Frequency-modulated Continuous-wave System Research Articles

3D imaging with large range dynamics and simultaneous accurate speed measurement

In this paper, we describe 3D imaging experiments performed using a frequency modulated continuous wave (FMCW) LIDAR. A detailed study of noise and signal statistics is provided in order to analyze the sensitivity of our FMCW LIDAR, and its capability to image complex scenes over a broad range (up to 3 km) is demonstrated. An image example is also given to illustrate that simultaneous measurement of the speed for each pixel gives additional information that can be used to help segmentation algorithms and target classification. Although 3D imaging using the FMCW system has already been demonstrated, the demonstrated range is generally limited to a few 100 m. In this paper, 3D imaging up to 3 km is achieved while preserving compactness and ruggedness to the various environmental conditions.

Interpolation linearization predistortion technology for FMCW LiDAR.

Frequency-modulated continuous-wave (FMCW) laser ranging technology is an important development direction of light detection and ranging (LiDAR) for the future. It has the advantages of high ranging accuracy, high resolution, wide range, and no ranging blind zone. A distributed feedback laser can be used as a high-quality light source in FMCW laser ranging systems because of its wide frequency modulation range, simple frequency modulation mode, and small package. Aiming at the nonlinear problem of the laser in the frequency modulation process, we present a novel, to our knowledge, predistortion algorithm based on interpolation linear fitting to enhance the linearity of the FMCW laser for LiDAR systems. The sweeping frequency curve of the laser is obtained using the Hilbert transform, and then the sweeping frequency curve is segmented and linearly fitted to calculate the interpolated driving current signals corresponding to linear frequency changes. Using this method, we achieved a nonlinearity error lower than 1e-7 for the swept-frequency signal and demonstrated that the ranging error is less than ±5c m at a distance of 100m in the FMCW system. In addition, we also demonstrated a 3D static object point cloud with high imaging quality. Compared with the iterative predistortion algorithm based on the function fitting, this method avoids fitting errors at the inflection points of the triangular swept-frequency signal and the complexity of multiple iterative calculations. It enables rapid generation of pre-distorted swept-frequency signals, making it particularly suitable for real-time applications of automotive LiDAR systems.

Frequency Domain Imaging Algorithms for Short-Range Synthetic Aperture Radar

In order to achieve miniaturization, short-range radar (SRR) generally adopts millimeter-wave (MMW) radar with a frequency-modulated continuous-wave (FMCW) system, which may make the stop–go–stop assumption in traditional synthetic aperture radar (SAR) imaging algorithms invalid. In addition, in order to observe a large enough area, SRR often needs a wide radar beam, which may cause serious range–azimuth coupling when using SRR for SAR imaging. The above two problems may make the traditional SAR imaging algorithm invalid in SRR SAR imaging. Taking the SRR SAR imaging application into account, traditional frequency domain SAR imaging algorithms are analyzed and improved in this paper. Firstly, the intra-pulse motion (IPM) caused by the FMCW system and the two-dimensional coupling (TDC) in the case of a wide beam are analyzed. Subsequently, the applicability of the range Doppler algorithm (RDA), the frequency scaling algorithm (FSA) and the range migration algorithm (RMA) for SRR SAR is analyzed. Then, improvement measures are put forward to address the aliasing and folding phenomena caused by the wide-beam problem in the FSA and RMA, respectively. Finally, the effectiveness of the proposed algorithm is verified using simulation data and real measured data collected using an MMW radar fixed on a slide rail.

Ultralinear 140-GHz FMCW signal generation with optical parametric wideband frequency modulation enabling 1-mm range resolution.

