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Application NoteInterference Mitigation on the AWR294x Transceiver

ABSTRACTThe AWR294x transceiver is an integrated radar-on-a-chip device having not just the RF, analog, and ADC
circuits but also a number of processing cores on the die. It has a specialized radar signal processing
accelerator (called the hardware accelerator or HWA) with features enabling the detection and mitigation ofradar-radar interference. This document introduces the mechanism of the most common kind of interference(called crossing interference), and shows how the Texas Instruments HWA can be configured to detect and heal
such interference in-line.
Crossing Interference typically results in an increase in the noise floor, which can cause missed detections.

Table of Contents
1 Introduction.............................................................................................................................................................................22 Crossing Interference.............................................................................................................................................................2
2.1 Performance Analysis for Crossing Interference................................................................................................................43 Localization and Mitigation on the Hardware Accelerator (HWA)......................................................................................5
3.1 Interference Statistics.........................................................................................................................................................6
3.2 Localization Module........................................................................................................................................................... 8
3.3 Mitigation............................................................................................................................................................................84 Summary................................................................................................................................................................................. 9
5 References.............................................................................................................................................................................. 9

List of Figures
Figure 2-1. Crossing Interference Causing a Glitch in the Time Domain Signal..........................................................................3
Figure 3-1. Different Accelerators Available in the HWA............................................................................................................. 5Figure 3-2. Interference Mitigation Flow...................................................................................................................................... 6Figure 3-3. Detecting Outliers Using Abs and Abs-diff Filters......................................................................................................7
Figure 3-4. Different Mitigation Approaches................................................................................................................................ 8Trademarks
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1 Introduction
The most common FMCW radar to FMCW radar interference happens when the transmitted chirp from a radar(called an aggressor) crosses the chirp of another radar’s receiver (called the Victim). In this context, the termcrossing is meant to indicate that the instantaneous RF frequencies of both the devices are equal at a certain
point of time. In such a case, the Aggressor’s signal will not be rejected by any of the filters on the Victim’sreceiver. The Aggressor's chirp will be down-converted, digitized and send out as part of ADC data.
In the ADC data, the duration during which the crossing occurs will have glitch – whose amplitude is related tothe distance between the Aggressor and the Victim, as well as the antenna gain of both the Aggressor and theVictim. Unmitigated, this glitch can result in unacceptably higher noise floor after range and doppler processing– hence identifying the region of the glitch (referred to as the process of localization), and ‘healing’ the damage(referred to as mitigation) are both vital signal processing steps necessary to preserve the SNR and maintainrobust performance.
The AWR294x transceiver is TI’s latest radar-on-a-chip which has, in addition to a C66x DSP (for proprietary
signal processing) and a R5F Arm processor (tracking, communication, and control, and so forth), a radar signal
process hardware accelerator (called the HWA). The HWA is equipped with hardware blocks to localize andmitigate interferers in-line during range processing.
This document discusses two topics, the mechanism of crossing interference and the methods to detect andmitigate such interference using the HWA. A more thorough introduction to FMCW radar interference can beseen in [1].

2 Crossing Interference
Crossing interference can only happen if the victim radar and the aggressor radar have chirp designs withdifferent slopes. In such a case, the two chirps can cross each other. When the crossing happens, the victim
will observe a transient interference event. The aggressor’s transmitted chirp is mixed with victim’s chirp anddownconverted. The energy of the aggressor is observable to the victim only if their frequency difference falls
into victim’s IF bandwidth. If the frequency difference is greater, then most of the energy will be filtered out by
analog and digital filters. As most of the FMCW radars encountered in the field will have different slopes, andchirp repeat rates, crossing interference is the most likely interference that an FMCW radar will have to face.
An example is given in Figure 2-1. As the aggressor's chirp crosses the victim’s transmitted chirp (top plot), theaggressor chirp's energy is observed as a chirp that rapidly moves through the IF bandwidth (second plot). In
time domain, the region affected by interference thus resembles a glitch (third plot).

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Crossing Interference

Figure 2-1. Crossing Interference Causing a Glitch in the Time Domain Signal
Finally, after a Fourier transform is applied on the ADC samples, in the frequency domain, these crossing
interferers typically increase the noise floor and reduce the SNR of strong targets and bury weak targets, thereby
affecting detection and creating momentary blind spots. The glitch duration (τGlitch) is governed by the victim’s IF
bandwidth and the slopes of the victim (slopevictim) and the aggressor (slopeaggressor). It is shown in Equation 1:
τGlitcℎ =BandWidtℎIF
slopeaggressor − slopevictim(1)

Note that the glitch duration is typically small. For example, if the IF bandwidth is 12 MHz and the differencein slopes is 40 MHz/μs, approximately 0.3 μs, or four samples of the final ADC output, would be affected byinterference.

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Crossing Interference

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2.1 Performance Analysis for Crossing Interference
The level of interference would depend on the distance between the radars (R), the antenna gain in the line
connecting the Aggressor Tx and the Victim Rx (AggtxAntGaindB + VicrxAntGaindB), the duration of time during which
the chirps ‘cross’ and the Aggressor transmit power (PAggtx).
The main effect of this interference would be to raise the noise floor. The increase of the noise floor after range
processing can be calculated as:affectedAdcSamp

NoiseIncIndB = PInterference + 10log10 totalNumAdcSamp − Pnoise(2)affectedAdcSamp

Where totalNumAdcSamp is the ratio of the number of ADC samples that were affected by interference to the total

number of adc samples. Pnoise is the native noise floor without any interference, and is given by the following
equation, where NF is the Victim Receiver’s noise figure, and BandWidthIF is the IF bandwidth.
Pnoise = -174° + NF + 10log10(BandWidthIF)(3)

PInterference is the interference power received at the victim and is given by the following equation:
PInterference = PAggtx + AggtxAntGaindB + VicrxAntGaindB − 10log10 4πRλ2(4)Where ƛ corresponds to the wavelength.

