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The corner reflector is an effective means of interference for radar seekers due to its high jamming intensity, wide frequency band, and combat effectiveness ratio. Properly arranging multiple corner reflectors in an array can form dilution jamming that resembles ships, substantially enhancing the interference effect. This results in a significant decline in the precision attack efficiency of radar seekers. Hence, it is critical to accurately identify corner reflector array. The common recognition methods involve extracting features on the high-resolution range profile (HRRP) and polarization domain. However, the former is constrained by the number of corner reflectors, while the latter is affected by the accuracy of polarization measurement, both of which have limited performance on the identification of corner reflector array. In terms of the evident variations in physical structures, there must be differences in their scattering characteristics. To highlight the differences, this paper proposes a new method based on the concept of mismatched filtering, which involves changing the frequency modulation slope of the chirp signal in the filter. Then, the variance of width and intervals within a specific scope are extracted as features to characterize these differences, and an identification process is designed in combination with the support vector machine. The simulation experiments demonstrate that the proposed method exhibits stable discriminative performance and can effectively combat dilution jamming. Its accuracy rate exceeds 0.86 when the signal-to-noise ratio is greater than 0 dB. Compared to the HRRP methods, the recognition accuracy of the proposed algorithm improves 15% in relation to variations in the quantity of corner reflectors. [ABSTRACT FROM AUTHOR]

This paper proposes a Long Range (LoRa) chirp transmission scheme with multiple transmit and receive antennas. The main goal is to increase the data rate of LoRa using multiple-input multipleoutput (MIMO) spatial multiplexing. Several coherent detectors are proposed and compared in terms of performance, assuming a channel with flat Rayleigh fading and additive white Gaussian noise. By leveraging on a convenient matrix-vector model, we show that the maximum-likelihood (ML) detector can be obtained with low complexity, when the number of transmit antennas is two or three. When the number of transmit antennas is four or five, we propose a transmission scheme that permits a near- ML detection with reduced complexity. We also propose linear and widely linear detectors that exploit the signal sparsity in the chirp domain. Simulation results confirm the effectiveness of the proposed MIMO LoRa transmission schemes and detectors. Simulated results also include the effect of imperfect synchronization and channel estimation errors on the proposed coherent detection.

Using physically separated multiple-input multiple-output (MIMO) systems for millimeter-wave measurement systems based on linear frequency chirps poses unique challenges for generating a modulated reference chirp to apply high coherence. The reference frequency chirp is crucial for the measurement accuracy of the overall system and should feature high bandwidth, low phase noise, and high linearity. For this reason, we present a novel architecture combining a fixed-integer phase-locked loop (PLL) with a fast-modulated frequency divider. Thus, modulated output frequencies of up to 2 GHz with an adjustable bandwidth of up to 1.75 GHz are achieved while maintaining low phase noise of −140 dBc/Hz at 1 MHz from the carrier at the center frequency. Synchronous programming and modulation of the fractional frequency divider is done by a new type of control utilizing fast transceivers in a field-programmable gate array (FPGA), which does not require back-synchronization to the frequency divider. Measurements with the novel reference frequency chirp generator combined with a V-band PLL reveal a low RMS linearity error of 0.67ppm of the reference chirp for a chirp duration of 1 ms and a bandwidth of 363 MHz.

A new on‐chip diagnostic tool is developed, allowing for real‐time full characterization of waveguided ultrashort optical pulses. This technique, called transverse frequency‐resolved optical gating (T‐FROG), relies on second harmonic generation (SHG) in a transverse (surface emitting) geometry. The T‐FROG is implemented on a thin‐film lithium niobate (LN) platform, demonstrating its versatility and consistency in accurately characterizing waveguided femtosecond pulses, including information about chirp and self‐phase modulation. In contrast to traditional FROG techniques, T‐FROG represents a significant improvement as it provides temporal amplitude and phase profiles of ultrafast optical pulses directly inside photonic integrated circuits. The real‐time in situ characteristics and dynamics of optical pulses offered by T‐FROG show promise for their potential applications in the design, testing, and optimization of ultrafast photonic integrated circuits. [ABSTRACT FROM AUTHOR]

This paper presents the design and implementation of a C-Band Frequency Generator developed for Space-borne Synthetic Aperture Radar. This Frequency Generator subsystem generates stable and coherent reference signals for all the sub-systems of C-Band Synthetic Aperture Radar payload. Frequency Generator based on frequency multiplication technique generates various coherent signals namely 500 MHz signal for digital clock, local oscillator (LO) signals of 900 MHz and 4500 MHz needed for receivers and chirp signal of 5400 ± 37.5 MHz. This chirp signal is generated by direct modulation of the full bandwidth baseband signal of DC-37.5 MHz at 4500 MHz and subsequently mixing with 900 MHz signal. Frequency generator unit is realized in a compact two-tier architecture, using a novel concept of full chirp modulation, resulting in 6◦ rms phase error in the transmit chirp signal along with in-band spurious rejection better than 20 dBc, whereas other coherent frequencies resulting in out of band spurious rejection better than 53 dBc against the specification of 40 dBc. [ABSTRACT FROM AUTHOR]

Spread spectrum techniques, such as the Chirp Spread Spectrum (CSS) used by LoRa technology, are important for machine-to-machine communication in the context of the Internet of Things. They offer high processing gain, reliable communication over long ranges, robustness to interference and noise in harsh environments, etc. However, these features are compromised by their poor spectral efficiency, resulting in a very low data transmission rate. This paper deals with a spectrally efficient variant of CSS. The system uses M-ary phase keying to modulate the data and exploits CSS's properties to transmit the modulated symbols as overlapping chirps. The overlapping of chirp signals may affect the system performance due to inter-symbol interference. Therefore, we analyse the relationship between the number of overlaps and the effect of inter-symbol interference (ISI), and we also determine the BER expression as a function of the number of overlaps. Finally, we derive the optimal number of overlapping symbols that corresponds to the minimum error probability. [ABSTRACT FROM AUTHOR]

