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A Study of Pattern Time Delay Shift Coding Communication in Underwater Acoustic Multipath Channel

Author: YinJingWei
Tutor: HuiJunYing
School: Harbin Engineering University
Course: Underwater Acoustics
Keywords: Underwater acoustic communication Pattern time delay shift coding scheme Time reversal mirror Fractional Fourier transform Mobile underwater acoustic communication Multiuser underwater acoustic communication
CLC: TB567
Type: PhD thesis
Year: 2007
Downloads: 688
Quote: 9
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Abstract


The underwater acoustic channel is random temporal and spatial varying, and reflection at the boundaries and ray bending are two fundamental mechanisms of multipath formation. The intersymbol interference (ISI) caused by the multipath channel is one of the primary barriers to underwater acoustic (UWA) communication. In order to acquire high information data rate at low bit error rate (BER), efficient techniques must be adopted to reduce the ISI.Pattern Time Delay Shift Coding (PDS) scheme makes use of the time delay shift values of the Pattern to code the information, and the duty cycle is less than 1 which could economize the system power. The PDS scheme adopts the code division and correcting-codes in order to enable every information-code to be against the ISI which is more robust against distortions, multipath fading and noise interference in the channel due to the larger bandwidth. A communication scheme is proposed which integrates the spread spectrum (SS) with PDS scheme called SS-PDS for short. SS-PDS scheme could achieve good performance of the spread spectrum and improve the information data rate, that could be competent for long-range UWA communication, and its multiple access working is valid for underwater acoustic networks.Time reversal mirror (TRM) is a fresh signal processing technology in underwater acoustic field recently which takes advantage of the heterogeneity of the medium to match the acoustic channel automatically without any transcendental knowledge. This dissertation analyses the theory of time reversal array processing and its focusing character in frequency- and temporal-domain. Traditionally, the time reversal mirror called ATRM is active, which needs monostatic sensor, and the signal propagates through the acoustic channel twice leading to low efficiency. Further, the array processing enhances the complexity, which could not satisfy the modem’s requirement of simple structure and low-power cost. In order to overcome these disadvantages and promote the TRM to be used in the UWA communication, the single-element passive time reversal mirror (PTRM) and virtual time reversal mirror (VTRM) are presented innovatively in this dissertation, the focusing gain of single-element TRM is calculated, and then ATRM, PTRM and VTRM are analogical analyzed. The proposed PTRM in this dissertation, which breaks the limitation of the waveform consistency of probe signal with information-code, is different from literatures.The applications of the single-element PTRM and VTRM to the UWA communication system based on PDS and SS-PDS scheme are able to focus the multipath signal and adaptively equalize the acoustic channel real time, further mitigate ISI and enhance SNR, which enable UWA communication with low-BER. The scheme for multiuser communication is presented based on TRM-PDS system, which is significant for realizing the underwater acoustic networks.The application of fractional Fourier transform (FRFT) in mobile UWA communication where large Doppler exists is studied according to FRFT theory and characters. Underwater acoustic channel parameters estimation, synchronous detection, and PDS scheme using FRFT are put forward.The communication range can be evidently enlarged by locating the modem at the depth corresponding to minimal sound velocity value and making use of convergence zone, according to the research on the physical characters of deep water channel. Based on this, the scheme for long-range deep water UWA communication is offered using TRM channel equalization and chaotic SS-PDS.Every key technology is simulated both under deep water and shallow water and validated by lake trials at last. Mobile UWA communication lake trials results further prove the feasibility and robustness of the single-element TRM and its application to PDS communication system in practice.

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CLC: > Industrial Technology > General industrial technology > Acoustic engineering > Underwater Acoustics > Underwater communication ( Sonar communication)
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