Description: Notes the passing of Stan Ehrlich, an active member of the IEEE and the Oceanic Engineering Society.

Description: Systematic measurements were performed to characterize shallow-water acoustic propagation channels for applications in the field of underwater communications. The survey was conducted in northern Europe and covers the continental shelf, Norwegian fjords, a sheltered bay, a channel, and the Baltic Sea. The measurements were performed in various frequency bands between 2 and 32 kHz. The outcome of the study is a variety of channels that differ in many ways, defying any attempt to define a typical acoustic communication channel. Miscellaneous forward propagation effects are presented, which are relevant to channel models for the design of modulation schemes, network protocols, and simulation environments.

Description: This special issues presents some of the work brought to a conference on underwater communication held in 2012 with the aim to present a snapshot of the state of the art in UW communication technologies, both acoustic and optical, and to set the stage for agreeing on some benchmark problems and models that can be used to equitably evaluate different coding schemes and protocols. The conference was followed by a workshop that attempted to create a roadmap for future channel modeling and standardization. This guest editorial presents an introduction to the conference papers you find in this issue, together with the consensus distilled out of the workshop discussions.

Description: There is anecdotal evidence that under conditions of moderate to high wind speeds (8–15 ${hbox {m}} cdot {{hbox {s}}^{ - 1}}$ ), clouds of bubbles entrained in the near-surface layer by breaking waves can create a benign underwater communications channel through the resonant absorption of forward-scattered sound, reducing reverberation times and the occurrence of high-intensity, Doppler-shifted arrivals. Current models for the effects of bubbles on surface-interacting sound show two effects: refraction of low-frequency sound due to reductions in sound speed near the surface and resonant absorption at higher frequencies. These models include uncertainty in the numbers and sizes of the largest bubbles present in the near-surface layer, and their dependence on wind speed. This uncertainty makes quantitative prediction of bubble effects in the underwater acoustic communications band of workhorse frequencies (10–30 kHz) difficult. The model calculations presented here show that resonant absorption associated with the largest bubbles is strongly frequency and wind-speed dependent. The frequency dependence can be explained by the concept of a bubble escape radius; this being the radius of a bubble for which turbulent fluid velocity fluctuations and bubble terminal velocity in the upper ocean boundary layer balance. Bubbles smaller than the escape radius tend to remain trapped by fluid turbulence while larger bubbles are lost to the surface through buoyant degassing. Calculation of the escape radius provides a means of estimating the lowest frequency at which resonant absorption can be expected for a given wind speed. Initial estimates suggest that resonant absorption at 10 kHz begins at 10-m wind speeds of around 8 ms $^{-1}$ , and significant surface bounce losses at frequencies lower than this are expected in the- range of wind speeds 13–20 $ {hbox {m}} cdot {{hbox {s}}^{ - 1}} $ .

Description: Sea-surface scattering by wind-generated waves and bubbles is regarded to be the main nonplatform related cause of the time variability of shallow acoustic communication channels. Simulations for predicting the quality of acoustic communication links in such channels thus require adequate modeling of these dynamic sea-surface effects. For frequencies in the range of 1–4 kHz , there is an important effect of bubbles on sea-surface reflection loss due to refraction, which can be modeled with a modified sound-speed profile (SSP) accounting for the bubble void fraction in the surface layer. The bubble cloud then acts as an acoustic lens, enhancing the rough-surface scattering by the resulting upward refraction. It is shown here that, for frequencies in the considered range of 4–8 kHz, bubble extinction, including both the effects of bubble scattering and absorption, provides a significant additional contribution to the surface loss. Model-based channel simulations are performed by applying a ray tracer, together with a toolbox for generation of rough sea-surface evolutions. This practical simulation framework is demonstrated to provide realistic results for both stationary and mobile communication nodes by capturing specific features observed in experiments, such as time variability, fading reverberation tails, and wind-speed dependence of the Doppler power spectrum.

Description: In the last several years, the Swedish Defence Research Agency (FOI, Stockholm, Sweden) has been engaged in developing a system for underwater (UW) acoustic communications for both point-to-point (P2P) and network applications. The basis of the system is a single-carrier (SC) scheme with recursive equalization on the receiver side. In this paper, we will give a motivation for our choice of SC by discussing some aspects of modulation and coding for UW communication systems from an information-theoretic point of view. The system is able to take advantage of the diversity offered by the multipath and/or by multiple receivers. Due to the great variations in the UW channel, reliable prediction of communication performance in terms of achievable range and data rate is nontrivial. This has motivated the development of a simulation tool called COMLAB, based on combining the communication system with hybrid ray-trace plane-wave time-domain modeling of sound propagation. The simulation tool is designed to account for environmental effects with significant influence on communication performance, including surface and bottom reflections, transmission loss, ambient noise, and ray-path-dependent Doppler shifts caused by moving source, receivers, or surface waves. Short descriptions of the communication system and the simulation tool are given. A comparison of the predicted performance with experiments made at the UAN'11 trials in the Trondheim fjord (Norway) is presented.

Description: In recent years, there have been notable technical advances in modulation schemes for underwater acoustic communications, and inexpensive commercial modems are now readily available. This has generated a renewed interest in modeling the effects of the underwater sound channel on the transmission of a known time series. The previously developed Virtual Timeseries Experiment (VirTEX) algorithm addressed the need for such models. It utilizes a sequence of ray-tracing computations on temporal snapshots of the environment. This approach can handle practical environments with arbitrary source, receiver, or sea-surface motion. While VirTEX can model the transmission of a known time series to any desired accuracy, its utility is offset by the computational resources required. In this paper, we present two new algorithms for modeling the propagation of a known time series in a restricted class of time-varying environments. The first algorithm can address steady motion of the source and/or receiver. The second algorithm can address a moving sea surface that satisfies some simple constraints. While more restrictive and less accurate than VirTEX, these new algorithms are significantly faster and more efficient. This makes them much more attractive for applications involving the modeling of extensive “what-if” scenarios. The algorithms can be implemented in software by postprocessing of the output from popular ray-tracing computer programs.

Description: Traditional channel models for communications research are designed for narrowband systems. Underwater acoustic communication systems use a bandwidth that is not small compared with the center frequency of the signal and qualify as ultrawideband (UWB) in a relative sense. In this paper, measurements and analysis of acoustic propagation effects demonstrate the shortcomings of narrowband channel models. These effects are frequency-dependent fluctuation rates and frequency-dependent attenuation, where the frequency dependence of the attenuation differs between paths. This frequency selectivity of the medium violates the assumption of uncorrelated taps and requires a UWB channel model. It is also shown that correlative channel sounders preserve wideband properties, which renders them suitable for UWB channel simulation based on the principle of direct replay.

Description: This paper discusses validation methods for underwater acoustic communication channel simulators, and validates direct and stochastic replay of underwater acoustic communication channels as implemented in a channel simulator called Mime. Direct replay filters an input signal directly with a measured time-varying impulse response, whereas stochastic replay filters an input signal with a synthetic impulse response consistent with the scattering function of the measured channel. The validation uses data from two sea experiments and a diverse selection of communication schemes. Good agreement is found between bit error rates and packet error rates of in situ transmissions and simulated transmissions. Long-term error statistics of in situ signaling are also reproduced in simulation when a single channel measurement is used to configure the simulator. In all except one comparison, the packet error rate in simulation is within 20% of the packet error rate measured on location. The implication is that this type of channel simulator can be employed to test new modulation schemes in a realistic fashion without going to sea, except for the initial data collection.