Interference Mitigation on the AWR294x Transceiver
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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 term crossing 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’s receiver. 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 to the distance between the Aggressor and the Victim, as well as the antenna gain of both the Aggressor and the Victim. 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 maintain robust 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 and mitigate interferers in-line during range processing.
This document discusses two topics, the mechanism of crossing interference and the methods to detect and mitigate such interference using the HWA. A more thorough introduction to FMCW radar interference can be seen in [1].
2 Crossing Interference
Crossing interference can only happen if the victim radar and the aggressor radar have chirp designs with different 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 and downconverted. 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, and chirp 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), the aggressor 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).
Figure 2-1 Crossing Interference Causing
a Glitch in the Time Domain SignalFinally, 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:
Note that the glitch duration is typically small. For example, if the IF bandwidth is 12 MHz and the difference in slopes is 40 MHz/μs, approximately 0.3 μs, or four samples of the final ADC output, would be affected by interference.
