NASA A/G Communications Technology Assessment Study

First Order Analysis of Required Bandwidth for the Next-Generation

Aeronautical Data Link

March 14, 2006

2

Background

• The Draft ICAO position for the World Radiocommunication Conference 2007 requests additional allocations for mobile route communications– These additional allocations could enable, among other things,

provisioning of a next-generation aeronautical communications system, specifically designed for data communications

– Eurocontrol and the FAA, with the support of NASA, have entered into a bilateral study to determine the user requirements for, the technical capabilities of, potentially applicable technologies, and the roadmap for attaining this next-generation communications system• User requirements, including expected number of users and required data

rate, are specified in the “Communications Operating Concept and Requirements for the Future Radio System” (abbreviated as the COCR)

• This paper presents the results of a first order analysis that estimates the required bandwidth for the next-generation A/G communications system– This may be useful to spectrum management professionals faced

with a requirement to justify the aforementioned reallocation of spectrum

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Analysis Process

Link Budget

Derive RadioCoverage

Using MLM data for all sectors provided by MITRE

Calculate PIACper radio site

Coverage above FL180 for US

Place En-routeRadio Sites

Account for channel access mechanism

Calculate requiredchannel rate

# Freq required

Freq reuse factor

Total BW

CalculateRequired BW

Use COCR loadingcalculations

Add VDL2 overhead

Estimate information throughput (service rate)

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Derive Basic Radio Coverage Area from Link Budget

• The first step is to presume some physical layer parameters, and derive the radio communications coverage area from link budget calculations

– Since this is a Future Radio System, to be deployed at L-Band, it is assumed that the physical layer of VDL M2 has been redesigned to be FSK modulation with a 100 Kbps data rate.

– Link Budget (shown below) closes at 160nmi

v05 L-Band link budgetL-Band link budget

Gr = 6.0dBi

1 Slant Range (nmi) 160.02 Ground Antenna Height (ft) 50.03 Frequency (MHz) 1024.04 Transmitter Power (watts) 25.05 Transmitter Power (dBm) 44.06 Transmit Antenna Gain (dBi) -4.07 Transmit Line Losses (dB) 3.08 Transmit EIRP (dBm) 37.09 Free Space Loss (dB) 142.1

10 Excess Path Loss (dB) 4.011 Receive Antenna Gain (dBi) 6.012 Receiver Line Loss (dB) 2.013 Receiver Signal Level (dBm) -105.114 Receiver Noise Figure (dB) 5.315 Receiver Noise Power Density (dBm/Hz) -168.716 Total System Noise Power in specified Data Rate (dBm) -118.717 Data Rate (kHz) 100.018 Theory Es/No for a BER of 0.001 9.019 Raised Cosine Filter Loss (dB) 1.820 Transmitter Implementation Loss (dB) 1.021 Receiver Implementation Loss (dB) 1.222 Required Es/No (dB) 13.023 Required Receiver Sensitivity (dBm) -105.724 Es/No Availabale (dB) 13.625 Residual System Margin (dB) 0.6

Assumes FSK modulation and data rate is selected to meet capacity requirements. A binary modulation was chosen due to the increased path loss at L-Band – the reduction in required Es/No more than offsets the increased noise due to larger BW

Calculated from statistics derived from multiple iterations of the IF-77 Electromagnetic Wave Propagation Model (Gierhart-Johnson) model for slightly rolling plains

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Place Radio Sites

• The second step is to develop a “lay down” for radio coverage in a region of interest– We have selected the continental United States, and show a notional radio

placement to provide coverage above FL180

Radio Site ID(arbitrary number, used for tracking in spreadsheet)

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Calculate PIAC per radio site

• The third step is to derive an estimate of the Peak Instantaneous Aircraft Count (PIAC) in each radio coverage volume

• To formulate this estimate, – Used 2020 PIAC number from MITRE MLM data

• This provides a number of aircraft in the en-route sectors of each center (ARTCC) as projected to the year 2020

– Assume uniform distribution within ARTCC

– Derive PIACs for each ARTCC above Flight Level 180 • This step is necessary as the radio coverage area (assumed) was for flight

level 180 and above, and the MITRE data sometimes includes aircraft below this flight level

– For each radio site estimate % ARTCC(s) coverage• This step allows an estimate of the percent of aircraft in each center that

fall in a particular radio site’s coverage volume

– Calculate PIACs for each radio site

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• Use COCR loading analysis to derive capacity requirements– COCR analysis includes message overheads through the Network layer

(assumes ATN OSI overheads), models arrival rates for each message type and treats the A/G network as an M/G/1 server

• Add VDL-2 message overhead to account for sub-network loading1

Derive Required Information Throughput

US Phase 2 ER High Density A/G Capacity Requirements Separate ATS, combined UL & DL (kbps) including VDL2 overhead

0.0

10.0

20.0

30.0

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60.0

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90.0

100.0

1 51 101 151 201 251 301 351 401 451 501 551

PIACs

Ca

pa

cit

y R

eq

uir

em

en

ts (

kb

ps

)1Loading is not particularly sensitive to the sub-network overhead. For example, inspection of the graph to the right does not show appreciable differences between the fully loaded message set and the COCR loading analysis, which leaves off at the network point of attachment.

This is not the case for channel access mechanism, which can significant impact the required data rate.

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Estimated Required Information Throughput

• Given the previously calculated PIAC values, and the transfer function between PIAC and loading (previous chart) map data requirements to radio sites as shown below.