We demonstrate ultralinear and ultrawideband frequency-modulated continuous-wave (FMCW) signal generation using an optical parametric wideband frequency modulation (OPWBFM) method. The OPWBFM method optically expands the bandwidths of FMCW signals beyond the electrical bandwidths of optical modulators via a cascaded four-wave mixing (FWM) process. Compared to the conventional direct modulation approach, the OPWBFM method simultaneously achieves high linearity and a short measurement time of the frequency sweep. On the other hand, it is also known that the OPWBFM method expands the phase noise of idlers as well as their bandwidths if an input conjugate pair has different phase noise. To avoid this phase noise expansion, it is crucial to synchronize the phase of an input complex conjugate pair of an FMCW signal using an optical frequency comb. For demonstration, we successfully generated an ultralinear 140-GHz FMCW signal by using the OPWBFM method. Moreover, we employ a frequency comb in the conjugate pair generation process, leading to the mitigation of phase noise expansion. By using a 140-GHz FMCW signal, we achieve a range resolution of ∼1 mm through fiber-based distance measurement. The results show the feasibility of an ultralinear and ultrawideband FMCW system with a sufficiently short measurement time.

Requirement Analysis and Teardrop-Based Design of High Antenna Isolation for FMCW Radar

Frequency-modulated continuous wave (FMCW) radar is widely used in automotive and consumer electronics because of its range, velocity, and angle measurement functionality. In an FMCW radar system, the isolation between transmitting (Tx) and receiving (Rx) subsystems affects the sensitivity of the FMCW system, which directly impacts the system’s overall performance in target detection. The factors that affect system performance include transmitter-to-receiver on-chip coupling and Tx-to-Rx antenna coupling. The on-chip isolation performance is basically fixed once a radar chip is given, but the antenna isolation performance depends on a designed antenna array. Usually, a targeted antenna requirement is first specified, and then the corresponding Tx and Rx antenna array is designed. However, there is no general principle or criteria for specifying a proper antenna isolation requirement in the existing research. In this paper, first, we reveal that the antenna isolation requirement should be set to be almost the same as the given on-chip isolation value, which is very significant as a general guideline in setting a targeted antenna isolation requirement. All current antenna isolation methods cannot reach the level of on-chip isolation in a compactly designed radar system. We further propose a teardrop-based method to provide high antenna isolation. The principle of an antenna isolation requirement and a novel antenna design using teardrops are both analyzed and demonstrated based on a representative 24 GHz FMCW radar. Our teardrop-shaped structure in the mouth of the conventional Vivaldi antenna achieved greater than 50 dB isolation, while the distance between the Tx and Rx antennas could be reduced to 2.1 mm.

Two-dimensional multi-layered SiN-on-SOI optical phased array with wide-scanning and long-distance ranging.

Silicon based optoelectronic integrated optical phased array is attractive owing to large-dense integration, large scanning range and CMOS compatibility. In this paper, we design and fabricate a SiN-on-SOI two-dimensional optical phased array chip. We demonstrate a two-dimensional scanning range of 96°×14.4° and 690 mW peak power of the main lobe. Additionally, we set up the time of flight (ToF) and frequency-modulated continuous-wave (FMCW) ranging systems by using this optical phased array chip, and achieve the objects detection at the range of 20 m in the ToF system and 109 m in the FMCW system, respectively.

Nonlinear error correction for Terahertz FMCW System by a new beat frequency estimation method.

Terahertz (THz) frequency modulated continuous wave (FMCW) technology is a means of nondestructive testing. The signal's nonlinearity is an unavoidable problem in the daily application of THz FMCW technology. The signal's nonlinearity will lead to the spectrum broadening of the FMCW's beat frequency (BF) signal, which degrades the range resolution and result in distance-measuring error. Traditional methods require additional hardware or require a lot of computation, which are not conducive to the miniaturization of the system and real-time measurement. A novel method for correcting the nonlinear error of THz FMCW technology has been proposed and demonstrated in this article. In the proposed method, the windowed Fourier transform (WFT) is introduced to estimate the BF corresponding to the measured target, according to the linearity distribution of voltage-controlled oscillator (VCO). In this way, the measured target's BF can be accurately estimated from the unprocessed BF signal with a poor linearity. From the estimated BF of the reference target, the non-linear compensation coefficients are calculated. With the non-linear compensation coefficients, the non-linearity of the output BF signal can be calibrated. The results of simulations and experiments show that the proposed method allows the range resolution of an FMCW system to reach the theoretical limit.

