Final Spectrum Report

8/2/2019 Final Spectrum Report

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Spectrum Task SA2-07Spectrum Study

Final Report

November 5, 2007

Brian Butka(Principal Investigator)

Jianhua Liu

(Co-Principal Investigator)

Thomas YangWilliam Barott

Jeff WagnerIlteris Demirikan


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Table of Contents

Executive Summary.............................................................................................................................. 1Introduction........................................................................................................................................... 2Report Overview................................................................................................................................... 2

Part 1: Radio Spectrum ......................................................................................................................... 3Chapter 1: Survey of Existing Navigation Aids................................................................................ 4Radio Navigation Aids Survey...................................................................................................... 4Global Satellite Navigation Systems ............................................................................................ 4Low-Frequency Terrestrial Navigation Systems........................................................................... 4Data Augmentation Systems......................................................................................................... 9Navigation System Accuracy Comparison ..................................................................................11

Chapter 2: Spectrum Allocation Overview..................................................................................... 13LF and MF Bands ....................................................................................................................... 14MF and HF Bands....................................................................................................................... 14VHF and UHF Bands.................................................................................................................. 16

SHF Band.................................................................................................................................... 20Chapter 3: Current Use and Potential Spectrum Availability ......................................................... 22Spectrum Allocations of Interest................................................................................................. 23Usage Analysis............................................................................................................................ 24Usage of the 960-1215 MHz Band ............................................................................................. 24Usage of the 5000- 5250 MHz Band .......................................................................................... 26Sensitivity and Range ................................................................................................................. 27Other Bands ................................................................................................................................ 28

Chapter 4: Summary and Recommendations.................................................................................. 31Part 2: Considerations for New Systems ............................................................................................ 32

Chapter 1: Antennas ........................................................................................................................ 33Characteristics for Airborne Antennas ........................................................................................ 33Other Constraints on Antenna Selection ..................................................................................... 35Minimizing the Impact of Antenna Requirements for New Communications Systems............. 36

Chapter 2: Link Losses ................................................................................................................... 37Molecular Absorption ................................................................................................................. 37Rain Absorption .......................................................................................................................... 38Other Absorption......................................................................................................................... 39Millimeter-Wave Opportunities .................................................................................................. 40Multipath and Doppler Effects.................................................................................................... 40Doppler Effect and Time Variance.............................................................................................. 41

Chapter 3: Channel Characteristics................................................................................................. 42Communications System Elements Impacted by Channel Characteristics................................. 42Multiple Access Techniques........................................................................................................ 42Modulation Schemes................................................................................................................... 43Evaluating Modulation Schemes for Aeronautical Communications......................................... 43

Chapter 4: Network Characteristics ................................................................................................ 45Air to Air Communications......................................................................................................... 45Sharing Spectrum with Airport Surface Uses ............................................................................. 45Example: Communication Modes for Secondary ASDE users.................................................. 45Relative Positions of Aircraft and Antenna Patterns................................................................... 47Motions of the Aircraft and Worst-Case Scenarios for the Communication Channels............... 48


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Chapter 5: Air to Satellite Communications ................................................................................... 51L-Band ........................................................................................................................................ 51S-Band......................................................................................................................................... 51Ku-band....................................................................................................................................... 51Ka-band....................................................................................................................................... 52

Commercial Solutions................................................................................................................. 52High Risk High Return Technology Laser Data Links......................................................... 53Chapter 6: Summary and Recommendations.................................................................................. 54

Part 3: Example Scenarios .................................................................................................................. 55Chapter 1: Frequency Scenarios ..................................................................................................... 56

Link Calculation.......................................................................................................................... 56Link Analysis .............................................................................................................................. 58Noise Floor and Received Signal Power .................................................................................... 59

Chapter 2: Aircraft Communications Scenarios ............................................................................ 61Example of Modulation Usage in C-Band.................................................................................. 63

Chapter 3: Summary and Recommendations.................................................................................. 68

Part 4: Summary and Conclusions...................................................................................................... 69Chapter 1: Report Summary ........................................................................................................... 70Criteria Determining Data Supported by Spectrum.................................................................... 70Available Useful Spectrum ......................................................................................................... 71Additional Spectrum Required ................................................................................................... 71

Chapter 2: Report Conclusions ....................................................................................................... 71Chapter 3: Recommendations for Future Research ........................................................................ 72

References........................................................................................................................................... 73Appendix 1: Glossary of Terms and Abbreviations ............................................................................ 74


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List of Tables

Table 1: Specifications of Three Global Network Satellite Systems .................................................... 5Table 2: Specifications of Two Long-Range Navigation Systems........................................................ 6Table 3: NDBs, VOR, and DME Specifications ................................................................................... 7Table 4: Local Airport System Specifications ...................................................................................... 8

Table 5: WAAS Specifications.............................................................................................................. 9Table 6: EU System Specifications..................................................................................................... 10Table 7: Safety Critical GPS Augmentation Systems..........................................................................11Table 8: Navigation System Accuracy Comparison ........................................................................... 12Table 9: LF and MF Allocations ......................................................................................................... 14Table 10: MF and HF Allocations....................................................................................................... 15Table 11: VHF and UHF Allocations Pt. 1 ......................................................................................... 16Table 12: VHF and UHF Allocations Pt. 2 ......................................................................................... 17Table 13: VHF and UHF Allocations Pt. 3 ......................................................................................... 18Table 14: VHF and UHF Allocations Pt. 4 ......................................................................................... 19Table 15: SHF Allocations Pt. 1.......................................................................................................... 20

Table 16: SHF Allocations Pt. 2.......................................................................................................... 21Table 17: Maximum Distance for Signal Reception........................................................................... 28Table 18: MiniMijet Satellite Date System Specifications................................................................. 53Table 19: The Data Rate of a 54 Mbps Link vs. Indoor and Outdoor Distance ................................. 58Table 20: Frequency Bands of Interests with Maximum Raw Available Bandwidth ......................... 59Table 21: Frequency Bands with No. of Possible Parallel Channels per Band .................................. 59Table 22: Minimum Power Required at Receiver vs. Data Rate for Netgear WG302 Access Point.. 60Table 23: Antenna Gains Used in Link Budget Calculations for Each Band ..................................... 60Table 24: Transmitter Power Required for 54 Mbps Link over 300 Nmi and Total Data Rate for Each

Band ............................................................................................................................................ 61Table 25: Aircraft Communications Scenarios ................................................................................... 62


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List of Charts and Figures

Figure 1: Allocated Bandwidth for Aeronautical Applications........................................................... 22Figure 2: Spectrum Allocated to Aeronautical Use with Potential for Future Data Links ................. 22Figure 3: Available Aeronautical Allocations below 6 GHz............................................................... 23

Figure 4: Maximum Detected Power Level as a Function of Frequency at 960-1215 MHz.............. 25Figure 5: Utilization of Band by Frequency at 960-1215 MHz.......................................................... 25Figure 6: Maximum Detected Signal Level at 5000-5250 MHz ........................................................ 26Figure 7: Utilization of Band by Frequency at 5000- 5250 MHz....................................................... 27Figure 8: NTIA Signal Levels for 108-132 MHz............................................................................... 29Figure 9: NTIA Signal Levels for 225- 400 MHz.............................................................................. 30

Figure 10: A Monopole Located on 1-, 2-, and 10-Diameter Disk Ground Planes [21]........ 34Figure 11: Voltage Radiation Patterns of a 1000 MHz Monopole on the Bottom of a C-141 Aircraft

[22].............................................................................................................................................. 34Figure 12: Voltage Radiation Patterns for the E-slot VOR Antenna on the L-1011 Aircraft [22]...... 34Figure 13: Antenna Installations on a Boeing 767 [23] ...................................................................... 35

Figure 14: Water Vapor and Oxygen Absorption vs Frequency ......................................................... 37Figure 15: Water Vapor Absorption vs. Altitude................................................................................. 38Figure 16: Worst-Case Rain Attenuation vs Frequency...................................................................... 39Figure 17: Sea-level Atmospheric Absorption for Frequencies above 10 GHz [9] ............................ 40Figure 18: Typical Relative Aircraft Positions of Two Aircraft .......................................................... 47Figure 19: Mounting of the Omni-Directional Antennas.................................................................... 48Figure 20: Distances of Two Side-to-Side Aircraft............................................................................. 49Figure 21: Multipath on an Omni-directional Receive Antenna at the Bottom of an Aircraft ........... 50Figure 22: The Data Rate of a 54 Mbps Link as a Function of Distance ........................................... 57Figure 23: OFDM-FDMA Performance for ATC Applications .......................................................... 64Figure 24: OFDM-CDMA Performance for ATC Applications.......................................................... 65

Figure 25: Doppler Shift vs. BER Plot for Rate LDPC Code......................................................... 66Figure 26: BER vs. Doppler Shift for Different Modulation Schemes for Rician Factor of 1 dB ..... 67


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Executive Summary

Our analysis indicates that a total data rate of over 1 Gbps exists using current aeronautical spectrumallocations. If all of the spectrum allocations over 5 GHz can be used for communications, well over 2 Gbpsis available. Reducing the range of the links will allow an even higher data rate to be achieved. The actual

data rate to any aircraft depends on the number of other aircraft sharing the spectrum.

