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Direct-sequence code-division multiple-access is considered for underwater acoustic communication networks. Unlike in the majority of spread-spectrum radio systems, intersymbol interference cannot be neglected, and time variability of the channel requires that receiver adaptation be performed at the chip, rather than the bit rate. Adaptive decision-feedback equalization, which has successfully been used for single-user underwater communications, is not directly applicable to spread-spectrum signals because of the delay in the despreading process and the lack of reliable chip decisions. To overcome this problem, a receiver is proposed which feeds back hypothesized, rather than the actual decisions. Numerical examples demonstrate the receiver's ability to cope with time varying channel distortions and preserve the processing gain when conventional, symbol-rate adaptive methods fail.

Underwater acoustic communications are a rapidly growing field of research and engineering, driven by the expansion of applications which require underwater data transmission without wired connections. In this chapter, we explore the problems of underwater acoustic communications in three parts. The first part presents an overview of modern applications in underwater data transmission and today’s achievements in this area. System requirements are reviewed, and propagation characteristics of underwater acoustic channels are given. It is shown that the majority of underwater acoustic channels are severely band-limited, with signal distortions depending on the link configuration, and ranging from benign to extreme ones caused by time-varying multipath propagation and signal phase variations. Examples of existing systems are given, with emphasis on the methods used for intersymbol interference mitigation. Most of these systems use noncoherent or a differentially coherent signal modulation and detection methods. Phase-coherent detection, which offers better efficiency in bandwidth utilization, is the subject of the second part of this chapter. In this part, the design of high-speed digital communication systems, which rely on powerful equalization and multiple sensor signal processing methods is treated.

Preface. 1. High-Speed Unterwater Acoustic Communications M. Stojanovic. 2. Synthetic Aperture Mapping and Imaging M.E. Zakharia, J. Chatillon. 3. Integrated Programmable Underwater Acoustic Biotelemetry System R.S.H. Istepanian. 4. Digital Underwater Voice Communications H. Sari, B. Woodward. 5. Applications of Neural Networks in Underwater Acoustic Signal Processing Z. Zhaoning, P.R. Nanjing. 6. Statistical Signal Processing of Echo Ensembles J. Penrose, T. Pauly. 7. Advanced Coding for Underwater Communications H. Junying, G. Haihong. Reference.

Underwater wireless communications over distances in excess of about 100 m are established using acoustic signals. Acoustic signals propagate as pressure waves, whose energy absorption limits the available bandwidth. As a result, existing technology provides bit rates on the order of several kilobits per second for transmission over distances on the order of several kilometers. Additional challenges are presented by multipath propagation that causes frequency selectivity, random time variation, and Doppler effects that occur due to low speed of sound (1500 m/s). This article overviews the development of acoustic modem technology, which evolved over the past several decades from noncoherent modulation/detection techniques to bandwidth-efficient phase-coherent modulation/detection. A survey of techniques for channel equalization in single-carrier systems as well as recent advances in multicarrier acoustic communications is also presented. Keywords: Acoustic; communications; coherent; equalization; channel estimation; phase synchronizations; multipath; Doppler; sparse channels; diversity combining; beamforming; multiuser detection; interference suppression; time reversal; multi-input–multi-output (MIMO) processing; multicarrier modulation; OFDM; adaptive modulation; underwater networks

In this work we explore the feasibility of a cognitive acoustic communication system that exploits a dynamic closed loop between the transmitter and receiver with the goal of maximizing the information throughput. Specifically, we design a power control mechanism and couple it with an adaptive modulation method based on orthogonal frequency division multiplexing (OFDM). We propose two methods: the first method is adaptive overall power adjustment in which the transmitter modifies the power gain to provide a target SNR at the receiver by exploiting a feedback link in a time-varying channel. The second method adaptively adjusts the modulation level on each individual sub-carrier to achieve a target bit error rate (BER) at the receiver. Crucial to both of these methods is the ability to predict the acoustic channel and the signal-to-noise ratio (SNR) one travel time ahead, which enables adaptive adjustment of the transmit power and modulation level. The performance of the proposed algorithms is demonstrated using an in-air test bed and further verified with real-time at-sea experiments conducted off the coast of Kauai, HI, in June 2011. Experimental results obtained using real-time at-sea experiments show significant savings in power, as well as improvement in the overall information throughput (bit rate) as compared to conventional, non-adaptive modulation with the same power consumption and target BER.

