Doppler-Resilient Schemes for Underwater Acoustic Communication Channels.

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

Harvesting time-frequency-space diversity with coded modulation for underwater acoustic communications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2009.Includes bibliographical references (leaves 172-180).The goal of this thesis is to design a low-complexity, high data-rate acoustic communications system with robust performance under various channel conditions. The need for robust performance emerges because underwater acoustic (UWA) channels have time-varying statistics, thus a coded modulation scheme optimally designed for a specific channel model will be suboptimal when the channel statistics change. A robust approach should use a coded modulation scheme that provides good performance in both additive white Gaussian noise (AWGN) and Rayleigh fading channels (and, consequently in the Rice fading channel, an intermediate channel model between the latter two). Hence, high data-rate coded modulation schemes should exhibit both large free Euclidean and Hamming distances. In addition, coded modulation is regarded as a way to achieve time diversity over interleaved flat fading channels. UWA channels offer additional diversity gains in both frequency and space; therefore a system that exploits diversity in all three domains is highly desirable. Two systems with the same bit-rate and complexity but different free Euclidean and Hamming distances are designed and compared. The first system combines Trellis Coded Modulation (TCM) based on an 8-PSK signal set, symbol interleaving and orthogonal frequency-division multiplexing (OFDM). The second system combines bit-interleaved coded modulation (BICM), based on a convolutional code and a 16-QAM signal set, with OFDM.(cont.) Both systems are combined with specific space-time block codes (STBC) when two or three transmit antennas are used. Moreover, pilot-symbol-aided channel estimation is performed by using a robust 2-D Wiener filter, which copes with channel model mismatch by employing appropriate time and frequency correlation functions. The following result was obtained by testing the aforementioned systems using both simulated and experimental data from RACE '08: the BICM scheme performs better when the UWA channel exhibits limited spatial diversity. This result implies that coded modulation schemes emphasizing higher Hamming distances are preferred when there is no option for many receive/transmit hydrophones. The TCM scheme, on the other hand, becomes a better choice when the UWA channel demonstrates a high spatial diversity order. This result implies that coded modulation schemes emphasizing higher free Euclidean distances are preferred when multiple receive/transmit hydrophones are deployed.by Konstantinos Pelekanakis.Ph.D

Doppler compensation algorithms for DSP-based implementation of OFDM underwater acoustic communication systems

In recent years, orthogonal frequency division multiplexing (OFDM) has gained considerable attention in the development of underwater communication (UWC) systems for civilian and military applications. However, the wideband nature of the communication links necessitate robust algorithms to combat the consequences of severe channel conditions such as frequency selectivity, ambient noise, severe multipath and Doppler Effect due to velocity change between the transmitter and receiver. This velocity perturbation comprises two scenarios; the first induces constant time scale expansion/compression or zero acceleration during the transmitted packet time, and the second is time varying Doppler-shift. The latter is an increasingly important area in autonomous underwater vehicle (AUV) applications. The aim of this thesis is to design a low complexity OFDM-based receiver structure for underwater communication that tackles the inherent Doppler effect and is applicable for developing real-time systems on a digital signal processor (DSP). The proposed structure presents a paradigm in modem design from previous generations of single carrier receivers employing computationally expensive equalizers. The thesis demonstrates the issues related to designing a practical OFDM system, such as channel coding and peak-to-average power ratio (PAPR). In channel coding, the proposed algorithms employ convolutional bit-interleaved coded modulation with iterative decoding (BICM-ID) to obtain a higher degree of protection against power fading caused by the channel. A novel receiver structure that combines an adaptive Doppler-shift correction and BICM-ID for multi-carrier systems is presented. In addition, the selective mapping (SLM) technique has been utilized for PAPR. Due to their time varying and frequency selective channel nature, the proposed systems are investigated via both laboratory simulations and experiments conducted in the North Sea off the UK’s North East coast. The results of the study show that the proposed systems outperform block-based Doppler-shift compensation and are capable of tracking the Doppler-shift at acceleration up to 1m /s2.EThOS - Electronic Theses Online ServiceIraqi Government's Ministry of Higher Education and Scientific ResearchGBUnited Kingdo