Design of Frequency-PRT Coupled Agile Waveform and Low-Complexity Signal Processing

The traditional probe–pass integration system embeds communication information into a radar waveform, which leads to a high level of waveform autocorrelation sidelobes and a poor false symbol rate at low signal-to-noise ratios. This article proposes a three-dimensional indexed modulation-based design method for probe–pass integration waveforms. This method realises communication information modulation and demodulation by simultaneously indexing orthogonal waveform selection, transmitting pulse PRI changes and carrier frequency changes in three dimensions, and applying compressed perception technology to solve the problems of PRI shortcuts and carrier frequency, resulting in a velocity term in the received waveform that cannot be accumulated by phase reference to realise velocity super-resolution. Finally, the radar detection performance and communication performance are simulated and analysed, and the simulation results reveal that the method proposed in this paper can not only satisfy the radar detection performance requirements but also achieve a lower unsigned rate on the basis of an improved communication rate.

Optimizing jamming strategies is crucial for enhancing the performance of cognitive jamming systems in dynamic electromagnetic environments. The emergence of frequency-agile radars, capable of changing the carrier frequency within or between pulses, poses significant challenges for the jammer to make intelligent decisions and adapt to the dynamic environment. This paper focuses on researching intelligent jamming decision-making algorithms for Intra-Pulse Frequency Agile Radar using deep reinforcement learning. Intra-Pulse Frequency Agile Radar achieves frequency agility at the sub-pulse level, creating a significant frequency agility space. This presents challenges for traditional jamming decision-making methods to rapidly learn its changing patterns through interactions. By employing Gated Recurrent Units (GRU) to capture long-term dependencies in sequence data, together with the attention mechanism, this paper proposes a GA-Dueling DQN (GRU-Attention-based Dueling Deep Q Network) method for jamming frequency selection. Simulation results indicate that the proposed method outperforms traditional Q-learning, DQN, and Dueling DQN methods in terms of jamming effectiveness. It exhibits the fastest convergence speed and reduced reliance on prior knowledge, highlighting its significant advantages in jamming the subpulse-level frequency-agile radar.

It is hypothesized that proportional changes in echo amplitude and carrier frequency might aid obstacle avoidance in cf bats by means of acoustic flow perception. This approach appears to be feasible in principle for single targets, however, obstacle avoidance, unlike prey capture in midair, is invariably prone to constitute a multiple target problem, which has to be addressed in assessing the hypothesis’ validity. In order to map a compound excitation pattern formed by superposition of responses to multiple echoes back into target space by means of an acoustic flow approach, instantaneous (carrier) frequency and amplitude together with their first-order derivatives must be recovered individually for each echo to be perceived as a separate entity. The primal sketch serving as a substrate for this operation is impoverished in that it is only preserving response envelopes. At the same time, the system design might capitalize on knowledge about the transfer functions of the filter bank’s channels as well as at least a rough guess about the echo spectrum. Consequently, it is attempted to explain compound excitation patterns as a sum of superimposed responses to individual echoes, which are known for any given frequency location to a scaling constant.

<p indent=0mm>Compared with urban application scenarios, Global Navigation Satellite System (GNSS) navigation receivers that are mounted on high-speed aircraft must handle a higher carrier frequency dynamic and output the measured result at a higher frequency. Both these factors adversely affect the accuracy of the carrier frequency measurement. Herein, we propose an adaptive carrier-tracking algorithm to measure the radial velocity by introducing a fuzzy logic controller in the carrier-tracking loop. The proposed algorithm adaptively adjusts the loop bandwidth as the carrier frequency changes, improving the velocity measurement accuracy while ensuring dynamic adaptability. Herein, the measured result of the loop is used to calculate the input variables of the fuzzy controller, and the output of the fuzzy controller is used to adjust the loop bandwidth. The fuzzy controller adopts a single input and single output Takagi-Sugeno model, which is easy to implement. The analysis and simulation are performed under fixed-, linear-, and sine-shaped Doppler frequency scenarios. The radial velocity between the aircraft and the receiver is set in the range of <sc>±10 km/s,</sc> and the radial acceleration is set in the range of <sc>±30<italic>g</italic>.</sc> To ensure the validity of the measurement results, we output the measurement results at a frequency of 1000 points per second. Simulation results demonstrate that when the input carrier-to-noise ratio is <sc>40 dB Hz,</sc> the velocity accuracy of the proposed algorithm is better than <sc>0.3 m/s</sc> (1<italic>σ</italic>).

