Characteristics Of Underwater Acoustic Channel Research Articles

Investigation of Characteristics of Underwater Acoustic Channel and Quality of Underwater Acoustic Communication in Line-of-Sight and Non-Line-of-Sight Environments

Investigation of Characteristics of Underwater Acoustic Channel and Quality of Underwater Acoustic Communication in Line-of-Sight and Non-Line-of-Sight Environments

Hamming Distance Optimized Underwater Acoustic OTFS-IM Systems.

The orthogonal time frequency space (OTFS) modulation technique can provide reliable communication in time-varying channels. Due to the dispersive characteristics of underwater acoustic channels, this paper proposes an OTFS-IM underwater acoustic communication system based on Hamming distance optimization to reduce the impact of dispersion in underwater acoustic communication. Firstly, the OTFS-IM underwater acoustic communication system is introduced, which introduces index modulation into the Delay-Doppler (DD) domain to make the OTFS system have stronger anti-Delay-Doppler capability. In contrast, since there is index sequence redundancy in a specific index combination, a Hamming distance optimization model is used to eliminate the redundant combination in the specific index combination sequence and further improve the bit error rate performance of the system. In addition, the Hamming distance optimized OTFS-IM underwater acoustic communication system is verified by simulation analysis. The results show that the proposed Hamming distance optimized OTFS-IM can achieve more reliable bit error rate performance.

DOIDS: An Intrusion Detection Scheme Based on DBSCAN for Opportunistic Routing in Underwater Wireless Sensor Networks.

In Underwater Wireless Sensor Networks (UWSNs), data should be transmitted to data centers reliably and efficiently. However, due to the harsh channel conditions, reliable data transmission is a challenge for large-scale UWSNs. Thus, opportunistic routing (OR) protocols with high reliability, strong robustness, low end-to-end delay, and high energy efficiency are widely applied. However, OR in UWSNs is vulnerable to routing attacks. For example, sinkhole attack nodes can attract traffic from surrounding nodes by forging information such as the distance to the sink node. In order to reduce the negative impact of malicious nodes on data transmission, we propose an intrusion detection scheme (IDS) based on the Density-Based Spatial Clustering of Applications with Noise (DBSCAN) clustering algorithm for OR (DOIDS) in this paper. DOIDS is based on small-sample IDS and is suitable for UWSNs with sparse node deployment. In DOIDS, the local monitoring mechanism is adopted. Every node in the network running DOIDS can select the trusted next hop. Firstly, according to the behavior characteristics of common routing attack nodes and unreliable underwater acoustic channel characteristics, DOIDS selected the energy consumption, forwarding, and link quality information of candidate nodes as the detection feature values. Then, the collected feature information is used to detect potential abnormal nodes through the DBSCAN clustering algorithm. Finally, a decision function is defined according to the time decay function to reduce the false detection rate of DOIDS. It makes a final judgment on whether the potential abnormal node is malicious. The simulation results show that the algorithm can effectively improve the detection accuracy rate (3% to 15% for different scenarios) and reduce the false positive rate, respectively.

Video Error-Resilience Encoding and Decoding Based on Wyner-Ziv Framework for Underwater Transmission

Limited by the characteristics of underwater acoustic channels, the video transmission applications targeting deep-sea detection and operation tasks are facing severe challenges such as network failure and high delay, resulting in loss of video details, color distortion, blurring, and even bit errors, which seriously affect decoding quality of the video transmission and reception. In order to solve the problems of deep-sea long-distance wireless communication, this paper proposes an improved Wyner-Ziv coding scheme (UnderWater-WZ) for video transmission through acoustic channels. The implementation process includes controlling error range by using MJPEG coding, combining motion compensation time interpolation with calibration information to generate high-quality side information. And intraframe quantization matrix is designed to weaken the change of video scene. The experimental results show that under the highest packet loss rate of 20%, this scheme can achieve 2.6~3.5 dB improvements in terms of video reconstruction compared to the previous methods, which is close to the error-free level.

