Doppler Frequency Shift Research Articles

Experimental study of acoustic superradiance from a rotating absorber

The concept that classic waves reflected from a rotating absorbing cylinder will be amplified is analogously linked to the Penrose superradiance that may extract energy from a rotating black hole. The superradiance of acoustic waves carrying orbital angular momentum (OAM) from a sound absorber has been demonstrated in a recently published experiment using two rotating microphones. The experiment showed that the waves were amplified by up to 30% when the rotation rate of the absorber satisfied the Zel’dovich condition. Here, we proposed an experimental method to detect acoustic superradiance by static microphones. We demonstrated that the acoustic waves transmitted through a perforated absorber were amplified (by up to 1000%) even when the Zel’dovich condition was not satisfied. The experiment also showed that when the Doppler-shifted frequency of an observer was 0 Hz, the acoustic amplitude was very weak regardless of the rotation rate of the absorber. Our work was worthwhile not only in the proposed experimental method that was well-suited for observing the acoustic OAM wave but also in the high amplification achieved at a low rotation rate of the perforated absorber, showing a great prospect in practical applications such as amplifying the information-carrying OAM waves for high-speed acoustic communication.

Computationally Efficient Implementation of Joint Detection and Parameters Estimation of Signals with Dispersive Distortions on a GPU.

The detector is an integral part of the device for receiving and processing radio signals. Signals that have passed through the ionospheric channel acquire an unknown Doppler shift and are subject to dispersion distortions. It is necessary to carry out joint detection and parameter estimation to improve reception quality and detection accuracy. Modern hardware base developing makes it possible to implement a device for joint detection and evaluation of signals based on standard processors (CPU) and graphic processors (GPU). The article discusses the implementation of a signal detector that allows for real-time operation. A comparison of implementations of algorithms for estimating the Doppler frequency shift through multiplication by a complex exponent and the fast Fourier transform (FFT) is performed. A comparison of computational costs and execution speed on the CPU and GPU is considered.

Photonic System for Simultaneous and Unambiguous Measurement of Angle-of-Arrival and Doppler-Frequency-Shift

A photonic system based on a two-channel structure is proposed to realize the simultaneous measurement of angle-of-arrival (AOA) and Doppler-frequency-shift (DFS) of microwave signal without direction ambiguity. The unambiguous AOA measurement can be acquired by recording the power of the output intermediate frequency (IF) signal and output radio frequency (RF) signal from two channels. The DFS can be determined from the frequency of the IF signal. Estimated experimental results show that a −66.44° to 66.44° AOA measurement with errors of less than ±2.2° and an unambiguous DFS measurement with errors of ±0.6 Hz are implemented.

3-D Target Localization and Motion Analysis Based on Doppler Shifted Frequencies

This article proposes two algorithms for the localization or motion analysis of a moving target, by using a number of stationed sensors observing the frequency of the emitted target signal. This is a challenging problem due to the complicated non-linear relation between the target location parameters and the observations. The first algorithm is closed-form through algebraic evaluation that is computationally efficient. The second is derived from semidefinite programming by relaxationthat is robust against noise. The situations of having target frequency known but erroneous and completely unknown are investigated, as well as the presence of sensor position errors. Sequential algorithms for motion analysis are proposed in addition to the two for reducing latency and complexity. The algorithms are analyzed and their performance is able to achieve the CRLB accuracy under Gaussian noise.

Multi-block Sparse Bayesian learning channel estimation for OFDM underwater acoustic communication based on fractional Fourier transform

In OFDM underwater acoustic communication, according to the sparse characteristics of underwater acoustic communication channel in the time domain, sparse Bayesian learning (SBL) is introduced to make the channel estimation. In order to reduce the use of pilots and improve the transmission efficiency, a sparse pilot structure is adopted, and a multi-blocks sparse Bayesian learning (MBSBL) channel estimation algorithm is proposed. Aiming at the problem of insufficient channel estimation of MBSBL algorithm in the condition of few pilots, the fractional Fourier transform is introduced to compensate the Doppler frequency shift, which further improves the channel estimation performance and output signal-to-noise ratio (SNR). Besides, the complexities of Orthogonal Matching Pursuit (OMP), SBL and MBSBL algorithms are summarized briefly. Simulation and sea trial data show that the proposed algorithm can save half the number of pilots and the channel estimation effect of proposed algorithm is better than that of OMP algorithm and SBL algorithm.