Chirp signals are currently widely used in broadband and spread spectrum communications due to their advantageous features, such as immunity to fading noise, low power consumption, consistent long-range transmission, and constant bandwidth. As a result, they are applied at the physical layer of the Internet-of-Things (IoT). This study proposes two techniques for encoding and decoding 4-cyclic shift chirp symbols, based on addition and subtraction operations. The proposed techniques have simple structures that can be easily implemented using analog circuits. The proposed encoding techniques reveal obviously the relationship between cyclic-shift chirp symbols and pulse modulating signals (PWM, PPM, and PAM), which is rarely discussed in prior research. Moreover, the circuit for encoding and decoding of the proposed technique is implemented by discrete commercial devices at low frequency (25-35 kHz) which is suitable for sonar and communication under water; however this proposed technique is not limited to only low frequency but is also capable of being used in high frequency band as well which experimental and simulation result show agreeing well with each other. [ABSTRACT FROM AUTHOR]

This work presents a new technique based on modulating the IR absorbance of each substance in a mixture in a chirped manner to reduce the effect of their partial spectral absorption overlap on the accuracy of determining their concentrations. This chirped spectral modulation CSM algorithm can deal with mixtures containing unknown substances rather than the substances whose concentrations are aimed. This novel algorithm, when compared to existing pattern recognition techniques, makes it easy to analyze the constituents of a mixture with high accuracy in the presence of traces of unknown components. It is found that the new algorithm can detect the presence of gas pollutants such as sulfur dioxide, carbon monoxide, carbon dioxide, nitrogen dioxide in a sample containing many other unknown polluting substances. This new algorithm is tested on air samples composed of predetermined percentages of air constituents and the results of calculations are compared with those of classical least squares CLS pattern recognition algorithm. The comparison showed that the new algorithm can detect down to very small traces of harmful gases such as NO2, and SO2, at least one order of magnitude less than those detected by the CLS approach. Finally, the new algorithm is used to examine collected air samples from an industrial zone, and in the middle and at the exit of a road tunnel in Riyadh area which showed that the percentages of sulfur dioxide, nitrogen dioxide, and carbon monoxide are well below the safe levels. [ABSTRACT FROM AUTHOR]

Generally, synthetic aperture radar(SAR)generates linear frequency modulation (LFM) signals by method of look‐up table (LUT), which requires calculating different LFM signals and storing these waveforms in advance. With the demand for multi‐mode, high‐resolution and wide swath of airborne SAR systems, the method of LUT is not available, because that requires huge storage resources. In order to solve these drawbacks in limited hardware resources, this letter proposes a novel LFM signal generator in real‐time processing. By taking advantage of high level synthesis (HLS) on FPGA, the proposed generator calculates the LFM signal simply using parameters of pulse width, bandwidth, and sampling frequency. Meanwhile, in order to compensate for the SAR system phase error, the proposed generator calculates phase error using a linear polynomial algorithm, which can compensate for the phase error in real time. The simulation results prove that this approximation can achieve a good effect. The performance of this method is implemented by experiments with an X‐band airborne SAR system. [ABSTRACT FROM AUTHOR]

The paper introduces a neuromorphic computational approach for breathing rate monitoring of a single person observed using a Frequency-Modulated Continuous Wave radar. The architecture, aimed at implementation in analog hardware to ensure high energy efficiency and to provide system operation longevity, comprises two main functional modules. The first one is a data preprocessing unit aimed at the extraction of information relevant to the analysis objective, whereas the second one is a pre-trained recurrent neural regressor, which analyzes sequences of incoming samples and estimates the breathing rate. To ensure compatibility with neural processing and to achieve simplicity of underlying resources, several solutions were proposed for the data preprocessing module, which provides range-wise space segmentation, selection of a bin of interest (comprising the dominant motion activity), and delivery of data to regressor inputs. To implement these functions, we introduce an appropriate chirp frequency modulation scheme, apply a neuromorphic filtering procedure and use a Winner-Takes-All network for extracting information from the bin of interest. The architecture has been experimentally verified using a dataset of indoor recordings supplied with reference data from a Zephyr BioHarness device. We show that the proposed architecture is capable of making correct breathing rate estimates while being feasible for analog implementation. The mean squared regression error with respect to the Zephyr-produced reference values is approximately 3.3 breaths per minute (with a deviation of ±0:27 in the 95% confidence interval) and the estimates are produced by a recurrent, GRU-based neural regressor, with a total of only 147 parameters. [ABSTRACT FROM AUTHOR]

