To facilitate the exploration and exploitation of underwater resources, autonomous systems and underwater acoustic networks (UANs) are deployed for tasks unsuitable for direct human intervention and exchange information between devices. To keep the reliability of information exchanged, direct-sequence spread spectrum (DSSS) communication is commonly adopted for this scenario. To simplify the channel equalization process in DSSS communication, a minimum-mean-square-error (MMSE) equalizer is often utilized. However, the characteristics of underwater acoustic channels and acoustic modem, including long delay spreads and limited computational resources, lead to high computational complexity for MMSE equalization, thereby reducing decoding efficiency. To address this challenge, we propose a refined MMSE equalizer, termed the efficient MMSE equalizer (EME). Unlike conventional MMSE methods, the EME approach involves initially despreading the received chip sequence, followed by equalizing on the noisy symbols. By reducing the size of the correlation matrix in the core computational step of MMSE equalization, our method significantly improves computational efficiency. We assess the computational complexity of the proposed EME approach in comparison to conventional MMSE equalization and validate its performance through simulations and experimental studies. The results demonstrate that the EME achieves a bit error rate (BER) performance comparable to that of conventional MMSE equalization under high signal-to-noise ratio (SNR) conditions, while significantly enhancing computational efficiency.

Single beacon passive positioning is crucial for the covert and rapid navigation of autonomous underwater vehicles (AUVs). Traditional methods, which assume a fixed beacon position, known sound speed, and error-free AUV heading, limit the applicability and accuracy of passive positioning systems. In this article, we proposed an angle-based passive positioning algorithm, underwater moving bearing angle of arrival (U-MBAOA), to solve the above problems. First, we replace the bottom-anchored fixed beacon with a mobile surface beacon and develop a dynamic positioning model to accommodate its movement. Then, to enhance the positioning accuracy, we parameterized the sound speed and derived a weighted least squares method to estimate the sound speed. Finally, we constructed an error model for heading errors and positioning under a straight-line trajectory and combined this model with U-MBAOA, achieving the estimation of heading errors in passive positioning for the first time. Through simulation analysis and sea tests, we confirmed that our algorithms outperform the existing algorithms.

This paper proposes a novel multi-cluster network system for autonomous underwater vehicle (AUV) swarm communication and positioning, addressing challenges of low throughput, high end-to-end delay, and significant ranging errors in traditional asynchronous underwater acoustic networks. The proposed system introduces a novel hybrid Medium Access Control (MAC) protocol, which combines intra-cluster Time Division Multiple Access (TDMA) with inter-cluster Frequency Division Multiple Access (FDMA) to enhance channel utilization. Additionally, the system introduces an adaptive time-offset estimation method based on network data exchange and the Ultra-Short Baseline Moving Ranging (USBL-MR) algorithm, which significantly improves network throughput and reduces delay and improves positioning accuracy. Field trials demonstrate that the system achieves a throughput of 398.3 bps with an end-to-end delay of 21.2 s. Furthermore, the ranging Root Mean Square Error (RMSE) has been reduced from 7.87 to 2.42 m, marking a 14% improvement in positioning accuracy.

Long propagation delay of acoustic signal has long been considered the nemesis of wireless underwater communication. This paper proposes a novel space-propagation-aware Medium Access Control (MAC) protocol for swarm of Autonomous Underwater Vehicles (AUV). We show that, with proper design, we can take advantage of propagation delay, and improve underwater channel utilization in a way that is not possible in terrestrial wireless networks. The proposed MAC protocol leverages a unique spatial-temporal property of acoustic medium, enabling a fully decentralized channel access for multiple underwater nodes, while maintaining zero packet collisions at high traffic. The results from simulations and real experiments show that the protocol outperforms contention-based, and contention-free channel access schemes in terms of scalability, and energy consumption, achieving up to 50% gain in network throughput. In addition, the proposed scheme does not require prior knowledge about network topology, and it can successfully operate in highly dynamic networks without synchronization overhead. This makes the protocol a practical, and efficient solution for densely deployed underwater networks, such as swarm of AUVs.