Motion Model of Floating Weather Sensing Node for Typhoon Detection

To improve the accuracy of typhoon prediction, it is necessary to detect the internal structure of a typhoon. The motion model of a floating weather sensing node becomes the key to affect the channel frequency expansion performance and communication quality. This study proposes a floating weather sensing node motion modeling method based on the chaotic mapping. After the chaotic attractor is obtained by simulation, the position trajectory of the floating weather sensing node is obtained by space and coordinate conversion, and the three-dimensional velocity of each point on the position trajectory is obtained by multidimensional linear interpolation. On this basis, the established motion model is used to study the Doppler frequency shift, which is based on the software and physical platform. The software simulates the relative motion of the transceiver and calculates the Doppler frequency shift. The physical platform can add the Doppler frequency shift to the actual transmitted signal. The results show that this method can effectively reflect the influence of the floating weather sensing node motion on signal transmission. It is helpful to research the characteristics of the communication link and the design of a signal transceiver for typhoon detection to further improve the communication quality and to obtain more accurate interior structure characteristic data of a typhoon.

Low-complexity OTFS-based hybrid precoding in mmWave high mobility systems

The integration of aircraft into air-to-ground (A2G) communications with a variety of applications has attracted much attention, and it is critical to establish reliable communication links in A2G systems. However, suffering from the multipath effects and high Doppler frequency shifts, the quality of the transmission links is severely degraded in high mobility scenarios. To overcome the performance degradation caused by the double selective channel, a new modulation technique called orthogonal time-frequency-space (OTFS) is proposed recently. Moreover, millimeter wave (mmWave) massive multiple-input-multiple-output (MIMO) precoding is a simple and effective method to improve the system performance. Unfortunately, most of the existing MIMO-OTFS studies are implemented on the full-digital architecture, which will lead to high power consumption and high hardware cost. Besides, with the assumption of the practical waveforms in OTFS systems, it is complicated to work out the precoding algorithm. The objective of this paper is to develop a low-complexity OTFS-based hybrid precoding algorithm with rectangular waveforms for mmWave A2G communications. We first establish the delay-Doppler (DD) domain signal transmission process in hybrid precoding and derive the equivalent baseband DD domain channel matrix. Next, a two-stage hybrid precoding algorithm is proposed, in which the analog precoder based on steering vector is designed, and the recursive digital precoding algorithm is explored. Then a matrix approximation-based strategy is discussed to further reduce complexity. Furthermore, the computational complexity of the proposed algorithm is analyzed. Finally, the numerical results show the lower complexity and superior bit error rate performance of the proposed algorithm compared with the existing OTFS algorithms in high mobility scenarios.

Ultra-High Mobility Analysis of MIMO Wireless System in Tunnel Scenarios

This letter studies the effect of ultra-high mobility on the performance of multiple-input multiple-output (MIMO) wireless communication system in tunnel scenarios. It is found that the Doppler frequency shift generated by the ultra-high mobility reduces the correlation coefficient. Importantly, it is observed that there exists a key inflection point distance in the ultra-high speed tunnel scene. When this distance is exceeded, the channel correlation of MIMO can be significantly reduced. Moreover, it is perceived that the inflection point distance is relatively stable and is less affected by the communication frequency. On this basis, we propose a high-quality communication area with low MIMO channel correlation. For the ultra-high speed of 720km/h, the inflection distance is about 95m, and the reduction of MIMO channel correlation at 3.5GHz and 28GHz can reach up to 52.7% and 41.2%, respectively. The findings are extremely useful for the applications of the mobile MIMO communication system in tunnel and railway scenarios.

Optimal nonlinear filtering of MPSK signals in the presence of a Doppler frequency shift

Objectives . Phase-shift keyed (PSK) signals are widely used in many telecommunication, communication, and cellular information transmission systems. Phase-shift keying provides a higher noise immunity than amplitude and frequency modulations do. An increase in the modulation order of such a signal leads not only to an increase in its spectral efficiency, but also to a certain decrease in the noise immunity of reception. To ensure a high noise immunity of reception of multiple phase-shift keyed (MPSK) signals, a demodulator should provide the coherence of the reference oscillation with the carrier. Ignorance of the frequency and phase of the received signal leads to significant energy losses. The purpose of this work was to synthesize and analyze algorithms for receiving MPSK signals with phase fluctuations caused by changes in the carrier frequency due to the Doppler effect against the background of white Gaussian noise. Methods . The problem was solved using the apparatus of optimal nonlinear filtering and methods of statistical radio engineering. Results . A demodulator was synthesized, which includes two interconnected units. One of them is a discrete symbol estimation unit, at the output of which a decision on the received symbol is issued, and the other is a phase-lock circuit. Analytical expressions were derived to estimate the characteristics of the receiver noise immunity as functions of the signalto-noise ratio and fluctuation parameters. It was shown that the synthesized quasi-coherent algorithm compensates well for the MPSK signal phase fluctuations caused by the instability of the master oscillator and the Doppler effect. Conclusions . Comparison of the results of this work with results obtained in the case of the absence of fluctuations in the initial phase showed that, at a high relative speed of the transmitter and the receiver (satellite radio channel), the energy loss is no more than 1 dB, and at lower speeds of the objects, it is about 0.2 dB and less.

