Xu Xiaomei

Identification of overlapped ultrasonic NDE echoes with adaptive deconvolution

Ultrasonic nondestructive evaluation (NDE) application, narrow bandwidth of ultrasonic transducer will cause highly overlapped echoes, limiting the characterization performance of inspection. The least mean square (LMS) adaptive algorithm has gained wide applications in adaptive filtering, control and signal processing due to its effectiveness, low computational complexity and simplicity. The purpose of this paper is to investigate the LMS algorithm based adaptive deconvolution techniques in order to improving the time resolution of ultrasonic nondestructive detection. The theoretical basis as well as the implementation is provided. Experimental results on physical ultrasonic NDE data conforms the resolution improvement of proposed method.

Comparison of the performance of LDPC codes over different underwater acoustic channels

The fast temporal variations, long multipath delay spreads, and severe frequency-dependent attenuations of underwater acoustic communication channels are extremely complex that impedes underwater acoustic (UWA) data transmission. To alleviate this problem, channel coding is indispensable in UWA communication system to increase the reliability. In this paper, performances of LDPC codes with different parameters over different underwater acoustic communication channels are presented, which is to know how to adjust the encoding and decoding parameters according to different underwater acoustic channels. The results indicate that LDPC codes have a good performance of the estimated BER on the order of 10−4 −10−5 either in positive sound velocity gradient (PSVG) underwater channel or negative sound velocity gradient (NSVG) one, even invariable sound velocity gradient (ISVG) one. In addition, whatever the encoding or decoding parameters is adopted, performance of the LDPC codes over the NSVG underwater channel is the best, compared with PSVG one and ISVG one. Furthermore, in the desired performance of BER of 10−5, the encoding and decoding parameters according to the three different underwater channels specifications are summarized.

Application of evolutionary neural network in impact acoustics based nondestructive inspection of tile-wall

The degradation of the bonding of the tile-wall structure has emerged as an urgent issue in metropolitan cities, with more and more tile-dropping accidents caused. In order to ascertain the bonding integrity of tile-walls, impact acoustics features obtained from the sound signals generated by controlled impact are employed to develop a rapid and effective nondestructive inspection technique. To facilitate the automatic interpretation, the multilayer artificial neural network (ANN) is used as a cost-effective classifier superior to traditional statistics methods. Nonetheless, the classical gradient descent based backpropagation (BP) training strategy of the multilayer neural network faces certain drawbacks, e.g., very slow convergence, easily getting stuck in a local minimum. In this paper, an evolutionary algorithm based training method is developed to train the ANN and perform the automatic classification of bonding integrity. The design, feature extraction approach, training and application of the proposed evolutionary neural network are presented. The classification results obtained experimentally from prepared sample slabs are presented and compared with that with BP algorithm, demonstrating the validity of the proposed methodology.

A frame synchronization method for underwater acoustic communication on mobile platform

In this paper, we propose a frame synchronization method for underwater acoustic communication on mobile platform. This new method involves transmitting signal based on hyperbolic frequency modulated (HFM) signal as a preamble of the frame, while the receiver uses a correlator which is matched to the transmitted signal. This method can provide us with good signal detection ability which is very important in underwater acoustic (UWA) channels, and can provide Doppler scale estimation for accurate synchronization. It can also provide multiple access capability for UWA communication network. We analyze and simulate the performance of the HFM signal. A contrast is made with the linearly-frequency modulated (LFM) signal compared to the LFM based method. The proposed method works with a robust correlation output of the Doppler Effect and is suited to handle the presence of dense multi-path channels.

Simulation Study of Multi-Path Characteristics of Acoustic Propagation in Shallow Water Wireless Channel

Multi-path interference due to boundary reflection in shallow water acoustic communication poses the major obstacle to reliable, high-speed underwater communication system. Based on the typical sound speed profiles of Taiwan Strait, this paper adopt fast-field program to simulate the delay time and amplitude fluctuation of multi-path under different channel conditions. Simulation results reveal the relationships between delay, amplitude and typical channel parameters, such as sound- speed gradient, distance, channel depth, which provides a valuable guide for the practical design of shallow-water wireless acoustic communication system.

