
necessary. The use of existing communications assets along with the implementation of
new technologies will be required to meet increased capacity demands and maintain
operational safety standards. A high level communications concept of use, rolled out in
three stages, is summarized as follows.
Near Term:
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Initially, the surface wireless communications network will meet fixed
communication requirements. No mobility requirements are addressed with this
initial version.
The communications system will initially transport a small number of critical
systems which include ASDE-X. ASDE-X information will be transported from
field sensors to the central processing system located at the Air Traffic Control
Tower.
Mid Term:
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Additional critical services are migrated onto the wireless network including
Surveillance, Navigation, Communications and monitoring systems.
For large airport facilities, wireless and wired technologies operate in a hybrid
configuration increasing the reliability and availability of services.
The wireless network enables System Wide Information Management (SWIM)
availability to most SWIM native systems.
Far Term:
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Mobility is added to the communications system. Communications directly to
the aircraft is possible utilizing the full capabilities of the 802.16e standard.
Migration of VHF communications for ATC and AOC to 5091-5150 MHz band
becomes possible.
3.0 – APPLICATION OF IEEE 802.16E TO AIRPORT SURFACE COMMUNICATIONS
The Eurocontrol-FAA Future Communications Study (FCS) performed assessments of
technologies applicable to future air-ground communications for aviation. In the first
phase of this study, a technology pre-screening effort conducted by ITT Industries for
NASA in support of the FCS concluded that IEEE 802.16 standard was the best available
technology alternative for airport surface communications:
“IEEE 802.16 provides the opportunity to utilize the MLS (extension band)
spectrum to support a broad scope of communications needs, both data and voice,
over the entire airport surface. Increased data rates on the airport surface that
might not be met by a future system in the DME band, could be met by a fully
COTS system base on 802.16e in the MLS (extension) band. The business case for
802.16 infrastructure may be driven by factors beyond ATS and AOC
communications. For example, airport authorities may desire to support airport
fixed services to support airport infrastructure. At current time, 802.16e mobility
addresses mobile speed less than 120 km/hr and so will not support aircraft on
landing and takeoff. This is an issue to address in future consideration of 802.16e
for aeronautical applications.”
The IEEE 802.16 link-layer has many desirable properties for use in airport surface
networking. The 802.16 specification has several configurable portions, some of which
have implications on airport surface environments. Here we present initial results of
examining the parameter profiles in the 802.16 specification which contain values that
can be expected to be supported in equipment obtained in the near-term. The profiles in
the protocol specification merely represent configurations that multiple vendors have
agreed to support; the protocol's operation is not constrained to these values. However,
having equipment custom-built is not feasible at this time, so current usage plans should
involve parameters that are supported in the profiles.
The 802.16 specification offers 5 air interface options, but the only one that is appropriate
for airport surface use is the WirelessMAN-OFDM option. The drivers for this
determination are the desire to operate in the 5 GHz band (other options only operate
above 11 GHz, or in unlicensed spectrum), and the desire for mesh mode operations to
cover dead spots that may not be viewable from a single base station located at the tower.
Other options only support the point-to-multipoint mode of operation, which would
require multiple base-stations sited at various locations to specifically cover all of the
dead spots from the tower. The mesh mode allows some subscriber stations to act as
surrogate base stations to extend coverage. Mesh is more flexible and adaptable and is
superior for the dynamic nature of the airport surface cnosidering its ability to support
path diversity.
The physical (PHY) profiles for WirelessMAN-OFDM Mesh networks include 3, 3.5,
5.5, and 7 MHz channels. A channelization scheme can select combinations of these in
order to reach the required aggregate data rates. There are also two duplexing options,
TDD and FDD. These appear to be functionally equivalent, although TDD usage is more
prevalent, with FDD intended mainly for low-cost subscriber stations that only support
half-duplex operations.
Security considerations for 802.16 are discussed in the next section.
4.0 - AIRPORT SURFACE SAFETY COMMUNICATIONS BANDWIDTH REQUIREMENTS
NASA GRC commissioned a study of airport surface communications requirements in
2004. The study was conducted by Trios Inc (now a part of SAIC Inc.) The study
focused on a “level 12” US airport, Dallas-Forth Worth (DFW). The study, which
analyzed current and planned communications loading for FAA air traffic operations,
airline operations, and airport (port authority) operations, concluded that approximately
114 Mbps would be required to support all operations. Another WGF-15 working paper
presented the methodology and results of this study.
Mitre Corporation extended this work in a 2006 study to estimate the total bandwidth
required to implement a safety-related wireless local are network for the airport surface.
Mitre concluded that the total bandwidth required was 60 MHz, assuming 20 MHz
channel bandwidths. This analysis included a subset of the applications analyzed in the
Trios/SAIC study, as well as some additional surveillance, automation and AOC data
categories.
One additional future airport surface application which will present a significant data load
to an airport surface communications network is wake vortex sensing. Researchers at
NASA’s Langley Research Center are performing research and development on wake
vortex detection systems and have provided the following information.
