Adaptive operation in underwater acoustic networks relies on channel prediction. This work models and predicts the large-scale variation of underwater acoustic channels by taking the channel signal-to-noise ratio (SNR) as a quality indicator, which is defined as the received SNR with a unit transmission power. The channel SNR process is modeled as a summation of an environment process which can be represented as a linear combination of a set of measurable environment parameters, and a Markov latent process that accounts for the contribution from unknown or unmeasurable physical mechanisms. Based on historical SNR measurements and available environment parameters, a recursive algorithm is developed to estimate the latent process and the combinational coefficients of environment parameters, which are then used for channel prediction. The algorithm is further extended to seasonal channels, where both a sequential channel prediction algorithm and a joint channel prediction algorithm are developed. The proposed channel model and prediction algorithms are validated via extensive simulations and experimental data analyses.

Point-absorbing converters are widely applied in exploiting of the wave energy. This paper will present some results of point absorber converters by experiment measurements in regular waves. Meanwhile, the research investigates the influence of take-off damping on the power output of wave energy converter. Results demonstrate the wave energy output of three-degree wave energy device is better than result of one-degree point absorber. The parameter, take-off damping, has a significant effect on the energy output of converters. Its result indicates that the damping parameter could be adaptively adjusted in different wave states, to capture the maximum power of waves.

Thermocline has a significant impact on marine fishery, navy submarine activity, ocean circulation and internal wave. In consideration of long range, low cost, and high maneuverability, the AUV (Autonomous Underwater Vehicle) is an ideal platform for ocean local precisely thermocline tracking. Combining with the advantage of the AUV yo-yo movement, this paper presents a simple and effective thermocline tracking method. According to the definition of the thermocline, we set a threshold, and in the real detect area, the threshold can change adaptively, thus we can draw the outline of the thermocline. Meanwhile, we present a method to determine bin size in order to solve the abruptness of robust and method's resolution. The simulation analysis was carried out using previous data of field test in the South China Sea.

Underwater acoustic channels usually suffer from the extended multipath propagation which causes inter symbol interference (ISI). At the receiving end, serious ISI would result in the decrease of the reliability of underwater acoustic communication systems. In this paper, Turbo equalization is applied to increase the reliability of communication systems. The realization of the channel equalizer in Turbo equalization is the linear minimum mean-square error (MMSE) estimate. To avoid performance loss, Turbo equalization and factor graph are combined together which provides a full-window method. Moreover, a modified extrinsic information feedback mechanism of turbo equalization is proposed. The reliability of communication systems is further enhanced by using this mechanism. Simulation results show that the proposed algorithm performs well with low complexity. This demonstrates the potential of the suggested algorithm in practice.

We present wireless optical communication system for data transfer in the underwater networks. We use the optical channel to facilitate the communication link in free space and under water. This work bypasses the limitations involved in the use of electromagnetic waves and acoustics for free space and underwater communication. The system shows that optical communication using light can be a viable solution for underwater data transmission applications that requires high data rate at the moderate distances. We have designed, implemented and tested our system in real time and provide the evaluations results.

Techniques for automating hologram processing, information extraction and construction video based on holographic data are discussed. The method for alive/dead particles identification is suggested. Experimental results are presented.

A model is developed for bistatic bottom reverberation in the deep ocean, which includes the propagation and scattering processes. The two-way propagations from the source to scatterer and the scatterer to receiver are described by ray theory. The sound speed profile in the deep ocean is taken into account for the accuracy. The scattering intensity is assumed to be a sum of the roughness-scattering and volume-scattering cross sections. The roughness-scattering intensity is formed by smooth interpolation between the Kirchhoff cross section near the specular direction and the perturbation cross section elsewhere. The model is tested against data acquired as part of the deep ocean research program in the south China sea. The comparisons between model prediction and measured reverberation data are presented. The results show that the numerical calculations from the bistatic bottom reverberation model fit the experimented data well.