RA coding joint adaptive equalization for underwater acoustic communications

Underwater acoustic (UWA) communication channels that vary from stationary with sparse arrivals to rapidly varying and fully reverberant present fading, multipath and refractive properties, greatly impede UWA data transmissions. In order to solve this problem, a receiver with the iterative soft-input/soft-output equalization and decoding that employs Repeat-Accumulate (RA) coding joint adaptive decision feedback equalizer (DFE) is proposed. By using computer simulations, a 5-path UWA channel model is built to establish the UWA communication system model with the proposed receiver structure. The error correction capability of RA codes is discussed and how the parameter affects on the performance of the adaptive DFE with LMS algorithm is analyzed. Simulation results show that the UWA communication system with the proposed channel coding scheme can get about 6dB coding gain comparing with that without channel coding, and the DFE with proper parameters can overcome the effects of the ISI and channel fading, lower the overall bit error ratio (BER). Furthermore, due to the receiver structure which performs in RA decoder with 10 iterations and in adaptive DFE with proper parameters, the reliability of the system can get significant improvement, demonstrating that the iterative soft-input/soft-output equalization and decoding which employs RA coding combined with the adaptive DFE, with algorithmic simplicity, will have good prospect in UWA communications.

Study on frequency coding anti-interference system for data transmission in shallow water acoustic channels

Achieving reliable underwater communication in shallow water acoustic channels is a difficult task because of the random time-varying nature of multipath propagation, severe amplitude fluctuation, and spatial variability of the channel conditions. This paper describes a new signal processing technique-frequency coding and decoding by means of real-time measurement of signal width, jamming and suppressing multipath interference and using redundant coder. The application of the technique to the model ZIY-1 status monitor for underwater system of scabed is introduced in this paper. The main principle, the technique specifications and the key techniques of the system are discussed here. Theoretical estimations and experimental results proved that the performance of the system is excellent. The method can be used for some other related low-data-rate data transmission detecting in shallow water acoustic channel.

Application analysis of QC-LDPC codes in shallow water acoustic channels

The shallow water acoustic (SWA) channel is one of the most challenging channels in wireless digital communications. In this paper, we propose a good performance, low complexity channel coding scheme using quasi-cyclic LDPC (QC-LDPC) codes to improve the communication system reliability for SWA channels. A system simulation model including QC-LDPC codes for the SWA channels is built. With the decoding algorithm of belief propagation (BP), the effects of QC-LDPC codes design parameters, such as girth, circulant permutation matrix size and decoding iterations are discussed. To verify the proposed approach, Quangang Xiaocuo harbor blasting data are used to set up a SWA channel model, and the performances of QC-LDPC codes over the channel model are analyzed. The results show that, with the same parameters, the proposed QC-LDPC channel coding strategy for SWA channels substantially outperforms the conventional (3, 6)-LDPC channel coding strategy, and with less complexity and easy implementation.

High-order modulation based on repeat-accumulate codes for underwater acoustic communications

Several characteristics of underwater acoustic (UWA) communication channels-limited bandwidth, extended multipath and severe fading-impede UWA data transmissions. To solve this problem, we propose to set up a receiver combining repeat-accumulate (RA) coding and high-order modulation, and calculate the channel soft information for RA decoding with high-order modulation. A multipath fading acoustic channel model for the southern Taiwan Strait channel is established based on BELLHOP ray model and its real sound speed profile. The paper also discusses the error correction capability of the different combination of RA coding and high-order modulation. Simulation results show that the system performance may be improved by using error correction coding; however, its data rate will be reduced. While the frequency efficiency grows significantly, the reliability cannot be improved remarkably.

Application of stochastic resonance theory in weak underwater acoustic signal detection

This paper researches the application of stochastic resonance theory in weak underwater acoustic signal detection, and derives the conditions to maximum switching rate of bistable system. Considering the character of underwater acoustic signals, this paper studies the bistable system parameter adjustment and signal frequency compression technology. Through digital signal processing experiments, it contrasts the stochastic resonance system and traditional digital filter, proves the superiority of stochastic resonance theory, which may be widely used in weak underwater acoustic signal detection.