Interference Mitigation Techniques in FMCW Automotive Radars

Sammanfattning: Radar has emerged as an important sensor for scenario perception in automated driving and surveillance systems. The exponential increase of radar units in traffic and their operating frequency limitations have given rise to the problem of mutual interference. Radar's performance degrades in the presence of interference, which can result in false alarms and missed detections. In the case of safety-oriented systems (such as automatic emergency braking, blind-spot detection and obstacle detection at level crossings), radar's degraded performance can result in accidents. Therefore, it is important to mitigate the effect of mutual interference to make modern radar applications safe and reliable. The goal of this work is to develop signal processing techniques for interference mitigation in frequency modulated continuous wave (FMCW) radars operating at 77-81 GHz.The thesis investigates radar interference suppression in the spatial domain, using antenna arrays. The interference is suppressed by placing notches in the antenna radiation pattern in the direction of the interference source by employing digital beamforming.The array aperture (size) determines the beam-width and notch resolution of the receiving antenna. Narrow notches are desirable since they lead to a smaller suppressed region in the radar's field of view. It is demonstrated that an extended virtual aperture in a multiple-input-multiple-output (MIMO) FMCW radar does not offer an improved notch resolution for interference suppression due to a non-coherent interference signal in the virtual aperture. Moreover, it is shown that the calibration mismatches of the receiving array completely change the final antenna beam-pattern compared to the theoretical one.Additionally, an adaptive beamforming approach of interference suppression based on the least mean squares (LMS) algorithm is presented, which is evaluated using outdoor measurements from a 77GHz FMCW radar. The results demonstrate that the proposed technique suppresses interference successfully, resulting in a signal to interference plus noise ratio (SINR) improvement. It is also shown that complex-baseband (IQ) receivers achieve better interference suppression compared to real-baseband receivers when spatial domain methods are employed.The final research publication deals with interference mitigation in the time-domain intermediate frequency signal. The disturbed samples in the received signal are detected, removed, and reconstructed based on an estimated autoregressive (AR) signal model. The baseband signal coherence in both fast- and slow-time makes it possible to perform signal reconstruction in both dimensions. With the help of outdoor measurements covering selected scenarios, it is demonstrated that by carefully selecting the signal reconstruction dimension, a better SINR and side-lobe suppression can be achieved.