Frequency Continuous Wave Research Articles

Phase coded waveforms for integrated sensing and communication systems

AbstractNowadays, especially with the developments in the automotive industry, it has become a highly popular topic for radar and communication systems to operate on the same platform and perform joint tasks to increase situational awareness. Sharing the hardware of the radar and communication systems and using a joint waveform for both functions eliminate interference problems between the systems, increasing the effectiveness of the joint task. In studies where radar signals are utilised for communication functions, continuous wave frequency modulation or intra‐pulse frequency modulation is generally employed to transmit communication signals. There are a few studies in the literature on joint radar and communication waveforms using phase modulation. In these studies, phase modulation is not employed directly for both functions. As a contribution to the literature, this study evaluates the usage of joint radar and communications waveforms, such as Barker sequences and Zadoff–Chu sequences as opposed to linear frequency modulation. We compare these waveforms terms of range‐speed detection and symbol error rate.

Comparative Study of GPR Acquisition Methods for Shallow Buried Object Detection

This paper investigates the use of ground-penetrating radar (GPR) technology for detecting shallow buried objects, utilizing an air-coupled stepped frequency continuous wave (SFCW) radar system that operates within a 2 GHz bandwidth starting at 500 MHz. Different GPR data acquisition methods for air-coupled systems are compared, specifically down-looking, side-looking, and circular acquisition strategies, employing the back projection algorithm to provide focusing of the acquired GPR data. Experimental results showed that the GPR can penetrate up to 0.6 m below the surface in a down-looking mode. The developed radar and the back projection focusing algorithm were used to acquire data in the side-looking and circular mode, providing focused images with a resolution of 0.1 m and detecting subsurface objects up to 0.3 m below the surface. The proposed approach transforms B-scans of the GPR-based data into 2D images. The provided approach has significant potential for advancing shallow object detection capabilities by transforming hyperbola-based features into point-like features.

Doppler Factor in the Omega-k Algorithm for Pulsed and Continuous Wave Synthetic Aperture Radar Raw Data Processing

Synthetic aperture radar (SAR) raw data do not have a direct application; therefore, SAR raw signal processing algorithms are used to generate images that are used for various required applications. Currently, there are several algorithms focusing SAR raw data such as the range-Doppler algorithm, Chirp Scaling algorithm, and Omega-k algorithm, with these algorithms being the most used and traditional in SAR raw signal processing. The most prominent algorithm that operates in the frequency domain for focusing SAR raw data obtained by a synthetic aperture radar with large synthetic apertures is the Omega-k algorithm, which operates in the two-dimensional frequency domain; therefore, in this paper, we used the Omega-k algorithm to produce SAR images and modify the Omega-k algorithm by adding the Doppler factor to improve the accuracy of SAR raw data processing obtained by the continuous wave and pulsed frequency modulated linear frequency modulated radar system from the surfaces of interest. On the other hand, for the case of unmanned aerial vehicle-borne linear frequency modulated continuous wave (LFM-CW) SAR systems, we added motion compensation to the modified Omega-k algorithm. Finally, the testing and validation of the developed Omega-k algorithm used simulated and real SAR raw data for both pulsed synthetic aperture and continuous wave radars. The real SAR raw data used for the validation of the modified Omega-k algorithm were the raw data obtained by the micro advanced synthetic aperture radar (MicroASAR) system, which is an LFM-CW synthetic aperture radar installed on board an unmanned aerial system and the raw data obtained by European remote sensing (ERS-2) satellite with a synthetic aperture radar installed.

