UNDERWATER COMMUNICATIONS WITH ACOUSTIC STEGANOGRAPHY: RECOVERY ANALYSIS AND MODELING

In the modern warfare environment, communication is a cornerstone of combat competence. However, the increasing threat of communications-denied environments highlights the need for communications systems with low probability of intercept and detection. This is doubly true in the subsurface environment, where communications and sonar systems can reveal the tactical location of platforms and capabilities, subverting their covert mission set. A steganographic communication scheme that leverages existing technologies and unexpected data carriers is a feasible means of increasing assurance of communications, even in denied environments. This research works toward a covert communication system by determining and comparing novel symbol recovery schemes to extract data from a signal transmitted under a steganographic technique and interfered with by a simulated underwater acoustic channel. We apply techniques for reliably extracting imperceptible information from unremarkable acoustic events robust to the variability of the hostile operating environment. The system is evaluated based on performance metrics, such as transmission rate and bit error rate, and we show that our scheme is sufficient to conduct covert communications through acoustic transmissions, though we do not solve the problems of synchronization or equalization.Lieutenant, United States NavyApproved for public release. Distribution is unlimited

A Study on Covert Underwater Communication System Using the LPI Based on Spread Spectrum

Researches for oceans are limited to military purpose such as underwater sound detection and tracking system. Among the various ocean technologies, for the purpose of personal or military security, underwater acoustic communications with low-probability-of-interception covert characteristics were received much attention recently. Covert underwater acoustic communication system is designed for the transmission signal is not intercepted by the unintended another receivers. Covert communication system must operate at low signal-to-noise ratio. Therefore, typical covert communication systems use a spread spectrum communication technology. Spread spectrum signals used to hide the transmitted signal by transmitting it at low power and making it difficult for an unintended listener to detect the signal in the presence of background noise. The problem for spread spectrum communications in an underwater acoustic channel is the multipath arrivals, which create severe interchip and inter symbol interferences. In order to improve performance in multipath underwater channel, this thesis proposed turbo equalization techniques based on RAKE receiver for spread spectrum communications. RAKE receiver was applied threshold comparison and weighted coefficient. In this thesis, the performance of the proposed transceiver model is analyzed. This thesis focused not only single user but also multiuser. Many earlier works focused on the single-user detection problem. They are often inspired by advances made in wireless radio communications and sometimes dedicated to the underwater acoustic communications. In a single user code acquisition algorithm was proposed. However, the growing interest in underwater communications led researchers to consider the multiuser detection and acquisition problem in an underwater communication network system. By expanding single user communication concept, this thesis proposed a multiuser access communication system based on turbo equalized RAKE model. Thesis analyzed the performance of the proposed multiple access communication for covert communication by simulations. Also number of users are fixed on two, underwater experiment was conducted and demonstrate that proposed model is very effective for covert multiuser underwater communications.목 차 List of Tables ii List of Figures iii Abstract iv 제 1 장 서론 1 제 2 장 은밀 수중 통신 기법 3 2.1 대역 확산 기법 4 2.2 은밀 수중 통신에 적합한 채널 부호화 기법 6 2.3 대역확산 기반 수중 음향 통신 모델 8 제 3 장 은밀 수중 통신에서 효율적인 복호 방식 제안 10 3.1 반복기반의 터보 등화 복호 방식 10 3.2 RAKE 기반의 터보 등화 복호 방식 13 3.3 시뮬레이션 성능 비교 분석 17 3.4 실제 수중 실험을 통한 성능 비교 24 제 4 장 은밀 다중 접속을 위한 송수신 구조 32 4.1 은밀 다중 접속 송신 구조 33 4.2 은밀 다중 접속 수신 구조 36 4.3 시뮬레이션 성능 분석 39 4.4 실제 수중 실험을 통한 성능 분석 41 제 5 장 결 론 46 감사의 글 38 참고문헌 40Maste

A Study on Weighted Multiband Communication Method Based on Iterative Coding for Long Range Underwater Acoustic Communication