The frequency bands allocated to aeronautical uses were surveyed and several bands were identified as beinguseful for future aeronautical data communication systems.

The 108-117.98 MHz and 328.6-335.4 MHz bands offer excellent communication range and reliability, butprovide a limited data rate. It is recommended that these bands be dedicated for high-reliability safety-criticalcommunications.

The 960-1215 MHz band offers high data rates, but requires unfeasibly large antenna sizes for long-distancecommunication links. It is recommended that the 960-1215 MHz band be used for scenarios with a large

number of aircraft within a small space, such as occurs near busy metropolitan airports. Given the highprobability of interference and data collisions due to the high density of users, we recommend that an OFDMsystem with FDMA and advanced coding techniques should be used for this application.

The 5000-5250 MHz and the 32300-33000 MHz bands offer high data rates and are suitable for both short-range and long-range communications. A single Ku band satellite can support data rates of over 1Gb/s andexisting commercial systems have demonstrated reliable data links between aircraft and satellites. Thecombination of Ku band satellite communications and a 32300-33000 MHz data link would provide a high-speed redundant communications link for trans-oceanic aircraft.

In the future, aeronautical communications will need to transmit data at ever increasing data rates. Although

increased data compression and improved modulation techniques will help slow the need for more bandwidth,it is clear that additional bandwidth allocations dedicated to aeronautical uses are necessary, as the frequenciesabove 70 GHz are starting to be used commercially for high-speed data links. It is recommended a proposal

be made for the FCC to dedicate some of the spectrum between 70 and 100 GHz for aeronautical use.

Before any of the bands evaluated in this study can be unequivocally recommended for future aeronauticalcommunication use, it will be necessary to conduct a measurement study of the current utilization of these

bands over a wide geographic range. Although the usage study reported in this work found several of therecommended bands to be lightly utilized, there is no guarantee that the results apply world-wide. It is clearthat further measurements are needed in this area.

The recommendations made in this report are based on information available to the public. No proprietaryinformation was used to produce this report.


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Introduction

We approached this task by first surveying aeronautical navigation aids, looking at both aids currently in useand aids intended for future use. Then, spectra assigned for aeronautical purposes were surveyed andspectrum allocations presently available and under-utilized or soon to become available were identified. Theidentified allocations were evaluated for their suitability to aeronautical data links. For these allocations,modulation techniques and antenna designs suitable for aeronautical communication links were identified.

One of the most difficult aspects of this task is to define a meaningful metric on which to base comparisons ofthe data rate capacity of competing spectrum allocations. Currently, a determining criterion is that it isdesirable to network data between aircraft at altitude and to provide for a line-of sight link for an aircraft ataltitude. The standard IEEE 802.11 network is similar in data rate to the desired aeronautical network and willtherefore be used as the metric against which the spectrum allocations will be measured.

Report Overview

This report begins with an overview of aeronautical spectrum, focusing on existing navigational aids, thecurrent spectrum usage, and the current spectrum availability.

It proceeds to consider the demands of new systems, including antennas, link losses, channel characteristics,network considerations, and the availability and practicality of air-to-satellite communications.

Once these aspects have been examined, it proceeds to provide example cases using the spectrum available indifferent ways using new technologies with common scenarios for aeronautical communications.

Finally, a summary of findings with conclusions and recommendations for future research is presented.


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Part 1: Radio Spectrum


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Chapter 1: Survey of Existing Navigation Aids

Radio Navigation Aids Survey

This chapter will survey existing operational radio navigation aids. It will examine both aids being

decommissioned and aids starting to become operational. For the purposes of this survey, radio-navigationaids can be classified in one of four categories: global navigational satellite systems (GNSS), low-frequencynavigational aids (Loran-C), aeronautical-only navigational aids (NDBs, VOR/DME), and data augmentationsystems that augment GNSS or Loran-C systems.

Global Satellite Navigation Systems

There are three global navigation satellite systems (GNSS) currently being deployed.1. The global positioning system (GPS) is operated by the US; it is fully deployed and being upgraded.2. The global navigation satellite system (GLONASS) is operated by Russia; it is fully deployed and

being upgraded.3. The Galileo positioning system (Galileo) is operated by the EU and is currently undergoing testing; it

is scheduled to become operational in 2012.

The International Civil Aviation Organization (ICAO) has recognized the GPS and GLONASS (Russian GPS)as the two principal candidates for the Global Navigation Satellite System; it is not anticipated that any of thespectrum allocated to global satellite navigation systems will become available for other uses. Table 1 on thefollowing page details key characteristics of the three global satellite navigation systems.

Low-Frequency Terrestrial Navigation Systems

Long Range Navigation (LORAN) is a long-range low-frequency navigation system by which hyperboliclines of position are determined by measuring the difference in the times of reception of synchronized pulsesignals from two fixed transmitters [1]; the LORAN-C receiver differs from LORAN-A in that time difference

measurements are increased in accuracy by utilizing phase comparison techniques in addition to relativelycoarse matches of pulse envelopes of received signals.

LORAN-C was originally developed to provide military users radio-navigation capability [2]. It wassubsequently selected as the radio-navigation system for civil marine use in the US coastal areas and has beenapproved by the Federal Aviation Administration (FAA) as a supplemental system in the National AirspaceSystem (NAS) for the en route and terminal phases of flight. It is also available for a precise frequency sourceto support precise timing applications.

The Department of Defense (DoD) has determined that LORAN is no longer needed as a positioning,navigation, or timing aid for military users and the FAA has determined that sufficient alternative navigational

aids exist in the event of a loss of GPS-based services; therefore, LORAN is not needed as a back-upnavigation aid for aviation users. The government continues to operate the LORAN-C system in the shortterm while evaluating the long-term need for the system [2].

In addition to the LORAN-C system used in the US and the EU, Russia and many former Soviet states use theSoviet-euqivalent CHAYKA system. There are several enhanced versions of LORAN currently indevelopment, including Enhanced Long Range Navigation (E-LORAN) in the US and EUROFIX in the EU.All the enhanced versions are backward compatible with LORAN-C and primarily aim to modulate the timingof data bits. Table 2 details the key characteristics of the primary LORAN systems.


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Table 1: Specifications of Three Global Network Satellite Systems


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Table 3: NDBs, VOR, and DME Specifications


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Table 4: Local Airport System Specifications

Navigation Aid Notes

The Federal Aviation Administration (FAA) has announced intentions to reduce very high frequency omni-directional radio range (VOR) stations to a minimal operation network while increasing the number ofdistance measuring equipment (DME) stations. In addition, instrument landing systems (ILS) will be phasedout. At first glance, it appears the bandwidth occupied by VOR and ILS stations will become available for

NGATS use; however, several proposed GNSS augmentation systems, such as LAAS and GBAS, will use thesame VHF spectrum (108-118 MHz).

Proposals have been presented to the FAA (August 2006) suggesting that an E-LORAN system could be usedto provide accuracy comparable to the LAAS system at a lower cost and without requiring the VHF spectrum.Should this proposal be accepted, the VHF spectrum would be available for NGATS use.

The 329-335 MHz band allocated to the ILS glide slope system appears to be available for NGATS use if ILSis eliminated.