Chirp sequence has been adopted in automotive applications for its simple generation and exible integration within radar-centric systems. Besides, recent studies have shown its ability to carry data between communicating vehicles in the surroundings. Since the parameters adopted from current automotive radar sensors can differ at the transmitter side dependent on the automotive supplier, the carrier alignment of the communication receiver of one of the communicated nodes might not concur with the one in the transmitter. This paper presents a novel two-stage synchronization method for communication-assisted chirp sequence (CaCS) signals. The proposed synchronization method applies a sequence of up- and down-chirp as a preamble to estimate frequency and time offsets during the transmission. The suggested synchronization scheme supports partial chirp modulation systems and can be adapted for similar radar-centric systems that employ chirp modulation. The former stage performs a coarse synchronization, reallocates the receive carrier frequency, and corrects eventual time offsets between the communication receiver from one CaCS-node and the transmitter of another node. The carrier allocation at the communication receiver side is based on a combination of spectrum sensing via short-time Fourier transforms and image processing to estimate the transmitting signal pattern (slope, frequency offset, and delay). The latter stage, in its turn, relies on range-Doppler estimation to perform a fine correction of time and frequency offsets and compensates residual offsets of the coarse synchronization stage. Furthermore, the paper analyzes the case of a multi-user scenario with mutual interference between the signals that affects the synchronization and communication data detection. Besides, measurements are provided based on two completely unsynchronized software-defined radios to validate the proposed method. The study also illustrates the in uence of the signal-to-noise ratio on the proposed method and verifies it with simulations in MATLAB. As a result, the offsets at the investigated CaCS-node are returned to recover the transmitted data correctly. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Harbin Institute of Technology. Social Sciences Edition / Haerbin Gongye Daxue Xuebao. Shehui Kexue Ban is the property of Harbin Institute of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Accurate measurement of flue gas velocity is of great significance for the safe production, energy saving, and emission reduction of coal-fired power plants. In this paper, a method of flue gas velocimetry based on a hyperbolic chirp-modulating acoustic source was proposed, and its superiority was verified by numerical simulations and field experiments. For an acoustic velocimeter, it is crucial to ensure the accuracy of the acoustic Time-of-Flight (TOF), which directly affects the flue gas velocity measurements. The improvement of the accuracy of estimation of the TOF directly determines the accuracy of the flue gas velocity measurement. Simulation on the different types of acoustic sources at different signal-to-noise ratios (SNRs) was conducted, and the results proved that the hyperbolic chirp signal is more accurate in the measurement of TOF. In the strong noise range of − 10 dB to − 20 dB, the average relative root mean square error (RRMSE) obtained using this hyperbolic chirp modulating acoustic source is 0.60 dB and 0.36 dB lower than obtained using the linear and logarithmic modulating modes, respectively. The average relative error (RE) is approximately 0.87 times that of the linear chirp. Power plant flue field experiments show that the measurement error using this hyperbolic modulating mode is about 0.52 times and 0.74 times that of using linear and logarithmic modulating modes, respectively. In harsh measurement environments with strong noise, the proposed acoustic source modulation scheme can significantly improve the measurement SNR and effectively enhance the accuracy and robustness of flue gas velocimetry. [Display omitted] • The proposed method is a flue gas velocimetry method based on a hyperbolic chirp modulating acoustic source. • This method can be applied to the non-contact measurement of velocity of gaseous media in coal-fired power plants. • The proposed method is able to iomprove the estimation accuracy of acoustic TOF. • The proposed method offers better noise resistance, and reduced attenuation. [ABSTRACT FROM AUTHOR]

Envisaged network scaling in the beyond 5G and 6G era makes the optical transport of high bandwidth radio signals a critical aspect for future radio access networks (RANs), while the move toward wireless transmission in millimeter-wave (mm-wave) and terahertz (THz) environments is pushing a departure from the currently deployed orthogonal frequency division multiplexing (OFDM) modulation scheme. In this work, the orthogonal chirp-division multiplexing (OCDM) waveform is experimentally deployed in a converged optical/mm-wave transmission system comprising 10 km analog radio-over-fiber (A-RoF) transmission, remote mm-wave generation and 2 m wireless transmission at 60 GHz. System performance is evaluated in terms of both bit error ratio (BER) and error vector magnitude (EVM) for a wideband 4 GHz 16 Gb/s signal and 128/256-Quadrature Amplitude Modulation (QAM) mobile signals compatible with 5G new radio numerology. OCDM is shown to outperform OFDM by offering enhanced robustness to channel frequency selectivity, enabling performances below the forward error correction (FEC) limit in all cases and exhibiting an EVM as low as 3.4% in the case of the mobile signal transmission.

Long-range (LoRa) communication has attracted much attention recently due to its utility for many Internet of Things applications. However, one of the key problems of LoRa technology is that it is vulnerable to noise/interference due to the use of only up-chirp signals during modulation. In this paper, to solve this problem, unlike the conventional LoRa modulation scheme, we propose a modulation scheme for LoRa communication based on joint up- and down-chirps. A fast Fourier transform (FFT)-based demodulation scheme is devised to detect modulated symbols. To further improve the demodulation performance, a hybrid demodulation scheme, comprised of FFT- and correlation-based demodulation, is also proposed. The performance of the proposed scheme is evaluated through extensive simulation results. Compared to the conventional LoRa modulation scheme, we show that the proposed scheme exhibits over 3 dB performance gain at a bit error rate of 10−4.

The development of Low-Power Wide-Area Networks is challenging in urban areas due to the terrain elevation changes, clutter losses or dense vegetation regions that attenuate the radio signals. To provide accurate coverage estimation, signal propagation models that integrate losses caused by reflections or attenuations should be used. In this study, we analyze two radio propagation models used for different urban environment configurations, Longley-Rice and ITU-R, to perform a coverage estimation of a LoRa communication network for large-scale deployments. According to our analysis results, validated by measurements, the Longley-Rice and ITU-R radio propagation models are suitable for an urban environment as they use vegetation path losses and can be adapted according to LoRa modulation requirements. Those propagation models are adjusted for real urban field measurements achieved from a point-to-point communication. The obtained results focus also on coverage optimization of a locally deployed LoRa network, considering the best gateway location for the optimum coverage. Thus, a low-cost deployment of the entire network is ensured by reducing the number of installed gateways.