Directional and High-Gain Ultra-Wideband Bow-Tie Antenna for Ground-Penetrating Radar Applications

Bow-tie antennas are utilized extensively in ground-penetrating radar (GPR) systems. In order to achieve sufficient penetration depth and resolution, the bow-tie antennas for GPR applications require low operating frequency, high gain, and excellent broadband. A novel ultra-wideband (UWB) bow-tie antenna with gain enhancement for GPR applications is proposed in this paper. First, a UWB bow-tie antenna with resistive loading is designed. The metal reflector and metamaterial loading make the bow-tie antenna directional, and loading the same metamaterial on the front side of the antenna further improves directional gain. After testing, the lowest frequency of the fabricated antenna is 317 MHz, the relative bandwidth is 98.6%, the peak gain in the frequency range is 9.3 dBi, and the size is only 0.38 λ at the lowest frequency. The proposed compact antenna takes both gain and bandwidth into consideration. Finally, in order to further verify the effectiveness of the proposed antenna in the GPR system, a stepped frequency continuous wave ground-penetrating radar (SFCW-GPR) system was built. The experimental results show that the designed antenna is suitable for the GPR system of deep penetration and high-resolution detection, which is beneficial to the imaging of underground structures.

An Efficient Full Wave Model for Investigating Layered Media Using Bi-Static Ground Penetrating Radar

Ground penetrating radar (GPR) is a popular non-destructive sensor for subsurface media investigation. Modelling of the GPR system has a significant role in the successful estimation of sub-surface media properties, imaging, and detection of buried object. A model should provide good accuracy and computational efficiency while taking care of numerous GPR scenarios. However, achieving accuracy and speed of computation is a difficult task. In this respect, analytical modellings are promising techniques to represent a specific GPR scenario. In this work, a simplified full wave model (FWM) for bi-static GPR has been developed to represent radar transceivers with layered media. The model has been derived from an existing full wave model (FWM) solution. The accuracy of the proposed model has been verified by comparing it with existing FWMs, and by laboratory experiments based on stepped frequency continuous wave (SFCW) GPR. The corresponding results show that the proposed bi-static model is as accurate as FWM and its computational efficiency is enormous in estimating the sub-surface media properties.

Evaluation Method of Noise Electromagnetic Radiation Interference Effect

In order to evaluate the viability of frequency equipment in complex electromagnetic environment, the electromagnetic radiation effect evaluation model of random noise is established based on the analysis of the blocking effect mechanism of frequency equipment. Through MATLAB calculation, the corresponding relationship table between the ratio of the critical interference field strength effective value of single frequency continuous wave and sinusoidal amplitude modulation wave and the key parameters of the model is given, and the prediction method of noise electromagnetic interference of frequency equipment is proposed. Taking a Beidou navigation receiver as the test object, the accuracy of the noise electromagnetic radiation blocking effect model is verified by single frequency continuous wave, random noise and narrowband noise electromagnetic radiation blocking effect test, and the maximum error is only 1.51 dB, which indicates that the model and prediction method proposed in this article can accurately evaluate and predict the electromagnetic radiation interference of random noise.

Sparse Reconstruction of SFCWSAR Based on an Approximate Observation Operator

The traditional compressed sensing SFCWSAR (Stepped Frequency Continuous Wave Synthetic Aperture Radar) sparse reconstruction algorithm consumes a lot of computer memory and cannot compensate the range migration in the same pulse group. Based on this, this paper proposes a SFCWSAR sparse reconstruction algorithm based on an approximate observation operator. First, the algorithm replaces the accurate observation operator with the approximate observation operator, which greatly reduces the computer memory consumption while the algorithm is running and realizes the compensation of the range migration in the SFCWSAR pulse group. Furthermore, the SFCWSAR sub-band echo data under full sampling conditions are used to modify the important parameter of the Doppler center frequency of the approximate observation operator, which significantly improves the reconstruction accuracy of the scene. The SFCWSAR data show that, compared with the conventional sparse autofocus algorithm, the proposed algorithm takes less memory and can reconstruct scenes with high accuracy.

Signal Calibration and Noise Reduction for Software-Defined Radio-Based GPR

This article studies signal calibration, frequency response estimation, and noise reduction for a software-defined radio (SDRadio)-based ground-penetrating radar (GPR). The radar signal generation utilizes the stepped frequency continuous wave (SFCW) method. The signal calibration takes advantage of the dual-channel structure of the SDRadio and solves the incoherent phase problem caused by phase mismatch between the local oscillators (LOs) in the transmitter and the receiver. The frequency response estimation involves two steps: First, the calibration measures the system’s intrinsic frequency response. Then it measures the frequency response of the target under the ground surface. The noise effect at the receiver is reduced using the ridge regression method. For performance evaluation, outdoor and indoor experiments are conducted, which show that the GPR A-scan waveform and B-scan image qualities can be effectively improved. Compared with the traditional method, the signal-to-clutter ratio (SCR) of the A-scan waveform is improved by around 7 dB and that of the B-scan image is improved by around 13 dB.

