TIME-VARYING DOPPLER COMPENSATION FOR UNDERWATER ACOUSTIC COMMUNICATION. PDF Download

Author: Seyedhabib Mirhedayati Roudsari
Publisher:
ISBN:
Category :
Languages : en
Pages : 0

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Book Description
This thesis focuses on time-varying Doppler compensation for underwater acoustic communications using an orthogonal frequency division multiplexing (OFDM) system. An underwater acoustic channel simulator is developed based on the geometry of the system deployment and by considering the statistics of the random variation of the channel. This channel simulator is capable of modelling any relative motion between the transmitter and receiver. The effect of the underwater acoustic (UWA) channel on OFDM system is studied by incorporating a model in system level simulation of the communication link. The time-varying Doppler compensation method is then introduced as an innovative method which is able to compensate for Doppler shift due to time-varying relative velocity between the transmitter and receiver. Finally, the proposed Doppler compensation method is integrated to an ultrasonic OFDM system and the communication reliability is evaluated through simulations and measurements in realistic deployment conditions. Specifically, for a distance of 150 m a net throughput after synchronization, channel estimation and decoding 760 bit/s was demonstrated with a bit error rate of 0.035 for a low SNR of 5.5 dB. More importantly, it will be shown that the assessment of the communication performance realized using simulations is very close to that of the measured performance.

Author: Ammar Ebdelmelik Abdelkareem
Publisher:
ISBN:
Category :
Languages : en
Pages : 136

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Author: Franz Hlawatsch
Publisher: Academic Press
ISBN: 0080922724
Category : Technology & Engineering
Languages : en
Pages : 457

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Book Description
As a result of higher frequencies and increased user mobility, researchers and systems designers are shifting their focus from time-invariant models to channels that vary within a block. Wireless Communications Over Rapidly Time-Varying Channels explains the latest theoretical advances and practical methods to give an understanding of rapidly time varying channels, together with performance trade-offs and potential performance gains, providing the expertise to develop future wireless systems technology. As well as an overview of the issues of developing wireless systems using time-varying channels, the book gives extensive coverage to methods for estimating and equalizing rapidly time-varying channels, including a discussion of training data optimization, as well as providing models and transceiver methods for time-varying ultra-wideband channels. - An introduction to time-varying channel models gives in a nutshell the important issues of developing wireless systems technology using time-varying channels - Extensive coverage of methods for estimating and equalizing rapidly time-varying channels, including a discussion of training data optimization, enables development of high performance wireless systems - Chapters on transceiver design for OFDM and receiver algorithms for MIMO communication channels over time-varying channels, with an emphasis on modern iterative turbo-style architectures, demonstrates how these important technologies can optimize future wireless systems

Author: Sadia Ahmed
Publisher:
ISBN:
Category : Doppler effect
Languages : en
Pages : 154

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Book Description
Water occupies three fourth of earth's surface. The remaining one fourth is land. Although human habitats reside on land, there is no denying of the vital connection between land and water. The future sustainability of human species on this planet depends on wise utilization of all available resources, including that provided by the vast water world. Therefore, it is imperative to explore, understand, and define this massive, varying, and in many areas, unexplored water domain.

