International Civil Aviation
Organization
ACP/WGF 20/WP 03
03/11/09
WORKING PAPER
AERONAUTICAL COMMUNICATIONS PANEL (ACP)
20TH MEETING OF THE WORKING GROUP F
Montreal, Canada, 24 March – 3 April 2009
Agenda Item 5 Development of material for ITU-R meetings
:
DRAFT REVISION TO REPORT ITU-R M.2120 INITIAL ESTIMATE OF NEW AVIATION
AM(R)S SPECTRUM REQUIREMENTS
Presented by Mike Biggs (US FAA)
Summary
To accommodate future growth in surface applications, the 5 000-5 030 MHz band has been
selected for evaluation as additional spectrum for the airport surface local area network (LAN)
currently being developed for operation in the 5091-5150 MHz band. This paper addresses the
spectrum requirement for such a system to determine whether the existing band (5091-5150
MHz) is sufficient, or whether additional spectrum (e.g., portions of 5000-5030 MHz) is
required. The results are incorporated in a draft revision to Report ITU-R M.2120. This material
is intended to also be submitted to the May 2009 meeting or ITU-R WP5B, and is shown as
revision marks to the original Report.
Action
The meeting is invited to consider this material and to provide any necessary comments and/or
corrections to facilitate its acceptance at the May 2009 ITU-R WP5B meeting.
DRAFT REVISION TO REPORT ITU-R M.2120
Initial estimate of new aviation AM(R)S spectrum requirements
(2009)
1
Introduction
As part of its work on Agenda item 1.6 of the 2007 World Radiocommunication Conference
(WRC-07), ITU-R considered allocations to the aeronautical mobile (R) service (AM(R)S) in
some portions of the 960-1 164 MHz and 5 000-5 150 MHz bands. Based on available studies,
two distinct types of aeronautical applications require AM(R)S spectrum. The first, like the
current VHF AM(R)S, requires longer propagation distances (e.g. out to radio line-of-sight),
moderate bandwidth, and a number of distinct channels to allow for sector-to-sector assignments.
The second type, supporting surface applications at airports including data links – is
distinguished by a high data throughput, however only moderate transmission distances and it is
expected that a single resource can be shared at multiple geographic locations. Based on the
technical characteristics of the spectrum being considered, it is expected that the former
applications will be accommodated in some portion of the 960-1 164 MHz band, while the latter
will be implemented in some portion of the 5 000-5 150 MHz band.
Administrations and the International Civil Aviation Organization (ICAO) performed studies in
order to scope the amount of spectrum needed for each frequency band. The results, while
preliminary, provide an order of magnitude of expected spectrum requirements. In addition, the
studies point to the difficulty of fitting a new system into spectrum that is currently utilized for
other systems, especially when aircraft integration is taken into account.
2
Aircraft system integration
Integration of radio services and related equipment onto an aircraft is one of the most difficult
challenges faced in implementing a new aviation system. A large number of radio frequency
(RF) links, many supporting safety of life functions, already exist on the aircraft.
This makes compatible introduction of a new system difficult. Separation in frequency, together
with tight filter and waveform spectral characteristics help, however in some cases sufficient
isolation between systems cannot be achieved. In those cases, one approach is to connect the
systems to a “blanking bus” such that when one system is transmitting, other system receivers
are effectively turned off. This approach is obviously facilitated by transmitters that have short
transmitter on-times.
The 960-1 164 MHz band is heavily used by aviation for systems such as distance measuring
equipment (DME), tactical air navigation systems (TACAN), secondary surveillance radar
(SSR), aircraft collision avoidance systems (ACAS), and automatic dependent surveillancebroadcast (ADS-B; either via 1090 Extended Squitter or the 978 MHz Universal Access
Transceiver (UAT)). As a result, introducing a new system in that band will prove challenging.
One recent success however, was the introduction of the UAT. Aircraft integration of that system
was facilitated by forcing the UAT aircraft transmissions to be very short in time. This was
accomplished through the use of a wideband (approximately 1 MHz) channel, though the per-
aircraft data rate was only about 400 bits/second. As a result, the UAT transmitter on-time was
only about 400 microseconds; an aircraft-throughput-to-bandwidth multiplier of 2 5001. Analysis
and test showed that the other on-board aircraft radiosystems could operate safely in the presence
of that type of UAT radiotransmission.
Based on this experience, one approach to providing aircraft compatibility for the new AM(R)S
system planned for the 960-1 215 MHz band may be to limit the on-time of system transmission.
