Peak-to-Average Power Ratio (PAR) Reduction for Acoustic OFDM Systems

Projecte fet en col.laboració amb el Massachusetts Institute of Technology (MTI

Performance evaluation of T-transform based OFDM in underwater acoustic channels

PhD ThesisRecently there has been an increasing trend towards the implementation of orthogonal frequency division multiplexing (OFDM) based multicarrier communication systems in underwater acoustic communications. By dividing the available bandwidth into multiple sub-bands, OFDM systems enable reliable transmission over long range dispersive channels. However OFDM is prone to impairments such as severe frequency selective fading channels, motioned induced Doppler shift and high peak-to-average-power ratio (PAPR). In order to fully exploit the potential of OFDM in UWA channels, those issues have received a great deal of attention in recent research. With the aim of improving OFDM's performance in UWA channels, a T-transformed based OFDM system is introduced using a low computational complexity T-transform that combines the Walsh-Hadamard transform (WHT) and the discrete Fourier transform (DFT) into a single fast orthonormal unitary transform. Through real-world experiment, performance comparison between the proposed T-OFDM system and conventional OFDM system revealed that T-OFDM performs better than OFDM with high code rate in frequency selective fading channels. Furthermore, investigation of different equalizer techniques have shown that the limitation of ZF equalizers affect the T-OFDM more (one bad equalizer coefficient affects all symbols) and so developed a modified ZF equalizer with outlier detection which provides major performance gain without excessive computation load. Lastly, investigation of PAPR reduction methods delineated that T-OFDM has inherently lower PAPR and it is also far more tolerant of distortions introduced by the simple clipping method. As a result, lower PAPR can be achieved with minimal overhead and so outperforming OFDM for a given power limit at the transmitter

시변 스파스 수중 음향 통신 채널 매개변수 추정

학위논문 (박사)-- 서울대학교 대학원 : 조선해양공학과, 2012. 8. 성우제.This dissertation addresses the problem of estimating the channel parameters of time-varying sparse underwater acoustic communication channels. A new method to estimate the channel parameters including arrival time delay, incidence angle, Doppler frequency, and complex amplitude of impinging wave components is presented. The new method exploits the sparse structure of the wideband underwater acoustic communication channel and is based on the matching pursuit which iteratively identifies multipath components by projecting the target signal on the columns of dictionary which are hypothesized by the channel parameters. Because of the large dimension of the parameter space, the size of dictionary can be prohibitively large especially when the parameter range is oversampled for effective sparse approximation. In order to prevent the dictionary from being too large, the parameter estimation is achieved in two stages which are the identification and the iterative estimation stages. In the identification stage, the initial parameter values are identified using a pre-computed dictionary of low coherence. In the next estimation stage, a coherent and redundant dictionary of the oversampled parameter range is constructed from the identified parameter values, and the channel parameters are estimated by projecting the residual signal onto the redundant dictionary. To reduce memory requirement and computational complexity caused by using the redundant dictionary, a space-alternating estimation scheme is introduced to separate the parameter search space. The space-alternating scheme limits the size of the redundant dictionary within practical extent and accordingly reduces the computational burden of the matrix-vector product required in the iteration. The performance of the new method is evaluated via Monte Carlo simulation and real channel measurement data analysis. The Monte Carlo simulation evaluates the resolution performance by resolving two paths of small parameter differences, and its result shows that the new method successfully decomposes multipath components whose parameter differences are merely a subfraction of the resolution limit of the classical correlation-based method. It is also applied to the experimental data obtained in the large scale water tank which is capable of making a surface gravity wave with designated wave parameter. The channel parameters under the time-varying regular surface wave condition is analytically derived from a simple reflection constraint, and the channel parameters by the new channel estimation method are compared with those analytic solution showing that the estimation results are consistent with theoretical expectation. Finally, it is applied to the real channel data of shallow water which were acquired at various transmitter-receiver ranges. The performance of the estimated channel parameters is evaluated indirectly via comparison of the channel characteristic functions which are the delay-Doppler-spread function, the angle-Doppler-spread function, and the power delay and angle profiles. The comparison result shows that the estimated channel parameters coincide well with the channel characteristic functions obtained by the matched filter and accordingly proves that the presented method gives consistent estimation result for the estimation of real channel parameters.ABSTRACT I TABLE OF CONTENTS III LIST OF TABLES V LIST OF FIGURES VI CHAPTER 1 INTRODUCTION 1 1.1 BACKGROUND 1 1.2 DEFINITIONS 3 1.3 PRIOR WORK 4 1.4 THESIS OUTLINE 8 CHAPTER 2 SPARSE CHANNEL ESTIMATION 9 2.1 ANGLE-DELAY-DOPPLER-SPREAD FUNCTION 9 2.2 ORTHOGONAL MATCHING PURSUIT 17 2.3 SPACE-ALTERNATING MATCHING PURSUIT 23 2.4 COMPUTATIONAL COMPLEXITY 27 CHAPTER 3 PARAMETER ESTIMATION OF SYNTHETIC CHANNEL 33 3.1 PERFORMANCE EVALUATION METHOD 33 3.2 MEAN SQUARE ERROR PERFORMANCE 36 CHAPTER 4 PARAMETER ESTIMATION OF REAL CHANNELS 42 4.1 WATER TANK CHANNEL EXPERIMENT 42 4.1.1 Surface Reflected Signal Model 48 4.1.2 Comparison between Data and Model 51 4.2 SHALLOW WATER CHANNEL EXPERIMENT 55 4.2.1 Estimation of Incidence Angle 58 4.2.2 Estimation of Doppler Shift 62 4.2.3 Delay and Angle Profiles 67 CHAPTER 5 CONCLUSIONS 76 REFERENCES 78 초 록 82 감사의 글 85Docto