Effects of particle precipitation on the polar mesospheric summer echoes observed by EISCAT VHF 224 MHz radar

We present the results from investigation of spectra of polar mesospheric summer echoes (PMSE) observed with EISCAT VHF 224 MHz radar during energetic particle precipitation events in the summer of 2019. We used the sudden enhancements in electron densities derived from the VHF observations above 90 km as indicators of particle precipitation. We find that the altitude extent of the PMSE increased along with an enhancement of the strength of the pre-existing PMSE. On some occasions, PMSE suddenly appeared during particle precipitation events. After the particle precipitation subsided, the PMSE intensities continued to be stronger for a few minutes and then decreased. At the altitudes where the maximum enhancement in PMSE backscatter occurred, there is no corresponding broadening in the spectral widths. Interestingly, the most intense PMSE echoes almost always coincided with the lower values of spectral widths both during particle precipitations and other times. In some cases, the spectral widths show well separated regimes between the upper and lower parts of the PMSE layers and this also happened irrespective of particle precipitation. The frequency Doppler shifts showed alternating upward and downward motions without much difference before and after the particle precipitation. Based on these observations, we conclude that the sudden enhancement in the electron densities during particle precipitation intensifies PMSE structures that match the Bragg scale of the incident radio wave thereby enabling an enhancement in the backscatter strength. Similarly, PMSE backscattering may get initiated in the altitudes in which the structures were already present but where electron density was not sufficient before the onset of precipitation. Spectral widths and Doppler velocities does not show sudden variations with particle precipitation because they depend on the neutral dynamics.

Joint Coordinated Beamforming and Power Splitting Ratio Optimization in MU-MISO SWIPT-Enabled HetNets: A Multi-Agent DDQN-Based Approach

This paper proposes a multi-agent double deep Q network (DDQN)-based approach to jointly optimize the beamforming vectors and power splitting (PS) ratio in multi-user multiple-input single-output (MU-MISO) simultaneous wireless information and power transfer (SWIPT)-enabled heterogeneous networks (HetNets), where a macro base station (MBS) and several femto base stations (FBSs) serve multiple macro user equipments (MUEs) and femto user equipments (FUEs). The PS receiver architecture is deployed at FUEs. An optimization problem is formulated to maximize the achievable sum information rate of FUEs under the constraints of the achievable information rate requirements of MUEs and FUEs and the energy harvesting (EH) requirements of FUEs. Since the optimization problem is challenging to handle due to the high dimension and time-varying environment, an efficient multi-agent DDQN-based algorithm is presented, which is trained in a centralized manner and runs in a distributed manner, where two sets of deep neural network parameters are jointly updated and trained to tackle the problem and avoid overestimation. To facilitate the presented multi-agent DDQN-based algorithm, the action space, the state space and the reward function are designed, where the codebook matrix is employed to deal with the complex transmit beamforming vectors. Simulation results validate the proposed algorithm. Notable performance gains are achieved by the proposed algorithm due to considering the beam directions in the action space and the adaptability to the Doppler frequency shifts. Besides, the proposed algorithm is shown to be superior to other benchmark ones numerically.

A Linear Chirp Wireless Transmission Method utilizing Doppler Effect

The Chirp Spread Spectrum(CSS) based wireless communication has been widely used in Wireless Sensor Network(WSN). These sensors generally have slow speed, and is becoming more demand for higher data rate. However, due to the low transmission rate of CSS, there are still many problems to be studied. A new modulation method based on the linear chirps is introduced in this paper. Unlike the BOK(Binary Orthogonal Keying) and DM(Direct Modulation) methods, this modulation technique is to implant Doppler frequency shift into the linear chirps. M-ary modulation is realized in a single pulse by this proposed modulation technique. Demodulation is accomplished by calculating the position of the compress pulse peak within the pulse duration, or by using different reference chirp signals in the matching filter.

Inter-spacecraft offset frequency setting strategy in the Taiji program.

