Acknowledgements Although the completion of a Ph.D. is looked upon as an individual accomplishment, it is rarely achieved without the help of many people. The research presented in this thesis is no exception. Potter, for their support and guidance in my academic pursuits. A/Prof. Ong Sim Heng has supported me through several projects over the past years, allowing my research interests in underwater acoustics and digital image processing an avenue for growth. A/Prof. John Potter introduced me to underwater acoustics and guided me through the learning process involved in theoretical and experimental aspects of the subject. Without his mentoring and support, I would not be pursuing independent research in this field today. I would also like to thank Prof. Chan Eng Soon, who first recognized my research interests during my undergraduate studies and helped me find numerous opportunities to further them. He has since supported me in several endeavors, eventually leading to my current research in underwater acoustics. I would like to express my gratitude towards the DSTA a and the ARL b for supporting most of the research presented in this thesis. experimental deployments and data collection in support of this project, for which I am grateful. kindly agreed to find time from their busy schedules to help monitor the technical progress during the course of the research. Their comments and feedback during the Ph.D. qualification defense proved valuable in helping define some of the key research directions for the project. Lastly, I would like to thank my lovely wife, Yun Ping, and my loving parents for their support, understanding and unconditional love. Without them, none of this work would have been possible.

Underwater acoustic communications in warm shallow water channels

In this study, we introduce a novel OFDMA-based MAC protocol for underwater acoustic sensor networks. The design approach of this protocol exploits the multi-path characteristics of a fading acoustic channel. The goal is to convert the channel into parallel independent acoustic sub-channels that undergo flat fading. Communication between node pairs within the network is then done using subsets of these sub-channels thus fully utilizing the available limited bandwidth while avoiding interference completely. We derive the mathematical model for optimal sub-channel selection. We also present the results of the experiments that we conducted to evaluate the new protocol and we compare these results to those of a MAC protocol that is based on the CDMA scheme.

An OFDMA-based MAC protocol for under water acoustic wireless sensor networks

In the past decades, researchers/scientists have paid attention to the physical layer of underwater communications (UWCs) due to a variety of scientific, military, and civil tasks completed beneath water. This includes numerous activities critical for communication, such as survey and monitoring of oceans, rescue, and response to disasters under the sea. Till the end of the last decade, many review articles addressing the history and survey of UWC have been published which were mostly focused on underwater sensor networks (UWSN), routing protocols, and underwater optical communication (UWOC). This paper provides an overview of underwater acoustic (UWA) physical layer techniques including cyclic prefix orthogonal frequency division multiplexing (CP-OFDM), zero padding orthogonal frequency division multiplexing (ZP-OFDM), time-domain synchronization orthogonal frequency division multiplexing (TDS-OFDM), multiple input multiple output orthogonal frequency division multiplexing (MIMO-OFDM), generalized frequency division multiplexing (GFDM), unfiltered orthogonal frequency division multiplexing (UF-OFDM), continuous phase modulation orthogonal frequency division multiplexing (CPM-OFDM), filter bank multicarrier (FBMC) modulation, MIMO, spatial modulation technologies (SMTs), and orthogonal frequency division multiplexing index modulation (OFDM-IM). Additionally, this paper provides a comprehensive review of UWA channel modeling problems and challenges, such as transmission loss, propagation delay, signal-to-noise ratio (SNR) and distance, multipath effect, ambient noise effect, delay spread, Doppler effect modeling, Doppler shift estimation. Further, modern technologies of the physical layer of UWC have been discussed. This study also discusses the different modulation technology in terms of spectral efficiency, computational complexity, date rate, bit error rate (BER), and energy efficiency along with their merits and demerits.

/pdf/a-survey-on-physical-layer-techniques-and-challenges-in-14mhk8qj.pdf

A Survey on Physical Layer Techniques and Challenges in Underwater Communication Systems

In the scenario of underwater acoustic communication, Orthogonal Frequency Division Multiplexing (OFDM) is widely used for its high spectral efficiency and high transmission rate. However, OFDM is sensitive to Doppler shift, which will generate inter-carrier interference to degrade bit error rate performance. Orthogonal Chirp Division Multiplexing (OCDM) consists of multiplexing a number of chirp waveforms that are mutually orthogonal with each other and share the same bandwidth and time slot, achieving good robustness against multipath and Doppler shift. This study reviews the concepts of Orthogonal Chirp Division Multiplexing and combines this multicarrier communication scheme with an underwater acoustic channel. Unlike previous studies analyzing within the signal domain, this study formulates the mathematical model of OCDM in shallow water acoustic channel and illustrates the reason why OCDM can outperform OFDM in both Doppler robustness and multipath robustness, which is a necessary and solid complement for OCDM in the further development of underwater acoustic communication. 