A Joint Design of Radar Sensing, Wireless Power Transfer, and Communication Based on Reconfigurable Software Defined Radio

This paper proposes a compact three-mode base station capable of performing radar sensing, communication, and wireless power transfer (WPT) in collaboration with indoor sensor networks. With regard to the wireless sensor node, the base station transmits two-tone signals in the downlink to support its operation and provides two-way communication. The sensor node sends uplink information through backscattering using the third order intermodulation (IM3) product of the rectification. In the radar mode, a single-tone continuous wave (CW) is used to monitor if there is a moving target in the static environment. If a speed is detected, the transmit signal to the node is stopped, while the single-tone CW excitation will continue until the speed of the target is zero, and then the base station transmits a stepped frequency continuous wave (SFCW) signal to measure the distance of the target. The repeat between the two radar waveforms continues until the target is undetectable within the detection range. The software defined radio PlutoSDR is adopted as the base station. The system can wirelessly supply power and bi-directionally communicate with a CO2 sensor node 2 m away. It gives a range resolution of 2.5 cm and a minimum detectable speed of 0.25 m/s in the radar mode.

A Highly Linear Ka-Band GaN-on-Si Active Balanced Mixer for Radar Applications

A highly linear active mixer in Gallium Nitride on Silicon (GaN-on-Si) monolithic microwave integrated circuit (MMIC) technology, operating in Ka-band for radar applications is introduced. It exhibits a local oscillator (LO) balanced topology of gate-pumped mixers based on high-electron mobility transistors (HEMT) and operates in down-conversion mode. Its design principles and the theoretical foundation of its linearity features resulting from both the high power handling capability of GaN-on-Si HEMTs and the corresponding high applied LO level are presented. An extensive characterization based on excitation of a prototype by broadband continuous wave signals and frequency modulated continuous wave signals validates both the analytical treatment of its linearity and the design of the prototype of the mixer. In the 35–40 GHz operating band, this prototype exhibits a state-of-the-art input referred third order intercept point that reaches 39.1 dBm, and a maximum input referred second order intercept point of 39.4 dBm, with a corresponding conversion gain of −5.2 dB, and LO level of 24 dBm.

Application of Continuous Wavelet Transform and Artificial Naural Network for Automatic Radar Signal Recognition.

This article aims to propose an algorithm for the automatic recognition of selected radar signals. The algorithm can find application in areas such as Electronic Warfare (EW), where automatic recognition of the type of intra-pulse modulation or the type of emitter operation mode can aid the decision-making process. The simulations carried out included the analysis of the classification possibilities of linear frequency modulated pulsed waveform (LFMPW), stepped frequency modulated pulsed waveform (SFMPW), phase coded pulsed waveform (PCPW), rectangular pulsed waveforms (RPW), frequency modulated continuous wave (FMCW), continuous wave (CW), Stepped Frequency Continuous Wave SFCW) and Phase Coded Continuous Waveform (PCCW). The algorithm proposed in this paper is based on the use of continuous wavelet transform (CWT) coefficients and higher-order statistics (HOS) in the feature determination of selected signals. The Principal Component Analysis (PCA) method was used for dimensionality reduction. An artificial neural network was then used as a classifier. Simulation studies took into account the presence of noise interference with signal-to-noise ratio (SNR) in the range from −5 to 10 dB. Finally, the obtained classification efficiency is presented in the form of a confusion matrix. The simulation results show a high recognition test accuracy, above 99% with a signal-to-noise ratio greater than 0 dB. The article also deals with the selection of the type and parameters of the wavelet. The authors also point to the problems encountered during the research and examples of how to solve them.

Numerical simulation of nonlinear optical gain modulation in a Raman fiber amplifier.

