Acoustic Communication Channel Research Articles

Adaptation of acoustic transmission rates to optimize unmanned underwater vehicle communications

A framework for on-line characterization of in-water in situ acoustic transmission conditions, and intelligent adaptation of transmission rates to these conditions, is implemented on-board an unmanned underwater vehicles (UUV). The objective is to optimize use of the acoustic communications channel during collaborative shallow water missions with other UUVs. The software database uses relatively little bandwidth to track the success of transmitted packets, providing operator data tracking in addition to communications layer visibility into current channel conditions. The rate selector chooses the optimal transmission rate based on an adaptive distance bin approach. Measurable results are improvements in bandwidth, reduction in modem power usage, and increased visibility into data success compared to traditional, constant-rate acoustic communication patterns. The algorithm, its implementation, and recent in-water validation results are presented.

High frequency underwater acoustic communication channel characteristics in the Gulf of Mexico

In the applications of underwater acoustic communications, higher carrier frequencies support wider bandwidth, which is preferable for achieving higher data rates. At the same time, due to stronger attenuation, higher frequencies lead to shorter communication ranges. During past decades, a large number of efforts were devoted to investigate acoustic communications for the band of 8-50 kHz, over the medium range of several kilometers and beyond. Several efforts focused on the very high frequencies, greater than 200 kHz, for short communication ranges, tens or hundreds of meters. Here we consider a frequency band that falls between the well-studied high frequencies 8-50 kHz, for example from the Kauai Island experiment series, and the “unknown” very high frequencies (200 kHz or higher). A high frequency acoustic experiment was conducted in the northern Gulf of Mexico in August, 2016 to examine the operating ranges, data rates, and performance of acoustic communication systems at the carrier frequencies of 8...

A Framework for Multiple Object Tracking in Underwater Acoustic MIMO Communication Channels

This work presents a computational framework for the analysis and design of large-scale algorithms utilized in the estimation of acoustic, doubly-dispersive, randomly time-variant, underwater communication channels. Channel estimation results are used, in turn, in the proposed framework for the development of efficient high performance algorithms, based on fast Fourier transformations, for the search, detection, estimation and tracking (SDET) of underwater moving objects through acoustic wavefront signal analysis techniques associated with real-time electronic surveillance and acoustic monitoring (eSAM) operations. Particular importance is given in this work to the estimation of the range and speed of deep underwater moving objects modeled as point targets. The work demonstrates how to use Kronecker products signal algebra (KSA), a branch of finite-dimensional tensor signal algebra, as a mathematical language for the formulation of novel variants of parallel orthogonal matching pursuit (POMP) algorithms, as well as a programming aid for mapping these algorithms to large-scale computational structures, using a modified Kuck’s paradigm for parallel computation.

Probabilistic Neighborhood-Based Data Collection Algorithms for 3D Underwater Acoustic Sensor Networks.

Marine environmental monitoring provides crucial information and support for the exploitation, utilization, and protection of marine resources. With the rapid development of information technology, the development of three-dimensional underwater acoustic sensor networks (3D UASNs) provides a novel strategy to acquire marine environment information conveniently, efficiently and accurately. However, the specific propagation effects of acoustic communication channel lead to decreased successful information delivery probability with increased distance. Therefore, we investigate two probabilistic neighborhood-based data collection algorithms for 3D UASNs which are based on a probabilistic acoustic communication model instead of the traditional deterministic acoustic communication model. An autonomous underwater vehicle (AUV) is employed to traverse along the designed path to collect data from neighborhoods. For 3D UASNs without prior deployment knowledge, partitioning the network into grids can allow the AUV to visit the central location of each grid for data collection. For 3D UASNs in which the deployment knowledge is known in advance, the AUV only needs to visit several selected locations by constructing a minimum probabilistic neighborhood covering set to reduce data latency. Otherwise, by increasing the transmission rounds, our proposed algorithms can provide a tradeoff between data collection latency and information gain. These algorithms are compared with basic Nearest-neighbor Heuristic algorithm via simulations. Simulation analyses show that our proposed algorithms can efficiently reduce the average data collection completion time, corresponding to a decrease of data latency.