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Data Augmentation Systems

The primary mode of GPS data augmentation in the United States is the Wide-Area Augmentation System(WAAS), specifications for which are given in Table 5. Table 6 provides specifications for GPS augmentationsystems currently in use by the European Union. Safety-critical GPS augmentation systems are presented inTable 7.

Table 5: WAAS Specifications


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Table 6: EU System Specifications


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Table 7: Safety Critical GPS Augmentation Systems

Navigation System Accuracy Comparison

Table 8 is included for reference. Comparing the accuracy of the competing navigation systems, one finds that

the accuracy of E-LORAN systems is equivalent to that of differential GPS. It appears that both GPS and E-LORAN systems can provide the navigation accuracy needed for navigation in the future, but E-LORANrequires substantially less spectrum to achieve the same level of accuracy.


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Table 8: Navigation System Accuracy Comparison


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Chapter 2: Spectrum Allocation Overview

The following collection of tables presents an overview of all spectrum currently available for aeronauticalcommunications, with the frequency range, bandwidth, current application, and a simple yes or no assessmentof whether it has an impact on the NGATS SBA project. More specific information on the current use of the

band is also included.

Throughout the tables, the term mobile refers to mobile communications and the term fixed refers tofixed communications.

Frequency bands are defined as follows:

Low Frequency (LF): 30 to 300 kHz (Ref Table 9)

Medium Frequency (MF): 300 to 3000 kHz (Ref Table 9 and Table 10)

High Frequency (HF): 3 to 30 MHz (Ref Table 10)

Very High Frequency (VHF): 30 to 300 MHz (Ref Table 11 and Table 12)

Ultra High Frequency (UHF): 300 to 3000 MHz (Ref Table 12, Table 13, and Table 14)

Super High Frequency (SHF): 3 to 30 GHz (Ref Table 15 and Table 16)

Extremely High Frequency (EHF): 30 to 300 GHz (Ref Table 16)


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LF and MF Bands

Table 9: LF and MF Allocations

MF and HF Bands

These are allocated for long-range or "over-water" systems, usually air-to-ground or ground-to-air. Usage isdecreasing. While these frequencies might be viable for a frequency-hopping system, many of these channelswill be unusable due to atmospheric conditions. These frequencies are generally for SSB voice. Datatransmission on these frequencies already exists with automatic-hopping systems like MIL-STD-188-110B.


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Table 10: MF and HF Allocations


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VHF and UHF Bands

Table 11: VHF and UHF Allocations Pt. 1


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In the following chart, the term Air-Traffic Control Beacon System (ATCRBS) refers to a system adopted bythe FAA for use in controlling air traffic over the United States; the aircraft carry identification transpondersdesigned to transmit an aircraft identity code, altitude, and additional message when interrogated by an air-traffic controller's equipment.

Mode-S is an augmentation of the ATCRBS in which each aircraft is equipped with a transponder that replieswhen interrogated with a discrete identity code. It is also known as ADSEL (in Britain); discrete address

beacon system or Mode-S (in the United States).

According to ICAO, the modes of operation for civilian flights are the A, C, and S.

Mode A is based on a 4-digit code using numbers between 0 and 7 assigned by the ATC and set by the pilotenabling identification and monitoring. Mode C transmits pressure altitude, read automatically from theaircraft altimeter. Mode S is triggered by a mode-S interrogation and can provide the particular informationthat is requested by the interrogation signal. For modes A and C, all aircraft receiving the interrogation signalwill reply, whereas mode S allows aircraft to be addressed individually. In modern ATC systems, the dataappear with alphanumeric characters in a tag or label linked to the flight position symbol on the radar screen.


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SHF Band

Table 15: SHF Allocations Pt. 1


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Table 16: SHF Allocations Pt. 2


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Chapter 3: Current Use and Potential Spectrum Availability

Knowing the spectrum allocations dedicated to aeronautical use, we begin the investigation into how muchspectrum is available and what data rates are achievable by dividing spectrum allocations for aeronautical

purposes into two bands. The first band contains the HF, VHF, and UHF bands as well as all frequencyallocations up to 5 GHz. The second band is all frequency allocations above 5 GHz. As shown in Figure 1

below, over 75% of the total bandwidth allocated to aeronautical applications is above 5 GHz.

Figure 1: Allocated Bandwidth for Aeronautical Applications

The bandwidth allocations below 5 GHz are heavily utilized; however, due to the decommissioning of somenavigation aids, there is some bandwidth available for use in this range. The allocations above 5 GHz tend to

be lightly used, but all of the allocations there have shared uses. Figure 2 (below) shows the spectrumallocations potentially useful for future aeronautical communication links.

Figure 2: Spectrum Allocated to Aeronautical Use with Potential for Future Data Links

It is apparent from the information in Figure 2 that most of the spectrum bandwidth that can be used for futurecommunication links is above 5 GHz. While the spectrum below 5 GHz is dedicated to aeronautical uses,

most of the spectrum above 5 GHz is shared. However, while roughly 2650 MHz of spectrum is availableabove 5 GHz, only 17 MHz of spectrum is available below 5 GHz. Just above the 5 GHz cut-off, there is asignificant amount of bandwidth dedicated to aeronautical uses. Figure 3 on the following page shows thecurrent allocations of bandwidth for aeronautical use in frequencies below 6 GHz.


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Figure 3: Available Aeronautical Allocations below 6 GHz

Dedicated frequency allocations are best for communications requiring high reliability. The VHF and UHFcomponents of the spectrum are provided by the ILS beacons, VOR, and the localizer and glide slope system.The majority of the bandwidth is available from the microwave landing system (MLS) allocation. The VHFand UHF allocations are not strongly affected by atmospherics or rain, and therefore seem to be best suited forair-to-ground communications. The MLS can be used for close-range air-to-ground communications orlonger-range air-to-air communications.

Spectrum Allocations of Interest

Analysis of the spectrum allocations yields the following allocations as pertinent to this discussion.

1. 74.8-75.2 MHzFormer ILS Beacons This is only 400 kHz of bandwidth, but it is dedicated toaviation use. This allocation appears best suited for communications requiring high reliability and lowlatency, such as safety critical communications. The bandwidth is sufficient to support sixteenconventional 25 kHz AM voice channels if desired.

2. 108-117.98 MHzVOR/ILS/LAAS This spectrum will continue to be shared since a minimal

operating network of stations is planned. However, it may be possible to have much of the spectrumdedicated to future uses by assigning the channels for the remaining VOR stations, in order tominimize the number of channels used.

3. 328.6-335.4 MHzGlide slope This is another prime piece of UHF spectrum best for air-to-groundcommunications.

4. 960-1215 MHzDME/TACAN At any given location, this band is primarily empty; a cognitive radiocould work around existing users.

5. 5-5.15 GHzMLS frequencies These are probably the prime spectrum for air-to-air communications,in part because antenna size is reasonable. 5.15-5.25 GHz is used for 802.11 networks and might beuseful as well.

6. 9-15 GHZASDE frequencies This band has a cluster of sub-bands for ASDEs; these sub-bands canbe used for air-to-air communications as a secondary allocation.

7. 15-30 GHz This range of frequencies has huge bandwidths. Many of these bands are shared withradars. However, the range of airport surface detection radars is intentionally small and the antennasare pointed towards the ground. It is anticipated these bands can be used for communications. The

possibility of sharing allocations with airport surface detection devices is discussed in a later section.

Several of the bands being considered for future aeronautical communication use have shared allocations. Thefollowing sections consider two of the most promising bands and analyzes these bands with respect to whatfrequencies are utilized and what percentage of time the frequencies are utilized.


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Usage Analysis

Future radio-navigation and communication systems must contend with existing users of the radio spectrum.Prior to establishing a new system within bands discussed in this report, it is necessary to determine thecurrent use and availability of these bands. Shared allocations that are infrequently used offer substantialresources for new users. However, frequencies containing safety critical services, or allocations that are

currently approaching capacity, are not good choices for new shared allocations.