To complete the sparse sampling of linear frequency modulation (LFM) signals and estimate the parameters, the authors proposed an improved finite rate of innovation (FRI) system to achieve the goal. First the authors build a mathematical model based on the Z‐transform, then they input the LFM signal into the modified FRI system and process the output of the system based on this mathematical model, which enables the estimation of the parameters of the LFM signal using only four sampling points. The error of the final experimental results can be controlled within 3%, which greatly improves the efficiency of LFM signal parameter estimation. [ABSTRACT FROM AUTHOR]

A new technique for obtaining switched wave packets using spectrally truncated chirped laser pulses is demonstrated experimentally and numerically by one-dimensional alignment of both linear and asymmetric top molecules. Using a simple long-pass transmission filter, a pulse with a slow turn-on and a rapid turn-off is produced. The degree of alignment, characterized by ⟨cos2θ2D⟩, rises along with the pulse intensity and reaches a maximum at the peak of the pulse. After truncation, ⟨cos2θ2D⟩ drops sharply but exhibits pronounced half and full revivals. The experimental alignment dynamics trace agrees very well with a numerically calculated trace based on the solution of the time-dependent Schrödinger equation. However, the extended periods of field-free alignment of asymmetric tops following pulse truncation reported previously are not reproduced in our work. [ABSTRACT FROM AUTHOR]

We study some fundamental properties of the special affine Fourier transform (SAFT) in connection with the Fourier analysis and time-frequency analysis. We introduce the modulation space M A r , s in connection with SAFT and prove that if a bounded linear operator between new modulation spaces commutes with A-translation, then it is a A-convolution operator. We also establish Hörmander multiplier theorem and Littlewood-Paley theorem associated with the SAFT. [ABSTRACT FROM AUTHOR]

Peculiarities of light absorption and localization in one-dimensional chirped photonic crystals (PC) are analyzed. It is demonstrated that the integral over a broad spectral interval energy density of light remains nearly constant despite the fact that chirping causes the PC to lose an ideal periodicity. It is also demonstrated that violation of the ideal periodic structure (chirping) can not only reduce integral absorption in a wide spectral interval but also lead to an increase in such absorption in a PC depending on the form of chirping and modulation of the imaginary part of the dielectric permittivity. Analysis of the evolution of the absorption spectra caused by changes in the chirping contrast revealed that transmission minibands characterized by substantial absorption appear within the photonic band gap at certain values of the chirping contrast. [ABSTRACT FROM AUTHOR]

Abstract Here, we address the problem of estimation of the parameters of a harmonic chirp model, often encountered in speech and music applications. This model was introduced recently by Christensen and Jensen [1] as an extension of a standard harmonic model. We propose two methods of estimation: the least squares estimation method and the approximate least squares estimation method. We establish the asymptotic properties of the least squares estimators as well as the approximate least squares estimators of the parameters of this model under the assumption of stationary errors. These asymptotic properties are proved analytically as well as corroborated through simulation experiments. We present two speech signal data sets and their analysis using both the estimation methods. The results show that the proposed methods perform reasonably well for estimating the unknown model parameters.

In terms of low power consumption and long-range communication—low-power wide-area networks (LPWAN) are suitable for wireless sensor networks. Long-range (LoRa) wireless communication is one of the standards of LPWAN. LoRa shares common frequency spectrum bands with both multiple transmitters, which are the sensors in the LoRa system (and those in the other system). Therefore, co-channel interference (CCI) degrades the packet delivery rate. To avoid CCI, the CCI power and the occurrence probability of CCI in the target channel are estimated, then the sensor decides whether to use the channel and where the occurrence probability of CCI is defined as the channel occupancy ratio (COR). If a large signal power is obtained at the receiver, the received signal can be demodulated because of the capture effect. The desired signal power must also be estimated for the capture effect. In this study, we propose an estimation scheme based on chirp modulation of LoRa under spectrum sharing among other systems. The proposed scheme estimates the desired signal power, CCI power, and COR. From the computer simulation results, we clarify the advantages of the proposed scheme in terms of estimation accuracy and packet delivery rate. [ABSTRACT FROM AUTHOR]

Abstract This paper demonstrates a photonic‐based wideband TTD beamforming will demonstrate network employing super‐Gaussian apodized chirped fiber Bragg grating (SGFBG) of different lengths (each having central wavelength of 1550 nm) such as 5, 4.5, 4, 3.5, 3, 2.5, 2, 1.5 cm as well as varying chirp rate, which can be used as variable TTD lines for controlling the radiation angle of PAA (the main lobe radiated by the PAA can be steered squint‐free between 0° and ± 69°) suitable for continuous beamforming at microwave frequencies in Ku‐band (12‐18 GHz). The experimental demonstration shows the effect of change in laser wavelength on delay incurred within the reflected light from the SGFBG. This delay in modulated optical signal will be converted into delayed microwave signal feed to respective element of PAA after passing through photodetector. Finally, the time‐delayed microwave signal feeded to each element of PAA will be responsible for tilt in microwave signal compared to the horizontal direction. Novelty of the proposed SGFBG as compared to linearly chirped fiber Bragg grating (LCFBG) is in terms of reduction in ripples in delay spectrum which will lead to exact delay introduction in the optical signal.