High-rate acoustic communication typically rely on coherent detection which requires sophisticated channel estimation, and may in turn suffer a penalty in performance when channel tracking is less than ideal (a situation that is often inevitable on time-varying channels). To improve the robustness of signal detection, orthogonal frequency division multiplexing (OFDM) is considered with differentially coherent detection. The resulting receiver has very low computational requirements, and a potential to outperform coherent OFDM detection when channel tracking becomes difficult. Differential encoding is applied in the frequency domain (across carriers) so that it does not require the channel to remain constant over consecutive blocks in time. Instead, it requires only that the channel transfer function change slowly between adjacent carriers, but this requirement coincides with the basic premises of OFDM system design. At the same time, closely spaced carriers promote bandwidth-efficiency. For extreme situations, when close carrier separation leads to insufficient temporal coherence within each OFDM block, a method of partial FFT demodulation can be used with differentially coherent detection. The ensuing receiver algorithm is cast into the multi-channel (spatial diversity) framework, and its performance is illustrated using synthetic, as well as experimental data.

The underwater acoustic channel is characterized as a rapidly fading time dispersive channel with Doppler effects. In order to achieve both power efficiency of coherent communications and diversity improvement from multipath propagation over such channels, we address the problem of joint optimization of synchronization and equalization techniques.

The channel capacity bounds of orthogonal frequency division multiplexing (OFDM) systems over the underwater (UW) acoustic fading channel as a function of the distance between the transmitter and receiver are investigated. The upper bound is obtained under perfect channel state information (CSI) assumption at the receiver. The lower bound is obtained under imperfect CSI at the receiver. The reduction from the upper bound is due to the linear minimum mean square prediction error. The UW channel deviates from the wide sense stationary and uncorrelated scattering (WSSUS) model commonly used for small bandwidths. Frequency‐selectivity is incorporated due to the acoustic propagation into each arrival path between the transmitter and receiver. This leads the UW channel to be modeled as a frequency‐dependent doubly spread fading channel characterized by the wide sense stationary and correlated scattering (WSS‐non‐US) fading assumption. Both Rayleigh and Ricean fading assumptions are investigated in the model. Results from the model show a gap between the upper and lower bounds that depends not only on the ranges and shape of the scattering function of the UW channel but also the distance between the transmitter and receiver. The model for the scattering function was confirmed from experimental data.

Orthogonal frequency division multiplexing (OFDM) offers simplicity of FFT‐based implementation with frequency‐domain channel equalization, and has emerged as a standard in many terrestrial systems. Its application to underwater systems, however, is challenged by the motion‐induced Doppler distortion, which creates nonuniform frequency offset in a wideband acoustic signal. To counteract this problem, recent research has focused on synchronization methods, demonstrating the feasibility of wideband OFDM in initial sea trials. We consider the design of a complete OFDM receiver based on three key techniques: adaptive carrier tracking, spatial diversity combining, and sparse channel estimation. In particular, we focus on time‐domain channel estimation, which provides a natural platform for channel sparsing. Sparsing of the impulse response, or discarding of its insignificant coefficients, is shown to offer performance gains on acoustic channels, since they are often naturally sparse. Decision‐directed operation, which is made possible by adaptive carrier tracking, yields a low overhead, and further improves the performance of channel estimation beyond that of nonadaptive pilot‐based schemes. The technique proposed is applied to experimental data recorded in a shallow water channel over 1 km. Results show excellent performance of coded OFDM signals, transmitted in the 19‐31 kHz band with a varying number of subcarriers.

Wireless information transmission underwater is one of the enabling technologies for the development of future ocean-observation systems, whose applications include gathering of scientific data, pollution control, climate monitoring, detection of objects on the ocean floor, and transmission of images from remote sites. Implicitly, wireless signal transmission is crucial for control of autonomous underwater vehicles (AUVs) which will serve as mobile nodes in the future information networks of distributed underwater sensors. Wireless communication provides advantages of collecting data without the need to retrieve instruments, and maneuvering underwater vehicles and robots without the burden of cables. It is most often accomplished using acoustic transmission, although other approaches including optics and electromagnetic fields are also being investigated for short-range high-bandwidth links.

A high bit‐rate acoustic link for underwater video transmission is examined. Currently, video encoding standards allow video transmission at bit rates on the order of 64 kbps. To provide an acoustic transmission capability that meets this bit‐rate requirement, we focus on the use of high‐level bandwidth‐efficient modulation methods. We consider the use of 16‐, 32‐, and 64‐differential amplitude and phase shift keying (DAPSK) with varying number of amplitude levels. DAPSK does not require explicit carrier phase synchronization at receiver, but instead relies on differentially coherent detection. It thus represents an alternative to conventional quadrature amplitude modulation (QAM) methods, whose high‐level constellations are sensitive to phase distortion induced by the channel. System performance is demonstrated experimentally, using 25 000 symbols/s at a carrier frequency of 75 kHz over a short vertical path. Excellent results were obtained, thus achieving bit rates as high as 150 kbps, which are sufficient for real‐time transmission of compressed video.

