Intelligent Automation, Inc.15400 Calhoun Drive, Suite 400

Rockville, MD 20855

Advances in Channel Modeling Tool Based on Bidirectional Analytic Ray Tracing and

Radiative Transfer (CBAR)Intelligent Automation Inc.

Rockville, MD. ITEA Workshop

Las VegasMay 14-16, 2019

This project is funded by the Test Resource Management Center (TRMC) Test and Evaluation/Science & Technology (T&E/S&T) Program through the U.S. Army Program Executive Office for Simulation, Training, and Instrumentation (PEO STRI) under Contract No. W900KK-11-C-0029. Distribution Statement A Distribution: Unlimited

Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Test Resource Management Center (TRMC) and Evaluation/Science & Technology (T&E/S&T) Program and/or the U.S. Army Program Executive Office for Simulation, Training, & Instrumentation (PEO STRI).Approval: AFFTC-PA-12461, 412 TW-PA-13524, 412TW-PA-19265

OverviewDetailed Description– Terrain Preparation– Antenna Pattern Generation– Route Generation– Channel Modeling– Communication Performance– User Interface– Channel Emulation

SummaryCurrent and Future Enhancements

Outline

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OVERVIEW

Predict bit error rate of air to ground telemetry communications link by incorporating the impact of the following factors:– Terrain– Aircraft position orientation and velocity– Antenna pattern of aircraft and ground stations– Receiver characteristics– Modulation scheme

Objective

CBAR in a nutshell

Ray Tracing &Physical optics

Channel Modeling

Deterministic:

Statistical:

ModelValidation

PerformanceEvaluation

Database/library

CBAR Software

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Low cost approach to studying telemetry link performance for different locations of ground stations, routes, modulation schemes, center frequency and antenna configurations. Ability to portend trouble spots during an aircraft flight when the telemetry link may not be available.As a planning tool to test various suitable locations of ground stations for a given test article trajectory.

Benefits of CBAR

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DETAILED DESCRIPTION-I(Terrain, Antenna, Route)

Terrain PreparationAntenna pattern generationRoute GenerationChannel modelingCommunication performance User interface

Seven Elements of CBAR

Based on elevation data downloaded from USGS and other sources a terrain profile is created.

Element# 1: Terrain

Radiation pattern of ‘antenna on platform’ is critical for telemetry channel modelingMethod of Moments (MoM) is used to model the EM problem of ‘antenna on platform’Antenna and platform are modeled togetherThe power splits and relative phase-shifts between the two antennas, and cable losses are accountedMutual coupling between two antennas (top and bottom)– Found to be minimal in the case of C-12, i.e. ~78dB– primarily due to blockage of the fuselage

Element# 2: Antenna

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Convert antenna pattern into ray tubes3D antenna pattern is represented as triangular mesh on a unit sphere surface– each mesh represents a discretized beam– gain of each beam is simply calculated as the antenna gain of its

center direction

Incorporation of antenna pattern

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Triangular Mesh Meshed Antenna Pattern

CBAR uses AGI’s route design library to generate routes based on waypoints, speed and altitude information.For flights changing altitude at a fixed rate of climb, IAI has created a Matlab code to find intermediary waypoint that can be used as input to CBAR.

Element# 3: Route

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Waypoints:

Generated route:

A highly efficient 3D ray-tracing algorithm– Bidirectional ray tracing launches rays from both transmitter and

receiver and path is found when two rays meet.– It employs computational geometry to accurately trace polygon

ray tubes and calculate shadowing and reflection.A set of physics-based EM scattering models– It also incorporates a range of EM scattering models such as

Physical Optics (PO), Physical Theory of Diffraction (PTD)– Multithreading is used to parallelize the scattering computation

across multiple CPU cores.

Element# 4: Channel Modeling

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Channel Modeling, cont.

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The channel modeling engineuses bi-directional analytic ray-tracing to estimate time-varying channel impulseresponse for the aircraft toground link.The channel modeling enginetakes into account terraincharacteristics and theantenna patterns of groundstation and aircraft.The time-varying frequencyresponse is computed bytracking the Doppler shiftsassociated with each ray tubethat is launched from theaircraft.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

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RT2Theoretical

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Bidirectional– Conventional RT shoots rays from Tx; only trace paths of

multiple reflections + one last hop of scattering– BART shoots rays from Tx and Rx; trace all paths of multiple

reflections + one hop of scattering + multiple reflections

Bidirectional Analytic Ray Tracing (BART)

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GO GO

PO

Tx Rx

Tx' Rx'

dx

Analytic– Describes 3D ray tubes as

polygonal cylinders (or cones) and precisely determines shadowing using geometric calculations of polygons

– Problem becomes scalable; independent of wavelength

– Analytic scattering model of polygon surface

– Breaks the wavelength-dependence of algorithm complexity

BART (cont…)

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Father Ray

Facet

Illumination Area

Passed Region

Child Ray

Blocked Region

The channel modeling engine uses physical theory of diffraction to capture the effects of diffraction.Even when a line of sight link is unavailable due to obstruction from buildings, it is possible to receive signal behind the building due to edge diffraction.