Fractional Fourier domain hopped communication method based on chirp modulation for underwater acoustic channels

To improve the data rate of underwater acoustic frequency-hopped communications, frequency hopping is applied to different orders of fractional Fourier domain (FrFD), to enable non-intrusive, bandwidth-limited acoustic communications. An FrFD frequency-hopped communication method based on chirp modulation, namely multiple chirp shift keying-FrFD hopping (MCSK-FrFDH), is proposed for underwater acoustic channels. Validated by both simulations and experimental results, this method can reach a bandwidth efficiency twice more than conventional frequency-hopped methods with the same data rate and anti-multipath capability, suggesting that the proposed method achieves a better performance than the traditional frequencyhopped communication in underwater acoustic communication channels. Results also show that in practical scenarios, the MCSKFrFDH system with longer symbol length performs better at the low signal-to-noise ratio (SNR), while the system with larger frequency sweeping range performs better at a high SNR.

LRP: Link quality‐aware queue‐based spectral clustering routing protocol for underwater acoustic sensor networks

SummaryRecently, underwater acoustic sensor networks (UASNs) have been considered as a promising approach for monitoring and exploring the oceans in lieu of traditional underwater wireline instruments. As a result, a broad range of applications exists ranging from oil industry to aquaculture and includes oceanographic data collection, disaster prevention, offshore exploration, assisted navigation, tactical surveillance, and pollution monitoring. However, the unique characteristics of underwater acoustic communication channels, such as high bit error rate, limited bandwidth, and variable delay, lead to a large number of packet drops, low throughput, and significant waste of energy because of packets retransmission in these applications. Hence, designing an efficient and reliable data communication protocol between sensor nodes and the sink is crucial for successful data transmission in underwater applications. Accordingly, this paper is intended to introduce a novel nature‐inspired evolutionary link quality‐aware queue‐based spectral clustering routing protocol for UASN‐based underwater applications. Because of its distributed nature, link quality‐aware queue‐based spectral clustering routing protocol successfully distributes network data traffic load evenly in harsh underwater environments and avoids hotspot problems that occur near the sink. In addition, because of its double check mechanism for signal to noise ratio and Euclidean distance, it adopts opportunistically and provides reliable dynamic cluster‐based routing architecture in the entire network. To sum up, the proposed approach successfully finds the best forwarding relay node for data transmission and avoids path loops and packet losses in both sparse and densely deployed UASNs. Our experimental results obtained in a set of extensive simulation studies verify that the proposed protocol performs better than the existing routing protocols in terms of data delivery ratio, overall network throughput, end‐to‐end delay, and energy efficiency.

Differential frequency hopping performance in doubly spread underwater acoustic communication channels

Underwater acoustic communication requires waveforms that are robust to the signal distortions caused by the channel. Many waveforms used for this purpose require the transmission of training symbols that span the inter-symbol interference (ISI) to compensate for these channel effects. Differential frequency hopping (DFH) is a fast frequency hopping, digital signaling technology that requires minimal information at the transmitter to communicate in the underwater channel. The relationships between the parameters of Doppler spread‐inducing underwater environments and DFH bit‐error performance are characterized. Wind speed determines the nature of the effect that the water surface imposes on acoustic DFH waveforms with low wind speeds resulting in an essentially flat, low‐absorption sea surface. In this case, strong surface reflections and little frequency spreading make ISI the dominant effect on the received waveforms. At high wind speeds, air bubbles in the surface layer absorb almost all energy incident...

MECHANICAL DESIGN OF AN AUTONOMOUS MARINE ROBOTIC SYSTEM FOR INTERACTION WITH DIVERS

SCUBA diving, professional or recreational, remains one of the most hazardous activities known by man, mostly due to the fact that the human survival in the underwater environment requires use of technical equipment such as breathing regulators. Loss of breathing gas supply, burst eardrum, decompression sickness and nitrogen narcosis are just a few problems which can occur during an ordinary dive and result in injuries, long-term illnesses or even death. Most common way to reduce the risk of diving is to dive in pairs, thus allowing divers to cooperate with each other and react when uncommon situation occurs. Having the ability to react before an unwanted situation happens would improve diver safety. This paper describes an autonomous marine robotic system that replaces a human dive buddy. Such a robotic system, developed within an FP7 project “CADDY – Cognitive Autonomous Diving Buddy” provides a symbiotic link between robots and human divers in the underwater. The proposed concept consists of a diver, an autonomous underwater vehicle (AUV) Buddy and an autonomous surface vehicle (ASV) PlaDyPos, acting within a cooperative network linked via an acoustic communication channel. This is a first time that an underwater human-robot system of such a scale has ever been developed. In this paper, focus is put on mechanical characteristics of the robotic vehicles.