Information Throughput Required

Radio Site ID

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• For a CSMA with collision detection with random backoff, a relationship between offered load and information throughput can be derived as S = Ge-2G

– Relationship is plotted below

– Using the above plot, actual channel BW required is ~5 times the information throughput (this is considered conservative for VDL M2 – published results indicate an achieved efficiency closer to 32%)

Calculate Required Channel Rate

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0.25

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0.35

0.40

0.45

0.50

0.01 0.1 1 10 100

Offered Traffic (G)

Th

rou

gh

pu

t (S

)

Peak value at ~18%

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Estimated Required Channel Rate

• Given the previously calculated information throughput, and transfer function (previous slide) we can calculate the required channel signaling rate.– Map shows required signaling rates between 122 and 430 kbps

Channel Rate Required

Radio Site ID

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Calculate Required BW

• Use the following steps to estimate required BW

– Assume a base radio data rate that is commensurate with serving a “common denominator” number of users – 100 kbps was selected

• Sensitivity analysis showed lower data rates were more efficient, but would not meet throughput requirements

– For each radio site, given the required data rate, scale by the base data rate and take the ceiling. This is the number of the frequencies needed for each radio site

– Count all radio sites frequencies to obtain total number of radio frequencies required

– Scale by frequency reuse factor to account for reuse data rate

– Multiply total number of frequencies by base data rate and efficiency (bps/Hz) to arrive at the BW required

• Derived BW required is approximately 10 MHz– Recall that this is just for en-route airspace,

above FL 180. As coverage is pushed down to the surface, more BW would be required

Radio site ID PIACService Data Rate

ChannelData Rate

NumberFrequencies

1 146 30.9 171.6 2 Effective Throughput 18%2 184 33.0 183.3 2 Base radio data rate (kbps) 1003 334 48.4 268.8 3 Total Number of frequencies 1244 271 40.7 226.2 3 Reuse factor 1.235 258 39.1 217.4 3 Number Frequencies 1016 261 39.5 219.5 3 Efficiency (bps/Hz) 17 237 36.6 203.1 3

8 321 46.8 260.1 3 Total BW required (kHz) 101009 574 77.3 429.5 5

10 167 32.1 178.2 211 361 51.7 287.0 312 210 34.3 190.8 213 397 56.0 311.2 414 163 31.8 176.9 215 138 30.4 168.9 216 83 26.9 149.2 217 387 54.8 304.5 418 216 34.6 192.4 219 74 26.2 145.6 220 370 52.8 293.1 321 83 26.9 149.2 222 83 26.9 149.2 223 242 37.2 206.5 324 127 29.7 165.3 225 120 29.3 162.9 226 83 26.9 149.2 227 125 29.6 164.6 228 208 34.2 190.2 229 132 30.1 166.9 230 79 26.6 147.6 231 98 27.9 155.0 232 90 27.3 151.9 233 123 29.5 163.9 234 182 32.9 182.7 235 79 26.6 147.6 236 77 26.4 146.8 237 77 26.4 146.8 238 47 24.1 133.9 239 189 33.3 184.7 240 65 25.5 141.9 241 77 26.4 146.8 242 135 30.2 168.0 243 217 34.7 192.7 244 23 21.9 121.8 245 116 29.1 161.5 246 74 26.2 145.6 247 111 28.7 159.7 248 117 29.1 161.8 249 191 33.4 185.3 250 195 33.6 186.5 251 91 27.4 152.3 252 110 28.7 159.3 253 47 24.1 133.9 254 23 21.9 121.8 2

Totals: 8988 1796.597346 9981.09636 124

*Reuse factor of 1.23 is very conservative. In essence, we have assumed that coverage is provided from FL180 through FL450, and that the reuse factor is being driven by aircraft at the highest altitudes.

*

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Closing Thoughts

• The predicted bandwidth required was for “green field” spectrum– The need to engineer for compatibility with existing systems (if an

overlay system in envisioned) will drive up spectrum requirements• Use of pulsed signaling with low duty cycle is a technique that might be

effective in promoting compatibility with DME equipment. This would drive up the spectrum requirements of the new system

– Some efficiencies can be gained by use of a deterministic channel access mechanism • Has the potential of reducing required bandwidth for the information

transfer requirements of the future radio system

• However, the considerations required for co-site compatibility still apply and will work to drive spectrum requirements up

• Predicted BW was only for en-route communications. Coverage was assumed to be required above FL180 only. Additional channels/BW would be required to push coverage down to the surface

NASA A/G Communications Technology Assessment Study

Supplemental

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Calculate PIAC per Radio Site

ZAB ZAU ZBW ZDC ZDV ZFW ZHU ZID ZJX ZKC ZLA ZLC ZMA ZME ZMP ZNY ZOA ZOB ZSE ZTL PIAC1 50% 1462 50% 10% 1843 30% 30% 5% 3344 30% 10% 5% 2715 50% 5% 2586 80% 2617 10% 40% 2378 10% 50% 5% 321

36 20% 7737 20% 7738 10% 4739 40% 18940 5% 20% 6541 20% 7742 5% 40% 13543 40% 10% 21744 10% 2345 50% 11646 10% 20% 7447 40% 11148 40% 11749 50% 19150 30% 19551 20% 9152 10% 20% 11053 10% 4754 10% 23

6 2 3 4 6 5 5 3 5 4 6 7 2 5 8 4 4 4 5 4100% 100% 140% 100% 100% 100% 100% 100% 100% 100% 110% 100% 100% 100% 100% 100% 100% 100% 110% 100%

PIAC 531 458 293 650 522 505 345 464 455 481 473 386 327 497 416 382 279 499 233 629