Few studies on the usage of spectrum have been published, and few of these constitute long-term broad-bandstudies. For example, data provided by the NTIA includes only minimum, mean, and maximum observedsignal strengths, without a statistical usage analysis. Additionally, the brevity of these studies is not conduciveto detection of weak or sporadic users. In 2005, Petrin reported on spectrum usage from 500 MHz to 7.2 GHz[10], after a rigorous long-term study at three separate locations. These locations included an urban site inAtlanta, a site in suburban Atlanta, and a site in rural North Carolina.

The study by Petrin surveyed two bands of interest to NGATS: from 960-1215 MHz (DME, TACAN, etc.)and from 5000-5250 MHz (MLS). We have processed the data from this study to report on two relevant

characteristics for each band in the geographical areas surveyed: maximum detected signal strength andpercent-utilization of the band.

Usage of the 960-1215 MHz Band

Data for the 960- 1215 MHz band is presented in Figure 4 and Figure 5. In Figure 4, the maximum power-level detected over the entire survey period is displayed as a function of frequency, using a channel bandwidthof 10 kHz; this power level is claimed by the author of the study to have 99.99% accuracy. Data for each ofthe three measurement sites is superimposed in this plot, and it is observed that, for these locations, much ofthis spectrum can be considered as white space: spectrum that never exhibited a signal.

The two largest continuous regions of white space, between 960- 1025 MHz and 1150-1200 MHz, comprise115 MHz of unused spectrum common to all three measurement locations. All the noncontiguous white-spacecombined comprises 198 MHz, or about 78% of this band. However, this amount includes guardbands forutilized channels.

The maximum power plot of Figure 4 indicates the frequencies on which usage was detected, but does notconvey the percentage of time these frequencies were in use. Figure 5 contains data on the detected utilizationof this band, indicated by the percentage of time for which a signal is detected at any given frequency. Forexample, 1140 MHz (the DME interrogator frequency for Atlanta Hartsfield Jackson International Airport) isobserved as being used almost 90% of the time. In contrast, traffic is observed on the transponder responsefrequency (secondary radar at 1090 MHz) less than 20% of the time.

This data indicates that new services could reuse the frequencies allocated to many existing services byestablishing the new service as a time-multiplexed secondary service. When this band is viewed in the contextof time-frequency utilization, multiplying channel bandwidth by the percentage of time the channel is in use,it is found that 99.7% of the capacity of the 960-1215 MHz band is unutilized. This data should be used withcaution, however. The detected utilization depends greatly on the measurement site and distance from thesources. For example, while the urban Atlanta measurement site indicates almost 90% usage of the Hartsfield1140 MHz DME channel, the suburban location indicates only 10% usage of this same channel. In the

preceding calculation, the maximum detected utilization from all three sites was used to determine frequencyusage.


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1000 1050 1100 1150 1200-105

-100

-95

-90

-85

-80

-75

-70

Frequency (MHz)

9

9.9

9%S

trongestPowerLevel(dBm-is

otropic)

Rural

Urban

Suburban

Figure 4: Maximum Detected Power Level as a Function of Frequency at 960-1215 MHz

1000 1050 1100 1150 12000

10

20

30

40

50

60

70

80

90

100

Frequency (MHz)

Percentoftimeused

Rural

Urban

Suburban

Figure 5: Utilization of Band by Frequency at 960-1215 MHz


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Usage of the 5000- 5250 MHz Band

A similar usage analysis was performed for the 5000- 5250 MHz band, using Petrins data obtained from allthree measurement locations. Figure 6 contains a plot of the maximum detected signal level (to 99.99%accuracy.) It is observed that 239 MHz of this band (95.6%) can be considered white space, as thesefrequencies never exhibited a signal during the measurement periods at any of the three locations. This offers

a substantial amount of unutilized spectrum that could be used for high-bandwidth aeronauticalcommunications.

Figure 7 contains data on the detected utilization of this band. An analysis of this figure indicates that 99.86%of the capacity of this band is unutilized.

5000 5050 5100 5150 5200 5250-130

-125

-120

-115

-110

-105

-100

-95

-90

Frequency (MHz)

99.99%S

trongestPowerLevel(dBm-isotropic)

Rural

Urban

Suburban

Figure 6: Maximum Detected Signal Level at 5000-5250 MHz


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5000 5050 5100 5150 5200 52500

10

20

30

40

50

60

70

80

90

100

Frequency (MHz)

Percentoftimeused

Rural

Urban

Suburban

Figure 7: Utilization of Band by Frequency at 5000- 5250 MHz

Sensitivity and Range

One concern in the interpretation of these measurements is accurate detection and representation of signals inthe environment area. Several factors contribute to this ability, including the sensitivity (noise floor) of the

receiver and the height of the antennas above ground level.

The sensitivity due to the noise floor is calculated using data provided in [10]. The mean noise floor of the960- 1215 MHz data is calculated to be -106 dBm / 10- kHz (In contrast to the maximum detected noise levelof -100 dBm / 10- kHz in Figure 6). Similarly, the mean noise floor of the 5000-5250 MHz band is claimed to

be -131 dBm / 10- kHz. Assuming an isotropically-radiating transmitter, we can derive the relationshipbetween the transmitter power and maximum allowable line-of-sight distance to obtain a minimum detectablesignal level. We define this minimum detectable signal level at 6 dB SNR, although Petrin claimed the abilityto detect much weaker signals. A summary of the results is contained in Table 17 for low power signal levelsof 1 mW (like handheld personal devices), medium power signal levels of 1W (like high-power handhelds),and high power signal levels of 100W (typical of aircraft transmitters.)


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Table 17: Maximum Distance for Signal Reception

As demonstrated in this table, the noise level does not inhibit these measurements. High power signals aredetectable for line-of-sight distances of up to hundreds of nautical miles, with lower power signals detectablewithin the local area.

The range of the measurement system is impacted to a greater degree by the height of the antennas, whichprovides a maximum line-of-sight distance to sources, dependent on their altitude. Using the range equationwith a reasonable height for the antennas in this study, we can estimate a line-of-sight range of 10 Nmi totargets at ground level, and a range of 100 Nmi or more for aircraft at altitude.

While both noise and antenna height contribute to the limited range of the detection system, we can concludethat this system was adequate for detecting signals of interest within the aeronautical communication bands

presented here.

Other Bands

Although National Telecommunications and Information Administration (NTIA) studies do not provide therigor of the previous data for determining actual spectrum usage, this data is still useful for bands not includedin the previous analysis. The NTIA has performed several short-term studies at various sites in the UnitedStates, and have included several bands of interest to this report. We include the data for the 108-133 MHZand 328-336 MHz bands below. We have selected data from the 1995-1996 study taken in San Diego,California [24].

A plot of the detected power levels in the 108-133 MHz band is shown below.


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Figure 8: NTIA Signal Levels for 108-132 MHz

This NTIA data indicates the primary problem with existing surveys; it indicates the maximum, minimum,and mean detected signal level, but gives no indication as to the actual usage or percentage of time thatsignals are detected. Despite the initial appearance that this spectrum is utilized to capacity (the maximumsample line, especially from 117-133 MHz), few of the channels exhibit high mean values, and many of thechannels have very low mean values. This could be indicative of low usage, or radios temporarily set tochannels not used in this geographic area (this band includes ATC communications). A more rigorous studywould help establish the regional viability for reuse of this spectrum.

The following figure contains similar data for the 225- 400 MHz band. Frequencies between 328-336 MHzare relevant to this study.


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Figure 9: NTIA Signal Levels for 225- 400 MHz

As noted in Figure 9, this data does not indicate actual use or differentiate actual signals from variations in thenoise floor. However, the relatively flat mean-measured-value between 325-335 MHz might be indicative ofsparse usage, and thus potential for incorporation of new cognitive radio systems.


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Chapter 4: Summary and Recommendations

While a first reading of the international frequency allocations would indicate sparse options for the creationof a new communications system, there are many opportunities for reuse and re-designation of the radiospectrum. The planned decommissioning of existing services makes large blocks of spectrum available fornew use. Furthermore, available usage data indicates that current services make very inefficient use,spectrally, spatially, and temporally, of current allocations, and that new services might effectively operate assecondary users in currently allocated spectrum, provided that the new cognitive radios can operate withoutinterfering with the primary users.