Author: Saed Daoud
Publisher:
ISBN:
Category :
Languages : en
Pages : 115

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Book Description
In this thesis we consider Orthogonal Frequency Division Multiplexing (OFDM) technique by taking into account in the receiver design the fundamental and unique characteristics of Underwater Acoustic (UWA) channels in the context of Relay-Assisted (RA) systems. In particular, OFDM technique is used to combat the problem of Intersymbol Interference (ISI), while to handle the Intercarrier Interference (ICI), a pre-processing unit is used prior to the Minimum Mean Squared Error (MMSE) frequency-domain equalization called Multiple Resampling (MR), which minimizes the effect of time variation. This pre-processor consists of multiple branches, each corresponds to a Doppler scaling factor of a path/user/cluster, and performs of frequency shifting, resampling, and Fast Fourier Transform (FFT) operation. As a suboptimal alternative to MR pre-processing, Single Resampling (SR) pre-processing is also used to reduce the effect of ICI in the system, and it consists of only one branch that performs frequency shifting, resampling, and FFT operation, which corresponds to one approximated resampling factor, that is a function of one or more of the actual Doppler scaling factors. The problem of bandwidth scarcity is considered in the context of Two Way Relaying (TWR) systems in an attempt to increase the bandwidth efficiency of the system, while the problem of fading is considered in the context of Distributed Space-Time Block Coding (D-STBC) to boost the system reliability. Also, joint TWR-D-STBC system is proposed to extract the advantages of both schemes simultaneously. Second, motivated by the fact that OFDM is extremely sensitive to time variation, which destroys the orthogonality between the subcarriers, we consider another candidate to UWA channels and competitor to OFDM scheme, namely, block-based Single Carrier (SC) modulation with Frequency Domain Equalization (FDE). We start by the Point-to-Point (P2P) systems with path-specific Doppler model and Multiple Access Channel (MAC) system with user-specific Doppler model. The Maximum Likelihood (ML) receiver in each case is derived, and it is shown that a MR pre-processing stage is necessary to handle the effect of time variation, as it is the case in OFDM. Different from OFDM, however, the structure of this pre-processing stage. Specifically, it consists of multiple branches and each branch corresponds to a Doppler scaling factor per path or per user, and performs frequency shifting, resampling, and followed by and integration. FFT operation is not a part of the pre-processor. The goal of this pre-processing stage is to minimize the level of time variation in the time domain. So, the output of the pre-processor will still be time-varying contaminated by ISI, and hence an equalization stage is required. To avoid the complexity of the optimum Maximum Likelihood Sequence Detector (MLSD), we propose the use of MMSE FDE, where the samples are transformed to the frequency domain by means of FFT operation, and after the FDE transformed back to the time domain, where symbol-by-symbol detection becomes feasible. Also, the channels are approximated such that all paths or all users have the same Doppler scaling factor, and the pre-processing stage in this case consists of only one branch and it is called SR. Having the basic structure of SC-FDE scheme, we then consider the corresponding schemes that are considered for OFDM systems, namely: TWR, D-STBC, and TWR-D-STBC schemes. A complete complexity analysis, bandwidth efficiency, and extensive Average Bit Error Rate (ABER) simulation results are given. It is shown that MR schemes outperforms its SR counterparts within a given signaling scheme (i.e., OFDM or SC-FDE). However, this superiority in performance comes at the expense of more hardware complexity. Also, for uncoded systems, MR-SC-FDE outperforms its OFDM counterpart with less hardware complexity, because in SC-FDE systems, FFT operation is not part of the MR pre-processor, but rather a part of the equalizer. Finally, under total power constraint, it is shown that TWR-D-STBC scheme serves as a good compromise between bandwidth efficiency and reliability, where it has better bandwidth efficiency with some performance loss compared to D-STBC, while it has better performance and the same bandwidth efficiency compared to TWR.

Author: Sheng Zhou
Publisher: John Wiley & Sons
ISBN: 1118693817
Category : Science
Languages : en
Pages : 498

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Book Description
A blend of introductory material and advanced signal processing and communication techniques, of critical importance to underwater system and network development This book, which is the first to describe the processing techniques central to underwater OFDM, is arranged into four distinct sections: First, it describes the characteristics of underwater acoustic channels, and stresses the difference from wireless radio channels. Then it goes over the basics of OFDM and channel coding. The second part starts with an overview of the OFDM receiver, and develops various modules for the receiver design in systems with single or multiple transmitters. This is the main body of the book. Extensive experimental data sets are used to verify the receiver performance. In the third part, the authors discuss applications of the OFDM receiver in i) deep water channels, which may contain very long separated multipath clusters, ii) interference-rich environments, where an unintentional interference such as Sonar will be present, and iii) a network with multiple users where both non-cooperative and cooperative underwater communications are developed. Lastly, it describes the development of a positioning system with OFDM waveforms, and the progress on the OFDM modem development. Closely related industries include the development and manufacturing of autonomous underwater vehicles (AUVs) and scientific sensory equipment. AUVs and sensors in the future could integrate modems, based on the OFDM technology described in this book. Contents includes: Underwater acoustic channel characteristics/OFDM basics/Peak-to-average-ratio control/Detection and Doppler estimation (Doppler scale and CFO)/Channel estimation and noise estimation/A block-by-block progressive receiver and performance results/Extensions to multi-input multi-output OFDM/Receiver designs for multiple users/Cooperative underwater OFDM (Physical layer network coding and dynamic coded cooperation)/Localization with OFDM waveforms/Modem developments A valuable resource for Graduate and postgraduate students on electrical engineering or physics courses; electrical engineers, underwater acousticians, communications engineers

Author: Baosheng Li
Publisher:
ISBN:
Category : Orthogonal frequency division multiplexing
Languages : en
Pages :

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Book Description
Underwater acoustic channels are wideband in nature due to the fact that the signal bandwidth is not neglibible with respect to the center frequency. OFDM transmissions over UWA channels encounter frequency-dependent Doppler drifts that destroy the orthogonality among OFDM subcarriers. In this paper, we propose a two-step approach to mitigating the frequency-dependent Doppler drifts in zero-padded OFDM transmissions over fast-varying channels: (1) non-uniform Doppler compensation via resampling that converts a "wideband" problem into a "narrowband" problems; and (2) high-resolution uniform compensation on the residual Doppler. Based on block-by-block processing, our receiver does not rely on channel dependence across OFDM blocks, and is thus desirable for fast-varying UWA channels. We test our receiver with data from a shallow water experiment at Buzzards Bay, Massachusetts. Our receiver achieves excellent performance even when the transmitter and the receivers have a relative speed up to 10 knots, where the Doppler drifts are several times larger than the OFDM subcarrier spacing.