A novel feature, based on the chirp z-transform, that offers an improved representation of the underlying true spectrum is proposed. This feature, the chirp MFCC, is derived by computing the Mel frequency cepstral coefficients from the chirp magnitude spectrum, instead of the Fourier transform magnitude spectrum. The theoretical foundations for the proposal, and the experimental validation using product of likelihood Gaussians, to show the improved class separation offered by the proposed chirp MFCC, when compared with basic MFCC are discussed. Further, real world evaluation of the feature is performed using three diverse tasks, namely, speech–music classification, speaker identification, and speech commands recognition. It is shown in all three tasks that the proposed chirp MFCC offers considerable improvements. • The true spectrum of the signal is desired. • Chirp spectrum can offer additional insight into the true spectrum. • Chirp MFCC is compared with basic MFCC. • Chirp MFCC offers better class separation in likelihood space. • Chirp MFCC performs better for speech and audio applications. [ABSTRACT FROM AUTHOR]

We investigate stimulated Brillouin scattering (SBS) suppressing capability of a piecewise parabolic waveform phase modulated fiber system by means of localized nonfluctuating source model. The optical spectra of piecewise parabolic phase modulated laser field are derived analytically. We prove that the parabolic phase modulated laser spectra have a discrete top-hat envelope, and the spectral linewidth can be changed by the product of modulation depth and chirp rate while the spectral line separation remains constant. Theoretically, we calculated the Brillouin gain spectra of piecewise parabolic phase modulation to visually characterize the SBS suppression capability. We define $L_{ch}$ as the characteristic fiber length. It is found that the Brillouin gain spectra broadened as fiber length $L$ increases for the case of $L < L_{ch}$ , while reach the widest linewidth and keep constant for the case of $L\geq L_{ch}$. We establish a piecewise parabolic phase modulation experimental setup to verify our simulation results. According to our model and experiments, we also made predictions for SBS suppression of GHz-level fiber system with several kilowatts output power and $\sim 10$ m fiber length. [ABSTRACT FROM AUTHOR]

Ocean of Things, consisting of multiple buoys distributed on the sea, is an acoustic radio cooperative wireless network that aims to acquire underwater information. In this paper, a deep neural network (DNN)-based receiver with data augmentation, termed chirp (C)-DNN, is developed for a buoy that uses chirp modulation-based underwater acoustic communications. To further solve the problem that the training data at a single buoy may not be sufficient, a federated meta-learning (FML) scheme is proposed to train the DNN-based receiver by exploiting the model parameters from multiple buoys. We analyze the convergence performance of FML and derive a closed-form expression for the convergence rate, accounting for the impacts of scheduling ratios, local epochs, and data volumes on a single node. Simulation results show that the proposed C-DNN receiver that is trained with sufficient data achieves better bit error rate performance and lower complexity than classical matched filter (MF)-based detectors. The proposed FML also outperforms the MF-based method after several communication rounds and achieves better generalization performance than federated learning based systems. [ABSTRACT FROM AUTHOR]

Photonic generation of a bandwidth multiplied linearly chirped signal with phase modulation capability is proposed and its applications in communication and radar systems are demonstrated. In the generator, a dual-polarization quadrature phase shift keying modulator is used to simultaneously perform bandwidth multiplication and phase modulation. The generator can be functionally expanded to a transceiver for wireless communication, which can be used to perform optical generation and coherent demodulation of a phase modulated linearly chirped signal. Meanwhile, the generator can be used to increase the radar detection distance by improving the signal time bandwidth product. The signal generation and application are verified by simulation. A linearly chirped signal with an instantaneous frequency of 8–12 GHz or 16–24 GHz is obtained by using a 4–6 GHz electrical driving signal. The proposed transceiver is demonstrated to perform 156.25-bit/s wireless communication. When the emitting power is 11 dBm and the communication distance is 1 km, the bit error rate can be better than 10−4. While for radar detection, the peak to sidelobe ratio of the compressed pulse is increased from 1.1 dB to 9.2 dB, through which the distance improvement can be verified.

Low earth orbit (LEO) constellations for enhancing global navigation satellite systems (GNSS) have been investigated and studied due to their higher floor power and rapid geometric changes. Compared with GNSS based on medium orbit (MEO) and geostationary (GEO) satellites, the higher Doppler frequency is one of the problems faced by the LEO enhanced GNSS. For the traditional BPSK signal, the complexity has been greatly improved in acquisition, especially in the cold start state, which means more requests on the signal design. The periodic chirp signal modulated by the pseudo‐noise code (chirp‐PC) is proposed as the ranging signal of LEO enhanced GNSS. The coupling characteristic of Doppler and delay in chirp‐PC can make the search frequency grid less than the traditional BPSK signal with the same acquisition performance, which can greatly reduce the complexity. However, the delay estimation obtained by the acquisition is biased. A tracking loop based on the chirp phase locked loop (CPLL) was proposed for removing the coupling of Doppler and time delay. The simulations show that the tracking loop can track chirp‐PC signals stably and unbiasedly, and the tracking accuracy is similar to that of the corresponding BPSK signal. Therefore, the chirp‐PC signal is a new choice for LEO enhanced GNSS with low acquisition complexity and equivalent tracking accuracy to BPSK. [ABSTRACT FROM AUTHOR]

Based on the Richards–Wolf vectorial diffraction theory, we investigate the tight-focusing properties of the chirp modulation linearly polarized circular Airy Gaussian vortex beams (C-LPCAiGVB) through high NA objective lens. Numerical results demonstrate that by setting different beam parameters, we can adjust the degree of freedom and beam width of C-LPCAiGVB vector light focus (including shape and depth of focus, etc.), and the light intensity distribution will exhibit nontrivial properties with different positions of the vortex pairs. In particular, we can control the focus of the beam by introducing the second-order chirp, which provides another way to manipulate the degree of freedom of the beam. Our results provide a new technical reference for optical trapping and manipulation and optical imaging.