Instant Machine Health Assessment Through the Use of Mobile Phones/PDAS

peer reviewed

Extracorporeal shock wave therapy (ESWT) achieves its therapeutic purpose mainly through the biological effects produced by the interaction of shock waves with tissues, and the accurate measurement and calculation of the mechanical parameters of shock waves in tissues are of great significance in formulating the therapeutic strategy and evaluating the therapeutic effect. This study utilizes the approach of implanting flexible polyvinylidene fluoride (PVDF) vibration sensors inside the tissue-mimicking phantom of various thicknesses to capture waveforms at different depths during the impact process in real time. Parameters including positive and negative pressure changes (P+, P-), pulse wave rise time ([Formula: see text]), and energy flux density (EFD) are calculated, and frequency spectrum analysis of the waveforms is conducted. The dynamic response, propagation process, and attenuation law of the shock wave in the phantom under different impact intensities were analyzed. Results showed that flexible PVDF sensors could precisely acquire the characteristics of pulse waveform propagating within the phantom. At the same depth, as the driving pressure increases, P+ and P- increase linearly, and [Formula: see text] remains constant. At the same driving pressure, P+, P-, and EFD decay exponentially with increasing propagation depth. At the same depth, the spectra of pulse waveforms are similar, and the increasing driving pressure does not cause significant changes in carrier frequency and modulation frequency. The research findings could provide a reference for developing ESWT devices, improving treatment strategies, and enhancing the safety of clinical applications.

In this study, the issue of phase discontinuity in multiple-input–multiple-output (MIMO) radar systems is analysed and described in detail, together with an efficient way for processing MIMO radar data by means of discrete-time Fourier transform (DFT) fast Fourier transform (FFT). In particular, here it is shown that the phase history of a point target illuminated by an MIMO radar shows a number of discontinuities because of the geometry of the antennas. It is also shown that these become negligible in the far range, whereas they represent an issue in the short range if not compensated properly because, in turn, they prevent DFT-based algorithms correctly focusing the received signals. It is shown that data from MIMO radars have to be pre-processed in order to allow an efficient DFT-based focalisation. For this purpose, together with a theoretical description of the problem, a sub-optimal algorithm is presented, discussed and tested against real data gathered by the Joint Research Centre (JRC) MIMO radar system.

A frequency-hopping communication receiver based on Rydberg atoms, whose carrier frequency changes in accordance with a predetermined sequence, is proposed for what we believe to be the first time and experimentally verified. Based on the coherent population trapping (CPT) effect of atoms, a five-level system is built under the assumption of a constant laser wavelength, where the carrier is the resonance band corresponding to different separated quantum states. Pseudo-random binary sequence (PRBS) and complex image signals undergo keying modulation to achieve frequency-domain hopping reception. In the experiment, a dynamic range of 62 dB is demonstrated. The digital transmission with a data rate of 1 Mbps is performed reliably, within a tunable bandwidth of 50 MHz, an instantaneous bandwidth of 700 kHz and a max hopping rate of 20000 hop/s. The proposed atom-based frequency-hopping receiver is suitable for secure communications, satellite uplinks, quantum cryptography, and high-precision scientific research.

This work aims to design and implement a secure frequency hopping spread spectrum system (FHSS) on an FPGA. Design is modelled using the Verilog HDL module. In this work, Xilinx ISE Design suite 14.4 software is used along with the ModelSim simulator and XST synthesizer. The core of the FHSS transmitter is a Direct Digital Synthesizer (DDS) which generate desired frequency signal. This is a very secure communication system because its carrier frequency changes at every fixed interval in a Pseudo-Random manner. In this work, the simulation and synthesis procedure of a frequency hopping binary frequency shift keying (FH/BFSK) spread spectrum system is designed and described.