Nonlinear optical gain modulation (NOGM) in a Raman fiber amplifier is numerically simulated with the generalized nonlinear Schrödinger equation. In the NOGM setup, a single frequency continuous wave seed laser is gain modulated into femtosecond pulses by an ultrafast pump, which can induce strong stimulated Raman scattering in a piece of single mode optical fiber. Different parameters regarding seed, pump and nonlinear gain medium (Raman fiber) are investigated in detail to find the best condition for higher Raman conversion efficiency. Simulated results reveal that the walk-off between pump and Raman pulses due to dispersion is one of the most important factors affecting the NOGM pulse's performance. Only when the speed of walk-off matches with the one of Raman conversion process can the conversion efficiency be optimized. This work offers a guild-line for the design of a fiber-based NOGM laser, which is able to produce wavelength-agile, femtosecond laser pulses with µJ-level pulse energy under more than 85% efficiency.

Polarimetric Chaotic Ground Penetrating Radar for Underground Pipes Detection

A polarimetric chaotic ground penetrating radar (GPR) is proposed to improve the detection accuracy of underground pipes. In this radar system, the chaotic signal is used as the probe signal and the multi-polarization detection mode is employed to gather the GPR data. Furthermore, the laplacian pyramid algorithm is applied to fuse multi-polarization components. The simulations demonstrate that by using the polarization chaotic GPR, the pipe responses are enhanced and their shape and distribution can be accurately obtained. Here, the information entropy, average gradient, spatial frequency, and standard deviation are introduced to quantitatively evaluate the imaging results. The corresponding experiment is consistent with the simulation. Additionally, to verify the performance of the polarimetric chaotic GPR, we compare the polarimetric chaotic GPR with the polarimetric stepped frequency continuous wave GPR. The experimental results show that the proposed radar improves the detection accuracy and enlarges the detection range.

Sensitivity Analysis of a 280–312 GHz Superheterodyne Terahertz Link Targeting IEEE802.15.3d Applications

This article presents the sensitivity analysis of a superheterodyne terahertz link specifically designed for applications using the IEEE802.15.3d standard. With its wide frequency range for LO input signals in conjunction with an IF frequency range spanning over the full <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> -band, frequency-domain multiplexing of RF channels in the frequency range between 285 and 312 GHz is implemented in line with the frequency standard. In view of determining optimum operating conditions and performance limitations of the MMIC-based THz transmitter and receiver the link is characterized by a back-to-back operation of the transmitter and receiver modules using WR-3.4 waveguide interconnects. Custom <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> -band frequency extensions provide the intermediate frequency continuous wave or modulated signals with data rates of up to 32 Gb/s using 32-QAM modulation. The linearity of the presented THz link enables the use of modulation schemes as high as 256-QAM with reduced bandwidths.

Characterization of thermal boundary resistance at solid–liquid interface based on continuous wave frequency domain thermal reflection method

Characterization of thermal boundary resistance at solid–liquid interface based on continuous wave frequency domain thermal reflection method

Research on Dual-Frequency Electromagnetic False Alarm Interference Effect of a Typical Radar

In order to master the position variation rule of radar false alarm signal under continuous wave (CW) electromagnetic interference and reveal the mechanism of CW on radar, taking a certain type of stepping frequency radar as the research object, theoretical analysis of the imaging mechanism of radar CW electromagnetic interference false alarm signals from the perspective of time-frequency decoupling and receiver signal processing. Secondly, electromagnetic interference injection method is used to test the single-frequency and dual-frequency electromagnetic interference effect of the tested equipment. The results show that under the single frequency CW electromagnetic interference, the sensitive bandwidth of false alarm signal is about ±75 MHz, and the position of false alarm signal irregularity changes. Under the in-band dual-frequency CW electromagnetic interference, the position of non-intermodulation false alarm signal is similar to that of single frequency. However, the distance difference of two non-intermodulation false alarm signals is regular. In addition, the positions of the second-order intermodulation false alarm signals of the tested radar are also regular, and its position changes with the change of the second-order intermodulation frequency difference.

