Electromagnetic Simulation Solutions

Benefits of Time-Domain Electromagnetic
Simulation for Automotive Radar
Trends in automotive safety are pushing radar systems to higher levels of accuracy and reliabletarget identification for applications like blind spot monitoring and cross traffic alert. Consequently,
the requirements for automotive radar sensors in frequency bands such as 24 GHz and 77 GHz
are becoming more stringent and engineers need to better understand how design decisions affectperformance. Time-domain electromagnetic simulation promotes rapid development by allowing
engineers to determine the performance of a fully detailed sensor model installed behind a piece offascia without needing to build prototypes and run tests in an anechoic chamber. In this article, theanalysis of a 25 GHz sensor will be discussed using Remcom’s XFdtd® Electromagnetic Simulation
Software (XF).

Analysis of RF Board
The RF board – a multilayer PCB containing the feeding structure and radiating elements – is crucial tothe design of any sensor because it is the starting point for target identification. Given its importance,engineers need a tool that helps them understand which structures drive its performance.
Often created in tools from Cadence® or Mentor Graphics®, these boards contain hundreds of traces,vias, and surfaces that are imported into XF as CAD files. Figure 1 shows the top and bottom layersof the RF board after import. This board is 88.5 mm x 57 mm x 1.4 mm, contains 188 objects, and has
microstrip structures as small as 0.22 mm.
A simulation of the RF board will generate the same results that canbe obtained from measurements: broadband S-Parameters or farfield gain and directivity. As a design tool, these system-level results
are not very informative. They are used to compare one design
against another or determine if a design meets its requirements.
A design engineer needs more than the standard system-level
results in order to understand a device and improve its design.
XF can also compute:•Parasitic coupling between conductors. S-Parameters andimages of frequency domain currents will confirm that parasitic
coupling exists, but they do little to identify and fix the problem.
Time-domain simulation results generated using the FiniteDifference Time-Domain (FDTD) method allow engineers to see
where the coupling occurs and redesign the layout to prevent it.

315 S. Allen St., Suite 416 | State College, PA 16801 USA | +1.814.861.1299 phone |Figure 1: Top and bottom layersof RF board+1.814.861.1308 fax | sales@remcom.com | www.remcom.com

Benefits of Time-Domain Electromagnetic Simulation for Automotive Radar••

Current distribution on ground planes. At 25 and 77 GHz, grounding structures are no longer
equipotential surfaces. As seen in Figure 2, the ground plane has strong currents – 10dB below themaximum – on its edges that need to be considered during the design.
Effects of secondary sources like the local oscillator (LO) line. Secondary sources can couple onto
other conductors and even generate unintended radiation. Both of these problems can be identified
and quantified through simulation.

Analysis of RF Board with Sensor
A properly designed RF board is an indicator of future
success, but there is more work to do before OEM
specifications are met. For starters, the RF board needs
to be placed in the sensor’s case and covered by aradome. These structures will change the performance ofthe antenna.
An expanded view of the full sensor model is shown in
Figure 3. The model contains the radome, RF board,digital board, packaging, data connector, and sensor casewhich bring the overall dimensions to 106 mm x 63 mm x21 mm.
An FDTD simulation includes all of the model’s complexity
so no simplifications to the sensor are needed. This gives
Figure 2: Grounding structures are notengineers a more realistic picture of how the sensor will
equipotential surfaces
perform if it is built. For example, the data connector isa relatively large structure that is not in the immediateproximity of the radiators so one may remove it from the simulation to reduce RAM requirements.
Including it in the simulation, however, increases accuracy because energy couples onto its pins and isthen radiated from these dipole-like structures.
The radome is one of the most important structuresof the sensor because it sits directly in front of theantenna arrays and can have a large impact on theantenna’s radiation patterns. By parameterizing theimported radome model, its design can be honed
to meet the desired performance. The results froma basic parameter sweep of the radome thickness
are shown in Figure 4. Parameterizing the geometry
and setting up the simulations can be accomplishedin minutes, which is significantly less than the time
it would take to create and measure five different
radomes in a lab.
Figure 3: Expanded view of full sensor

Benefits of Time-Domain Electromagnetic Simulation for Automotive Radar

Analysis of Sensor Behind Fascia
Ultimately, the installed sensor’s performance
will dictate the sensor’s ability to accuratelyidentify targets. Here, engineers areinterested in understanding how mounting
brackets, paint color, and curves in the fascia
will degrade the antennas’ radiation pattern.
Models of fascia, obtained from an OEM, canbe imported into XF like any other CAD model.
Figure 5 shows an example of a fascia model
included with the sensor. The corresponding
simulation space is 195 mm x 204 mm x 74mm.
Application and design engineers benefit
from simulation because it allows them toidentify the optimal placement of a sensor
behind a fascia or troubleshoot problems with
Figure 4: Far-field gain results for different
radome thicknesses
an installation. Similar to parameterizing theradome’s thickness, the sensor’s location
relative to the fascia can be parameterized.
This, coupled with the ability to visualize trapped modes between the radome and fascia, allow
engineers to understand which aspects of the installation are impacting the results.

Run Time and Memory Requirements
The ability to complete simulations in a timely
manner is an important factor in determining
the usefulness of a simulator. Combining
graphics processing unit (GPU) technology
and FDTD allows engineers to perform
multiple design iterations much faster thanpreviously possible.
The chart in Figure 6 compares the memory
requirements and run time for threesimulations: RF board only, RF board with fullsensor, and sensor with fascia. The microstrip
structures in the RF board generated aminimum cell size of 0.037 mm and theFigure 5: Sensor mounted behind fascia

Benefits of Time-Domain Electromagnetic Simulation for Automotive Radar864

RAM (GB)

2Run Time (hr)

0RF BoardSensor

FasciaFigure 6: Run time and memory requirements

grid definition around the RF board wasmaintained as the problem size grew withthe additional geometry. For the benchmark,XF utilized four NVIDIA® GPUs with theKepler architecture.
GPUs provide a massively parallelized
computing platform with 2,800 cores
per card. The FDTD algorithm efficientlyutilizes this parallelization and 50x speed
improvements over CPUs are commonlyachieved. This combination enables fullsensor with fascia simulations to completein under seven hours.

SummaryEngineers are pushing the limits of sensor technology in order to meet OEM requirements andimprove transportation safety. FDTD simulation provides the tools they need to understand anantenna’s performance. At the board level, sources of parasitic coupling or variations in ground
potential can be identified and mitigated. This type of analysis carries through to optimizing radome
structures and determining the best location for a sensor behind a fascia. Coupled with GPUtechnology, engineers are able to perform this analysis in hours, thus reducing the overall
development time.

To learn more about XFdtd’s Automotive Radar Capabilities, please visit:www.remcom.com/automotive-radarTo learn more about WaveFarer Automotive Radar Simulation Software, please
visit: www.remcom.com/wavefarer-automotive-radar-software
Request a free trial at:www.remcom.com/free-trial-request-form
Contact sales at:
sales@remcom.com

NVIDIA and CUDA are trademarks and/or registered trademarks of NVIDIA Corporation in the United States and other countries. Cadence
and the Cadence logo are registered trademarks of Cadence Design Systems, Inc. Mentor Graphics is a registered trademark of Mentor
Graphics Corporation.

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315 S. Allen St., Suite 416 | State College, PA 16801 USA | +1.814.861.1299
phone | +1.814.861.1308 fax | sales@remcom.com | www.remcom.com
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