Performance Evaluation of the Complex-Coefficient Adaptive Equalizer Using the Hilbert Transform

In underwater acoustic communication, the transmitted signals are severely influenced by the reflections from both the sea surface and the sea bottom. As very large reflection signals from these boundaries cause an inter-symbol interference (ISI) effect, the communication quality worsens. A channel estimation-based equalizer is usually adopted to compensate for the reflected signals under the acoustic communication channel. In this study, a feed-forward equalizer (FFE) with the least mean squares (LMS) algorithm was applied to a quadrature phase-shift keying (QPSK) transmission system. Two different types of equalizers were adopted in the QPSK system, namely a real-coefficient equalizer and a complex-coefficient equalizer. The performance of the complex-coefficient equalizer was better than that of two real-coefficient equalizers. Therefore, a Hilbert transform was applied to the real-coefficient binary phase-shift keying (BPSK) system to obtain a complex-coefficient BPSK system. Consequently, we obtained better results than those of a real-coefficient equalizer.

Frequency-selective fading statistics of shallow-water acoustic communication channel with a few multipaths

The bit error rate of an underwater acoustic communication system is related to multipath fading statistics, which determine the signal-to-noise ratio. The amplitude and delay of each path depend on sea surface roughness, propagation medium properties, and source-to-receiver range as a function of frequency. Therefore, received signals will show frequency-dependent fading. A shallow-water acoustic communication channel generally shows a few strong multipaths that interfere with each other and the resulting interference affects the fading statistics model. In this study, frequency-selective fading statistics are modeled on the basis of the phasor representation of the complex path amplitude. The fading statistics distribution is parameterized by the frequency-dependent constructive or destructive interference of multipaths. At a 16 m depth with a muddy bottom, a wave height of 0.2 m, and source-to-receiver ranges of 100 and 400 m, fading statistics tend to show a Rayleigh distribution at a destructive interference frequency, but a Rice distribution at a constructive interference frequency. The theoretical fading statistics well matched the experimental ones.

Link Scheduling Method for Underwater Acoustic Sensor Networks Based on Correlation Matrix

The unique characteristic of the underwater acoustic communication channel force development of new protocols for underwater acoustic sensor networks. Especially, the pretty slower propagation speed of acoustic signals in underwater should be taken into consideration. This paper constructs a link scheduling model on medium access control (MAC) layer for multihop underwater acoustic sensor networks. The proposed model employs correlation matrix to describe the conflicts relationship among links and uses propagation delay to generate conflict matrix for collision detection. In order to minimum the frame length, a power control strategy is introduced and aimed to reduce the effect of link interference. Then, a heuristic algorithm is presented to solve the conflict-free problem based on the model. Simulation results show that the link scheduling method and the power control strategy can effectively improve network performance with respect to throughput and end-to-end delay.

수중음향통신을 위한 훈련 신호 구간의 비트 오차율에 기반한 레이크 수신기

ABSTRACT In the underwater acoustic communication channels, a multipath reflection becomes the cause of obstacle. To solve this problem, a rake receiver has been required for which one could take the time diversity. However, there is a concern about using incorrect path to recover signals with a high weighting value as underwater acoustic communication channels have severe time-variant property. In order to prevent these problem, a rake receiver is proposed which is based on BER(bit error rate) train sequence duration. The performance is evaluated through lake trials; there are three methods that are a proposed rake receiver, a conventional rake receiver, and a non-rake receiver. As a result, the number of bit errors in the proposed rake receiver, that of bit errors in the conventional rake receiver, and that of bit errors in the non-rake receiver is 8, 45, and 72, respectively. 키워드 : 레이크 수신기, 수중음향통신, 비트 오차율, 호수 실험 Key word : Rake Receiver, Underwater Acoustic Communication, Bit Error Rate, Lake Trial