Acoustic communications systems used on autonomous underwater vehicles (AUVs) provide supervisory control, access to real‐time data, and also allow multiple vehicles to cooperate in undertaking adaptive sampling missions. However, the use of acoustic systems on AUVs presents special challenges because of limited space for optimal placement of transducers, and potential conflicts with other acoustic systems such as side‐scan sonars and transponders. In addition, radiated and structure‐borne acoustic interference from thrusters and actuators reduces the sensitivity of on‐board receivers. Recent work in acoustic communications and AUVs has included combining some navigation functions into communications equipment, development of operating modes that remove conflicts between different subsystems, design of vehicle components to avoid or remove interference, and other approaches to improving performance. While these efforts have been successful for specific installations, many challenges remain. This talk addresses problems and solutions for supervised and completely autonomous multi‐vehicle communications to support complex AUV missions. Also presented are recent results which demonstrate that acoustic communications can be used successfully on a variety of AUV platforms for many different applications. [Work supported by ONR.]

A broadband acoustic communication link between a user equipped with a single transmit/receive element and a station equipped with an array is considered. System optimization is conducted to obtain transmit/receive filtering techniques that provide best performance over a multipath channel under varying system constraints. Two classes of systems are considered: those that aim for complete intersymbol interference suppression and those that use equalization at the receiver side. Both systems are considered with or without the possibility to implement a perfect channel feedback. Transmit/receive filters are derived analytically, and performance of each scheme is assessed through its data detection SNR. Performance of various techniques is compared analytically, using an example of a shallow water channel, where transmission at high bit rate results in extensive delay spread.

Latest achievements in underwater acoustic communications are leading the way to modern instrumentation for acoustical oceanography by eliminating the need for tethering or diver’s contact with submerged sensors. To meet the various telemetry needs, an acoustic link must provide both reliability and a high rate of data transfer, requirements that are often conflicting. The first approximation of an acoustic channel is that of a time‐invariant multipath communication channel. It is well known that by increasing the transmission rate on such a channel, multipath propagation causes longer intersymbol interference, ultimately limiting system performance. Recent experimental results obtained with a newly developed receiver algorithm, however, showed a seemingly surprising result: an increase in transmission rate resulted in improved system performance. An explanation for this phenomenon is found in the time variation of the ocean multipath. Notably emphasized in the fully saturated shallow water channels, higher transmission rates allow finer sampling of the rapidly varying channel, and therefore better tracking. Experimental results obtained in the Woods Hole harbor are presented showing a dramatic improvement in performance of QPSK coherent detection over a one‐mile range as the data rate is increased from 5 to 30 kilobits‐per‐second. A theoretical analysis based on stochastic channel modeling supports these observations.

Sound is the best form of energy for broadcasting communication signals in the ocean. However, the physical channel is impaired by nonwhite noise, frequency‐selective absorption, a refractive medium, a sloping seafloor, a dynamic sea‐surface, and scattering at the boundaries. Temporal variability of these influences and motion of the link terminals introduce fluctuations in the received signal and noise fields. These oceanographic and geometric factors produce measurable effects on underwater acoustic communication systems. The relevant direct measures of these channel effects include multipath spreading, Doppler shifts and spreading, coherence time, and noise statistics. Measuring and understanding these effects support optimization of both modulation and demodulation parameters. Such channel adaptation strategies enhance quality of service for the communication link. Theory, simulation, and ocean experiment illustrate link improvements. [Work funded by ONR 32, the SSC‐SD Independent Research Program, and the Navy SBIR Program.]

Underwater acoustic communications have received much attention in recent years leading to the development of powerful and reliable receiver algorithms for signal processing in a variety of ocean environments. With the feasibility of high‐speed coherent underwater communications established, the focus of current research is shifting to more demanding communication scenarios, as encountered in underwater acoustic networks. The problem of signal detection in highly variant shallow water environments, in the presence of strong cochannel interference from other acoustic modems in the network is addressed. The signals of different users occupy the same bandwidth, making the respective channel responses and the underlying data symbols sequences the only distinction among distorted replicas observed at the receiver. The performance of a single‐sensor decision‐feedback equalizer (DFE) is compared to the performance of a centralized multiuser receiver that jointly performs adaptive equalization and interference cancellation, and a decentralized multi‐sensor DFE that performs spatial signal combining and multichannel equalization. Performance at different receiver structures was tested on real data collected at a one‐mile range in the Woods Hole harbor, from two closely separated sources. Excellent results were obtained at signal‐to‐interference ratios as low as −10 dB, showing the performance improvement of proposed techniques over DFE in a network scenario.