Analysis of Doppler and Multipath on Orthogonal Chirp Division Multiplexing in Shallow Water Acoustic Channel

The underwater acoustic (UWA) communication of underwater unmanned vehicle (UUV) is relating to the communication among UUVs or between UUV and hoster. Then the UWA communication becomes more difficult due to its complex acoustic environment, variety of ranges and the limitation of the space and energy of UUVs. In this paper a dynamic modulation and bandwidth optimization (DMBO) UWA communication technique is presented, which is suitable for the UWA communication in far, medium and near ranges. According to the different acoustic channels the modulation and bandwidth can be optimized to overcome the disadvantage of single scheme UWA communication which performance is restricted by the longest communication range and the worst UWA channel to use lower data rates. DMBO can achieve higher data rates in the different communication ranges and UWA channels. Experiments in a lake were conducted and its performance is investigated. Experimental results show that the data rate reach 9090 bps and 640 bps in the ranges of 5 km and 25 km respectively with the bit error rate below 10-4

Dynamic Modulation and Bandwidth Optimization for UUV Underwater Acoustic Communication System

An underwater digital acoustic communications system based on multicarrier modulation technique is presented. This technique is relatively robust in a strong multipath fading environment and has been successfully tested in a shallow water channel at a data rate up to 10 kbps over 1 km. The test system using 48 carrier frequencies for transmitting 48 parallel bits of data in each packet. The data stream is encoded into a sequence of packets. These packets are encapsulated by headers, synchronisation and training signals. In order to reduce the multipath fading problem, an adaptive channel equalisation scheme with a LSM algorithm is used. The multipath fading problem can be further improved by a redundant carrier assignment and wavelet based MFM scheme. The advantage of this scheme is that the fading is reduced without affecting the channel capacity.

Underwater acoustic modem using multicarrier modulation

With its superiorities of low cost, high flexibility and deployment convenience, small-size autonomous underwater vehicles (AUVs) have been extensively applied to perform a variety of undersea missions. While underwater acoustic (UWA) communication provides a practical way to establish a wireless link, it still poses a significant challenge due to the strict limitations of a small-size AUV platform in terms of load capacity, energy supply and cost. Orthogonal frequency division multiplexing (OFDM) has drawn extensive attention due to its high data rate capability and relative robustness to multipath, the performance of which is unfortunately sensitive to the widespread Doppler effect. While efficient Doppler compensation is significantly crucial for UWA OFDM mobile communication, most of the conventional approaches are conducted using software resampling, thus rendering a huge burden on memory and calculation capability as well as a considerable processing delay. In this paper, from the perspective of hardware completion an UWA OFDM communication payload based on STM32F407 processor is designed and implemented to facilitate agile Doppler compensation with low computational overhead. In particular, after estimating Doppler by calculating the time compression or extension of the preamble signal, Doppler compensation is performed by directly adjusting the direct digital frequency synthesizer (DDS)-driven sampling rate of the analog-to-digital converter (ADC). As the Doppler is compensated parallel to the ADC acquisition, processing delay and memory requirement can be avoided. Finally, hardware-in-loop (HIP) simulation is performed to demonstrate the effectiveness and superiority of the proposed system. The results show that the designed system has the potential to achieve an effective communication rate of 3.19 kbps with the admissible implementation overhead. Future work will entail the integration and performance evaluation of the proposed UWA OFDM communication payload on a practical small-size AUV platform.

/pdf/r-d-of-an-innovative-ofdm-communication-payload-for-small-2y6rmg7v.pdf

R&D of an Innovative OFDM Communication Payload for Small-Size AUV

The interest in detection of buried objects in oceans, especially in shallow sea, is increasing due to their utilization in ocean security and exploration, geophysical surveys, and military operations, etc. The traditional detection methods for the buried objects using acoustic waves and blue-green lasers fail in shallow sea environment due to their own disadvantages. The electromagnetic waves, however, offer strong immunity to various kinds of ocean environmental disturbances, and then allow for fully detection solution. In this paper, an active detection method for the buried objects in shallow sea is presented. Broadband and low frequency (lower than 10 kHz) electromagnetic waves radiated from a horizontal current loop impinge upon the buried objects, and then generate scattered fields containing the target's information, which are received by a magnetic sensor. By analyzing the received signal, the specific information of the buried object can be extracted to realize the detection purpose. Some representative simulation results are presented and verify the effectiveness of the proposed detection method.

The Detection of Buried Objects in Shallow Sea with Low Frequency Electromagnetic Waves

Cross-correlation quasi-gradient Doppler estimation for underwater acoustic OFDM mobile communications

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Papers

Underwater acoustic modem using multicarrier modulation

R&D of an Innovative OFDM Communication Payload for Small-Size AUV

The Detection of Buried Objects in Shallow Sea with Low Frequency Electromagnetic Waves

Cross-correlation quasi-gradient Doppler estimation for underwater acoustic OFDM mobile communications