Doctor of Philosophy

dissertationThe demand for high speed communication has been increasing in the past two decades. Multicarrier communication technology has been suggested to address this demand. Orthogonal frequency-division multiplexing (OFDM) is the most widely used multicarrier technique. However, OFDM has a number of disadvantages in time-varying channels, multiple access, and cognitive radios. On the other hand, filterbank multicarrier (FBMC) communication has been suggested as an alternative to OFDM that can overcome the disadvantages of OFDM. In this dissertation, we investigate the application of filtered multitone (FMT), a subset of FBMC modulation methods, to slow fading and fast fading channels. We investigate the FMT transmitter and receiver in continuous and discrete time domains. An efficient implementation of FMT systems is derived and the conditions for perfect reconstruction in an FBMC communication system are presented. We derive equations for FMT in slow fading channels that allow evaluation of FMT when applied to mobile wireless communication systems. We consider using fractionally spaced per tone channel equalizers with different number of taps. The numerical results are presented to investigate the performance of these equalizers. The numerical results show that single-tap equalizers suffice for typical wireless channels. The equalizer design study is advanced by introducing adaptive equalizers which use channel estimation. We derive equations for a minimum mean square error (MMSE) channel estimator and improve the channel estimation by considering the finite duration of channel impulse response. The results of optimum equalizers (when channel is known perfectly) are compared with those of the adaptive equalizers, and it is found that a loss of 1 dB or less incurs. We also introduce a new form of FMT which is specially designed to handle doubly dispersive channels. This method is called FMT-dd (FMT for doubly dispersive channels). The proposed FMT-dd is applied to two common methods of data symbol orientation in the time-frequency space grid; namely, rectangular and hexagonal lattices. The performance of these methods along with OFDM and the conventional FMT are compared and a significant improvement in performance is observed. The FMT-dd design is applied to real-world underwater acoustic (UWA) communication channels. The experimental results from an at-sea experiment (ACOMM10) show that this new design provides a significant gain over OFDM. The feasibility of implementing a MIMO system for multicarrier UWA communication channels is studied through computer simulations. Our study emphasizes the bandwidth efficiency of multicarrier MIMO communications .We show that the value of MIMO to UWA communication is very limited

Robust frequency-domain turbo equalization for multiple-input multiple-output (MIMO) wireless communications

