Mutual interference between automotive frequency-modulated continuous-wave (FMCW) radar systems has been a concern over recent years. Several interference mitigation (IM) techniques have been proposed to mitigate this phenomenon, which is deemed to grow in severity as more systems are deployed on the road. In this article, an inexpensive technique, based on well-known moving target indicator (MTI) processing, is proposed to separate interference from target signals. It exploits the contrast after stretch processing between uncorrelated FMCW interference (sparse and chirp-like) and beat signals (stable sinusoids). The interference is therefore marked and subtracted. This eliminates interference at the cost of introducing a distortion dependent on the relative radial velocity of the targets. To validate the proposed approach, called MTI-IM, numerical simulations and experiments with commercial-grade radars have been performed, with comparisons between MTI-IM and other IM techniques. Results show good capabilities to fight the uncorrelated interference coming from more than one interfering radar. This is achieved at reduced computational cost, which is a key limiting factor in automotive systems.

The problem of precipitation detection using Frequency Modulated Continuous Wave (FMCW) radar under strong sectorial interference is addressed. The effect of such strong interferences in the case of an FMCW scanning radar is presented. Three signal-processing pipelines (two reflectivity-based and one Doppler-based) are proposed. The performances of all these pipelines are analyzed and compared. The morphology-based pipeline performs better for higher signal-to-noise ratios (> −15dB), whereas the entropy-based pipeline performs better in the case of lower SNRs (< −15dB). On the other hand, the circular variance-based masking technique is computationally very efficient. The proposed techniques are applied to simulated and real X-band fast-scanning radar data.

The challenge of avoiding aliasing in the Doppler spectrum for precipitation is addressed. A novel integrative signal processing approach has been proposed to address the research gaps from various disciplines. The proposed approach consists of several steps. First, an aperiodic way of sampling the echoes (aperiodic sampling refers to aperiodic pulse train in the context of radar echoes in slow time) has been proposed by which the maximum unambiguous Doppler frequency (velocity) is enhanced. Second, the Doppler spectrum moment estimation is performed with the help of a parametric form of its covariance. The performance of the moment estimation is assessed by the bias and the variance in the estimated counterparts. The theoretical variance for the parameter estimation is also derived. An aperiodic pulse train design recommendation has been proposed for adequately and unambiguously estimating the Doppler moments for one extended target (like precipitation). Finally, a spectrum reconstruction technique is implemented after the moment estimation on simulated radar echo samples for a realistic precipitation-like event. The comparison with the other approaches proves its superiority for parameter estimation and Doppler spectrum reconstruction.

The high-resolution vertically pointed Doppler FMCW radar PARSAX was used for the investigation of turbulence in isolated convective clouds. The radar measures reflectivity, the mean Doppler velocity, and the spectrum width in clouds that are crossing the radar beam due to horizontal wind. The measured spectrum width is used to separate the air velocity from the sedimentation velocity of cloud drops in every reflecting volume. The transverse structure functions and the vertical velocity spectra were estimated for convective clouds which have different evolution stages and cloud top heights. The slopes of these structure functions and spectra, characterizing the kinetic energy transfer, were obtained. Three layers with different regimes of cascade energy transfer were found with an increase in the height. The first layer with a spectra slope of -1 was observed in the atmospheric boundary layer up to the height of 2 km. A layer with a classic Kolmogorov slope of -5/3 was observed above this layer, at a height of 2...5 km. At the upper part of the cloud appears a layer that tends to the Bolgiano-Obukhov slope of -11/5 generated by buoyancy forces influencing that part of the cloud. The measured spectra also made it possible to non-rigorously estimate the profiles of the turbulence kinetic energy, turbulent dissipation rate, and associated with velocity fluctuations turbulent diffusion coefficient.

A far-field calibration method based on broadside measurements with a co-polarized and a cross-polarized reference dihedral target is demonstrated for the first time for an asymmetric ±45o linearly polarized 77 GHz frequency modulated continuous wave (FMCW) multiple-input multiple-output (MIMO) automotive radar. A novel extension of the calibration technique for off-broadside targets' phase response is proposed by using an optimized progressive phase compensation. Rotated dihedral and trihedral targets are used for validation. On broadside, the standard deviation of the channels' phase difference is nearly zero degrees for reference targets and under 5o for validation targets. The calibration for off-broadside targets shows that the phase relation in the target's scattering matrix can be retrieved, the accuracy of which is dependent on the target's type and angular position.

This article focuses on the experimental validation of probing signals designed to enable radar operation in spectrally crowded environments using an S-band software defined radar (SDR). The tested waveforms ensure spectral coexistence between the sensing system and frequency-overlaid emitters, while optimizing radar performance. This is achieved through a bespoke notching of the radar signal spectrum to control the amount of interference injected by the radar in each shared frequency interval. In addition, some relevant features of the probing signal that influence radar performance are controlled via a similarity index, describing the maximum allowable distance between the spectrally notched waveform and a prescribed radar signal. In a first stage, the study is aimed at verifying whether the transmit and receive chains of the SDR system impair the spectral and temporal features of the designed waveforms. Subsequently, the tested signals are radiated into the environment to investigate their effectiveness to detect targets in the illuminated scene, as well as to ensure spectral coexistence in the presence of frequency-overlaid emitters. The results demonstrated that by exploiting the designed radar probing signals, the SDR system is capable of sharing spectrum with other radio frequency wireless systems while also allowing to detect both stationary and moving targets.

The problem of aliasing in precipitation Doppler spectrum with uniform pulse repetition time is addressed. This work focuses on de-aliasing such Doppler spectra using non-uniform sampling techniques, namely, Log-periodic and Periodic non-uniform sampling. These techniques reduce the ambiguous main lobes caused by aliasing (by going beyond the observable frequency limit) into ambiguous sidelobes that are distinguishable from the original spectra. The SNR is further enhanced by using an Iterative Adaptive Approach (IAA) algorithm. The performance of Doppler moment estimation is presented after applying the IAA algorithm on simulated precipitation-like radar echoes. However, the ambiguous sidelobe suppression is highly dependent on the spectral width of the Doppler spectra.

This paper investigates the possibility of transmitting waveforms designed to enable spectral coexistence between radar and other Radio Frequency (RF) wireless systems via a Software Defined Radar (SDR). The design technique tested in this study nominally enables the placement of notches in the spectrum of the synthesized probing radar signal. Their widths and depths are set during the design stage so as to accounting for the interference into each shared frequency interval, allowing for spectral coexistence. At the assessment stage, the synthesized signal is used with the PARSAX radar system, an SDR capable of operating in the S frequency band. The analysis first focuses on studying the compliance of the signal generated by the PARSAX radar with its theoretical counterpart. Subsequently, open-air experiments are conducted in the presence of stationary and moving targets. The results show that the spectral characteristics of the probing radar signal adhere well to the theoretical spectral mask, and prove the system ability to detect both stationary and moving targets.