Adaptive leakage signal cancellation algorithm in heterodyne FMCW system

To mitigate the leakage signal that obstructs the target information extraction in miniatured frequency modulated continuous wave (FMCW) radar system, an adaptive leakage signal cancellation algorithm is proposed in this paper. Without any help of the additional auxiliary path, all the processing for the intermediate frequency (IF) signal can be conducted in the digital domain. The basic principle to cancel leakage signal is to produce a cancelling signal, which has the same amplitude and anti-phase with the leakage signal. Differently, in this paper, the input phase parameter of the proposed method can be arbitrary and it can be modified by the corresponding corrected function. Besides, to acquire the phase noise in the leakage signal conveniently when generating cancelling signal, the information geometry theory is applied to transform the complex phase noise model into statistical characteristics calculation of the input signal. The effectiveness of the model and the proposed algorithm is verified by simulated results. Furthermore, a real FMCW system is designed to collect the practice echo signals. The resulting data denotes that the leakage signal in the received signal is greatly suppressed and the leakage signal power drops by −15.92 dB.

Modified FMCW system for non-contact sensing of human respiration

Small displacement detection capability becomes an important key for developing non-contact sensor for human respiration based on radar system. Frequency modulated continuous wave (FMCW) radar has been widely studied and applied for many applications. With respect to the conventional perspective, large bandwidth is needed for detecting the small displacement related to the human respiration. Meanwhile, extracting the respiration pattern from Doppler response has a drawback in identifying the small displacement location. In this paper, the modification on FMCW system was proposed for obtaining the capability in detecting the human respiration and its distance from the radar. Detecting the phase value of the low pass filter (LPF), output from conventional FMCW was investigated and applied as a modification concept. The pattern, rate and amplitude of respiration are extracted from phase detector output. Beat frequency detection is still elaborated for synthesising the reference signal for phase detection. The result shows that the modified FMCW system proposed the capability of detecting the rate and amplitude respiration and location of the target.

FMCW ISAR Autofocus Imaging Algorithm for High-Speed Maneuvering Targets Based on Image Contrast-Based Autofocus and Phase Retrieval

From the perspective of radar signal processing, the pulse-Doppler radar signal model is mostly based on the stop-and-go model due to the short pulse propagation time. Unlike the pulse-Doppler radar, the frequency modulated continuous wave (FMCW) radar takes a long time in the process of signal transmission to echo reception. During this period, the target cannot be considered to be stationary during the acquisition of the entire sweep echo, so the stop-and-go assumption in the FMCW system is no longer valid, which means that the target's motion related to the fast time must be taken into account. In the inverse synthetic aperture radar (ISAR) imaging scenario of high-speed maneuvering targets, the frequency offset correction introduced by the target's motion becomes reasonably complicated that it is difficult to restore the usual ISAR signal processing. In this work, a new method based on phase retrieval and image contrast-based autofocus (ICBA) is proposed to address the complex problem of motion compensation for FMCW ISAR systems. To cope with the fast time-varying characteristics of the phase error and the higher number of phase terms relative to pulsed radars, the proposed algorithm performs fine compensation with higher accuracy, resulting in high-resolution and autofocus imaging. Experimental results demonstrate the effectiveness of the proposed method.

Improved FMCW Radar System for Multi-Target Detection of Human Respiration Vital Sign

Frequency Modulated Continuous Wave (FMCW) radar system has been developed and applied for various needs. Based on the conventional FMCW radar concept, a large bandwidth is needed to detect small displacements in the chest wall or abdomen related with respiratory activity. To overcome the need for large bandwidths in detecting vital respiratory signs, several improvements to the FMCW system are proposed in this paper. The phase-detection concept has been elaborated in improving the capability of FMCW to detect the small displacement. In developing multi-target detection capability, range detection capability through beat frequency output needs to be combined with the phase-detection method. Theoretical and simulation studies were performed to investigate the concept of combining range detection and phase detection for detecting respiration on multi-target. The results show that the proposed method is well-performed in detecting the multi-target respiration in high noise reflection.