The implications of such an approach are explored later in this paper. In contrast, for the
AM(R)S system planned for the 5 GHz band, the only other near-frequency on-board system
would be the microwave landing system (MLS). Frequency management will be used to ensure
protection of the MLS, and in addition, since the AM(R)S will be used just for surface
operations, the MLS will likely not be in use during the time periods when the AM(R)S is active.
As a result, it is not expected that a similar bandwidth multiplier to facilitate on-board integration
will be required for the 5 GHz AM(R)S system.
3
Radiocommunications operating concepts and requirements
In conjunction with ICAO, a joint study was initiated to identify potential future
radiocommunications technologies to meet AM(R)S safety and regularity of flight
communications requirements; in particular supporting Air Traffic Services (ATS) and safety
related Aeronautical Operational Control (AOC) communications. That Future Communications
Study (FCS) had two main activities:
a)
to identify the future requirements based on emerging global future Air Traffic
Management (ATM) concepts taking into account the needs of civil aviation and State
aircraft; and
b)
to identify the most appropriate technologies to meet these communication requirements.
The results of the first activity are assessed in a document entitled the Communication Operating
Concepts and Requirements (COCR).
That COCR assisted in completion of the second activity by allowing key requirements to be
matched against candidate technologies – existing or future. To achieve this goal the COCR
identified the requirements placed on the communications that take place through the aircraft and
ground radios. In developing the COCR an approach was taken to make it technologyindependent.
The operational requirements were drawn from the ATM and AOC operating concepts expected
to be implemented in the highest density airspace regions of the world to achieve the required
capacity, safety and security. In particular, the ICAO Global ATM Operating Concept and the
International Air Transport Association (IATA) ATM Roadmap were considered.
4
AM(R)S for portions of the 960-1 164 MHz band
The COCR was also used as a starting point for assessing new AM(R)S spectrum requirements.
In particular, the COCR provided insight into future air/ground and air/air communications –
functions that might be accommodated in a portion of the 960-1 164 MHz band. Under the
1 It should be noted that while this per-aircraft bandwidth multiplier may seem large, a single UAT channel is
utilized to accommodate all aircraft, making the overall UAT system very spectrum efficient.
assumptions of the COCR, the maximum per-aircraft data throughput was estimated to be
approximately 20 000 bit/s.
The next task involved trying to determine the amount of spectrum required to accommodate a
full airspace of aircraft, each transmitting 20 000 bit/s. In order to accomplish that, the process
outlined in Fig. 1 was utilized. Important simplifying assumptions were that the communications
were only occurring above 18 000 feet, “clean” spectrum could be used (i.e. the new AM(R)S
would not share spectrum with other systems), and no account was made of the aircraft
integration problems noted above. Under those assumptions, the study estimated that
approximately 10 MHz was required for those en route communications. Later calculations using
the same clean spectrum/ignoring integration assumptions estimated that for the full aeronautical
domain approximately 15 MHz would be needed.
FIGURE 1
Analysis process
As stated in the study however, this is not the complete answer. In particular, the new AM(R)S
system will have to share spectrum with other aviation systems, and as noted above aircraft
integration issues must be considered. As a result, some form of bandwidth multiplier will be
required. Since the particular technology to be used for the future communications system has
not been defined, no final studies can be accomplished.
One estimate that was made, however, based on the UAT development success noted above, was
to assume the aircraft transmissions needed to be accomplished in the same 400 s deemed
acceptable for UAT. Under that approach, for a per-aircraft throughput of 20 000 bit/s, a 50 MHz
channel would be needed. Noting that the data throughput estimate did not take into account any
of the error correction coding that was required to ensure UAT operation in its dense interference
environment, that 50 MHz estimate was increased to 60 MHz. This value will need to be
confirmed as the new communications system is developed.
5
AM(R)S for portions of the 5 000-5 150 MHz band
Estimates for surface spectrum requirements were determined in a different manner as the COCR
did not address that domain. Instead, throughput estimates are based on a review of current
surface communications requirements at a major airport2 in the United States. Those throughput
requirements are then assumed satisfied using the candidate surface system technology (based on
the Institute of Electrical and Electronics Engineers (IEEE) Standard 802.16e), and an overall
spectrum requirement is derived.