Physical Theory of Diffraction

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Test case includes two elevated point sources (isotropic antenna pattern) over a real terrain surface of 10km by 10km centered at (34.9369°, 118.0628°) (near Edwards AFB)Compared with UTD-based commercial software

Validation of Channel Modeling

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0.6 0.65 0.7 0.75 0.8 0.85 0.90

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CIR

RT2Wireless Insite

2 2.05 2.1 2.15 2.2 2.25 2.3 2.35 2.4-160

-150

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freq /GHz

CIR

/dB

RT2Wireless Insite

RT2

UTD

RT2

UTD

Result Comparison

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Captures more multi-paths

CPU time MemoryRT2 4min 189MBUTD 36min 968MB

Comparison

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Ray paths in RT2 Ray paths in UTD

Computation Resources

Channel model is defined as a time-varying, wideband channel with Doppler spread.– 2D matrix format 𝐻𝐻(𝑓𝑓, 𝑡𝑡) with the first dimension defined as

frequency and the second dimension defined as time– at any time step 𝑡𝑡0, 𝐻𝐻(𝑓𝑓, 𝑡𝑡0) represents the channel response in

frequency domain at that particular time instanceDoppler-shift based extrapolation– given the instant velocity of the aircraft, the Doppler of each ray

path can be estimated by taking the projection of the velocity vector on the ray direction.

– for each path, we can predict its variations in vicinity time steps– the extrapolated channel can be calculated by incorporating a

Doppler phase term in each path

Channel Impulse Response

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Communication performance module incorporates the time-varying channel impulse response to predict bit error rate performance (BER).Supported modulation schemes are – SOQPSK-TG– PCM/FM– CPM– OFDM based on WiMax 802.16d, etc.

The user can select either Monte-Carlo based approach or look-up table.The Monte-Carlo based approach captures the effects of multipath and synchronization errors.Look-up table provides fast outputs based on received SNR.The communication performance module also generates multipath profile.

Element# 5: Communication Performance

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The user interface allows user to (a) setup a scenario, (b) run the simulation, (c) visualize the results, and (d) reporting.

Element# 6: User Interface

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One of requirements of CBAR is interfacing with a channel emulator such as Spirent S5500. The channel emulator takes fading file that can be replayed.In order to generate the fading file, we first prepare a text file that is compiled using a Spirent proprietary software to produce a fading file that can replayed.CBAR generates the text file for every mission. The fading text file contains the time-varying multipath response, with up to 24 multipath components.The time stamp within each file is 0.2 ms, and the total duration is 1 second, i.e., 5000 lines per file, or multiples thereof.

Element# 7: Channel Emulation

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We generated a sample file with only two rays that are at delays of 50 ns and 100 ns. We generated data for a duration of 10 seconds.Real part of the first 500 samples of the first and second multipath components (at delay of 50 ns).

Channel Emulation Example

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0 100 200 300 400 500-1

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Time steps, 0.2 ms

Sig

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The following screenshot is a typical output from a Spirent Emulator

Typical Spirent Output

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CBAR is a sophisticated yet easy-to-use T&E tool that allows a user to study air-to-ground communications link accounting for several factors, including, terrain, aircraft antenna pattern, modulation scheme, etc.The software has been validated with some test flight data, but validation is a unending process.The capabilities of CBAR can be expanded further in many ways.CBAR may find use at other MRTFBs besides Edwards AFB.CBAR has several niche capabilities that allow it to compete on a commercial front after some modifications.

Summary

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Since the delivery of CBAR, we added new features to the underlying software under a Navy SBIR effort (Contract No: N6833517C0641, POC: Frederick Werrell).

These features include the following:– Attenuation effects due to rain,– 3D spatial link availability calculations, and– Clutter due to propagation over ocean at different sea states.

New Features

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As part of another ongoing Navy SBIR effort (Contract Number: N68335-18-C-0528, POC: Robert Rumbaugh), IAI will transform CBAR into a radar propagation modeling tool.

We will add effects such as sea clutter, ducting, and time-variations due to target and ocean movement.

We will automate and optimize the terrain processing for speeding up the computations.

Future Enhancements

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IAI would like to thank the following people for their technical, contractual and managerial support during this project:– Tom Young– Mark Radke– Kip Temple– Mark Santiago– Liz Walden– Thomas O’Brien– Peter Weed– Michael Thomasson– Michael Rice– Erik Perrins– Scott Kujiraoka– Marnita Harris– Glenda Torres– Dean Reiser

Acknowledgements

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