Although valuable usage information has been presented evaluating two bands of interest, information onactual spectrum usage is incomplete as opposed to allocated spectrum usage, information on which is freelyavailable. To the best of the authors knowledge, no existing studies provide the level of detail demonstrated

by Petrin in documenting the use of spectrum relevant to aeronautical communications. The differencebetween the data from Petrin and that from the NTIA illustrates this.

The authors recommend that prior to the selection (or rejection) of any shared band for NGATS, a wide

geographic study be performed to evaluate actual usage and a metric for expected value from a potentialallocation should be determined. This study should consist of observations taken over a period of severalmonths at many locations using sensitive receivers located at high elevations. If possible, airbornemeasurements should also be taken to supplement and confirm ground-based observations.


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Part 2: Considerations for New Systems


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Chapter 1: Antennas

Characteristics for Airborne Antennas

Airborne applications add many more constraints to antenna design and usage than most other applications.

Additional consideration must be given to flight safety, as well as regulatory requirements such asredundancy, the aircrafts interaction with the antennas performance, and the extreme environments to whichthe antenna is exposed.

Many of these considerations, such as triboelectric charging of the airframe surfaces that may lead to coronadischarge damage, or protection from lightning strike damage, will not be discussed here as they have a largereffect on the communication system design than on the communication channel characteristics.

Since aircraft come in vastly different sizes, fly at different air speeds and altitudes, are constructeddifferently, and have very different operational and safety constraints, it is difficult to generalize on the designor feasibility of an all-encompassing airborne communications system.

In particular, airframe size has a large effect on the development and selection of an antenna for thecommunications system. The size of the airframe with respect to a wavelength of the radiated wave plays amajor role in determining an antennas operational characteristics such as radiation pattern, impedance, andefficiency. Typically, a monopole antenna will tend to radiate in a more omni-directional pattern when the

ground-plane upon which it is mounted is relatively small in size compared to a wavelength (). To highlight

this point, the radiation pattern for a single monopole antenna vertically mounted on differently sizedground-planes is shown in Figure 10 [21]. The figure shows the radiation pattern for the same antenna

mounted on circular ground-planes that are 1, 2, and 10 in diameter. The pattern for the 10 disk (larger

airframe) has many more lobes and tends to be more directional compared to that for the 1 disk (smallerairframe). If the ground-plane is not symmetrical, as would be the case for a monopole mounted on the top orthe bottom of an aircraft, then the radiation pattern would also vary around the horizontal plane as well as the

vertical plane. This effect is shown for a bottom-mounted monopole on a C-141 in Figure 11 and for theradiation pattern of a tail-mounted slot antenna on an L-1011 in Figure 12 [22].


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Figure 10: A Monopole Located on 1-, 2-, and 10-Diameter Disk Ground Planes [21]

Figure 11: Voltage Radiation Patterns of a 1000 MHz Monopole on the Bottom of a C-141 Aircraft [22]

Figure 12: Voltage Radiation Patterns for the E-slot VOR Antenna on the L-1011 Aircraft [22]


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As can be readily seen in the figures, a single antenna cannot provide omni-directional coverage. Therefore, ifa true omni-directional system is required to enable aircraft to communicate with all other nearby aircraft,even those at different altitudes, directionality of the antennas becomes a problem. A typical solution to this

problem is to utilize multiple antennas placed around the body of the aircraft to provide full coverage;however, the multiple antennas can interact with each other, forming very complex radiation patterns similar

to arrayed antennas, but much harder to predict or control because of non-uniformity of spacing andsurroundings.

Other Constraints on Antenna Selection

Even if a functional communication system is feasible in terms of antenna considerations, realization of sucha system may be greatly affected by other issues such as system cost, airframe structural integrity, and powerrequirements.

The existing communication systems onboard modern aircraft can be very complex, as shown in Figure 13[23]. This figure depicts the almost two-dozen antennas placed around the airframe of a Boeing 767commercial airliner. Most of these systems were incorporated into the initial design of the plane and thus are

part of the initial capital expense of purchasing a 767.

Figure 13: Antenna Installations on a Boeing 767 [23]

Modifying this or any other existing aircraft for the purpose of replacing antennas or of mounting newantennas can be very costly. Not only do the antennas themselves have to be mounted, but cable may have tobe run from the transceiver to the antenna and power might need to be supplied to the transceiver itself. Sincethe requirements of any newly incorporated systems may force their antennas to be mounted in locationswhere no cable raceways exist, possibly requiring disassembly of the inner shell and/or drilling though

bulkheads or airframe supports to run cable or power, reinforcement of the airframe itself may be required toenable it to maintain its structural integrity. Not only may this be a prohibitively expensive process, but theadded weight may also affect the functionality of the aircraft, which would be unacceptable for many aircraft,

particularly those for military applications.


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In addition to the cost and complexity difficulties of the systems themselves, there may also be powerproblems depending of the system power requirements. Smaller planes, older planes, or planes whoseelectrical systems are already overburdened by existing equipment might not be able to provide the powerrequired to operate a long range, two-way, digital communication system in their present state. It may be

possible to upgrade the existing generators and feeder cables with higher capacity versions, but that wouldfurther add to the cost, weight, and installation and performance issues described above.

Minimizing the Impact of Antenna Requirements for New Communications Systems

The cost of all of these modifications to antennas may be prohibitive, and prohibitive costs would hamper theacceptance of any Next Generation Air Transportation System for aircraft communications. Older or existingaircraft might be excluded from the system, despite its inherent benefits. If this happens, the new systemwould have to be designed to accommodate aircraft operating with legacy systems, the number of whichwould ideally decrease over time as they are decommissioned for non-communications issues.

To mitigate the impact of aircraft using legacy communications systems, it is recommended new aircraft bedesigned and built with the next generation air-traffic system in-mind. Routing cables, sizing the bulkheads,

providing power, and mounting antennas are all easier and cheaper during the initial design and constructionphase of an aircraft, and the effects of future additions can be minimized if the plane already accommodatessuch additions by design. Early specification of the system type, frequency band, and characteristics may becritical if such a system is ever to be realized. The exact details may not be known for many years, but timelydecision making and forward thinking may allow aircraft manufacturers to plan and incorporate modificationsthat would accommodate the potential system requirements of an airborne Internet into their aircraft with theidea that long term planning is always cheaper than last minute modifications.


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Chapter 2: Link Losses

Practical communications systems exhibit many loss mechanisms beyond the bulk free-space path losses. Theatmosphere plays a significant role in the loss exhibited by communications links, with primary losses due tomolecular and rain absorption. Additionally, multipath-fading effects can reduce the strength of acommunications signal in air-ground as well as air-air links (by reflections from the aircraft structure). Thesemechanisms must be considered in the design of a new communications system.

Molecular Absorption

Transmissions at frequencies above 10 GHz are subject to atmospheric absorption due to water vapor andoxygen. Figure 14 below shows the absorption/km versus frequency for the atmosphere at sea level.

Figure 14: Water Vapor and Oxygen Absorption vs Frequency

As can be seen, the water vapor absorption peaks around 22 GHz and the oxygen absorption peaks around 60GHz. While communications links operating in this frequency range exhibit a large amount of attenuation dueto absorption, this attenuation can actually be beneficial. Distant and potentially interfering signals will besignificantly attenuated, possibly improving the signal-to-interference level in a short-range communicationssystem. The highest frequency currently allocated for aeronautical uses is around 32 GHz. From the plotabove, we see that water vapor dominates the absorption losses for frequencies below 48 GHz.

Figure 14 paints an incomplete picture for the purposes of aeronautical communications. As the density of theatmosphere decreases with altitude, so does the effect of attenuation due to the constituent gases. Figure 15compares the absorption due to water vapor between sea level and 30,000 ft of altitude. It can be seen thatattenuation at high altitudes is roughly a factor of five lower than that at sea level. As a result, high-frequency

bands too lossy for terrestrial communications are well-suited for long-range air-to-air communications.Atmospheric losses will be included in the examples in part 3 for the 32 GHz link, using an estimate of 0.009dB/km, or about 5 dB for a 300 Nmi path.