Underwater sound speed profile parameters estimation in asynchronous network

AbstractThe technology of underwater wireless acoustic sensor networks (UWSNs) plays an important role in many commercial and military applications in underwater. In UWSNs, there exist many challenges posed by the specific characteristics of underwater acoustic channel, such as sound propagation speed is a function of depth with unknown parameters. Besides, time synchronization of anchor nodes is a crucial part of the applicable network. Time synchronization of anchor nodes as well as estimation of sound speed parameters considering an isogradient profile is explored here. While the previous efforts do not take into account these factors, this article assumes only one synchronized anchor node in the network and enables joint synchronization of the other anchor nodes and the estimation of the propagation delays between the anchor nodes using a weighted linear least square solution. Then, it uses the estimated propagation delays to estimate the unknown parameters of the sound speed profile using a conventional Gauss‐Newton algorithm with approximately three iterations to converge. Validation of the proposed method is done by the numerical simulations and to demonstrate the effectiveness of the proposed method we compare the results with Cramer‐Rao bound average.

High-Reliability Underwater Acoustic Communication Using an M-ary Cyclic Spread Spectrum

Multipath propagation, frequency selective fading, low-propagation velocity, and narrow bandwidth are all characteristics of underwater acoustic channels. Doppler shifts and diffusions can occur as a result of the low transmission speed of an acoustic signal, which can be caused by the movement of ocean currents or the transceiver. Furthermore, frequency selective fading and excessive noise interference can disrupt underwater acoustic communication on a continual basis. Because of its high anti-interference ability and high confidentiality, spread spectrum technology is commonly adopted in underwater acoustic communications. Although the direct sequence spread spectrum method has a low data rate, it is advantageous in a multipath propagation channel environment or an environment with a low signal-to-noise ratio (SNR). This advantage is suitable for long-distance transmission or LPD (Low Probability of Detection) communication, and the direct sequence spread spectrum method is applied. In this paper, we propose a highly reliable M-ary cyclic spread spectrum technique by superimposing the M-ary spread spectrum, an extension of the direct sequence spread spectrum technique, and cyclic shift keying. Furthermore, by estimating the Doppler frequency using M-ary spread spectrum codes and performing synchronization correction of the ensuing symbol based on the estimation results, higher performance can be attained. Simulations and experiments showed that the M-ary cyclic spread spectrum method can reduce Doppler estimation and synchronization error accumulation while maintaining a high data rate. Furthermore, the MCSS method had a lower bit error rate than the standard spread spectrum method.

Adaptive estimation of sparse channel based on modified RLS for coherent underwater acoustics communications

A new channel estimation approach used in coherent underwater acoustic communication system is presented in this paper. The proposed method consists of two steps: (1) obtain the initial channel estimation based on least square (LS) and the sparse characteristics of underwater acoustic channels; (2) track the channels using the modified recursive least squares (RLS). In the first step, the threshold value to set all coefficients below it to zero is deduced. In the second step, a modified RLS based on the idea of RLS and Kalman filter terminology is proposed. Experimental results show that the proposed method can track underwater acoustic channels well. The lower symbol error rate (SER) and higher output SNR can be achieved.

Fundamentals and Advancements of Topology Discovery in Underwater Acoustic Sensor Networks: A Review

With the extensive application of underwater acoustic sensor networks (UANs) in various fields such as commerce, marine environmental research, and national defense, the need for an autonomous and well-organized underwater acoustic network has been increasing. Topology discovery is a crucial step in constructing an underwater acoustic network, and node discovery and topology establishment are the essential components of the topology discovery process in UANs. This paper introduces the characteristics of underwater acoustic channels and networks and highlights their influences on topology discovery. We discuss the topology discovery protocol development in terrestrial networks (i.e., duty-cycle ad hoc network, Internet of things). The main focus of this paper is to study the topology discovery protocols of UANs. This paper also classifies and introduces the existing topology discovery protocols and compared their advantages and disadvantages to understand the current topology discovery methods. Furthermore, we also discuss the topology discovery protocol’s influence on different layers’ functions in the UAN protocol stack. Analyze the current research challenges in this field, followed by important open issues in UAN protocol development, which provide new opportunities for further research.