To interpret single-shot measurements of rotational revival patterns in molecular gases excited by an ultrashort laser pulse, an analytical description of the probe pulse modulation by the impulsively excited medium is developed. A femtosecond pump laser pulse prepares a rotational wavepacket in a gas-phase sample, and the resulting periodic revivals are mapped into the frequency domain by using a substantially chirped continuum probe pulse. Since the standard approximate descriptions of probe pulse propagation are inapplicable (such as the slowly varying envelope approximation and the slowly evolving wave approximation), we propose an approach capable of incorporating both the substantial chirp of the pulse and the temporal dispersion of the medium response. Theory is presented for the case where the frequency change of the probe during the probe pulse duration is comparable with the carrier frequency. Analytical expressions are obtained for the probe signal modulation over the pump-probe interaction region and for the resulting heterodyned transient birefringence spectra. The approach is illustrated using the case of nitrogen gas. [ABSTRACT FROM AUTHOR]

Here, we address the problem of estimation of the parameters of a harmonic chirp model, often encountered in speech and music applications. This model was introduced recently by Christensen and Jensen [1] as an extension of a standard harmonic model. We propose two methods of estimation: the least squares estimation method and the approximate least squares estimation method. We establish the asymptotic properties of the least squares estimators as well as the approximate least squares estimators of the parameters of this model under the assumption of stationary errors. These asymptotic properties are proved analytically as well as corroborated through simulation experiments. We present two speech signal data sets and their analysis using both the estimation methods. The results show that the proposed methods perform reasonably well for estimating the unknown model parameters. [ABSTRACT FROM AUTHOR]

In this paper, we investigate multi user chirp spread spectrum with noncoherent detection as a continuation of our work on coherent detection. We derive the analytical bit error ratio (BER) expression for binary chirp spread spectrum (BCSS) in the presence of multiple access interference (MAI) caused by correlation with other user signals because of either asynchronism or Doppler shifts, or both, and validate with simulations. To achieve this we analyze the signal cross correlations, and compare traditional linear chirps with our recently-proposed nonlinear chirps introduced and with other nonlinear chirps from the literature. In doing so we illustrate the superior performance of our new nonlinear chirp designs in these practical conditions, for the noncoherent counterpart. [ABSTRACT FROM AUTHOR]

In ISAR imaging, echo signals of complex manoeuvring targets have to be modelled as multi‐component cubic phase signals (m‐CPS). In this condition, the Doppler diffusion will seriously affect the quality of ISAR imaging. Herein, a complex manoeuvring target imaging method based on local polynomial transform‐fast chirp Fourier transform (LPT‐FCFT) is proposed. In this method, the instantaneous frequency and the quadratic chirp rate of the echo signal are estimated by local polynomial transformation, then the chirp rate and the amplitude are estimated by fast chirp Fourier transform, and finally the high‐resolution ISAR image of the complex manoeuvring target is obtained by CLEAN technology. Compared with the traditional method, the proposed method has the advantages of less computational complexity, clearer imaging results and better anti‐noise performance. Simulation results indicate the effectiveness and superiority of the proposed method. [ABSTRACT FROM AUTHOR]

The ever-expanding growth of the electronics and communications industries present new challenges for researchers. One of these challenges is the generation of the required bandwidth signal over a specific time frame that is used in a variety of contexts, particularly radar systems. To improve the range resolution in the radar along with better SNR, it is necessary to reduce the signal bandwidth and increase the peak power. There are some restrictions for narrowband signals like power limitation, pulse shaping, and the production of unwanted harmonics. So as a solution pulse compression techniques are suggested. Pulse compression is a process that modulating the transmitted pulse to achieve a wideband signal and then at the receiver, the received signal correlates with the transmitted pulse to achieve narrowband representations of data. Chirp is the most common signal used in pulse compression. The chirp signal is produced using linear frequency modulation. In this study, we attempted to add an amplitude modulation to the chirp signal and evaluate its performance by implementation on FPGA. The outcome signal is called chirplet and simulation will show that it enhance target detection and image quality in imaging radars like SAR. [ABSTRACT FROM AUTHOR]

The development of adaptive imaging techniques is contingent on the accurate and repeatable characterization of ultrasonic image quality. Adaptive transmit frequency selection, filtering, and frequency compounding all offer the ability to improve target conspicuity by balancing the effects of imaging resolution, the signal-to-clutter ratio, and speckle texture, but these strategies rely on the ability to capture image quality at each desired frequency. We investigate the use of broadband linear frequency-modulated transmissions, also known as chirps, to expedite the interrogation of frequency-dependent tissue spatial coherence for real-time implementations of frequency-based adaptive imaging strategies. Chirp-collected measurements of coherence are compared to those acquired by individually transmitted conventional pulses over a range of fundamental and harmonic frequencies, in order to evaluate the ability of chirps to recreate conventionally acquired coherence. Simulation and measurements in a uniform phantom free of acoustic clutter indicate that chirps replicate not only the mean coherence in a region-of-interest but also the distribution of coherence values over frequency. Results from acquisitions in porcine abdominal and human liver models show that prediction accuracy improves with chirp length. Chirps are also able to predict frequency-dependent decreases in coherence in both porcine abdominal and human liver models for fundamental and pulse inversion harmonic imaging. This work indicates that the use of chirps is a viable strategy to improve the efficiency of variable frequency coherence mapping, thus presenting an avenue for real-time implementations for frequency-based adaptive strategies. [ABSTRACT FROM AUTHOR]

In terms of low power consumption and long-range communication—low-power wide-area networks (LPWAN) are suitable for wireless sensor networks. Long-range (LoRa) wireless communication is one of the standards of LPWAN. LoRa shares common frequency spectrum bands with both multiple transmitters, which are the sensors in the LoRa system (and those in the other system). Therefore, co-channel interference (CCI) degrades the packet delivery rate. To avoid CCI, the CCI power and the occurrence probability of CCI in the target channel are estimated, then the sensor decides whether to use the channel and where the occurrence probability of CCI is defined as the channel occupancy ratio (COR). If a large signal power is obtained at the receiver, the received signal can be demodulated because of the capture effect. The desired signal power must also be estimated for the capture effect. In this study, we propose an estimation scheme based on chirp modulation of LoRa under spectrum sharing among other systems. The proposed scheme estimates the desired signal power, CCI power, and COR. From the computer simulation results, we clarify the advantages of the proposed scheme in terms of estimation accuracy and packet delivery rate.