In this presentation the issues concerning the impact of channel structure and dynamics on the design and performance of a decision feedback equalizer (DFE) type of the receiver for coherent underwater acoustic (UWA) telemetry is discussed. The receiver, developed as a part of the feasibility analysis of coherent communications over UWA telemetry channels, consists of a fractionally spaced DFE and a passband digital phase‐locked loop. The adaptive algorithm for updating the receiver parameters is a combination of the recursive least‐squares algorithm for the equalizer tap weights and a second‐order phase update for carrier recovery. The receiver jointly performs channel equalization, symbol timing, and carrier phase synchronization, using the minimum mean‐squared error criterion. Besides processing the single‐channel signal, the receiver structure and the algorithm are extended to the multichannel case in which coherent diversity combining of different spatially distributed signals is performed. This rece...

Information-theoretic throughput scaling laws are analyzed in an underwater acoustic network with n regularly located nodes on a unit square, in which both bandwidth and received signal power can be severely limited. A narrow-band model is assumed where the carrier frequency is allowed to scale as a function of n. We first characterize an attenuation parameter that depends on the frequency scaling as well as the transmission distance. In the dense network having unit area, a cut-set upper bound on the capacity scaling is then derived. We show that there exists either a bandwidth or a power limitation, or both, according to the path-loss attenuation regimes, thus yielding the upper bound that has three fundamentally different operating regimes. In the dense network, we also describe an achievable scheme based on the simple nearest-neighbor multi-hop transmission. The operating regimes that guarantee the order optimality are identified, where frequency scaling is instrumental towards achieving the order optimality in the regimes.

In Filtered Multitone Modulation (FMT), the bandwidth is split into a number of subbands in which single-carrier signals are modulated onto separate carriers with very little spectral overlap, making the waveform resilient to intercarrier interference. Demodulation is followed by a bank of parallel equalizers, one for each subband. The length of each equalizer is less than what would be needed in a single-carrier system operating over the same total bandwidth. A channel-estimation-based decision-feedback equalization method using a two-step procedure (estimation/fusion) is proposed for FMT. In this structure, channel estimates are used to cancel postcursor intersymbol interference from input signals prior to linear equalization. Parallel channel estimates of adjacent subbands are then fused, i.e. linked to a common underlying multipath model which exploits frequency correlation to improve upon the channel estimates. The sparse nature of underwater acoustic channels may be exploited by setting to zero all but the significant channel taps, thus further relieving equalization of the estimation noise. The performance of underwater FMT is assessed via simulation and using real data transmitted over 800 m in shallow water at rates of 2–6 kbit/s. The results are compared to OFDM and single-carrier QPSK modulation operating at similar bandwidth efficiencies.

A high bit rate acoustic link for video transmission over an underwater channel is investigated. The key to achieving this objective lies in two approaches: use of efficient data compression algorithms and use of high-level bandwidth-efficient modulation methods. Currently available video encoding standards allow video transmission at bit rates as low as 64 kbps. While this rate is still above the limit of commercially available acoustic modems, prototype acoustic modems based on phase coherent modulation/detection have demonstrated successful transmission up to 30 kbps over a deep-water vertical path. To bridge the final gap and provide acoustic transmission capability needed for near real-time video, we focus on the use of high-level bandwidth-efficient modulation methods. An experimental system, based on discrete cosine transform and Huffman entropy coding for video compression, and variable rate M-ary QAM was implemented. Phase-coherent detection is accomplished by decision-directed synchronization and adaptive equalization. System performance is demonstrated experimentally, using 25000 symbols/sec at a carrier frequency of 75 kHz over a short vertical path. Excellent results were obtained using modulation methods of 16, 32 and 64-QAM, thus achieving bit rates as high as 150 kbps, which are sufficient for real-time transmission of compressed video.

It is known that we can always 3-triangulate (i.e. divide into tetrahedra with the original vertices) convex polyhedra but not always non-convex ones. Polyhedra topologically equivalent to ball with p handles, shortly p-toroids, cannot be convex. So, it is interesting to investigate possibilities and properties of their 3-triangulations. Here we study the minimal number of necessary tetrahedra for the triangulation of a 3-triangulable p-toroid. For that purpose, we developed the concept of piecewise convex polyhedron and that of its connection graph.

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUG The effect of increased populations concentrated in urban areas, coupled with the ongoing threat of climate change, means that society is becoming increasingly vulnerable to the effects of extreme precipitation. The study of these events is therefore a key topic in climate research, in their physical basis, in the study of their impacts, and in our adaptation to them. From a meteorological perspective, the main questions are related to the definition of extreme events, changes in their distribution and intensity both globally and regionally, the dependence on large-scale phenomena including the role of moisture transport, and changes in their behavior due to anthropogenic pressures. In this review article, we address all these points and propose a set of challenges for future research. Agencia Estatal de Investigación | Ref. RTI2018‐095772‐B‐I00 Xunta Galicia | Ref. D431C 2021/44