Modelling and Simulation on Acoustic Channel of Underwater Sensor Networks

The reliability of the modelling system mainly depends on the simulation of underwater acoustic channel characteristics. The reliability of simulator is improved because of the use of Bellhop in NS‐Miracle. World Ocean Simulation System (WOSS) can retrieve the data of marine environment by accessing the database of seabed depth, sound speed profile and seabed sediment, and transmit them to Bellhop simulator automatically, so that the model is closer to the actual underwater acoustic transmission channel than not using WOSS. In order to verify the reliability of NS2/NS‐Miracle simulation system with WOSS, a centralized underwater sensor network with five nodes is simulated on the integrated simulation system. The characteristic empirical model, Bellhop Ray‐Tracing model, and WOSS combined with Bellhop model are, respectively, adopted to simulate underwater acoustic channel. The results of three types of simulation, such as average throughput, average delay, and packet error rate, and simulation time are very close under the same condition. It proves that the accuracy of integrated simulation system is as excellent as that of NS‐Miracle. However, WOSS can automatically acquire the actual sea environment parameters and provide them to the simulator, which can improve the authenticity of the simulation system. Furthermore, three MAC protocols, Aloha‐CS, CSMA/CA, and DACAP, are simulated on the integrated simulation system under the same condition including ocean environment, network topology, and parameters. The results show that the performance of CSMA/CA is greater than the other protocols in such networks. It also proves that the integrated simulation system can accurately simulate the relevant characteristics of the MAC protocol.

LEER: Layer-Based Energy-Efficient Routing Protocol for Underwater Sensor Networks

Acoustic signals are used for communication in underwater sensor networks (UWSNs) because radio signals attenuate heavily when propagating in water, while optical signals have large scattering in water. Data transmission in UWSNs faces great challenges due to the characteristics of underwater acoustic channels. Moreover, high energy consumption and long latency bring about increased challenges for the designs of routing protocols in UWSNs. In this paper, we propose a routing protocol called Layer-based Energy-Efficient Routing (LEER) protocol to address the route failure problem with greedy routing, as well as the long end-to-end delay and high energy consumption problems. In LEER, each node extracts the layer field information from hello packets received and updates its own layer to avoid the problem of routing a packet to a void area. All nodes with the LEER protocol forward packets to a sink node without the need for any location information. Simulation results show that the LEER protocol outperforms the depth-based routing (DBR) protocol in terms of the delivery rate and end-to-end delay.

Optimal geometric configuration of sensors for received signal strength based cooperative localization of submerged AUVs

Cooperative localization is one of the recent techniques that are being utilized for localization/navigation of Autonomous Underwater Vehicles (AUVs). Received Signal Strength (RSS) based cooperative localization is relatively unexplored field and only a little literature is available about it. Proper positioning of the sensors to maximize the observability of the AUVs is very critical for cooperative localization. In this paper, a method for estimating optimal geometric configuration of sensors is presented for 3D localization of targets using RRS measurements. An evaluation function based on Fisher Information Matrix (FIM) theory has been derived keeping in view the underwater acoustic channel characteristics. Most of the real world constraints such as acoustic waves characteristics in water, multiple-AUVs system and computation complexity have been considered. The proposed evaluation function has been solved using successive optimization algorithm to obtain optimal positions of the sensors. The algorithm ensured that the computation complexity should remain limited even when number of sensor AUVs is increased. Various simulation examples are then presented to calculate optimal formation for different systems/situations. Finally, efficacy of proposed method has been proved using Extended Kalman Filter (EKF) based simulations to compare the localization error between optimal formations and random formations of the sensors.

Research on Development and Application of Underwater Acoustic Communication System

Underwater acoustic communication technology is the core technology in the marine field, occupying a very important position, and has an important supporting role for underwater observation operations and military activities. Underwater acoustic technology is currently the most widely used and longest underwater wireless communication technology. With the increasing demand for development, the complexity of underwater acoustic channels, underwater environment and other factors make the communication speed, communication distance and communication stability of underwater acoustic communication greatly restricted. Despite these limitations, underwater acoustic communication is still valued by professionals from various countries and industries, and has been developed and applied to some extent. This article summarizes the basic content of underwater acoustic communication technology, leads to its general structure as a communication system, the current development of underwater acoustic communication technology, and the characteristics of underwater acoustic communication systems due to the characteristics of underwater acoustic channels. This article summarizes the development and some applications of the underwater acoustic communication system and the full text and some prospects for the future underwater acoustic communication.