Deep reinforcement learning for cognitive active-sonar employment

We introduce a framework to leverage deep reinforcement learning (RL) for active sonar employment, wherein we train an RL agent to select waveform parameters, which maximize the probability of single-target detection. We first simulate raw sonar returns of targets and clutter in reverberation and noise using a physics-based sonar-simulation model, the Sonar Simulation Toolkit (SST), then process the resulting signatures into network inputs via an in-house signal and information processing model of an archetypal antisubmarine warfare (ASW) processing chain. We demonstrate that the trained RL agent is able to appropriately select between continuous wave (CW) and hyperbolic frequency modulated (HFM) waveforms depending on target trajectory, as well as select an optimal bandwidth and pulse length trade-off (when constrained by a constant time-bandwidth product), when presented with sonar returns from a reverb-limited or noise-limited environment.

Silent speech command word recognition using stepped frequency continuous wave radar

Recovering speech in the absence of the acoustic speech signal itself, i.e., silent speech, holds great potential for restoring or enhancing oral communication in those who lost it. Radar is a relatively unexplored silent speech sensing modality, even though it has the advantage of being fully non-invasive. We therefore built a custom stepped frequency continuous wave radar hardware to measure the changes in the transmission spectra during speech between three antennas, located on both cheeks and the chin with a measurement update rate of 100 Hz. We then recorded a command word corpus of 40 phonetically balanced, two-syllable German words and the German digits zero to nine for two individual speakers and evaluated both the speaker-dependent multi-session and inter-session recognition accuracies on this 50-word corpus using a bidirectional long-short term memory network. We obtained recognition accuracies of 99.17% and 88.87% for the speaker-dependent multi-session and inter-session accuracy, respectively. These results show that the transmission spectra are very well suited to discriminate individual words from one another, even across different sessions, which is one of the key challenges for fully non-invasive silent speech interfaces.

Ground penetrating radar inspection of a large concrete spillway: A case-study using SFCW GPR at a hydroelectric dam

Hydroelectric dams are subject to aging and degradation. The concrete structure under investigation here had visible signs of spalling, cracking, and erosion, but it was unclear how damaged the spillway was below the surface. The asset owners wanted to identify delamination and voids 60 cm or larger so they could mitigate the effects. While most ground penetrating radar (GPR) inspections conducted on dams have been completedusing low frequency ranges between 16 MHz and 500 MHz, these frequencies may overlook small, but critical defects. Our study is the first to use a small hand-held sized stepped frequency continuous wave (SFCW) GPR to acquire information at frequencies ranging up to 4 GHz to evaluate its effectiveness on such a large asset. The two main questions we aimed to answer are: (1) how will a small GPR unit perform on a large, irregular concrete surface, and (2) could the higher frequencies provide more detailed images of small cracks and defects. The study provided useful results in real-time which supported a repair and refurbishment program. The high frequency system allowed for detailed visualization and measurement of reflections events consistent with expectations for delamination and voids. The investigation also identified other non-critical defects that can be targeted for ongoing monitoring programs.

Clutter removal in millimeter wave GB-SAR images using OTSU’s thresholding method

In this study, a filtering method based on the threshold value of normalized synthetic aperture radar (SAR) data is proposed to eliminate clutter in millimeter wave ground based synthetic aperture radar (GB-SAR) images. In the proposed method, first, stepped frequency continuous wave SAR data are reconstructed by using the back-projection algorithm and focused complex SAR data are obtained. Then, the amplitude values of the complex SAR data are normalized and the best threshold values to distinguish the target from clutter is determined by the OTSU’s thresholding method. Next, a filter mask is created that cancels all data below the computed threshold values. The mask matrix is finally multiplied with the resulted GB-SAR data to eliminate all clutter from the image. With the proposed technique, the best threshold value is determined automatically by directly processing the raw data without converting the SAR data into any RGB images. The proposed technique is validated through real GB-SAR experiments that were carried out in the frequency band of 78-81 GHz. In the experiments, challenging GB-SAR data are obtained using high cluttered background materials, and very successful filtering operations are performed with the proposed technique.