Path Loss & Fading - IEEE Xplore

UNIVERSITY of SOUTH CAROLINA Department of Electrical Engineering

INITIAL RESULTS FOR AIRFRAME SHADOWING IN L- AND C-BAND AIRGROUND CHANNELS Ruoyu Sun (Ph.D. candidate) David W. Matolak (Ph.D., Professor) ICNS 21-23 April 2015

University of South Carolina

Outline • Introduction & Motivation • Measurements – Measurement Equipment – Measurement Site

• Airframe Shadowing Results – – – – –

Shadowing Event Conditions Shadowing Depth and Duration Aircraft Antenna Diversity Gain Small Scale Fading Spatial Correlation

• Conclusion University of South Carolina

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Introduction & Motivation • Unmanned Aircraft System (UAS) use is growing & will continue to grow rapidly • UAS integration in the National Airspace System (NAS) via a reliable & safe link: Control & Non-payload Communication (CNPC) • New spectrum allocated for CNPC at ITU World Radiocommunications Conference (WRC-12) – 61 MHz in C-band (5030-5091 MHz) & – 17 MHz in L-band (960-977 MHz) University of South Carolina

Introduction & Motivation (2) • NASA Glenn Research Center (GRC) project: Unmanned Aircraft Systems (UAS) Integration in the National Airspace System (NAS)

• University of South Carolina (USC) subproject: Air-Ground (AG) Channel Measurements & Modeling University of South Carolina

Introduction & Motivation (3) • Previous studies on the AG channel – Sparse – Difference frequency bands – Different environments • Existing models do NOT include – Airframe shadowing – Shadowing by large obstacles (e.g., trees, buildings) for small UAS GS University of South Carolina

Introduction & Motivation (4) • Large scale path loss models Narrowband Channel Models

– Friis free space – Log-distance – Two-ray

• Airframe shadowing – Duration of shadowing – Depth of shadowing

• Fading: distribution of signal envelope – Ricean (K-factor) – Rayleigh – Nakagami (m-factor)


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Measurement Equipment • DS-SS Stepped Correlator • Dual-Band SIMO • ~ 3000 Power Delay Profiles (PDPs) per sec

Rx LNA Cable Frequency Signal Maximum Delay Transmit Tx Amplifier Loss Gain Span Band Bandwidth Power (W) (dB) Span (μs) (dB) (dB) (MHz) (MHz) 204.6 10 -15.5 4 L 5 960-977 20.46 10 7 30 7.5 C 50 5000-5150 University of South Carolina

Measurement Equipment (2) S-3B Viking aircraft with four antennas on bottom

Tx (GS)

Rx (Aircraft) University of South Carolina

Transportable GS with 20 m tower

Measurement Site • Over Sea: Oxnard, CA • Over fresh water: Cleveland, OH • Mountainous: Telluride, CO

• Urban: Cleveland, OH • Suburban: Cleveland, OH, Palmdale, CA & Latrobe, PA • Hilly: Latrobe, PA & Palmdale, CA


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Measurement Site (2) The most comprehensive AG channel characterization in history!

In total 82 flight tracks ~316 million PDPs ~146 GB raw data! Oxnard, CA Over Sea (Pacific Ocean)

Cleveland, OH Over Fresh water (Lake Erie) Urban (downtown Cleveland)

Telluride, CO Mountainous (Rocky Mountain)

Palmdale, CA Suburban (Palmdale township) Desert (Mojave Desert) Hilly (San Gabriel Mountain) University of South Carolina

Latrobe, PA Suburban (Latrobe township) Hilly (Appalachian Mountain)

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Measurement Site (3) • • • •

Over Lake Erie Start from Pink circle End at Black square Red oval indicates where airframe shadowing occurred

• • • • •

hGS = 20 m, hAircraft = 580 m Elevation ∠ 1.7° ~ 1.8° d = 20.3 ~ 21.8 km V = 76.5 m/s ~ 2750 PDPs/s for each of 4 Rxs


Shadowing Event Conditions • Shadowing event occurred only if all 3 conditions were satisfied  Abs(∠ Roll) > 5°  Abs(∠ Heading－ ∠ Azimuth) > 30 °  Pr,free space－ Pr,measured > 5 dB

• ∠ Roll up to -29.7 °


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Shadowing Depth • Shadowing depth (max shadowing loss) in L-band – L-band Rx1: 25.5 dB – L-band Rx2: 34.4 dB

• Multiple aircraft antennas provide diversity gain


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Shadowing Depth (2) • C-band Pr sometimes < noise floor – limits accuracy of shadowing depth

• Shadowing depth in C-band – C-band Rx1 > 30.4 dB – C-band Rx1 > 27.5 dB


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Shadowing Duration • Shadowing duration – 36.5 ~ 41.9 seconds, can be a disaster for the CNPC link – Depends on antenna location, airplane outline, velocity & maneuver Rx1

Distance (km)

L-band

Rx2

Rx1

36.5

37.3

41.9

39.8

Max Min Mean Median

30.4 0 9.1 6.3

27.5 4.2 17.2 16.9

25.5 2.6 10.3 9.6

34.4 5.0 14.2 12.9

Standard Deviation

7.4

6.4

4.4

5.9

Max Min

21.8 20.5

21.8 20.5

21.8 20.1

21.8 20.3

Shadow Duration (s)

Shadow Loss (dB)

C-band


Rx2

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Spatial Diversity Gain • Multiple aircraft antennas diversity gain  Less than 5 dB outside shadowing area  20.3 dB in L-band (968 MHz)  15.7 dB in C-band (5060 MHz)


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Small Scale Fading


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Spatial Correlation

Mean Median Max Standard deviation


Shadowing Area Whole FT C-band L-band C-band L-band 0.42 0.59 0.23 0.26 0.85 0.91 0.56 0.64 1.00 1.00 1.00 1.00 0.72

0.59

0.77

0.76

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Conclusion • Air-Ground channel measurements & models – New spectrum (L-band, 960-977 MHz & C-band 5030-5091 MHz), aiming for CNPC link – Previous studies on airframe shadowing are sparse • Measured shadowing results – Collected over the Lake Erie – Dual-band SIMO channel sounder • Airframe shadowing event – Aircraft making a “U-turn” – Roll angle up to 30° University of South Carolina

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Conclusion (2) • Airframe shadowing can severely downgrade the CNPC link  L-band (968 MHz) shadowing depth 34 dB  C-band (5060 MHz) shadowing depth > 30 dB  Airframe shadowing duration ~ 40 seconds  L-band Ricean K factor -20 dB in shadow (14 dB outside) • Multiple aircraft antennas mitigate the airframe shadowing Outside Shadow Inside Shadow  Diversity gain

< 5 dB

 Spatial correlation

0.9


20.3 dB in L-band 15.7 dB in C-band

0.6 20

Future Work • Develop airframe shadowing models using more shadowing events • Evaluate airframe shadowing by ray-tracing • Analyze other types of shadowing in AG channel – Mountain ridge shadowing – Building/tree shadowing for small UAS University of South Carolina

Questions?

Thank You! University of South Carolina