Phase-Coded FMCW Automotive Radar: System Design and Interference Mitigation

A frequency-modulated continuous-wave (FMCW) radar takes advantage of stretch processing (dechirping, deramping) to reduce the sampling requirements of radar systems such as automotive radars. When applying phase coding into FMCW radar systems for interference mitigation or joint communication, preservation of this benefit is not possible, due to the nature of this process. This paper proposes a new phase coded FMCW system as well as a processing strategy based on a group-delay filter to deal with unaligned phase-coded radar return signals. Moreover, this paper proposes an improved interference mitigation method based on a phase-coded linear- frequency-modulated (LFM) continuous waveform and points out the improvements in the signal-to-interference ratio (SIR). In the proposed mitigation technique, unlike the traditional approach where RF spectrum spreads, the beat frequency spectrum of the FMCW radar spreads, thus the bandwidth of RF hardware stays the same as a classical LFM continuous wave radar. The success of the proposed phase coded frequency-modulated continuous-wave (PC-FMCW) radar is illustrated in simulations and increased interference mitigation performance is validated by experiments.

드론용 W-대역 레이다 고도계

In this study, we propose a W-band frequency modulated continuous wave(FMCW) radar altimeter that can measure the altitude based on the frequency differences of transmitted and received signals. This W-band FMCW system is powered by an altitude control algorithm, which we propose to help prevent collisions of drones with obstacles in real deployment by measuring the relative altitude. It is shown that this algorithm enables the drone to be positioned within a 3 % error of altitude from the desired input height. The chip used in the W-band transmitter and receiver was fabricated using a 65-nm CMOS process, and a horn antenna was directly fed by incorporating an embedded waveguide feeder into the chip[1]. The clutter spectra observed in terrains including soil, grass, and calm lake water were measured and compared, confirming the reflectivity characteristics of various surfaces of different water contents.

Harmonic Synthetic Aperture Radar Processing

We review the basic concept of frequency-modulated continuous wave (FMCW) and describe how such a radar system can be used in a harmonic radar concept. It is argued that synthetic aperture radar (SAR) for FMCW harmonic radar can be implemented by carefully choosing the wavenumbers in the mixing part of the FMCW concept. It is also argued that the following SAR processing is an extension of conventional SAR processing when applied to a harmonic FMCW system.

Precision and accuracy testing of FMCW ladar-based length metrology.

The calibration and traceability of high-resolution frequency modulated continuous wave (FMCW) ladar sources is a requirement for their use in length and volume metrology. We report the calibration of FMCW ladar length measurement systems by use of spectroscopy of molecular frequency references HCN (C-band) or CO (L-band) to calibrate the chirp rate of the FMCW sources. Propagating the stated uncertainties from the molecular calibrations provided by NIST and measurement errors provide an estimated uncertainty of a few ppm for the FMCW system. As a test of this calibration, a displacement measurement interferometer with a laser wavelength close to that of our FMCW system was built to make comparisons of the relative precision and accuracy. The comparisons performed show <10 ppm agreement, which was within the combined estimated uncertainties of the FMCW system and interferometer.

Design and Development of A FMCW Ground Based Imaging Radar System

This paper describes the design and development of a ground based Frequency Modulated Continuous Wave (FMCW) radar system in Multimedia Univeristy (MMU), Malaysia. In this project, a ground-based fully polarometric, C-band, high bandwidth linear FM-CW and real time imaging radar system is to be designed and constructed. The system should have the capability to measure the complex scattering matrices of distributed targets using FMCW system and obtained fully polarimetric signals which can be used to provide more accurate identiflcation and classiflcation of the geophysical media. The purposed system hardware consists of four major sections: an antenna system, a radio frequency (RF) subsystem, an intermediate frequency (IF) electronic, and a data acquisition unit (DAU). The RF section is constructed in-house from several RF components, which include voltage-controlled oscillator, high power amplifler, directional coupler, RF switches, band-pass fllter, isolators, and mixers. In RF section, the received signal is mixed with a portion of the transmitted signal to produce low frequency IF signals. The IF signals are pre-processed in IF section, before they are digitised in DAU. A mobile personnel computer with analog to digital converter card is used to store the measurement data and process the data on real time basics. In paper, the high level design will be discussed and detail design parameters will be presented. It followed by radar electronics design, which outlined the detail in radar transmitter and receiver.