As an example, assume the following parameters:• Aggressor power (dBm), PAggtx = 10 dBm• Aggressor antenna gain, AggtxAntGaindB = 10 dB• Victim antenna gain, VicrxAntGaindB = 10 dB• Victim Noise Figure, NF = 14 dB• IF Bandwidth, BandWidthIF = 10 MHz• Wavelength, ƛ = 3.9 mm• Distance between Aggressor and Victim = 10 meters• Number of samples affected = 10 %Then,• Interferer power, PInterference = -60 dBm• Thermal Noise power, Pnoise = -90 dBm• Increase in the noise floor, NoiseIncIndB = 20 dB
A 20 dB increase in the noise floor results in missed detections, unless the glitch is located and removed.

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Localization and Mitigation on the Hardware Accelerator (HWA)

3 Localization and Mitigation on the Hardware Accelerator (HWA)
With the theory in place, the methods available on devices like the AWR294x for localization and mitigation ofinterference are discussed. Localization is defined as the process of finding interfered regions in ADC data, while
Mitigation refers to the process of healing the damaged region.
The radar hardware accelerator [2] (or HWA) is collection of accelerators for common radar signal processing
options. These accelerators are divided into four different cores (Figure 3-1): FFT, CFAR, Compression/
Decompression, and Local maxima. The interference modules are part of the FFT Engine – and are placed
before the FFT engine. This is so that the process of interference mitigation is performed prior to and inline withthe FFT. All modules are streaming engines – every cycle they take in one sample as input and give one sample
as output. Hence, the additional cost of interference mitigation is only a few additional cycles.

The different interference mitigation modules are seen in the pre-processing section (encircled in blue).

Figure 3-1. Different Accelerators Available in the HWA
The modules related to interference are:• Interference Statistics Module - computes statistics on the ADC data and generates thresholds.
• Interference Localization Module – uses the thresholds to find outliers.
• Interference Mitigation Module - removes outliers using one of three methods.
The process of interference mitigation is a two pass process (Figure 3-2):
1. In the first pass, statistics are computed on the ADC. Interference Localization and Mitigation is a two passmethod.
2. In the second pass, the statistics are used to generate thresholds. Then, samples that cross the threshold
are marked as corrupted/interfered with in the interference localization module. This information is thenprovided to the interference mitigation module.

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Localization and Mitigation on the Hardware Accelerator (HWA)

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Figure 3-2. Interference Mitigation Flow

3.1 Interference Statistics
Two kinds of statistics are computed on the ADC data [2]. These are:••

The sum of absolute values of the adc data, absSum = ∑Ni = 1 xi where xi corresponds to the adc samples inthe chirp.
The sum of the first difference of adc data, diffAbsSum = ∑Ni = 2 xi − xi − 1 , .

These are then scaled down to generate means, and then scaled up to generate two thresholds (AbsThresholdand AbsDiffThreshold).

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Localization and Mitigation on the Hardware Accelerator (HWA)

The abs-diff filters suppress the low-frequency tone and help make the glitch more visible.

Figure 3-3. Detecting Outliers Using Abs and Abs-diff Filters
The advantage of calculating the first difference is that it suppresses low-frequency reflections like the bumper
reflection or the antenna coupling effect, allowing weaker glitches to be easily found.

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3.2 Localization Module
Localization is performed by finding outliers in ADC data [2]. Using the thresholds set from the interference
statistics, these outliers can be found Samples that cross this threshold in energy can be marked as having
been affected by interference. Note that options are available to use either the ‘absThreshold’ and/or the‘absDiffThreshold’ to detect the outliers.
In the 2944, each sample is marked by an interference indicator bit (or IIB), which is zero if below threshold and1 if it is above. This sequence can then be provided to a Hysteresis module which optionally provides hysteresis.
Finally this IIB bitstream is send to the mitigation module, where it is used to identify interferer locations.

3.3 Mitigation
A number of mitigation methods are available in the HWA 2.0 [2]. The simplest method of mitigation is to replace
the region of interference with zeros (Figure 3-4 - top). However, that has the side effect of creating large
sidelobes that might bury weak targets. A better approach is to blank out with a window. A smoothing window
is used to zero the samples that have been affected by interference. This leads to lower sidelobes and better
detectability of weak targets (Figure 3-4 - middle).
An even better approach would be to perform linear interpolation in the blank region, using the last good ADC
sample before interference and the first good ADC sample after the period of interference. Since the strongest
reflectors are likely to be closer to the radar and hence have lower frequencies, this approach works well inmany cases (Figure 3-4 - bottom).

Figure 3-4. Different Mitigation Approaches
Mitigation is an active area of research and more complex mitigation schemes that are possible than the three
we’ve discussed. However, as the mitigation scheme becomes more complex, one has to weigh the amount ofMIPs consumed against the benefit gained by the more complex scheme.

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Summary

4 Summary
Crossing interference can lead to detection failures due to an unacceptable increase in the processed signals
noise floor. In ADC data, these interference events look like glitches. Localizing and mitigating these glitches isvital to maintain robust radar performance.
The AWR294x transceiver has multiple tools to find and localize interference, including specialized modules in
the radar signal processing to allow inline removal of these interferers.

5 References
1. Sriram Murali, Karthik Subburaj, Brian Ginsburg and Karthik Ramasubramanian, “Interference Detection inFMCW Radar Using A Complex Baseband Oversampled Receiver”
2. Texas Instruments: AWR294x Technical Reference Manual v0.7 Technical Reference ManualSWRA725 – NOVEMBER 2021
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