We propose and demonstrate a frequency modulated continuous-wave (FMCW) light detection and ranging (LiDAR) system, which employs double-sideband modulation combining with injection-locking to generate triangular linear frequency modulation light source. A fiber laser working as master laser is modulated by a Mach-Zehnder modulator to produce two first-order sidebands with tuning range of 8–14 GHz, one of which is extracted and amplified by a slave distributed feedback laser. A large carrier suppression ratio up to 20 dB is realized. The experiment results show the spatial resolution of the proposed LiDAR is 2.5 cm, equaling to the theoretical value. The velocity measurement is also performed by extracting Doppler shift. Finally, by combining the proposed LiDAR with 2-axis mechanical galvanometer scanner, the 3D imaging with high precision is realized, and the real scene is well restored. The proposed LiDAR system realizes pure linear frequency modulation without complex linearization algorithm or clock sampling circuit, and has advantages of simple structure and high precision. [ABSTRACT FROM AUTHOR]

Summary: Low‐power wide area networks (LPWAN) applications are emerging as a new paradigm, over Internet of Things (IoT) platform. LoRa (long range) is playing a key role in information exchange between motes. This paper illustrates a mathematical optimization technique of LoRa modulation, i.e., frequency shift chirp modulation. Mathematical optimization for low power and LoRa has been validated with customized hardware designed for agricultural application. The performance analysis with channel modulation is mentioned in the networks. LoRa link budget concerning various obstacles calculated and losses are noted during testing. In context to the agricultural application, customized motes have been deployed in an agricultural field for field monitoring. The mathematical model is validated using a hardware device. Further data collection and data analysis are mentioned. Code optimization techniques are implemented to improve the overall efficiency of the device. Localization of sensor nodes also discussed to cover all the area of coverage. A comparative analysis is a showcased between SNR (signal‐to‐noise ratio) and RSSI (received signal strength indicator). [ABSTRACT FROM AUTHOR]

Due to the extra degrees of freedom, special affine Fourier transform (SAFT) has achieved a respectable status within a short span and got versatile applicability in the areas of signal processing, image processing, sampling theory, quantum mechanics. However, due to its global kernel, SAFT fails to obtain local information of non-transient signals. To overcome this, in this paper, we introduce the concept of novel special affine wavelet transform (NSAWT) and extend key harmonic analysis results to NSAWT analogous to those for the wavelet transform. We first establish some fundamental properties including Moyal's principle, Inversion formula and the range theorem. Some Heisenberg type inequalities and Pitt's inequality are established for SAFT and consequently Heisenberg uncertainty principle is derived for NSAWT. [ABSTRACT FROM AUTHOR]

The sensitivity of underwater propagation models to acoustic and environmental variability increases with the signal frequency; therefore, realizing accurate acoustic propagation predictions is difficult. Owing to this mismatch between the model and actual scenarios, achieving high-frequency source localization using model-based methods is generally difficult. To address this issue, we propose a deep learning approach trained on real data. In this study, we focused on depth estimation. Several 18-layer residual neural networks were trained on a normalized log-scaled spectrogram that was measured using a single hydrophone. The algorithm was evaluated using measured data transmitted from the linear frequency modulation chirp probe (11–31 kHz) in the shallow-water acoustic variability experiment 2015. The signal was received through two vertical line arrays (VLAs). The proposed method was applied to all 16 sensors of the VLA to determine the estimation performance with respect to the receiver depth. Furthermore, frequency-difference matched field processing was applied to the experimental data for comparison. The results indicate that ResNet can determine complicated features of high-frequency signals and predict depths, regardless of the receiver depth, while exhibiting robust environmental and positional variability. [ABSTRACT FROM AUTHOR]

Based on the three-step model of high harmonic generation, we propose a scheme to improve the harmonic spectra and to produce the intense water window attosecond pulses. It is found that, firstly, by introducing the single chirp and two chirps modulations, the instantaneous frequency on the falling part of the laser field is decreased. As a result, the electron can obtain larger energy during the acceleration-recombination process, showing a broad harmonic plateau in the water window region. Secondly, by properly adding a third controlling IR or UV pulse, the ionization process can be controlled and a larger ionization probability can be found, which leads to the enhancement of the harmonic intensity by 1 or 2 orders of magnitudes, respectively. Most important, using this scheme, the harmonic plateau is only contributed by the intense single harmonic emission peak, which can support the generation of an intense 36 as pulse. [ABSTRACT FROM AUTHOR]

The productivity and quality of laser micromachining depend on multiple laser parameters that are intricately correlated. For these optimizations, a quick survey of laser parameters is vital. Recently, the authors developed a Yb-doped fiber chirped-pulse amplification system that can control various laser parameters in a wide range (pulse duration: 0.4–400 ps, repetition: single shot to 1 MHz, etc.). In this work, using this laser system, percussion microdrilling of three types of advanced ceramics, AlN, Al2O3, and Y2O3-doped ZrO2, was explored. In the case of the microdrilling of the Al2O3 ceramic, the ablation volume increased about 2–3 times as the pulse repetition increased from 100 Hz to 1 MHz. This suggests a different mechanism because the volume removal became dominant at 1 MHz. Scanning electron microscope observation confirmed a drastic melt formation at 1 MHz. From these, there is an additional volume removal due to the heat accumulation by multipulse irradiation on the Al2O3 ceramic at a higher repetition rate. It was also found that the variation of ablation volume with the pulse duration and fluence exhibited a big difference among these ceramics. A comprehensive survey of ultrafast laser ablation of ceramics was demonstrated. [ABSTRACT FROM AUTHOR]