Target detection method using multipath information in an underwater waveguide environment

Interface scattering in the ocean environment consistently results in reverberation and multipath echo, thereby decreasing the detection probability of moving targets. In this work, a new detection method which fully uses underwater acoustic channel characteristics to realise high detection probability for moving targets is proposed. First, this method combines fractional Fourier transform and fractional convolution theory to build a delay time-Doppler shift matrix related to multipath echoes. Second, the insensitivity of linear modulated frequency signal and distribution of multipath echo analysis are utilised to detect the moving target in a severe environment. Simulation results show that the proposed method can obtain higher detection probability and more accurate parameter estimation than the traditional method. Experimental results also verify that the proposed method has favourable performance even in strong reverberation environment.

Anti-Multipath Orthogonal Chirp Division Multiplexing for Underwater Acoustic Communication

Multipath due to reflections of the sea surface, seabed, and obstacles, as well as inhomogeneity within the ocean, is an important characteristic of underwater acoustic channels. Mutual interference among multiple paths causes severe amplitude fading and frequency selective fading. The guard interval is an effective anti-multipath method, but an excessively long guard interval will reduce the data rate of multi-carrier underwater acoustic communication. In this paper, we propose a new anti-multipath multi-carrier communication method based on orthogonal chirp division multiplexing (OCDM) that uses chirp signals for carrier modulation. As OCDM exploits the multipath components for diversity gain, the system robustness is improved. The new method also adds a data pick-based rake receiver for maintaining good communication performance, even at short guard intervals. We detail the implementation and parameter selection of the anti-multipath OCDM system and compare its performance with the traditional orthogonal frequency division multiplexing (OFDM) scheme using simulations; under a severe multipath simulation condition (the delay spread is longer than the guard interval), the anti-multipath OCDM achieves a bit error rate (BER) of $10^{-6}$ , while the OFDM has a BER floor of $10^{-3}$ . The simulation results verify the feasibility of the proposed method and the superiority of its anti-multipath performance.

M-ary Cyclic Shift Keying Spread Spectrum Underwater Acoustic Communications Based on Virtual Time-Reversal Mirror.

Underwater acoustic communications are challenging because channels are complex, and acoustic waves when propagating in the ocean are subjected to a variety of interferences, such as noise, reflections, scattering and so on. Spread spectrum technique thus has been widely used in underwater acoustic communications for its strong anti-interference ability and good confidentiality. Underwater acoustic channels are typical coherent multipath channels, in which the inter-symbol interference seriously affects the performance of underwater acoustic communications. Time-reversal mirror technique utilizes this physical characteristic of underwater acoustic channels to restrain the inter-symbol interference by reconstructing multipath signals and reduce the influence of channel fading by spatial focusing. This paper presents an M-ary cyclic shift keying spread spectrum underwater acoustic communication scheme based on the virtual time-reversal mirror. Compared to the traditional spread spectrum techniques, this method is more robust, for it uses the M-ary cyclic shift keying spread spectrum to improve the communication rate and uses the virtual time-reversal mirror to ensure a low bit error rate. The performance of this method is verified by simulations and pool experiments.

OSPG-MAC: An OFDMA-Based Subcarrier Pregrouping MAC Protocol for Underwater Acoustic Wireless Sensor Networks

In underwater acoustic wireless sensor networks (UAWSNs), designing media access control (MAC) protocols is highly challenging because of their significant influence on network performance. It is a great challenge to design suitable MAC protocols for UAWSNs because of the specific characteristics of underwater acoustic channels, such as low communication rates, large propagation delays, and limited available bandwidth. Orthogonal frequency division multiple access (OFDMA) is a special case of OFDM-based multicarrier modulation in which multiple user symbols are transmitted simultaneously using different subcarriers with overlapping orthogonal frequency bands. In this paper, we propose an OFDMA-based subcarrier pregrouping MAC protocol for UAWSNs, termed OSPG-MAC. To make it easier to allocate subcarriers and avoid conflict, we group all the subcarriers into fixed subchannels in advance and allocate them in terms of these subchannels. To maximize the total throughput on each subchannel, a bit and power allocation algorithm for the protocol is proposed, termed the OSPG-MAC algorithm. Simulation results show that the performance of OSPG-MAC is better than that of classic underwater MAC protocols, including SFAMA and T-Lohi.