For controlling the beat frequency of heterodyne interferometry so that the Taiji program can detect gravitational waves in space, an offset frequency setting strategy based on a linear programming algorithm is proposed. Considering factors such as Doppler frequency shift, phase-locking scheme, laser relative intensity noise, and phase detector bandwidth, inter-spacecraft offset frequency setting results suitable for the Taiji program are obtained. During the six years of running the detection process, the use of frequency bounds in the range of [5 MHz, 25 MHz] showed that offset frequencies will remain unchanged for a maximum of 1931 days. If the upper and lower bounds are adjusted, and the relative motion between spacecraft is further constrained, the offset frequencies do not need to change during the time of the mission. These results may provide insights into selecting the phase detector and designing operation parameters such as orbit and laser modulation frequency in the Taiji program.

Study on the High Accuracy and Fast Acquisition of Satellite Signals Based on the Blind Source Separation Technique

To address the problems of slow acquisition speed and low accuracy faced by existing grid search-based satellite acquisition methods in complex scenarios, this study proposes a high accuracy and fast satellite signal acquisition method based on blind source separation. The proposed method first adopts wavelet threshold denoising to reduce the noise in the overlapped satellite signal received by the receiver so as to improve the signal-to-noise ratio of the satellite signal. On this basis, a subspace estimation method is introduced to construct a blind acquisition algorithm for satellite signals, which achieves high accuracy and fast solution for the Doppler frequency shift and code phase shift of satellite carrier signals and is able to recover the unobservable individual source signals. The effectiveness of the proposed method is verified through experiments and compared against the traditional grid search type acquisition method, which confirmed that the method is suitable for the acquisition requirements of weak signals and has a good engineering application value.

Complex moving target recognition based on optical phase-locked loop in LIDAR

At present, laser vibration measurement technology is widely used based on the micro-Doppler effect. Micro-Doppler information contains a specific target feature, which provides an effective method for target recognition. Complex translation of the target is not conducive to detecting and collecting of the beat signal and increases the difficulty of extracting vibration information. In this paper, we propose to utilize an optical phase-locked loop (OPLL) operating in the carrier tracking mode at the receiver of coherent LIDAR, which can adaptively compensate the variable Doppler frequency shift (DFS) caused by the complex translational to obtain a detectable beat signal with the stable carrier frequency. On this basis, the micro-Doppler information can be accurately obtained from the variable DFS.A laboratory experiment for vibration targets with our OPLL method has been conducted, and it revealed that our approach worked well to extract the micro-Doppler information, thus the complex vibration contains two vibration frequencies of 100 Hz and 300 Hz, with amplitudes of 48.1 μm and 44.3 μm, respectively. This micro-motion information extraction method has universal applicability in recognizing the complex moving target, such as space debris recognition.

ОСОБЛИВОСТІ ВАРІАЦІЙ ХАРАКТЕРИСТИК ВЧ-РАДІОХВИЛЬ В ІОНОСФЕРІ ВПРОДОВЖ СОНЯЧНОГО ЗАТЕМНЕННЯ 21 ЧЕРВНЯ 2020 р. НАД КИТАЄМ

Subject and Purpose.The study of the effect that each new Solar eclipse (SE) has on radio wave characteristics is an actual scientific and technical issue. The purpose of the present work is to analyze the variations in Doppler spectra (DS), Doppler shift of frequency (DSF), and in the reflected wave amplitude (RWA) that were observed during the SE of June 21, 2020 over the People’s Republic of China. Methods and Methodology.The observations of HF radio wave characteristics were made using the Harbin Engineering University multi-frequency multipath coherent radio system. The temporal variations in DS, DSF of the main ray and RWA are analyzed further. The variations in the Doppler frequency shift (DSF) were subjected to a systematic spectral analysis that involved joint application of the windowed Fourier transform, adaptive Fourier decomposition, and the Morlet mother-function-based wavelet transformation. Results. The SE was accompanied by DS diffuseness resulting from an increase in the number of rays. The DSF temporal variations were observed to be bi-polar and asymmetrical, with extreme DSF magnitudes varying from –11 to –40 mHz and from 22 to 56 mHz. The duration of processes with negative DSF values varied from 50 to 80 min, and the duration of processes with positive DSF changed from 30 to 80 min. The multi-hop propagation (from two to five hops) took place along all propagation paths, with a 360 to 560-km one-hop range. The 4–5-min period quasi-periodic DSF variations showed 20–50 mHz amplitude, and the 8–18-min period variations exhibited 40–100 mHz amplitude. The relative amplitudes of the 4–5 min period quasi-periodic variations in the electron density were observed to be in the 0.3–6.2% range, and the amplitudes of the 8–18 min period variations were found to be in the 1.1–21.7% range. A decrease in the electron density along different propagation paths was observed to vary from –(12–16)% to –(20–26)%.