Recently, underwater acoustic communication is an essential technology for underwater communication in marine research, and its application field is expanding. In addition, with the development of military AUV capable of long-distance movement, there is an increasing need to develop a technology capable of reliably communicating over a long distance for efficient marine surveillance. In the long distance acoustic communication, as the transmission distance increases, the bandwidth decreases and the throughput efficiency decreases. Multiband transmission technique is an efficient method to improve the transmission distance and performance in long-range underwater acoustic communication. However, sometimes the performance of multiband is lower than that of single bands. This is because in the transceiver model using the conventional multiband transmission technique, signals of different frequency bands are combined with the same weight and input to the decoder, so that performance degradation of a specific frequency band affects the total band. Accordingly, this thesis propose a weighted multiband transceiver model to improve the performance of the multiband transceiver model. In the weighted multiband transceiver model, the transmitter uses a convolution code and a turbo code of 1/3 coding rate, and receiver uses a decision feedback equalizer to compensate for multipath distortion after compensating for frequency and phase offsets in each band. And, by applying the turbo equalization technique, the equalizer and the decoder is connected to each other to update the information iteratively to improve performance as the number of iteration increases. In addition, the threshold detector adds weights by setting threshold values through preamble BER(Bit Error Rate) of each band. The weighting method according to the threshold value for each band can improve the performance by reducing the influence of the band having low performance in the total band. Simulation results show that the performance improves as the number of bands increases when the multiband transmission technique is applied. The performance of the proposed weighted multiband transceiver model analyzed through short and 90 km long-range sea experiments. In the short-range sea experiment, it confirmed that the performance improved with the increase of the number of bands, and that the performance was improved by applying the proposed weighted multiband structure to the data with low performance when the number of bands was four. In addition, the 90 km long-range sea experiment applied a weighted multiband transceiver model for data that did not completely correct errors within 5 iterations in the conventional multiband. As a result, it confirmed that the performance is further improved when the weighted multiband is applied.List of Tables ii List of Figures iii Abstract iv 제 1 장 서론 1 제 2 장 수중음향통신에서 고려되는 전송 기법 3 2.1 채널 부호화 기법 4 2.2 단일밴드 전송 기법 6 2.3 다중밴드 전송 기법 9 제 3 장 가중화된 다중밴드 통신 12 3.1 임계값 및 가중치 설정 알고리즘 12 3.2 가중화된 다중밴드 기반의 송·수신구조 15 제 4 장 시뮬레이션 및 실험 결과 분석 17 4.1 시뮬레이션 17 4.2 단거리 해양 실험 20 4.3 장거리 해양 실험 26 제 5 장 결 론 35 참고문헌 37 감사의 글 40Maste

Analysis of and techniques for adaptive equalization for underwater acoustic communication

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2011Underwater 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.This work would not have been possible without support from the O ce of Naval Research, through a Special Research Award in Acoustics Graduate Fellowship (ONR Grant #N00014-09-1-0540), with additional support from ONR Grant #N00014-05- 10085 and ONR Grant #N00014-07-10184

Spread-spectrum techniques for environmentally-friendly underwater acoustic communications

PhD ThesisAnthropogenic underwater noise has been shown to have a negative impact on marine life. Acoustic data transmissions have also been shown to cause behavioural responses in marine mammals. A promising approach to address these issues is through reducing the power of acoustic data transmissions. Firstly, limiting the maximum acoustic transmit power to a safe limit that causes no injury, and secondly, reducing the radius of the discomfort zone whilst maximising the receivable range. The discomfort zone is dependent on the signal design as well as the signal power. To achieve these aims requires a signal and receiver design capable of synchronisation and data reception at low-received-SNR, down to around −15 dB, with Doppler effects. These requirements lead to very high-ratio spread-spectrum signaling with efficient modulation to maximise data rate, which necessitates effective Doppler correction in the receiver structure. This thesis examines the state-of-the-art in this area and investigates the design, development and implementation of a suitable signal and receiver structure, with experimental validation in a variety of real-world channels. Data signals are designed around m-ary orthogonal signaling based on bandlimited carrierless PN sequences to create an M-ary Orthogonal Code Keying (M-OCK) modulation scheme. Synchronisation signal structures combining the energy of multiple unique PN symbols are shown to outperform single PN sequences of the same bandwidth and duration in channels with low SNR and significant Doppler effects. Signals and receiver structures are shown to be capable of reliable communications with band of 8 kHz to 16 kHz and transmit power limited to less than 170.8 dB re 1 μPa @ 1m, or 1W of acoustic power, over ranges of 10 km in sea trials, with low-received-SNR below −10 dB, at data rates of up to 140.69 bit/s. Channel recordings with AWGN demonstrated limits of signal and receiver performance of BER 10−3 at −14 dB for 35.63 bit/s, and −8.5 dB for 106.92 bit/s. Piloted study of multipath exploitation showed this performance could be improved to −10.5 dB for 106.92 bit/s by combining the energy of two arrival paths. Doppler compensation techniques are explored with experimental validation showing synchronisation and data demodulation at velocities over ranges of ±2.7m/s. Non-binary low density parity check (LDPC) error correction coding with M-OCK signals is investigated showing improved performance over Reed-Solomon (RS) coding of equivalent code rate in simulations and experiments in real underwater channels. The receiver structures are implemented on an Android mobile device with experiments showing live real-time synchronisation and data demodulation of signals transmitted through an underwater channel.UK Engineering and Physical Sciences Research Council (EPSRC): PhD Doctoral Training Account (DTA)

