Received Signal In Time Domain Research Articles

Interference Compression and Mitigation for Automotive FMCW Radar Systems

Millimeter-wave (mm-wave) frequency-modu- lated continuous wave (FMCW) radars are increasingly being deployed for scenario perception in various applications. It is expected that the mutual interference between such radars will soon become a significant problem. Therefore, to maintain the reliability of the radar measurements, there must be procedures in place to mitigate this interference. This article proposes a novel interference mitigation technique that utilizes the pulse compression principle for interference compression and mitigation. The interference in the received time-domain signal is compressed using an estimated matched filter. Afterward, the compressed interference is discarded, and the signal is repaired in the pulse-compressed domain using an autoregressive (AR) model. Since the interference spans fewer samples after compression, the signal can be restored more accurately in the compressed domain. Real outdoor measurements show that the interference is effectively suppressed down to the noise floor using the proposed scheme. A signal to interference and noise ratio (SINR) gain of approximately 14 dB was achieved in the experimental data, supporting this study. Moreover, the results indicate that this method is also applicable to situations where multiple interference sources are present.

Time-Domain to Delay-Doppler Domain Conversion of OTFS Signals in Very High Mobility Scenarios

In Orthogonal Time Frequency Space (OTFS) modulation, information symbols are embedded in the delay-Doppler (DD) domain instead of the time-frequency (TF) domain. In order to ensure compatibility with existing OFDM systems, most prior work on OTFS receivers consider a two-step conversion, where the received time-domain (TD) signal is firstly converted to a time-frequency (TF) signal (using an OFDM demodulator) followed by post-processing of this TF signal into a DD domain signal. In this paper, we show that the spectral efficiency (SE) performance of a two-step conversion based receiver degrades in very high mobility scenarios where the Doppler shift is a significant fraction of the communication bandwidth (e.g., control and non-payload communication (CNPC) in Unmanned Aircraft Systems (UAS)). We therefore consider an alternate conversion, where the received TD signal is directly converted to the DD domain. The resulting received DD domain signal is shown to be not the same as that obtained in the two-step conversion considered in prior works. The alternate conversion does not require an OFDM demodulator and is shown to have lower complexity than the two-step conversion. Simulations reveal that even in very high mobility scenarios, the SE and symbol error rate performance of the alternate conversion is invariant of Doppler shift and is significantly better than the performance achieved with two-step conversion (which degrades with increasing Doppler shift).

Doppler-warping transform and its application to estimating acoustic target velocity

When there appears the relative motion between the sound source and the receiver, the received tonal signal will produce Doppler shift, and the Doppler information is relevant to the motion parameters of the target. According to the acoustic Doppler shift frequency, we propose the Doppler-warping transformation. The phase linearization of the Doppler signal can be realized by using the transformation. Then, we deduce the warping operator and propose an algorithm for estimating the target motion parameters by using the proposed transformation. Firstly, the Doppler-warping operator under different motion parameters is constructed. Secondly, the time resampling of the received time-domain signal is performed by using the operator. Then, the spectral function of the transformed signal is calculated. Finally, the spectral function entropy is minimized to estimate the objective parameter. The simulation results show the effectiveness of the proposed method. For a low signal-to-noise ratio of sea trial data, where the real speed of a fishing boat is 4.5 m/s, the result from the traditional minimum mean square error (MMSE) velocity estimation method is 5.2 m/s, and the estimation error is 15.56%. The proposed Doppler-warping transform method can estimate the target velocity more accurately, specifically, it is 4.7 m/s and its corresponding estimation error is 4.44%.

Micro-crack detection of nonlinear Lamb wave propagation in three-dimensional plates with mixed-frequency excitationyg**Project supported by the National Natural Science Foundation of China (Grant Nos. 61571222, 61602235, and 11474160) and the Six Talent Peaks Project of Jiangsu Province, China.

We propose a nonlinear ultrasonic technique by using the mixed-frequency signals excited Lamb waves to conduct micro-crack detection in thin plate structures. Simulation models of three-dimensional (3D) aluminum plates and composite laminates are established by ABAQUS software, where the aluminum plate contains buried crack and composite laminates comprises cohesive element whose thickness is zero to simulate delamination damage. The interactions between the S0 mode Lamb wave and the buried micro-cracks of various dimensions are simulated by using the finite element method. Fourier frequency spectrum analysis is applied to the received time domain signal and fundamental frequency amplitudes, and sum and difference frequencies are extracted and simulated. Simulation results indicate that nonlinear Lamb waves have different sensitivities to various crack sizes. There is a positive correlation among crack length, height, and sum and difference frequency amplitudes for an aluminum plate, with both amplitudes decreasing as crack thickness increased, i.e., nonlinear effect weakens as the micro-crack becomes thicker. The amplitudes of sum and difference frequency are positively correlated with the length and width of the zero-thickness cohesive element in the composite laminates. Furthermore, amplitude ratio change is investigated and it can be used as an effective tool to detect inner defects in thin 3D plates.