Multiple target detection for stepped multiple‐frequency interrupted CW radar

AbstractThis paper proposes a multiple target detection system using stepped multiple frequency ICW (Interrupted Continuous Wave) radar. The proposed system can obtain a short observation time compared to a conventional up/down sweep FMCW (Frequency‐Modulated Continuous Wave) system or a two‐frequency CW system with less computational complexity than a two‐dimensional super‐resolution system by using a new transmission frequency sequence intended for short‐range targets and a single‐dimension super‐resolution system that preprocesses a target velocity detection process through the use of an ordinary Fourier transformation. In addition, the proposed system can obtain a desired target range and velocity resolution with a transmission frequency bandwidth less than a pulse compression system or an FMCW system. The proposed system can also avoid pairing malfunctions which lead to problems with FMCW systems. Range separation of multiple equal‐velocity targets, a principal problem in two‐frequency CW systems, can also be obtained. Computer simulations show that as a radar system, the proposed system is expected to improve the above‐mentioned problems of conventional radar systems at practical input SN values. © 2007 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 90(10): 49–59, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.20388

Using a discrete thermal model to obtain a linear frequency ramping in a frequency-modulated continuous-wave system

The lasing wavelength of a single-section distributed feedback (DFB) laser diode can be modulated by modulating the drive current. This makes it possible to utilize the DFB laser diode in a frequency-modulated continuous wave (FMCW) range and velocity measuring system. In FMCW, the frequency of the laser is ramped, and the frequency difference between the reflected wave and a local-oscillator wave is monitored. For maximum performance the frequency ramping should be linear. Due to thermal phenomena, a linear ramping of the current seldom results in a linear ramping of the optical frequency. We have derived a discrete thermal model, using resistors and capacitors, of our laser module. The thermal model was then used as a starting point to model the frequency behavior of the laser and to derive modulation currents that resulted in a linear frequency ramping at some different modulation frequencies.

Design, fabrication, and application of precise SAW delay lines used in an FMCW radar system

An inexpensive frequency-modulated continuous-wave (FMCW) radar system is presented in this paper, which, nevertheless, meets all industrial requirements. The FMCW radar uses a low-cost nonlinear voltage-controlled oscillator (VCO), operating at an IF of 2.45 GHz to generate the frequency modulation of the radar system. This VCO signal is applied twice, first to generate the radar transmitter signal at 24 GHz, and then it is fed to a surface acoustic wave (SAW) delay line. The SAW delay line generates a fixed delay time, which corresponds with a fixed radar distance. Thus, all systematic nonlinearities and stochastic phase errors of the FMCW system can be monitored and, afterwards, can be compensated for in real time. This linearization technique leads to a significant enhancement in dynamic range for a FMCW radar system. For this FMCW system, SAW delay lines with a linear phase characteristic have been designed using a linear optimization program. The delay line consists of two chirped and weighted interdigital transducers. For high volume, low-cost, and high-yield production of the required SAW structures, with linewidths down to 0.3 /spl mu/m, technological improvements had to be achieved, especially in photolithography. Based on these design and fabrication techniques, delay lines at 2.45 GHz operating at the fundamental and third harmonic with bandwidths up to 800 MHz have been realized.

Multiple-target frequency-modulated continuous-wave ranging by evaluation of the impulse response phase

A microwave frequency-modulated continuous wave (FMCW) radar for precision ranging of multiple targets is described. The evaluation of the FMCW raw data is based on a fast Fourier transform (FFT). In contrast to customary techniques, we additionally evaluate its phase. The phase ambiguity is resolved on the basis of the more approximate envelope evaluation. The FMCW system does not need separate phase measurements, however, the phase information is extracted from the FMCW data. In comparison with conventional FFT techniques the evaluation of "response phase" enhances measurement accuracy or, which is equivalent, allows one to reduce the sweeping bandwidth while maintaining accuracy. The phase processing implies a Hilbert transform, which leads to small phase errors. The algorithm can be extended, such that the phase measurement is essentially error-free. One makes use of the virtual range finder, where measurement data are compared with synthetically prepared data to achieve the best fit.