Experimental Multipath Delay Profile of Underwater Acoustic Communication Channel in Shallow Water

<p>The shallow water channel is an environment that is of particular interest to many research fields. An underwater acoustic channel is characterized as a multipath channel. Time-varying multipath propagation is one of the major factors that limit the acoustic communication performance in shallow water. This study conducts two underwater acoustic experiments in Tanjung Balau, Johor, Malaysia. A transducer and a hydrophone are submerged at different depths and separated by different distances. Linear frequency modulated (LFM) pulses are chosen as the main transmit signal for the experiments. The cross-correlation between the transmitted and received signals represents the impulse response of the channel (multipath profile). The results show that the amplitude of the successive paths will not rapidly decline, and vice versa, when the distance between the sender and the receiver increases. Moreover, the time difference between the different paths will be small in the case of distance increase. In other words, the successive paths will converge in time.</p>

Simulation and modeling of hydro acoustic communication channels with wide band attenuation and ambient noise

Acoustic waves are the preferred medium for long-range underwater communications. Increasing number of innovative methods for underwater communication using acoustic waves appears. Performance of underwater communications for the new methods needs to be evaluated through simulation. Simulation of underwater acoustic communications is challenging due to many impediments, including attenuation, multipath propagation, noise and Doppler spread. In this paper, a baseline time domain simulation model is extended to several frequency-domain models. The proposed frequency models, including two incoherent models and a coherent model, take multipath attenuation and ambient noise into account. An incoherent linear fitting model and a coherent model are simulated and compared with a theoretical reference and the baseline time model. The proposed incoherent models are also compared with one another. Simulation shows that the incoherent linear fitting model produces results similar to the multi-frequency-merge-path model, but requires less computation time. In addition, the proposed coherent model is compared with field experimental data. The coherent model with color noise, in the frequency domain, can match closely the bit error rates of the field experimental data.

Alternative Approach for Combination of Fingers in Underwater Acoustic Communication

An underwater acoustic communication channel is time varying and has multipath propagation with bottom and surface reflection. A rake receiver has time diversity effects in multipath propagation environments. We have to recognize the correct path for a rake receiver, as this affects communication performance in the underwater acoustic channel. In this paper, we propose a more reliable rake receiver based on BER (bit error rate) of training sequence duration. We conducted simulations and lake trials to evaluate the performance of the proposed method. When the channel coding technique was not applied, the uncoded BER performances of the proposed method, the conventional method, and the nonrake method in the lake trial were 0.016, 0.088, and 0.141, respectively.

A Novel Cooperative Opportunistic Routing Scheme for Underwater Sensor Networks.

Increasing attention has recently been devoted to underwater sensor networks (UWSNs) because of their capabilities in the ocean monitoring and resource discovery. UWSNs are faced with different challenges, the most notable of which is perhaps how to efficiently deliver packets taking into account all of the constraints of the available acoustic communication channel. The opportunistic routing provides a reliable solution with the aid of intermediate nodes’ collaboration to relay a packet toward the destination. In this paper, we propose a new routing protocol, called opportunistic void avoidance routing (OVAR), to address the void problem and also the energy-reliability trade-off in the forwarding set selection. OVAR takes advantage of distributed beaconing, constructs the adjacency graph at each hop and selects a forwarding set that holds the best trade-off between reliability and energy efficiency. The unique features of OVAR in selecting the candidate nodes in the vicinity of each other leads to the resolution of the hidden node problem. OVAR is also able to select the forwarding set in any direction from the sender, which increases its flexibility to bypass any kind of void area with the minimum deviation from the optimal path. The results of our extensive simulation study show that OVAR outperforms other protocols in terms of the packet delivery ratio, energy consumption, end-to-end delay, hop count and traversed distance.

Doppler Estimation Based on Frequency Average and Remodulation for Underwater Acoustic Communication

Underwater acoustic communication channel is time varying and has Doppler effect due to wind, sea surface roughness, and platform motion. We have to recognize the changing channel state and apply it to communication technique for increasing transmission efficiency in the underwater acoustic channel. In this paper, we propose more reliable Doppler estimation method based on frequency average and remodulation. We conducted the simulation and sea trial to evaluate the performance of the proposed method. When the channel coding technique was not applied, the uncoded bit error rate performance of the proposed method was improved about maximum 50.3% compared with conventional method.