The past decades of development of underwater acoustic communications (acomms) have produced many advances in physical layer algorithms, in hardware and firmware capabilities tailored to the underwater environment, and in concepts for networked sensors. In general, most deployed systems and most conceptual networks have assumed a strictly half duplex channel wherein all links must be shared over a bandwidth-constrained channel. This has led to an almost universal insistence on avoiding collisions among receptions, which might be achieved by TDMA schemes or by adapting other network concepts derived from (RF) mobile communications technology. For long duration deployment of underwater systems, any requirement for tight TDMA access forces the modem developer to use power-hungry high accuracy clocks, thus quickly draining precious battery resources. While networked acknowledgements at the speed of light make perfect sense, the latency of the acoustic channel demands excessive consumption of time to enable the network simply to know that a message has ultimately been received. There are many studies which attempt to devise minimally intrusive ARQ approaches, but ultimately acknowledgements in some form are necessary. In this paper we revisit some older technology, combine it with new capabilities in hardware, and add some signal processing insights to support discussion of a back channel providing asynchronous, multi-access command and control of a network, with supervised use of a second acoustic channel for high data rate, priority, and/or large data volume transfer. The two contending modulation schemes for asynchronous signalling have long been frequency hopping and direct sequence (DS), both of which are (typically) non-coherently processed spread spectrum methods. For a given bandwidth and data rate in additive white Gaussian (AWGN) the performance of the two is virtually identically. However, in a dynamic, multipath-rich environment, the well-recognized requirement for power control for DS signalling, given the constraint of channel latency, makes FH signalling the preferred choice. Justification for this is found throughout the development of the (NATO) JANUS underwater protocol, which itself is a frequency hopping scheme. In the context of power control, it is possible to transfer the burden of interference mitigation from the multiple transmitters to the receiver at a single modem by judicious spectral whiting and clipping of too loud interference. This not only compensates for variable strength transmissions, but for external sources of interference. Our anecdotal experience shows that a single modem operating this way in an 8 kHz band at a center frequency of 39 kHz can readily accommodate asynchronous reception of at least five messages from irregularly-spaced transmitters. Hardware is now available to support integration of two or more modems into a single system, especially to provide dual frequency acomms. From the hardware/system perspective the main constraint is providing sufficient spectral isolation between the two channels. In our experience, this requires physical separation and spectral filtering between the transducers to overcome high power sidelobe interference which is at best 50 dB below the level of the primary channel. However, given that these constraints are accommodated, we are positioned to consider development of media access control (MAC) and networking systems that benefit from this full duplex approach. We describe an FH signal as defined for the JANUS application and show how it can be used in multi-access applications. We provide an overview of the physical infrastructure we deploy to accommodate the two frequency bands, and we describe a possible network infrastructure utilizing these capabilities. Our goal is to promote discussion of alternatives to conventional half duplex TDMA and conflict avoidance acomms schemes.

Automated environmental monitoring in marine environments is currently carried out either by small-scale robotic systems, composed of one or few robots, or static sensor networks. In this paper, we propose the use of swarm robotics systems to carry out marine environmental monitoring missions. In swarm robotics systems, each individual unit is relatively simple and inexpensive. The robots rely on decentralized control and local communication, allowing the swarm to scale to hundreds of units and to cover large areas. We study the application of a swarm of aquatic robots to environmental monitoring tasks. In the first part of the study, we synthesize swarm control for a temperature monitoring mission and validate our results with a real swarm robotics system. Then, we conduct a simulation-based evaluation of the robots' performance over large areas and with large swarm sizes, and demonstrate the swarm's robustness to faults. Our results show that swarm robotics systems are suited for environmental monitoring tasks by efficiently covering a target area, allowing for redundancy in the data collection process, and tolerating individual robot faults.

In this paper, we propose an Error Tolerant Dual-Hydrophone Localization method, i.e., ET-DHL, in underwater sensor networks (USNs). To reduce the impact of the node uncertainty, measurement uncertainty, poor link quality and long latency for underwater localization problem, ET-DHL adds bit-level probability to process the binary sequence. Different from our previous work PCM, each dual-hydrophone node in ET-DHL leverages different probabilities based on TDOA to distinguish which half-plane the source lies in, and ET-DHL takes the sum of expectations of all small regions by normalizing all weighted probability to localize the source. The proposed design is evaluated through extensive simulations, evaluation results demonstrate that ET-DHL can effectively locate the acoustic source with good robustness for node location error, node angle error and measurement uncertainty.