The 802.16e standard specifies the air interface for broadband wireless access (BWA) systems,
including mobile subscribers moving at vehicular speeds. The standard is also flexible in that it
can be implemented in bands below 10 GHz and with channel bandwidths from 1.25 MHz to 20
MHz. The approach followed for the spectrum requirements study involved:
1. Cataloging the potential message traffic for the airport network. For this study two
implementation phases were assumed: up-to 2020 (Phase 1) and beyond 2020 (Phase 2);
2. Determining the airport data rate (bits/sec) requirement to satisfy that message traffic;
3. Developing assumptions on the distribution of network participants3;
4. Developing assumptions on the network channel structure (for this analysis channel
bandwidths of 10 and 20 MHz were considered) and channel loading;
5. Estimating spectral requirements based on the above assumptions.
Table 1 contains the estimated spectral requirements for an airport surface network at a major
airport supporting the assumed applications. As previously mentioned, two different channel
bandwidths (10 MHz and 20 MHz) were considered. For the 20 MHz bandwidth case omnidirectional antennas were assumed for the base stations, while for the 10 MHz case sectorized
(3 sectors per base station) antennas were assumed.
TABLE 1
Spectral Requirements EstimatesNote 1
Implementation Phase
Channel BW = 20 MHz
Maximum
Estimated
Estimated Data
Spectral
Rate (Mbps) for Requirements
Channel BW = 10 MHz
Maximum
Estimated
Estimated Data
Spectral
Rate (Mbps) for a Requirements
2
For this study Dallas-Fort Worth airport in the United States of America was examined.
3
Since the IEEE 802.16e standard supports adaptive modulation and coding, channel throughput in bits
per second will vary depending on the signal to noise ratio (SNR) for the particular communications
path under consideration. In order to provide a representative mix of SNR’s the AM(R)S subscriber
units were assumed uniformly distributed across the airport surface.
Phase 1 (2020)
Phase 2 (post 2020)
a large airport
60.8
72.1
(MHz)
100
120
large airport
67
78.3
(MHz)
100
110
Note 1: These values are estimated based on specific parameters and assumptions. Further study discussed below
could serve to increase or decrease these estimates.
Certain observations should be made on the estimates in Table 1 and should be further
investigated, if necessary:
– Refining the assumption that all the subscriber stations are uniformly distributed over the
airport surface, as well as taking advantage of advanced features of the 802.16e standard
such as “smart” antennas, may serve to reduce the required spectrum. Care must be taken
however to ensure that the use of any such enhancements does not impact compatibility
with other services using the bands. For example, the addition of multiple base stations
on an airport, base stations with sectorized antennas, and/or the use of directional
antennas for certain high data rate user applications could raise the available carrier-tonoise (C/N) facilitating the use of higher-rate modulations and reduced spectral
requirements. Such use might also reduce compatibility with other users of the spectrum
which , in turn, may reduce the number of airports at which a given portion of spectrum
is available. This could impact the overall spectral requirements.
– The surface network spectrum requirements are scenario-dependent, however the
methodology utilized for this study could be applied to other aviation scenarios as well.
In particular detailed analyses are needed to determine data traffic at airports and to
determine the impact of sectorized antennas on fulfilling those traffic requirements.
– The impact of the channel loading factor should be analyzed in detail. The value of 0.75
used for this study is fairly ambitious and needs verification, through modeling and
simulation, by using traffic models for the aeronautical applications. It is unlikely that it
would move in any direction other than down, and it should be noted that using a lower
value could serve to increase the amount of spectrum required.
– While header compression was not assumed for this analysis, and in the end state header
compression may reduce spectrum required, such reduction may be small and would not
change the overall conclusion, if that is the case.
– The broadcast/multicast features of the IEEE 802.16e standard should be further
explored, however it is anticipated that they would require a more complicated frame
structure which could increase the overhead, so it is not expected to reduce overall
spectrum requirements.
– While a more detailed analysis could refine the traffic distribution between the forward
and reverse links, it may or may not impact the total spectrum requirement.
Although further studies may change the estimate, as discussed above, the results of studies todate indicate that the estimated airport surface network spectral requirements will exceed the
59 MHz (i.e., 5091-5150 MHz) allocated for that function at WRC-07, and may be closer to 100110 MHz.
6
Conclusion
While studies to determine appropriate technologies to support the future communication system
continue, initial estimates of new AM(R)S spectrum requirements based on the aforementioned
models and assumptions have been determined. Those estimates take into account evolving
aeronautical applications, and integration of a new system on an aircraft, and are:
–
–
approximately 60 MHz in some portion of the 960-1 164 MHz band; and
approximately 100-110 MHz in some portion of the 5 000-5 150 MHz band.
These estimates will be refined as studies progress, including investigation of any spectral
efficiencies offered by the items delineated in Section 5 above.