Acquisition of Time and Doppler Shift in WFRFT Encrypted Chaotic Direct Sequence Spread Spectrum System

Weighted fractional Fourier transform encrypted chaotic direct sequence spread spectrum (WFRFT-CD3S) signal is aperiodic and complex Gaussian distributed. The aperiodicity and complex Gaussian distribution are conducive to the covert transmission of information but also make it difficult to acquire the WFRFT-CD3S signal. An acquisition method implemented in two steps is proposed to acquire the Doppler-shifted WFRFT-CD3S signal. First, a bidirectional correlation search method is proposed to align the received signal with the local spreading sequence in the time domain. The correlation peak attenuation caused by the Doppler frequency shift is eliminated by differential. Second, the accumulated phase angle caused by the Doppler frequency shift is obtained by the differential correlation value between the aligned received signal and the local spreading sequence. Then, the Doppler frequency shift is estimated by the accumulated phase angle. The detection probability, the false alarm probability, and the root mean square error (RMSE) of the Doppler shift estimation are analyzed theoretically and simulated. The theoretical results are verified by simulation. Theoretical results and simulations show that the detection probability is higher than 0.999, the false alarm probability is less than 0.001, and the RMSE of Doppler shift estimation is lower than 63 Hz when the signal-to-noise ratio is higher than -7.3 dB.

Simple Doppler Frequency Shift Measurement Scheme Based on Microwave Photonics

Based on the theory of microwave photonics, we propose a new scheme to measure the Doppler frequency shift (DFS) in a radar system. This scheme is a single-channel system based on dual-parallel Mach–Zehnder modulator (DPMZM). And the frequencies of the transmitted signal and the reference signal are added together to form a combined signal. Then we add the combined signal and the echo signal to the two sub-modulators (sub-MZMs) of DPMZM, respectively. By measuring the beat signal output by the photoelectric modulator (PD), the DFS can be measured, whose error is maintained at −0.8Hz to 0.8Hz. The DFS’s direction is determined by comparing the frequencies of the obtained signal and the reference signal. The system has the advantages of wide operating frequency range, simple structure, low cost, and easy integration.

A Filterless Photonic Approach for DFS and AOA Measurement Using a Push-Pull DPol-MZM

We report a novel simple photonic approach for measuring the Doppler frequency shift (DFS) and the angle of arrival (AOA) of a microwave signal in a radar detection system. The echo signals combined with a reference signal are applied to a dual-polarization Mach-Zehnder modulator (DPol-MZM), and a low-frequency electrical signal is generated after beating between the two signals in a low-speed photodetector. The DFS without direction ambiguity can be calculated by the value of the low-frequency signal. A phase difference is introduced between two echo signals which are sent to two sub-MZMs of the DPol-MZM respectively. The AOA is determined by the peak power of the down-convert signals, and a simple calibration is used to remove the amplitude of the echo signals and the reference signal by switching the output polarization states of the system. Compared with previously reported works, the proposed method carries out a comprehensive and multifunction measurement system based on a filterless and simple structure. Experimental results demonstrate the DFS errors within ± 0.5 Hz for a 15 GHz echo signal with a frequency offset of ± 100 kHz, and AOA measurement errors are within ± 1° for a range of 16° – 82°

Simulation of the influence of negative internal factors on performance indicators of the synchronization system of a coherent demodulator of satellite telecommunications in permanent and transient modes of carrier frequency estimation

The subject of study in the article is the system of phase synchronization of radio technical devices of communication technology. The purpose of the article is theoretical research in the direction of development, analysis and improvement of known and synthesis of new phase synchronization schemes, which are characterized by high interference resistance, accuracy and speed of operation with a simple design. The problem to be solved is to investigate the possibilities of closed and combined synchronization systems in terms of operation in conditions of internal noise and disturbances against the background of external additive Gaussian noise and Doppler frequency shift. The article presents a model that allows you to investigate the influence of negative internal factors on the performance indicators of the synchronization system of the coherent demodulator of satellite telecommunications in stable and transient modes of carrier frequency estimation. With its help, the article analyzes closed-type synchronization systems when operating under the influence of negative internal factors and shows its inconsistency in minimizing the dispersion of the phase error and increasing the dynamics during tracking of the carrier frequency. The process of synthesis of open communication in the combined synchronization system has been clarified and analytical dependencies have been proposed, which allow to clarify the methodology of synthesis of open communication for the combined synchronization system, taking into account the influence of internal negative factors. It was established that under the influence of negative internal factors for the combined synchronization system, by selecting the parameters of the proportionalintegrating filter, it is possible to achieve an expansion of the noise bandwidth of the input signal and thereby ensure the necessary dynamics of the system and achieve the preservation of the optimal value of the dispersion of the phase error in it.