4.1 - WAKE VORTEX DATA THROUGHPUT REQUIREMENTS
To meet forecasted air traffic growth, research efforts aimed at reducing separation in the
terminal airspace approach and departures are ongoing. To increase airport throughput,
research in wake turbulence during arrival and departure stages of flight are being
investigated. Accurate measurement of wake vortex turbulence will enable aircraft
spacing based on actual wake vortex presence, rather than the more conservative
procedures now in place using estimates of potential wake vortex based on aircraft type.
The FAA and NASA are conducting tests and evaluation of wake vortex algorithms at
Denver and St. Louis international airports. Airports with closely spaced parallel
runways (CPRS) stand to benefit the most from this research effort. Reduced aircraft
spacing will enable more all-weather use of existing runways. Furthermore, for airports
that are “land locked”, it may be possible to increase the number of runways for a given
amount of land.
To enable deployment of new wake detection technologies, communication systems that
enable optimal wake sensor location at the airport are essential. Today’s wired airport
communication media lack the flexibility required to deploy sensors at optimal locations.
Wireless communications provide the data throughput and flexibility required to enable
deployment of wake turbulence detection technologies. Wake turbulence detection
communication requirements are:
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Air Traffic Control Tower to Sensor: 1.544 Mbps
Air Traffic Control Tower to LIDAR: 20 Mbps
Communications to LIDAR is required for system optimization only and does not
represent a continuous data requirement.
4.2 - EXAMPLE WAKE DETECTION SYSTEM
Figure 1 shows an example Wake Detection System for a two runway configuration. It is
anticipated that one system composed of two sensors will be required to detect wake
turbulence for the entire airport facility. Each sensor requires communications to a
central processor located at the Air Traffic Control Tower (ATCT).
The configuration shown in Figure 1 will require a total throughput of 3.088 Mbps.
2-5 mi
12 L
30 R
Wake Sensors
12 R
30 L
2-5 mi
Figure 1 – Example Work Vortex Turbulence Detection System
4.3 - SECURITY REQUIREMENTS
The use of wireless communications for certain safety critical applications will require
sufficient security to be designed into the system. Table 1 indicates possible security
level requirements for airport surface wireless network applications.
Table 1 – Airport Surface Communications Network Requirements Summary
Application
Category
Surveillance
Weather
Navigation
and Landing
Automation
Electronic
Flight Bag
Application
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AOC
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ASR-9 + Mode
S
ASR-11
ASDE-X
DBRITE
LLWAS
AWOS/ASOS
TDWR
ITWS
WSP
RVR
Wake Vortex
ILS
PAPI
ALSF
ATC Voice
and Data
CTAS
ETMS
Software
Loading
Electronic
Library
Update
Graphic
Weather
AOC voice and
data
Fixed or
Mobile

Fixed
Security
Requirement
- Network
 High
Security
Requirement
- Physical
 High
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Fixed
Fixed
Fixed
Fixed
Fixed
Fixed
Fixed
Fixed
Fixed
Fixed
Fixed
Fixed
Fixed
Mobile
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High
High
High
Medium
Medium
Medium
Medium
Medium
Medium
Medium
Medium
Medium
Medium
High
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High
Medium
High
High
High
High
High
High
High
Medium
High
Medium
Medium
High
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Fixed
Fixed
Mobile
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High
Medium
Medium
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High
Medium
Low
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Mobile
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Medium
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Low
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Mobile
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Medium
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Medium
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Mobile

High

High
The network security requirement refers to requirements for ensuring that transmitted
data remains sufficiently secure. In Table 1, High Network Security requires
authentication, encryption, replay protection, anti-clogging; Medium Network Security
requires authentication. Low Network Security would require no special measures,
however all of the applications listed will require at least authentication.
The physical security requirements indicate the criticality of maintaining communications
links in the presence of interference, whether intentional or non-intentional. High
physical security requires extremely high resistance to interference. Physical
countermeasures or mitigation techniques such as highly directional antennas, spread
spectrum, and path and channel diversity would be required. Medium physical security
would require some resistance to interference, such as path and channel diversity. Low
physical security would require no special measures.
4.3.1 – SECURITY OVERHEAD CONSIDERATIONS FOR 812.16
In 802.16, security overhead that reduces the link capacity available for application data
comes from three components:
1) Initial authentication between base stations and subscriber stations,
and establishment of connections and security associations;
2) Periodic rekeying throughout the life of a connection;
3) Per-frame inclusion of security headers and trailers.
The station authentication procedures only occur when a subscriber station first joins the
network, and thus only affects the latency in establishing initial connectivity or in reestablishing connectivity with other base stations in a mobility scenario. Thus, it is
negligible as a component of the required bandwidth. The periodic re-keying of security
associations occurs infrequently in comparison to actual data traffic, and can also be
considered negligible.