This letter proposes a novel time delay estimation method based on sparse optimisation of the cross‐correlation function to improve the estimation accuracy of delay parameters of chirp signals in multipath environments. In this method, the time delay estimation model is converted into a correlation‐function based model to estimate the parameter of exponent signals with frequency information, and the covariance matrix of such correlation function is solved by a sparse iteration optimisation algorithm abbreviated as CCF‐SPICE. Experimental simulations show that the proposed delay estimation algorithm has a sharper peak with less power leakage compared with the existing SPICE algorithm. Its performance is better than the SPICE algorithm and conventional MUSIC algorithm, especially under low SNR in multipath environments. The MSE performance of the proposed CCF‐SPICE algorithm is closer to Cramer–Rao bounds. [ABSTRACT FROM AUTHOR]

In this work, a self‐oscillating parametric optical frequency comb generator is proposed and experimentally demonstrated. It consists of an electroabsorption modulated laser (EML) based dual‐loop optoelectronic oscillation loop and a parametric frequency mixer. In the experiments, a 20 GHz radio‐frequency signal is produced to drive the EML. The output pulses from the EML are amplified to work as pump seeds for the frequency mixer. In the mixer, a length of high non‐linear fibre (HNLF) is firstly used to introduce chirps to the pump pulses, a section of single mode fibre is then employed for pulse compression, and finally another HNLF is applied for parametric frequency mixing. When the pump power is 28 dBm, a nine‐line comb with a frequency spacing of 20 GHz and an optical signal‐to‐noise ratio >15 dB is generated. More comb lines can be obtained when another frequency mixer is cascaded. [ABSTRACT FROM AUTHOR]

In this paper, a novel M-ary chirp modulation scheme is proposed on the basis of the energy concentration property of chirp signals in fractional domain. In the proposed scheme, chirp signals with different phases are multiplexed in the same time-frequency bandwidth through reverse chirp-rate to increase information rate. In addition, fractional filters based on fractional Fourier transform (FRFT) are designed to separate chirp signals of different chirp-rates in the receiver. Moreover, the theoretical performance of fractional filters and the error probability of M-ary chirp system are derived. Both theoretical analysis and simulation prove that the proposed scheme outperforms M-ary quadrature amplitude modulation (QAM) system in the anti-noise performance. [ABSTRACT FROM AUTHOR]

The monitoring and the management of smart grid require an advanced communication infrastructure. The time synchronization is among the most important services such an infrastructure should offer. Several solutions are available to disseminate the time information in a distributed system; for instance, most of the times a global positioning system (GPS) receiver is used to recover accurate time information. However, GPS receivers need a clear view of the sky. Other solutions, like the use of a network-based synchronization mechanism, require a dedicated communication infrastructure, which is not always feasible for economical reason. Thus, the use of the power grid itself to disseminate a time signal is extremely interesting. The aim of this article is to investigate the possibility to distribute a time signal over a medium voltage (MV) grid using a chirp-based modulation. The timing data are coded using IRIG-B time code. Such an approach does not require the installation of a dedicated infrastructure, and it is noninvasive, because the signal is transmitted on a different band with the respect to power line communication (PLC). The effectiveness of the approach has been experimentally evaluated on a test MV line available in a laboratory. Although the tested chirp-based symbol has a limited frequency bandwidth (80 kHz), the jitter in the identification of the start of frame is on the order of 7.5 $\mu \text{s}$ , fulfilling the requirements of the most smart grid applications. [ABSTRACT FROM AUTHOR]

This article develops a method for mitigating the negative effects of obstacles in narrowband, time-division multiple access (TDMA), ultrasonic localization systems. The method builds upon the robust Bayesian classifier for ultrasonic localization (RoBCUL) algorithm that utilizes an iteratively reweighted least-squares (IRLS) scheme. This algorithm has the advantage of low computational cost but loses performance in the presence of obstacles. The improved version of the RoBCUL algorithm presented in this article uses a statistical test applied after each iteration of the regression, using a weighted-residual vector calculated from the weight matrix and residual vector. The technique was tested using experimental data with its performance being quantified by its ability to correctly classify all the signals received during a single TDMA cycle. The extended version performed significantly better in all obstacle scenarios than the original, correctly classifying 100% of TDMA cycles in the scenarios with no obstacles, 99.9% with one obstacle, and 98.7% with two obstacles. [ABSTRACT FROM AUTHOR]

We theoretically and numerically study the propagation dynamics of a Gaussian beam modeled by the fractional Schrödinger equation with different dynamic linear potentials. For the limited case α = 1 (α is the Lévy index) in the momentum space, the beam suffers a frequency shift which depends on the applied longitudinal modulation and the involved chirp. While in the real space, by precisely controlling the linear chirp, the beam will exhibit two different evolution characteristics: one is the zigzag trajectory propagation induced by multi-reflection occurring at the zeros of spatial spectrum, the other is diffraction-free propagation. Numerical simulations are in full accordance with the theoretical results. Increase of the Lévy index not only results in the drift of those turning points along the transverse direction, but also leads to the delocalization of the Gaussian beam. [ABSTRACT FROM AUTHOR]