A Collision-Free Graph Coloring MAC Protocol for Underwater Sensor Networks

Employing contention-based medium access control (MAC) protocols in underwater sensor networks (UWSNs) is typically costly. This is due to the unique characteristics of underwater acoustic channels, such as long propagation delay, limited bandwidth, and high bit error rate. As a consequence, the contention-based (handshaking and random access-based) MAC protocols do not perform as efficiently as expected. The collision-free approach is therefore considered to achieve high performance by avoiding the collisions at the MAC layer in order to improve energy efficiency, throughput, and fairness. In this paper, we propose, inspired by the graph coloring techniques, a novel energy-conserving and collision-free reservation-based MAC protocol, called GC-MAC, for UWSNs. GC-MAC employs time-division multiple access (TDMA)-like approach by assigning separate time-slots, colors, to every individual sensor node in every two-hop neighborhood. Sensors with the same colors can thus transmit at the same time with no chance of collision. GC-MAC is also able to address the near-far effect, spatial-temporal uncertainty, and hidden/exposed node problems, without requiring code-division multiple access (CDMA) or power adjustment for collision avoidance. The network coverage and connectivity is then discussed to show the effectiveness of using cubes to cover a 3D underwater environment. Our extensive performance study shows that GC-MAC performs well by avoiding collisions to achieve better throughput and energy-efficiency performance compared with those of contention-based protocols. There is also a significant improvement in terms of packet delivery ratio and fairness among the nodes under different operational conditions.

Parametric Underwater Transmission Based on Pattern Time Delay Shift Coding System

The technique of pattern time delay shift coding (PDS) underwater acoustic (UWA) communication based on parametric array is presented in this paper, which is easy to be implemented and robust in the spatiotemporal variable ocean environment. The parametric array can generate low-frequency, broadband, and high-directivity beam with small-aperture. The high directivity reduces the impact of time variant characteristics of UWA channel especially multipath effects and improves the reuse rate of underwater acoustic channel at the same time. The wide bandwidth allows high rate communications. The sea trial results show that it can be employed to combat multipath propagation in shallow water and achieve very low bit error rate (BER). The theoretical research and sea trial verify the feasibility and effectiveness of the proposed UWA method.

Underwater Distributed Antenna Systems: Design Opportunities and Challenges

Underwater systems for ocean exploration and monitoring consist of a mix of geographically distributed stationary infrastructure and mobile units. The stationary infrastructure could be bottom-anchored nodes and surface buoys that are connected to a control center via cables and in-air radio links. Both the stationary and mobile units can be equipped with acoustic communication modules for underwater wireless data transmission. With the backbone (cable or radio) connection, those acoustic communication modules naturally form an underwater DAS. Similar to the DAS in terrestrial radio networks, the underwater DAS supports larger acoustic communication coverage and higher network throughput compared to the CAS. Furthermore, the characteristics of underwater acoustic channels, such as the frequency-dependent signal absorption loss and the low sound speed in water, lead to unique design challenges and unforeseen opportunities in the underwater DAS. This article examines both theoretical and system design issues pertaining to the underwater DAS, including operation strategies, communication algorithms and mobile node positioning, and pinpoints future research directions to fully realize its potentials.

Underwater acoustic communication and the general performance evaluation criteria

Driven by the huge demand to explore oceans, underwater wireless communications have been rapidly developed in the past few decades. Due to the complex physical characteristics of water, acoustic wave is the only media available for underwater wireless communication at any distance. As a result, underwater acoustic communication (UAC) is the major research field in underwater wireless communication. In this paper, characteristics of underwater acoustic channels are first introduced and compared with terrestrial communication to demonstrate the difficulties in UAC research. To give a general impression of the UAC, current important research areas are mentioned. Furthermore, different principal modulation-based schemes for short- and medium-range communications with high data rates are investigated and summarized. To evaluate the performance of UAC systems in general, three criteria are presented based on the research publications and our years of experience in high-rate short- to medium-range communications. These three criteria provide useful tools to generally guide the design and evaluate the performance of underwater acoustic communication systems.