Thus, the per-frame security components are the primary contributors to the bandwidth
needed for security functions. Although multiple data encryption algorithms are
available, we assume that Advanced Encryption Standard (AES) in Counter with Cipher
Block Chaining (AES CCM) mode will be used, as this seems to be the most supported
and recommended option. In this case, a 4-byte counter is pre-pended to frames and an
8-byte trailer containing an integrity-check value is appended. The amount of overhead
that these frame additions imply is dependent on the size of the payload. A small payload
(that could realistically be generated by a real-time audio application, for instance) would
be 60 bytes. In this case, about 17% of the channel would be used by security overhead,
after the Media Access Control (MAC) and other 802.16 overhead is accounted for.
When larger frames are generated by applications, then the security overhead is reduced.
For instance, on a 1200 byte payload (the IPv6 minimum MTU (maximum transmission
unit)), the 802.16 security overhead is less than 1%. Assuming a worst-case of small
frames generated by real-time traffic, adding 20% to the bandwidth estimate should be
sufficient to allow for optimally securing the channel.
4.3.2 - ADDITIONAL BANDWIDTH REQUIREMENTS FOR SECURITY
There are seven applications in Table 1 that are considered high security requirement.
Based on data rate requirements estimated in the Trios/SAIC and Mitre reports, these
applications can represent up to 60% of the total data load. Hence, the additional security
overhead of 20% will increase the total data throughput requirement, and thus the total
bandwidth requirement, by as much as 12%.
5 - CONSIDERATION FOR CHANNELIZATION OF THE 5091-5150 MHZ BAND FOR
AIRPORT SURFACE COMMUNICATIONS
There are several parameters affecting the choice of channelization schemes for an airport
surface network. These include fixed vs. mobile applications, security requirements, data
rate requirements, and physical location and grouping of network users. Also, as
discussed in section 3, current planned implementations of 802.16e by vendors include 3,
3.5, 5.5, and 7 MHz channel bandwidths.
Mobile applications are primarily ATC, AOC and EFB, and require relatively low data
rates (10s to 100s of kbps). The secure applications for voice and data could be handled
by a medium size channel (3.0 to 5.5 MHz). Path diversity for increasing physical
security and ensuring loss of line-of-sight due to physical blockage, is achieved through a
mesh network configuration. A single channel for these applications can be standardized
at all airport locations, minimizing on-board equipment requirements.
High data rate, point-to-point connections requiring high security links would have
directional antennas – separate channels for this purpose could be reused between
different locations. For these highly secure pt-to-pt links, several channels can be
assigned, enabling the use of channel diversity for interference and jamming mitigation.
Network management techniques can assign channels to manage load, and also to
mitigate possible channel jamming and interference. As airports have significantly
different configurations of equipment, depending on size, runway configurations, etc., the
selection of channel sizes could be allowed to vary between airport installations to
accommodate these differences.
According to the bandwidth requirements analyses previously cited, 3.0 MHz is probably
the minimum size channel to handle most pt-to-pt data requirements. 7.0 MHz is the
maximum channel size available in current or planned vendor offerings. This would
enable between 7 and 17 channels for these fixed applications.
The effectiveness of possible channelization schemes will be investigated in future
months within the NGATS-CNS National Test Bed, described in another WGF-15
working paper.
6 – SUMMARY AND CONCLUSION
This paper extended the results of previous analyses of the data rates and bandwidths
required for a future airport surface wireless network. In particular, requirements for
wake vortex sensors and for provisioning of security features were described. The paper
also reviewed the design parameters for a network based on the IEEE 802.16e standard,
and in particular discussed considerations for subdividing the 5091-5150 MHz band into
operational channels for an airport surface network. The selection of channelization
schemes will affect the efficiency of use of the spectrum in meeting airport surface
communications requirements.
The requirement for 60 MHz bandwidth to support a future airport surface network, as
developed in previous analyses, represents a minimum, as this paper has noted additional
requirements not included in those analyses.
7 – REFERENCES
Wideband Wireless Channel Characterization of the MLS Band (5000-5150 MHz)
FINAL Report, D. Matolak, Ohio University, May 2006 (summarized in WGF-15
Working Paper).
Technology Assessment for the Future Aeronautical Communications System, ITT
Industries, May 2005. (ICAO ACP WGC-9)
Surface CNS Network Requirements Study Final Report, SAIC, September 2004
(summarized in WGF-15 Working Paper).
Preliminary Analysis of the Spectral Requirements of Future ANLE Networks, I.
Gheorghisor, Y. S. Hoh, Mitre Corporation, March 2006.
Compatibility Analysis of Airport Wireless Local Area Networks and Satellite Feeder
Links in the 5091-5150 MHz Band, I. Gheorghisor, Y. S. Hoh, , F. Box, Mitre
Corporation, May 2006 (http://spacecom.grc.nasa.gov/icnsconf/).
Testbed for Prototype C-band Airport Surface Wireless Network, WGF-15 Working
Paper.