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Figure 15: Water Vapor Absorption vs. Altitude

A possible concern at these frequencies is the effect of high-altitude cirrus clouds, which have been found todepolarize radio waves. Reflections from the flat, horizontally-oriented surfaces of the crystals making upthese clouds cause abrupt phase reversals of a radio signal, thus degrading the performance of dual-

polarization systems. Although most ice crystals are too small to substantially scatter or depolarize signals atfrequencies below 32 GHz, this mechanism needs further study.

Rain Absorption

Aircraft at altitude fly above most storms and avoid most rain-related signal losses. However, largethunderstorms can force significant amounts of water vapor as high as 50,000 ft into the atmosphere; theabsorption losses due to water vapor can become significant in localized areas. Because of the potential forcommunications disruptions caused by storms, frequencies above 5 GHz cannot guarantee reliablecommunications over long distances. It is possible that if aircraft are in a network the signals can be linkedusing nodes outside of the storm. The high-altitude losses due to water vapor require further investigation.However, it is possible to use the ground-level values for rain attenuation as a starting point for evaluating thisloss mechanism.

The ITU defines the relationship between the attenuation due to rain, R (dB/km), and rain rate,R (mm/hr) forlow altitude rainfall as:

= kRR (ITU definition) (1)


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The frequency-dependent coefficients kand are given by ITU-R regulations. Figure 16, shown below,indicates the expected worst-case attenuation due to rain as a function of frequency for horizontal links; it is

plotted for horizontally and vertically polarized signals, assuming a horizontal link of the given distancethrough rainfall of 50 mm/hr.

0 5 10 15 20 25 30 35 40 45 500

2

4

6

8

10

12

14

16

Frequency (GHz)

Attenuation

(dB)

1 km, vertical

1 km, horizontal

10 km, vertical

10 km, horizontal

Figure 16: Worst-Case Rain Attenuation vs Frequency

Rain attenuation increases sharply with frequency; at worst, a 5 GHz signal experiences 0.1 dB/kmattenuation, while signals above 35 GHz might experience more than 10 dB/km of attenuation in a worst-casescenario.

Other Absorption

There are several other possible loss mechanisms, such as reflections off of atmospheric layers, troposphericscintillation, and interference due to signal ducting. The effects of these mechanisms require further study, asthey might significantly impact high frequency links. It has been suggested that peak fading due totropospheric scintillation in Ka band might approach 5 dB [8]. For the purposes of the analyses in part 3, alink margin of 5 dB will be used to account for the worst-estimated effects at 32 GHz. These mechanisms are

second-order at lower frequencies, and will be neglected.


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Millimeter-Wave Opportunities

Figure 17 below shows the standard sea-level atmospheric absorption for frequencies above 10 GHz.

Figure 17: Sea-level Atmospheric Absorption for Frequencies above 10 GHz [9]

Millimeter-wave frequencies exhibit high losses due to the high-frequency resonances of atmospheric gasses.Below 100 GHz, the peak absorption is located within the 60 GHz V-band, significantly limiting theusefulness of this band for terrestrial communications links. However for aircraft traveling at altitude andabove most of the atmosphere, the losses at these frequencies are significantly lower. It may be possible to use

the 60 GHz V-band for air to air communications. Additionally, aircraft can accommodate high-powertransmitters to overcome these losses.

It is noted that commercial systems are already available for W-band, where lower losses enable 1 Gb/s datarates over millimeter wave links. An example of a commercially available system is the PPC-1000 by ELVA-1. The system offers 1 Gb/s point-to-point full duplex connections operating using 71-76 GHz and 81-86 GHzchannel pairings. The system utilizes a 60 cm antenna and has a range of 2.5 miles through rain utilizing only10 mW of transmitter power. As commercial data link applications begin to utilize millimeter-wave spectrum,the need for aeronautical frequency allocations above 32 GHz becomes clear.

Multipath and Doppler Effects

Multipath fading, Doppler shift, and time-dependent channel characteristics all affect the quality of anaeronautical channel [13 & 20]. These effects are explained below.


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Multipath Propagation

In mobile communications, the received signal is often a superposition of a line-of-sight path signal andmultiple reflected waves coming from different directions. The reflected waves travel a longer distance thanthe line-of-sight signal; therefore, they arrive at the receiver with a time-delay. The received signal is spreadin time, and the channel is considered time-dispersive. The multipath spread is the time delay between the

line-of-sight component and the arrival of the latest scattered component. The inverse of multipath spread isthe coherence bandwidth of the channel. If the bandwidth of the signal is larger than the coherence bandwidth,the channel is considered frequency selective; otherwise, the channel is frequency flat (all frequencycomponents of the received signal are affected the same way by the channel.)

Stochastic Multipath Model

Since geometric analysis of the multipath channels is often impossible, stochastic models are usually used toillustrate the distribution of the channel parameters. For aeronautical channels, LOS plus the scattered wavesresults in a Rician fading, i.e., the received signals magnitude follows the Rician distribution.

The Doppler Power Spectrum Density (PSD) for aeronautical channels is approximated by a symmetricalGaussian distribution centered at origin. However, if the angles of arrival for reflective components are notuniformly distributed, the Gaussian PSD is unsymmetrical and shifted away from the origin.

Doppler Effect and Time Variance

This effect shifts the frequency of the received signal due to the relative movement between the transmitterand the receiver. The amount of shifting between the transmitted and received frequencies, named Doppler

Frequency, fD, is dependent on the angle of arrival of the signal relative to the heading of receivers motion.

fD=fD, maxcos()

Where the maximum Doppler frequencyfD, maxis:

fD, max=v/c*fc

Where v is the receivers speed,fc is the carrier frequency and c is the speed of light.

Since the reflected waves arrive from different directions relative to the receivers heading, they undergodifferent Doppler shifts. This results in a continuous distribution of frequencies in the spectrum of the signal,often called Doppler Spectrum.

The relative movements of the transmitter and receiver also indicate that communications channel effects willvary with time, creating what is termed a time-variantcommunications channel.


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Chapter 3: Channel Characteristics

Communications System Elements Impacted by Channel Characteristics

The channel characteristics affect the design of numerous items in communication systems, including:

Modulation Schemes,

Forward Error Correction Schemes,

Antenna Characteristics,

Power Spectrum and Bandwidths,

Attainable Rates and Latencies and Message Block Sizes,

Adaptation Algorithms for Allocating, and

Authentication and Security Measures and Performance.

Modulation schemes and antenna characteristics are discussed in more detail below, as they are key elementsin determining the amount of bandwidth actually required for any given aeronautical purpose.

Multiple Access Techniques

One possible solution to the issue of limited available bandwidth for aeronautical purpose is to allow multipleaccess.using certain modulation techniques

Multiple access techniques permit a large number of users to share the communication resources of a wirelesschannel. There are four basic types of multiple access:

1. Frequency Division Multiple Access (FDMA) operates by dividing the bandwidth of achannel equally among a number of users requiring access to the channel.

2. Time Division Multiple Access (TDMA) allows all users access to all available spectrum,but users are assigned specific time intervals.

3. Code Division Multiple Access (CDMA) is a spread spectrum modulation. Users areallowed to use the available spectrum whenever they transmit, but their signals must beencrypted with a specific code to distinguish them from other signals.

4. Space Division Multiple Access (SDMA): Users share the spectrum and the spatialdistribution of user terminals are exploited through the use of directional antennas thatminimize interference between users.

Orthogonal Frequency Division Multiplexing (OFDM) is an important FDMA technique adopted for wirelesslocal area networks (LANs) that fall under IEEE 802.11a, which operates in the 5 GHz band and offersinformation rates ranging from 6 Mbps up to 54 Mbps. OFDM is adopted primarily to combat the wirelesschannels frequency-selective characteristics. In OFDM systems, a number of narrow band carriers aretransmitted in a synchronous fashion. Since each carrier has a bandwidth that is significantly less than thecoherent bandwidth of the channel, the channel appears to be a flat-fading one to each carrier.

CDMA offers many advantages such as increased tolerance to interference, low probability of detection orinterception, increased tolerance to multipath, and increased range capability. Direct Sequencing (DS) and


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Frequency Hopping Spread Spectrum (FHSS) are the mainstream spectrum techniques used with CDMA. In aDS modulator, a narrowband signal is processed to be spread over a much wider bandwidth. Each user isassigned a unique spreading signature that makes the users communications approximately orthogonal toother users. Spreading the signal makes the original narrow band signal more robust to potential channeldegradations and to interference. The transmitted energy remains the same, but the signal spectrum is often

below the noise floor. Each user signal appears as a noise to any receiver that does not utilize the signals

spreading signature. Also, since timing error (which directly corresponds to range error) is inverselyproportional to the signal bandwidth, DS-CDMA can be used to measure distance or terminal location.