This dissertation investigates single carrier frequency-domain equalization (SC-FDE) with multiple-input multiple-output (MIMO) channels for radio frequency (RF) and underwater acoustic (UWA) wireless communications. It consists of five papers, selected from a total of 13 publications. Each paper focuses on a specific technical challenge of the SC-FDE MIMO system. The first paper proposes an improved frequency-domain channel estimation method based on interpolation to track fast time-varying fading channels using a small amount of training symbols in a large data block. The second paper addresses the carrier frequency offset (CFO) problem using a new group-wise phase estimation and compensation algorithm to combat phase distortion caused by CFOs, rather than to explicitly estimate the CFOs. The third paper incorporates layered frequency-domain equalization with the phase correction algorithm to combat the fast phase rotation in coherent communications. In the fourth paper, the frequency-domain equalization combined with the turbo principle and soft successive interference cancelation (SSIC) is proposed to further improve the bit error rate (BER) performance of UWA communications. In the fifth paper, a bandwidth-efficient SC-FDE scheme incorporating decision-directed channel estimation is proposed for UWA MIMO communication systems. The proposed algorithms are tested by extensive computer simulations and real ocean experiment data. The results demonstrate significant performance improvements in four aspects: improved channel tracking, reduced BER, reduced computational complexity, and enhanced data efficiency --Abstract, page iv

Orthogonal frequency division multiplexing with amplitude shift keying subcarrier modulation as a reliable and efficient transmission scheme for self-mixing receivers

A new receiving scheme for self-mixing receivers is presented that overcomes the disadvantages of the self-heterodyne concept. Generally speaking, the self-mixing receiver offers immunity to phase noise and frequency offsets, especially at very high frequencies, since it does not require radio frequency local oscillators. Our proposed technique eliminates the drawbacks of the self-heterodyne transmission scheme, which are the poor power efficiency and the strong dependence on the continously transmitted carrier. A nonlinear system of equations is constructed that describes a phase retrieval problem for the reconstruction of the original transmit signal before self-mixing. Two different solution strategies, with restrictions in time and frequency domain, are presented. As a consequence, the self-mixing equation system is shown to be solvable with some a-priori information about the transmit signal. With this novel approach, the transmitted information is distributed over the full available bandwidth, and there is no special dependence on a certain subcarrier for the down-conversion. The general performance, regarding bit error ratio over signal to noise ratio, is improved by at least 2 dB as compared to the self-heterodyne transmission scheme. In the case of frequency selective channels, e.g. multi-path propagation, this improvement is shown to be much larger, because the presented approach is able to reconstruct the received subcarriers without the necessity of receiving all subcarriers

Underwater Wireless Video Transmission using Acoustic OFDM

The current project aims to design and implement an acoustic OFDM system for underwater video transmissions. The thesis work combines a theoretical part, whose objective is to choose the appropriate techniques to deal with the characteristics of the targeted channel, and a practical part regarding the system deployment and experimental test

HIGH RESOLUTION TIME-OF-ARRIVAL RANGING OF WIRELESS SENSOR NODES IN NON-HOMOGENOUS ENVIRONMENTS

Wireless Sensor Networks (WSN) have emerging applications in homogeneous environments such as free space. In addition, WSNs are finding new applications in non-homogeneous (NH) media. All referred applications entail location information of measured data or observed event. Localization in WSNs is considered as the leading remedy, which refers to the procedure of obtaining the sensor nodes relative location utilizing range measurements. Localization via Time-of-Arrival (ToA) estimation has received considerable attention because of high precision and low complexity implementation, however, the traditional techniques are not feasible in NH media due to frequency dispersion of transmitted ranging waveform. In this work, a novel and effective ToA-based ranging technique for localization in NH media consisting of frequency dispersive sub-media is proposed. First challenges of ToA estimation in NH media regarding frequency dispersion is investigated. Here, a novel technique which improves ToA estimation resolution at fixed bandwidth via maximum rising level detector (MRLD) technique is discussed. The MRLD receiver utilizes oversampling and multiple correlation paths to evaluate with high resolution the path corresponding to the maximum rising level of matched filters output. In order to achieve higher resolution, a novel and effective ToA estimation is introduced that incorporates orthogonal frequency division multiple access (OFDMA) subcarriers. In the proposed technique, pre-allocated orthogonal subcarriers are utilized to construct a ranging waveform which enables high performance ToA estimation in dispersive NH media in frequency domain. Here, we show that each frequency component of propagated waveform is received with different time delay and phase which dramatically increases the number of unknowns in the received signal system model. Then, we propose a novel idea based on frequency domain analysis of the transmitted OFDMA subcarriers to reduce the number of unknowns exploiting feasible approximations. Finally, the proposed ToA technique is applied multiple times at different carrier frequencies to create a system of linear equations which can be solved to compute the available sub-mediums thickness and range. Simulation results prove that the proposed technique offers high resolution range measurements given simulated ToA estimation error at different signal to noise ratio regimes in NH media