In orthogonal frequency-division multiplexing (OFDM) systems, it is generally assumed that the channel response is static in an OFDM symbol period. However, the assumption does not hold in high-mobility environments. As a result, intercarrier interference (ICI) is induced and the system performance is degraded. A simple remedy for this problem is the application of the zero-forcing (ZF) and minimum mean square error (MMSE) equalizers. Unfortunately, the direct ZF method requires the inversion of an NxN ICI matrix, where N is the number of subcarriers. When N is large, the computational complexity can become prohibitively high. As for the direct MMSE method, in addition to an NxN matrix inverse, it requires an extra NxN matrix multiplication, making the required computational complexity higher compared to the direct ZF method. In this dissertation, we first propose a low-complexity ZF method to solve the problem in single-input-single-output (SISO) OFDM systems. The main idea is to explore the special structure inherent in the ICI matrix and to apply Newton's iteration for matrix inversion. With our formulation, fast Fourier transforms (FFTs) can be used in the iterative process, reducing the complexity from O(N^3) to O(Nlog_2 N) . Also, the required number of the iteration is typically one or two. We also analyze the convergence behavior of the proposed method and derive the theoretical output signal-to-interference-noise-ratio (SINR). For the MMSE method, we first reformulate the MMSE solution in a way that the extra matrix multiplication can be avoided. Similar to the ZF method, we then exploit the structure of the ICI matrix and apply Newton's iteration to reduce the complexity of the matrix inversion. For a multiple-input-multiple-output (MIMO) OFDM system, the required complexity of the ZF and MMSE methods becomes more intractable. We then manage to extend the proposed ZF and MMSE methods for SISO-OFDM systems to MIMO-OFDM systems. It turns out that the computational complexity can be reduced even more significantly. Simulation results show that the performance of the proposed methods is almost as good as that of the direct ZF and MMSE methods, while the required computational complexity is reduced dramatically. Finally, we explore the application of the proposed methods in mobility-induced ICI mitigation for OFDM multiple access (OFDMA) systems, and in carrier frequency offset (CFO) induced ICI mitigation for OFDMA uplink systems. As that in OFDM systems, the proposed methods can reduce the required computational complexity, effectively.

A direct sequence spread spectrum (DSSS) signal is commonly used in inter-satellite link (ISL) in navigation constellation; however, acquisition is a challenging task because of large-scale relative movement between satellites. In general, a signal search (carrier frequency and code phase) is implemented using two-dimensional partitioned fast Fourier transforms (FFTs), which is limited by the memory of the processor. Nevertheless, the carrier's continuity is damaged by the circular shift. Noncoherent integration is performed to circumvent this problem. However, the acquisition sensitivity will be reduced because of the square loss. In this paper, we analyze the processing flow of traditional noncoherent integration method and the new presented coherent integration method with compensation. Besides, the comparison of computation cost between the new approach and the conventional method is done. Numerical results show that the coherent method can improve the sensitivity of acquiring weak signals and save about 60% computation cost as well.

A simple algorithm is proposed for carrier frequency synchronization after PN code acquisition using FFT (fast Fourier transform) in a direct-sequence code division multiple access mobile satellite communications system. The proposed algorithm shows almost the same performance as the phase differential algorithm under additive white Gaussian noise channel environments. The proposed algorithm, however, largely reduces hardware complexity when a two-dimensional search algorithm is employed for PN code timing acquisition and coarse Doppler shift compensation since the proposed algorithm shares the FFT operator with the two-dimensional search.

Demodulation of micro fiber-optic Fabry–Perot interferometer using subcarrier and dual-wavelength method

Velocity ambiguity is one of the main challenges in accurately measuring velocity for the future Geostationary Spaceborne Doppler Weather Radar (GSDWR) due to its short wavelength. The aim of this work was to provide a novel velocity dealiasing method for frequency diversity for the future implementation of GSDWR. Two different carrier frequencies were transmitted on the adjacent pulse-pair and the order of the pulse-pair was exchanged during the transmission of the next pulse-pair. The Doppler phase shift between these two adjacent pulses was estimated based on the technique of the frequency diversity pulse-pair (FDPP), and Doppler velocity was estimated on the sum of the Doppler phase within the adjacent pulse repetition time (PRT). From the theoretical result, the maximum unambiguous velocity estimated by FDPP is only decided by the interval time of the two adjacent pulses and radar wavelength. An echo signal model on frequency diversity was established to simulate echo signals of the GSDWR to verify the extension of the maximum unambiguous velocity and the accuracy of the velocity estimation for FDPP used on GSDWR. The study demonstrates that the FDPP algorithm can extend the maximum unambiguous velocity greater than the Stagger PRT method and the unambiguous range and velocity are no longer limited by the chosen value of pulse repetition frequency (PRF). In the Ka band, the maximum unambiguous velocity can be extended to 105 m/s when the interval time is 10 μs and most velocity estimation biases are less than 0.5 m/s.