A combination of OFDM and CDMA is referred to as OFDM-CDMA, or Multi-Channel CDMA (MC-CDMA.) The MC-CDMA transmitter can be constructed by concatenating a DS-CDMA spreader and anOFDM transmitter. Therefore, the same data symbol is transmitted over several carriers. As opposed to DS-CDMA, MC-CDMA can handle simultaneous users without the use of highly complex interferencesuppression techniques. The MC-CDMA encoder is more efficient and results in a better BER performancethan OFDM.

Modulation Schemes

Two classes of modulation/demodulation schemes can be used for the air-to-air high data ratecommunications in the 10 GHz range. In situations using directional antennas, where the channels experienceflat time-varying fading, the single-carrier single differential modulation/demodulation schemes can be used.For situations using omni-directional antennas, where the channels experience frequency-selective time-varying fading, the multi-carrier differential modulation/demodulation schemes can be used. A more detailedanalysis of the modulation schemes is presented in the following section.

Evaluating Modulation Schemes for Aeronautical Communications

BER, SNR, and Link Budget Analysis

The communication system performance and requirements are usually specified by the probability of a singlebit-error in transmission, known as the Bit-Error-Rate (BER). The BER requirement for most aeronautical andsatellite applications is between 10-2 and 10-5, with certain critical data requiring 10-6.

BER is broadly determined by the Signal-to-Noise Ratio (SNR). The functional relationship between BERand SNR is different for every type of modulation scheme, but in general it has a waterfall-like shape. Oncethe modulation scheme is chosen, the system BER requirement dictates a particular operating point on theBER vs. SNR curve. The actual operating point must correspond to a less-than-or-equal-to BER requirement.The purpose of the link budget analysis is to calculate and tabulate the useful signal power and the interferingnoise power available at the receiver, and to make sure that the system operating point meets the BERrequirement.

Performance Enhancement and SNR Improvement

Among the various parameters affecting SNR, including hardware improvement, advanced modulationschemes, and coding,, the most cost-effective ways of improving the SNR arriving at the receiver should betaken. The system does not know and does not care where the SNR improvement comes from, as long assynchronization is achieved and Inter-Symbol Interference (ISI) has been equalized; these assumptions implythat multi-path fading should be mitigated. Error-correction coding is a cost-effective method for providingreduction in electronics cost and improving error performance.


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Fading Channels

As already introduced, the assumption that propagation takes place over ideal free space is often invalid,because the propagation takes place in the atmosphere and near the ground. A signal travels over multiplereflective paths, which causes fluctuations in the received signals amplitude, phase, and angle of arrival. This

phenomenon is known as multipath fading. Fading also includes scintillation, which is caused by physical

changes in the propagation medium, such as variations in the electron density of the ionosopheric layers thatreflect high frequency radio signals. Scintillation is of concern only at high frequencies, such as Ka-band.

Depending on the frequency selectivity and time variation of the fading channel, the system performance canbe adversely affected because of ISI distortion, pulse mutilation, irreducible BER degradation, high Dopplershift, loss in SNR, and Phase Lock Loop (PLL) failure. The BER curve is shifted to the right (worsened)

because of the degrading effects of fading. To combat fading, measures such as adaptive equalization, spreadspectrum, OFDM, pilot signals, diversity, and error correction coding should be implemented.


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Chapter 4: Network Characteristics

Air to Air Communications

Due to the large number of aircraft in the air, it is difficult to support bi-directional high data ratecommunications using only satellite communications; because of this, a major aspect of implementing the

next generation air traffic system is enabling full bandwidth high data rate communications between aircraftfor aircraft operations and passenger applications.

Sharing Spectrum with Airport Surface Uses

When aircraft are over land, it might be possible to use cellular-based air-to-ground communications toachieve sufficient data rates. In this situation, ground base stations, which are connected using ground fibernetworks, would be used to communicate with aircraft. By reducing the size of the cell (range of coverage),we can increase the reuse factor of the frequency, thereby accommodating a larger number of aircraft with afixed amount of bandwidth.

When aircraft are over the ocean, ground base stations are no longer available. In this situation, other aircraft

would be utilized as relays to provide needed communications. This makes air-to-air communications animportant issue, as high data rate air-to-air communications require large bandwidths, which are not currentlyallocated for aeronautical uses. To address this problem, it might be possible to use the spectrum allocated toother applications (called Primary) to accommodate necessary air-to-air communications (called Secondary.)This is only permissible if the operation of the Secondary does not affect the performance of the Primary andif the Secondary can successfully operate while the Primary is in operation.

As a sample case, consider that there is more than 1 GHz of bandwidth allocated to airport surface detectionequipment (ASDE)1, which is only used near airports. If air-to-air communications are directed away from theairports, or if they occur far away from the airports (such as on trans-oceanic flights), air-to-aircommunications on ASDE-allocated bands would not affect ASDE. Similarly, ASDE would not affect these

air-to-air communications. As a result, it would be possible to use ASDE as the Primary, and to use the air-to-air communications as the Secondary.

Example: Communication Modes for Secondary ASDE users.

For air-to-air communications, two communication modes can be used: point-to-point and broadcast.

A broadcast mode is needed if the information is generated from one aircraft with the intention of informingall other aircraft within the range of the air-to-air communication. Automatic Dependent Surveillance-Broadcast (ADS-B) is an example of broadcasting, although its bandwidth is not as wide as is consideredhere. Real-time updating of weather info is another example of broadcasting. High-resolution large-coverageweather images will need to be transmitted using high data rate broadcast communications.

Point-to-point communications are used when an aircraft intends to send information to a specific aircraft. Ifthe information source (transmitting aircraft) and information sink (receiving aircraft) are not within line-of-sight, relay from intermediate aircraft nodes would be needed, relaying the message using the point-to-pointmethod. It is anticipated that the primary method of air-to-air communications will be point-to-pointcommunications.

1 The 10 GHz band for ASDE includes the following sub-bands: 9-9.2 GHz; 15.4-15.7 GHz; and 15.7-16.6 GHz.


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Communication modes dictate the antenna patterns to be used on the aircraft. For point-to-pointcommunications, directional antennas, which can provide high gain and reduce unnecessary interferenceswith ASDE, are preferred for both the transmitter and the receiver. For broadcasting, omni-directionalantennas are preferred for their ability to quickly establish the link. At a minimum, the transmitting antennashould be omni-directional for broadcast communication modes. Optimally, the receiving antenna should also

be omni-directional.

To reduce interference to and from ASDE and to increase antenna gain, a vertical pattern with a narrow beamis preferred for the aircraft antennas. The horizontal pattern must be determined using several factors.

1. Prior Information on Relative Positions of the Aircraft: Point-to-point communications usingnarrow beam antennas require prior information on relative positions of the aircraft to determine thehorizontal pattern. Without this information, only omni-directional antennas can be used fortransmitting and receiving all communications, as in ADS-B. This information could be obtained byusing low data rate communications with omni-directional antennas on a much lower frequency, suchas in the VHF/UHF band. (Note that lower data rates allow lower signal-to-noise ratios and thusrequire a lower antenna gain.) With this information, we can use directional antennas at both

transmitter and receiver sides for high data rate communications.

2. Tracking of the Beam: If an antenna has the mechanism to change the pointing direction of thebeam, narrow beam patterns can be used and are preferred; otherwise, more antennas must be used.

3. Modulation Scheme: Conventional single-carrier modulation schemes require flat fading channels.To maintain the flatness of the channel, narrow beam antennas must be used to exclude multi-pathscaused by the reflections from the aircraft structure. On the other hand, modern multi-carriermodulation schemes works well on frequency-selective fading channels. As such, we can use wide

beam antennas or even omni-directional antennas which can lead to the frequency-selective fadingchannel due to the multi-paths caused by the reflections from the aircraft structure. (There is a largeroverhead for the multi-carrier modulation schemes.) Modulation schemes for air-to-air high data ratecommunications are discussed below.