Impulse Noise Correction in OFDM Systems

In orthogonal frequency division multiplexing (OFDM) systems, the impulse noise causes catastrophic accuracy degradation since the impulse noise affects all the subcarriers in a symbol due to the fast Fourier transform (FFT) operations at the receiver. Potential causes of impulse noise include erasure channel, power switching, and circuit failure in integrated circuits. In this paper, from a practical observation, a novel iterative impulse error correction scheme is proposed. This scheme is referred to as the impulse noise location and value search algorithm, which is based on the crucial observation of the relationship of the impulse noise and the symbol constellation. In a 512-FFT OFDM system at 25 dB additive white Gaussian noise signal-to-noise ratio, for quadrature amplitude modulation (QAM)-4 and QAM-8 modulation, simulation results show that our proposed novel scheme can effectively correct impulse errors that corrupt up to 20.7 % and 13.9 % of the received time-domain signal at known locations. In addition, without the knowledge of impulse noise location, the proposed scheme still can correct at least 9.96 % of the received time-domain signal for QAM-4 modulation.

Restoration of ultrasonic NDE images

The problem of determining the ultrasonic reflectivity function resulting from density changes in a material is posed as an image restoration problem. The received time-domain signal is modeled as resulting from convolution of the acoustic pulse with the reflectivity function with additive noise. A new deconvolution method is used to estimate the reflectivity function. The method operates by an iterative maximum-a-posteriori (MAP) optimization method that not only estimates the deconvolved function but incorporates the a prior knowledge that the material is relatively uniform, except for step discontinuities. Finally, an annealing algorithm is added that allows the optimization to avoid many local minima. Initial results, based on application of the method to synthetic signals and to real signals from a nondestructive evaluation sensor, are presented. >

A helicopter-borne eight-channel ranging scatterometer for remote sensing. I. System description

For pt.II see ibid., vol.31, no.1, p.170-9 (1993). HUTSCAT, a helicopter-borne dual-frequency FM-CW scatterometer, is described. The HUTSCAT measures the backscattering properties of a target with a range resolution of 65 cm. The real-time ranging capability is obtained by performing the fast Fourier transform (FFT) to the received time-domain signal. The measurement is made simultaneously at eight channels (VV, HH, HV, and VH modes of polarization at 5.4 GHz and 9.8 GHz). The scatterometer measures the radar return spectrum for eight channels in 16.6 ms, which corresponds to an along-track distance of 0.33 m for the helicopter speed of 20 m/s. The radar system has been designed for remote sensing of forests, sea ice, and snow.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Multiple phase-screen calculation of the temporal behavior of stochastic waves

An analytical/numerical technique is presented that provides a solution to the parabolic wave equation and allows for direct calculation of the received time-domain signal after propagation of a wide-bandwidth waveform through a turbulent ionized medium. The results are applicable to the case of satellite communication or to radar observation through a frequency-selective transionospheric propagation channel. A number of examples of the application of multiple phase-screen (MPS) techniques are given that enhance intuitive understanding of wave propagation. Numerical predictions of scattering through a striated barium cloud are compared to experimental observations. Good agreement is also shown between MPS calculations and theoretical results for cases in both weak and strong scattering regimes.

Time Domain Equalization Method for DFTS-OFDM Signal without GI under Highly Mobile Environments

In highly time-varying fading channel, the Discrete Fourier Transform Spreading Orthogonal Frequency Division Multiplexing (DFTS-OFDM) signal would be damaged significantly by the inter-channel interference (ICI) due to the loss of orthogonality among subcarriers which leads a fatal degradation of bit error rate (BER) performance. To solve this problem, this paper proposes a time domain equalization (TDE) technique in conjunction with a time domain channel impulse response (CIR) estimation method for the DFTS-OFDM signal without using a guard interval (GI). The features of proposed method is to employ a time domain training sequence (TS) both for the estimation of time domain CIR at every sampling time and for removing the inter-symbol interference (ISI) incurred in the multipath fading channel. The proposed method also employs the TDE with a maximum likelihood (ML) estimation method in the demodulation of received time domain signal at every symbol instead of using the conventional Minimum Mean Square Error-Frequency Domain Equalization (MMSE-FDE) method. This paper presents various simulation results to demonstrate the effectiveness of proposed demodulation method for the DFTSOFDM signal without GI as comparing with the conventional MMSE-FDE and TDE methods both of usingGI under highly mobile environments.