Performance Analysis and Optimal Design of Multichannel Equalizer for Underwater Acoustic Communications

Adaptive equalization is a widely used method of mitigating the effects of multipath propagation and Doppler spreading in underwater acoustic communication channels. While the structure of a multichannel equalizer and least-squares-based adaptation algorithm are extensively used in practice, little is known in how to choose the number of sensors, separation between them, and lengths of the constituent filters such that the equalization performance is optimized. This paper studies the problem of optimal multichannel equalizer design in the context of time-varying underwater acoustic communication channels. In the first part, the paper presents a theoretical characterization of the equalization performance when the number of symbols that can be received in the time period over which the channel can be considered time invariant is limited. This result is then used to develop an understanding that the optimal number of equalizer coefficients is a tradeoff between the minimum mean squared error (MMSE) requirement for longer constituent filters and the insight that the limit on the number of stationary observations also limits the number of filter coefficients that can be effectively adapted. In the second part, the paper develops a theoretical model for wideband arrivals impinging upon an array of sensors of the multichannel equalizer. This model is used to develop an understanding that the optimal sensor separation is a tradeoff between the requirement for long aperture which improves resolution, and the fact that the grating lobes, caused by spatial undersampling, limit the equalizer’s ability to estimate the transmitted signal from the received signal.

On the Achievable Rate of a Class of Acoustic Channels and Practical Power Allocation Strategies for OFDM Systems

In this paper, we consider a class of single-input–multiple-output (SIMO) underwater acoustic communication channels, where each propagation path can be characterized by a complex-valued Gaussian block-fading model. The capacity of such channels is computed and analyzed using three power allocation strategies: waterfilling, uniform, and on–off uniform power allocation across the signal bandwidth. Our analysis considers the effects of imperfect channel estimation, delayed feedback, and pilot overhead, which are found to contribute to about 1 (b/s)/Hz loss from 4 (b/s)/Hz at 20-dB signal-to-noise ratio (SNR) for the experimental channel. We find that given the long feedback delays associated with acoustic channels, all-on uniform power allocation, which does not require feedback and is simple to implement, emerges as a justified practical solution that outperforms the other strategies. Furthermore, when considering acoustic-specific propagation effects, such as frequency-dependent attenuation and colored noise, considerable gain can be achieved by selecting the frequency band according to the attenuation pattern and the available transmit power, e.g., at least 6-dB gain for a 10-km link when compared to transmission over a preselected frequency band of 10–15 kHz.

A Robust, Opportunistic Clock Synchronization Algorithm for Ad Hoc Underwater Acoustic Networks

Proliferation of deployed sea-going autonomous platforms, such as autonomous underwater vehicles (AUVs), unmanned surface vehicles (USV), and sensor nodes anchored to the seabed, make the deployment of true underwater acoustic networks more and more feasible. An important feature of any network is the ability to synchronize the clocks of the participants, for the purpose of, e.g., time-slotted media access control (MAC) and navigation. Terrestrial clock synchronization protocols, such as the well-established network time protocol (NTP), are not readily applicable to underwater acoustic networks, because of long propagation times, low packet delivery success rates, communication ranges that vary over time in an unpredictable manner, and, in the presence of mobile nodes, the ad hoc nature of the composition of the network. This paper proposes a continuous estimation of internode clock offset and drift, based on the continuous exchange of modem packets, possibly containing transmission and reception timestamps. The proposed solution takes explicitly into account the limitations of the acoustic communication channel and network node mobility. This robust, opportunistic clock synchronization (ROCS) is robust against modem reset, and will work even if packet delivery success rates are not optimal or if no communication is possible for extended periods of time. Experimental results are given from the COLLaborative Asw Behaviours–Next Generation Autonomous Systems (COLLAB-NGAS14) campaign, held October 19–31, 2014, off the west coast of Italy. During the sea trial, the proposed clock synchronization algorithm was deployed and successfully tested within an underwater acoustic network composed of mobile and fixed nodes.