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Relative Positions of Aircraft and Antenna Patterns

Figure 18: Typical Relative Aircraft Positions of Two Aircraft

The typical relative aircraft positions of two aircraft are head to head, head to tail, head to side, tail to tail, tailto side, and side to side, as shown in Figure 18. These relative positions affect the mounting locations ofdirectional antennas; in this analysis, we assume that the same antenna serves both transmit and the receivefunctions. In order to ensure the possibility of point-to-point communications, up to four directional antennasmay be needed: one at the head, one at the tail, and one on each side.

Due to the difference in altitude of the aircraft routes in the head-to-head or tail-to-tail cases, the antennas onthe two aircraft will have difficulty maintaining line-of-sight if the aircraft get too close. In this case, omni-directional antennas can be used for point-to-point communications since the low antenna gain of the omni-

directional antenna is not a big concern. In this case, two omni-directional antennas are needed on eachaircraft: one on the top and the other at the bottom of the frame, as shown in Figure 19.


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Figure 19: Mounting of the Omni-Directional Antennas

Motions of the Aircraft and Worst-Case Scenarios for the Communication Channels

The relative motion of the aircraft can be in the same direction (on the same route with head-to-tail position),the opposite direction (on the opposite route with head-to-head position or tail to tail position), or crossing (onopposite parallel routes with side to side position). These relative motions affect channel characteristics, suchas the Doppler frequency and the time rate of the time-varying fading channel, as detailed later. The followingcalculations assume a frequency of 10 GHz.

Maximum Doppler Frequency

Consider two aircraft flying head-to-head, each cruising at a speed of 900 km/hr. Then, the approaching speed

of the aircraft is

500=v m/s,

which corresponds to a maximum Doppler frequency of

==

vfmax 16.67 kHz,

at a 10 GHz carrier frequency for which the wavelength = 0.03m. Considering the wide bandwidth of highdata rate communications, about 50 MHz, this maximum Doppler frequency is not too large.

The majority of the Doppler shift can be corrected by using frequency synchronization. The remainder of thecorrection, as well as the channel amplitude gain change caused by the changing of the distance between thetransmitter and the receiver, can be seen as the time-varying channel, which needs to be handled by usingdifferential modulation/demodulation schemes. For slow time-varying channels, we can use single differentialmodulation/demodulation; for fast time-varying channels, we have to use double differentialmodulation/demodulation. The following section examines the maximum Doppler frequency change rate toexplore the possibility of using single differential modulation/demodulation for channel change.


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Maximum Doppler Frequency Change Rate

Consider two side to side aircraft flying on two routes separated by dmeters, cruising at the speedsconsidered above, as shown in Figure 20.

d

vt

r

Figure 20: Distances of Two Side-to-Side Aircraft

If time tis counted zero at the time of their shortest distance, d, then at time t, their separation is

22 )(vtdr += .

The 1st and 2nd derivatives ofrwith respect to time tare, respectively:

,)( 22

2'

vtd

tvr

+=

.))(( 2/322

22''

vtd

vdr

+=

Obviously, ''r attains its maximum at t= 0. Ifd= 5 km, then we have the maximum Doppler frequencychange rate as


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67.1max

2'''

max ====d

vf

d

vrf

kHz/s.

To see the impact of the maximum Doppler frequency change rate on the single differentialmodulation/demodulation scheme, consider the following case. Suppose the data rate is 50 Mbps and the

modulation is differential binary phase-shift keying (DBPSK). Then the symbol duration is =sb 20 ns.

During this time period, the channel phase change caused by the Doppler frequency change rate is

4'

max 101.22== fsb rads,

which can be neglected for the single differential demodulation. As a result, single differentialmodulation/demodulation is appropriate.

Time Delay Spread for the Case of Omni-directional Antennas

To estimate the time delay spread, consider an aircraft with the engine span of 30m and the antenna mountedin the middle of the two engines, at the bottom of the aircraft, as shown in Figure 21.

Omnidirectional antenna

Direc

tpath

Refle

ctedp

ath

Figure 21: Multipath on an Omni-directional Receive Antenna at the Bottom of an Aircraft

In this case, the maximum time delay spread is

10030

==c

mp ns,

which is 5 times =sb 20 ns. In this case, the channel experiences Rician frequency-selective fading, and

multi-carrier modulation/demodulation schemes, such as OFDM (orthogonal frequency-divisionmultiplexing), should be used.


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Because of the relative motion between the aircraft, the angles of reflections change with time, and thechannel also experiences time varying fading. To handle the time-varying problem with the frequency-selective fading cannel, we need to use differential OFDM modulation/demodulation schemes.

Note that for the case of directional antennas, frequency-selective fading is still a concern for large aircraft

with complicated structures.

Chapter 5: Air to Satellite Communications

Satellite communications normally operate in broadcast mode, where data is sent to many receiverssimultaneously. The uplink communication to the satellite is often from ground based stations.

L-Band

L-band contains the only current global satellite solution for aeronautical use. L-band satellites were designedto provide voice and very limited data on a global basis to serve maritime users, remote areas, and search and

rescue operations. L-band is usually used for low-Earth-orbit satellites (LEOs) because of the need to havebroad beamwidth antennas that dont have to track the satellites.

L-band has limited capacity for multiple users. The L-band satellites are limited to 34 MHz by allocation toMobile Satellite Service (MSS). Today, the capacity is limited to approximately 30 Mbps. Even in advancedgeneration L-band satellites, the total capacity will still be limited to around 100 Mbps for all users. Evenassuming all of the bandwidth in this future system is allocated to aerospace, only about thirty 3 Mbps linkscan be supported by a satellite.

S-Band

S-band is quite similar to L-band. Hence, the total throughput is limited. Every satellite can deliver about 80Mbps, which is comparable to advanced L-band satellites.

Ku-band

Ku-band is the most commonly used band today for fixed satellite services (FSS). The capacity of a single Kutransponder can be between 36-72Mbps. A single satellite can provide an average of 1-2Gbps. This means asatellite could support around three hundred aircraft with a bit rate of 3-6 Mbps. It is currently possible tolease a 54 MHz bandwidth Ku transponder for $150K/month.

Given that the satellites and the ground infrastructure already exist, airborne communications using Ku-bandsatellites appear to make the most sense in the near future. There are hurdles to overcome for plane-to-satellite

use since the existing Ku-band SATCOM systems were designed for terrestrial use, where the end user andthe energy level is fixed; the service is usually regional and there are no limitations on the antenna size.

It is expected that Ku-band satellite coverage will grow, as demand for airborne broadband becomes a reality.Since Ku-band satellite service is a competitive market, the current Ku-band coverage (which already coversmost existing routes) is expected to become global.


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Ka-band

Aeronautical SATCOM has much wider service coverage area than terrestrial aeronautical communications.World-wide satellite communication services for commercial aircraft are currently provided in L-band [11],which can provide only voice and low-speed data communications because of its limited frequency

bandwidth. We now look at the feasibility of Ka-band SATCOM for aeronautical use, since this band has a

wider available frequency band and has a higher equivalent isotropically radiated power (EIRP) for the same-size antenna.

There are several barriers to be overcome if the Ka-band aeronautical satellite communications system is to bedeployed:

1. High Rainfall Attenuation: This can be eliminated for most of the flight by flying above clouds.

2. Doppler Frequency Shift: The Doppler frequency shift in the Ka-band is more than ten timesgreater than that in the L-band. To compensate for this Doppler shift, an open-loop control system inan Aeronautical Earth Station (AES) can be installed.

3. Precise Antenna Tracking Capability: The beam-width of the Ka-band antenna is narrower thanthat of lower frequency antennas, so requires a more precise antenna tracking capability. This could

be achieved using altitude and location information from the aircrafts on-board navigation system.

As described in [25], Japans Communications and Broadcasting Engineering Test Satellite (COMETS) wasused to test the feasibility of Ka-band use for aeronautical SATCOM in 1999. A Ka-band Active Phased-ArrayAntenna (APAA) for a COMETS mobile terminal, which can receive a 21 GHz signal transmitted fromCOMET

Final Spectrum Report

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