Progress in underwater acoustic telemetry since 1982 is reviewed within a framework of six current research areas: (1) underwater channel physics, channel simulations, and measurements; (2) receiver structures; (3) diversity exploitation; (4) error control coding; (5) networked systems; and (6) alternative modulation strategies. Advances in each of these areas as well as perspectives on the future challenges facing them are presented. A primary thesis of this paper is that increased integration of high-fidelity channel models into ongoing underwater telemetry research is needed if the performance envelope (defined in terms of range, rate, and channel complexity) of underwater modems is to expand.

/pdf/the-state-of-the-art-in-underwater-acoustic-telemetry-25guddsizz.pdf

The state of the art in underwater acoustic telemetry

This chapter contains sections titled: Introduction Communication Architecture Basics of Underwater Communications Physical Layer Medium Access Control Layer Network Layer Cross-Layer Design Experimental Platforms UW-Buffalo: An Underwater Acoustic Testbed at the University at Buffalo Conclusions References

https://www.eng.buffalo.edu/wnesl/papers/Wiley_AdvancesUWAN_2013.pdf

Advances in Underwater Acoustic Networking

In this paper the combined finite element-boundary element method is applied to the analysis of the sound-structure interaction around a multi-layered anechoic baffle. The baffle is made of three layers: an aluminium backing supports a second layer of highly butyl rubber. The third layer, loaded polyurethane, presents a regular grid of steep shaped cones with a fluid-matching function. The viscoelastic materials are fully modelled into the finite element method using linear integral constitutive relationships. The combined FE-BE algorithm is based on a partial application of the Burton and Miller Helmholtz gradient formulation to overcome non-uniqueness problems. Examples of analysis of acoustic scattering in a range of frequencies typical of underwater communications are given together with a practical application of the structure.

http://ieeexplore.ieee.org/iel3/4919/13571/00624163.pdf

Combined finite element-boundary element analysis of a viscoelastic anechoic panel for underwater applications

This research work focuses on the analysis of multi-layered, anechoic tiles for underwater applications, especially in the field of communications.
It is firstly shown how the sound absorbing properties of viscoelastic materials can be modified and enhanced by the proper use of fillers, such as lead oxide and mica. Successively, a new method for identifying the viscoelastic frequency-dependent properties of such materials from experimental data is presented. The method is based on a variational method analogous to the Hamilton Principle. It allows calculating hard-to-find properties such as the complex viscoelastic response functions and the complex Poisson ratio.
After the materials properties have been determined, it is shown how they can be incorporated into the combined finite-element-boundary element method to provide accurate numerical solutions to the acoustic scattering problem.
A tile made of three layers, a reflecting aluminium layer, an absorbing butyl rubber layer and a matching layer made of a regular grid of polyurethane cones is finally analysed in several scattering and geometrical configurations.
The scattering patterns produced by a plane wave incident on the tile are plotted, discussed and compared with experimental data obtained from in-tank scattering measurements of a model tile.

Finite element-boundary elements modelling of acoustic scattering from viscoelastic anechoic structures

Underwater acoustic (UWA) communications have drawn attention to many researchers in recent years. The major obstacles to reliable UWA communications include limited bandwidth, long multipath delay and large Doppler shifts of the UWA channel. Due to the hostile UWA environment, implementation of a high-speed UWA communication system often requires advanced modulation and signal processing techniques. The multicarrier modulation in the form of orthogonal frequency-division multiplexing (OFDM) has many attractive features which make it a potential candidate for UWA communications. However, the OFDM signal has a high peak-to-average-power ratio (PAPR) and hence is highly susceptible to nonlinearities in the communication link. For combating channel nonlinearities, knowledge of the nonlinear channel is essential. In this paper, we propose a novel method for the estimation of nonlinear channels in OFDM UWA communications. Compared to conventional methods, the proposed method requires a significantly smaller amount of data to achieve the channel estimation. This makes the proposed method suitable for time-varying UWA channels. The effectiveness of the proposed method is demonstrated by applying it to estimate the nonlinear channel of a simulated OFDM UWA communication system.

Frequency-domain channel estimation for nonlinear multicarrier underwater communication systems

Accurate in-situ studies of the oceans requires an efficient, fast and reliable method of transmitting information to and from dedicated scientific payloads. The current generation of underwater telemetry systems tend to use proven, low data rate, 'discrete-component' designs. Although these systems perform well in static environments, their performance rapidly degenerates when the environment becomes more dynamic. The next generation of underwater telemetry systems must be able to adapt automatically to the changing environment and counteract time-varying effects by means of flexible signal processing techniques and detection algorithms. This goal is hard, if not impossible, to realise using conventional 'discrete component' designs. This paper presents a vertical communication system that uses an 'Analogue Devices' digital signal processor (DSP) at its heart. This DSP controls the transmission of data and operative commands to and from a seafloor unit, using multi-phase, 2, 4 or 8, differential phase shift keying (DPSK). This system is considered to be the first step towards a completely autonomous, adaptive communications system.

The design and testing of a DSP, half-duplex, vertical, DPSK communication link

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The design and testing of a DSP, half-duplex, vertical, DPSK communication link