Communication and time distortion

Communication systems always suffer time distortion. At the physical layer asynchrony between clocks and motion-induced Doppler effects warp the time scale, while at higher layers there are packet delays. Current wireless underwater modems suffer a significant performance degradation when communication platforms are mobile and Doppler effects corrupt the transmitted signals. They are advertised with data rates of a few kbps, but the oil and gas industry has found them useful only to around 100 bps. In our work, time-varying Doppler is explicitly modeled, tracked and compensated. Integrated into an iterative turbo equalization based receiver, this novel Doppler compensation technique has demonstrated unprecedented communication performance in US Navy sponsored field tests and simulations. We achieved a data rate of 39kbps at a distance of 2.7km and a data rate of 1.2Mbps at a distance of 12m. The latter link is capable of streaming video in real-time, a first in wireless underwater communication. Time distortion can also be intentional and be used for communication. We explore how much information can be conveyed by controlling the timing of packets when sent from their source towards their destination in a packet-switched network. By using Markov chain analysis, we prove a lower bound on the maximal channel coding rate achievable at a given blocklength and error probability. Finally, we propose an easy-to-deploy censorship-resistant infrastructure, called FreeWave. FreeWave modulates a client's Internet traffic into acoustic signals that are carried over VoIP connections. The use of actual VoIP connections allows FreeWave to relay its VoIP connections through oblivious VoIP nodes, hence keeping the FreeWave server(s) unobservable and unblockable. When the VoIP channel suffers packet transfer delays, the transmitted acoustic signals are time distorted. We address this challenge and prototype FreeWave over Skype, the most popular VoIP system

Analysis of and techniques for adaptive equalization for underwater acoustic communication

Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science; and the Woods Hole Oceanographic Institution), 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 203-215).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.by Ballard J. S. Blair.Ph.D

Development of an acoustic communication link for micro underwater vehicles

PhD ThesisIn recent years there has been an increasing trend towards the use of Micro Remotely Operated Vehicles (μROVs), such as the Videoray and Seabotix LBV products, for a range of subsea applications, including environmental monitoring, harbour security, military surveillance and offshore inspection. A major operational limitation is the umbilical cable, which is traditionally used to supply power and communications to the vehicle. This tether has often been found to significantly restrict the agility of the vehicle or in extreme cases, result in entanglement with subsea structures. This thesis addresses the challenges associated with developing a reliable full-duplex wireless communications link aimed at tetherless operation of a μROV. Previous research has demonstrated the ability to support highly compressed video transmissions over several kilometres through shallow water channels with large range-depth ratios. However, the physical constraints of these platforms paired with the system cost requirements pose significant additional challenges. Firstly, the physical size/weight of transducers for the LF (8-16kHz) and MF (16-32kHz) bands would significantly affect the dynamics of the vehicle measuring less than 0.5m long. Therefore, this thesis explores the challenges associated with moving the operating frequency up to around 50kHz centre, along with the opportunities for increased data rate and tracking due to higher bandwidth. The typical operating radius of μROVs is less than 200m, in water < 100m deep, which gives rise to multipath channels characterised by long timespread and relatively sparse arrivals. Hence, the system must be optimised for performance in these conditions. The hardware costs of large multi-element receiver arrays are prohibitive when compared to the cost of the μROV platform. Additionally, the physical size of such arrays complicates deployment from small surface vessels. Although some recent developments in iterative equalisation and decoding structures have enhanced the performance of single element receivers, they are not found to be adequate in such channels. This work explores the optimum cost/performance trade-off in a combination of a micro beamforming array using a Bit Interleaved Coded Modulation with Iterative Decoding (BICM-ID) receiver structure. The highly dynamic nature of μROVs, with rapid acceleration/deceleration and complex thruster/wake effects, are also a significant challenge to reliable continuous communications. The thesis also explores how these effects can best be mitigated via advanced Doppler correction techniques, and adaptive coding and modulation via a simultaneous frequency multiplexed down link. In order to fully explore continuous adaptation of the transmitted signals, a real-time full-duplex communication system was constructed in hardware, utilising low cost components and a highly optimised PC based receiver structure. Rigorous testing, both in laboratory conditions and through extensive field trials, have enabled the author to explore the performance of the communication link on a vehicle carrying out typical operations and presenting a wide range of channel, noise, Doppler and transmission latency conditions. This has led to a comprehensive set of design recommendations for a reliable and cost effective link capable of continuous throughputs of >30 kbits/s