Author: Samar Kaddouri
Publisher:
ISBN:
Category : Adaptive signal processing
Languages : en
Pages : 120

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Book Description
The underwater channel poses numerous challenges for acoustic communication. Acoustic waves suffer long propagation delay, multipath, fading, and potentially high spatial and temporal variability. In addition, there is no typical underwater acoustic channel; every body of water exhibits quantiably different properties. Underwater acoustic modems are traditionally operated at low frequencies. However, the use of broadband, high frequency communication is a good alternative because of the lower background noise compared to low-frequencies, considerably larger bandwidth and better source transducer efficiency. One of the biggest problems in the underwater acoustic communications at high frequencies is time-selective fading, resulting in the Doppler spread. While many Doppler detection, estimation and compensation techniques can be found in literature, the applications are limited to systems operating at low frequencies contained within frequencies ranging from a few hundred Hertz to around 30 kHz. This dissertation proposes two robust channel estimation techniques for simultaneous transmissions using multiple sources and multiple receivers (MIMO) that closely follows the rapidly time-varying nature of the underwater channel. The first method is a trended least square (LS) estimation that combines the traditional LS method with an empirical modal decomposition (EMD) based trend extraction algorithm. This method allows separating the slow fading modes in the MIMO channels from the fast-fading ones and thus achieves a close tracking of the channel impulse response time fluctuations. This dissertation also outlines a time-varying underwater channel estimation method based on the channel sparsity characteristic. The sparsity of the underwater communication channel is exploited by using the MIMO P-iterative greedy orthogonal matching pursuit (MIMO-OMP) algorithm for the channel estimation. Both techniques are demonstrated in a fully controlled environment, using simulated and experimental data. To test the proposed channel estimation techniques, an acoustic model for a MIMO channel is developed using the method of images applied to a completely closed three-dimensional duct with a pressure release surface boundary and five rigid walls. The MIMO simulated channel provides the strength and delay of all echoes forming the channel. Both simulation and experimental results show a signicant improvement in the estimation of the channel impulse response, thus validating the two proposed algorithms.

Author: Ali Bassam
Publisher:
ISBN:
Category :
Languages : en
Pages : 0

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Book Description
In this thesis, a time-domain Doppler estimator, tracker, and compensator using shift-orthogonal OFDM pilot sequences are designed based on the theory presented and tested using simulations and real-life experiments. OFDM signals are assumed and used throughout the dissertation, and a shift-orthogonal OFDM pilot sequence is designed. The UWA channel is analyzed and a simplified theoretical model is presented that holds under achievable conditions. The Doppler estimator is then developed, which uses an approach reminiscent of existing differential demodulation techniques. The estimator is developed with the flexibility of handling any Mach number provided the designer has the liberty of adjusting the signal's bandwidth, carrier frequency, or preamble size. The CRLB for the estimator is derived. The Doppler estimator performance is compared against existing estimators in literature and is shown to outperform most existing estimators in terms of MSE, with the added features of Doppler tracking and low computational complexity. The Mach number estimates also have a closed-form expression. A Doppler tracker is developed that reduces the size of the Mach number estimates' array while simultaneously tracking significant changes in the Mach number. A novel and practical Doppler compensator based on the proposed estimator is developed, which compensates the Doppler effect in two stages. The first stage involves resampling, which takes in the estimates at the tracker output and applies a form of time-varying resampling which we call block-by-block resampling. The second stage involves estimating the residual Doppler shift with the proposed Doppler estimator, followed by eliminating the residual shift via a simple phase rotation. The proposed tracker and compensator subsystem is shown to outperform most existing compensators in terms of MSE, while also being more computationally efficient. Each element of the proposed receiver subsystem is tested in simulations and the results are shown to agree with theory. Finally the full receiver subsystem is tested in real undersea experiments at various ranges, bandwidths and power levels, and the subsystem is shown to yield minimal residual errors when tracking and compensating the Mach number.

Author: Ballard Justin Smith Blair
Publisher:
ISBN:
Category : Acoustic models
Languages : en
Pages : 215

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Book Description
Underwater wireless communication is quickly becoming a necessity for applications in ocean science, defense, and homeland security. Acoustics remains the only practical means of accomplishing long-range communication in the ocean. The acoustic communication channel is fraught with difficulties including limited available bandwidth, long delay-spread, time-variability, and Doppler spreading. These difficulties reduce the reliability of the communication system and make high data-rate communication challenging. Adaptive decision feedback equalization is a common method to compensate for distortions introduced by the underwater acoustic channel. Limited work has been done thus far to introduce the physics of the underwater channel into improving and better understanding the operation of a decision feedback equalizer. This thesis examines how to use physical models to improve the reliability and reduce the computational complexity of the decision feedback equalizer. The specific topics covered by this work are: how to handle channel estimation errors for the time varying channel, how to use angular constraints imposed by the environment into an array receiver, what happens when there is a mismatch between the true channel order and the estimated channel order, and why there is a performance difference between the direct adaptation and channel estimation based methods for computing the equalizer coefficients. For each of these topics, algorithms are provided that help create a more robust equalizer with lower computational complexity for the underwater channel.