Underwater acoustic networks (UWANs) play a vital role in the Internet of Underwater Things (IoUT), enabling critical functions, such as communication, data collection, and navigation. Among the applications of the IoUT, localizing a mobile node (MN) via a UWAN is particularly promising. However, the existing localization algorithms are ineffective when faced with an insufficient number of time of arrival (TOA) measurements for an unsynchronized MN due to the presence of sparsely deployed anchor nodes and signal reception issues. Although tracking methods can offer position predictions, their accuracies are compromised over time due to the movement of MNs. To overcome these challenges, we propose a methodology that combines the departure time of a beacon signal (DOB) with limited TOA measurements. This methodology enables an MN to be localized using a TOA-based method, which typically requires fewer measurements than a time difference of arrival (TDOA)-based method. Based on this methodology, we introduce an algorithm called localization with insufficient TOA measurements (LITM), which comprises two subalgorithms: one for estimating and tracking the DOBs and the other for localizing MNs through a closed-form solution. Together, these subalgorithms provide accurate MN position estimates under the constraint of insufficient TOA measurements. To validate the performance of our proposed algorithm, we conduct both simulation studies and sea experiments. The results demonstrate the superior effectiveness and position estimation accuracy of our algorithm compared to those of the existing methods.

Underwater wireless sensor networks (UWSNs) can provide services to the ocean. The deployment is one of the key problems in UWSNs. Optimizing networks power consumption and coverage has always been a huge challenge in deployment. Existing deployment models do not consider the asymmetric-link phenomenon and power control. In this paper, a new multi-objective optimization deployment model with power control in UWSNs is proposed to obtain a deployment scheme, which reduces the power consumption, and the asymmetric-link phenomenon is considered. At the same time, due to the unsatisfactory optimization performance and stability of some current algorithms, this paper proposes an algorithm named Crow-Colony Search Optimization Algorithm (C-CSOA) to optimize the deployment scheme. In this algorithm, we adopt the framework of Colony Search Optimization Algorithm (CSOA). In addition, we combine the advantages of Crow Search Algorithm (CSA) and CSOA to improve the optimization performance of the algorithm. We conducted simulation experiments, and the results indicate that: First, the model we proposed is feasible. Second, an efficient deployment scheme can be obtained by C-CSOA. Third, when reaching the predetermined network coverage, the total power consumption of UWSNs is significantly reduced (nearly 23.93% in average). By comparing with various advanced optimization algorithms, it is showed that C-CSOA has advantages of the good optimization performance and the small standard deviation.

In recent years, the underwater sensor network has emerged as a promising solution for a wide range of marine applications. The underwater wireless sensors are usually designed to operate in open water, where eavesdropping can be a serious issue. Existing work either utilizes cryptography that is computationally intensive or requires expensive hardware. In this paper, we present a coordinated multi-point transmission based protocol to improve network security. The proposed protocol dynamically pairs sensors for coordinated communications to undermine the eavesdroppers’ capability. Our preliminary results indicate that the underwater sensor network security can be enhanced using the proposed method, especially in applications where cryptography or special hardware are not suitable.

The transmission scheduling is a critical problem in radio frequency (RF) energy harvesting communications. Existing transmission strategies are mainly designed based on a classic model, in which the harvested energy is assumed pre-determined and considered as prior knowledge in offline approaches. In this extended abstract, we challenge this assumption showing that the harvested energy is affected by the transmission scheduling and becomes unknown and not pre-determined. In the new model, we add a feedback line from the data transmission to the harvested energy. It properly indicates the interplay between the energy harvest and the data transmission but challenges the transmission scheduling in the meantime. We formulated the optimal transmission scheduling based on the new model and advocate a recursive solution.

Cognitive acoustic (CA) is emerging as a promising technique for environment-friendly and spectrum-efficient underwater communications. Due to the unique features of underwater acoustic networks (UANs), traditional spectrum management systems designed for cognitive radio (CR) need an overhaul to work efficiently in underwater environments. In this paper, we propose a receiver-initiated spectrum management (RISM) system for underwater cognitive acoustic networks (UCANs). RISM seeks to improve the performance of UCANs through a collaboration of physical layer and medium access control (MAC) layer. It aims to provide efficient spectrum utilization and data transmissions with a small collision probability for CA nodes, while avoiding harmful interference with both “natural acoustic systems”, such as marine mammals, and “artificial acoustic systems”, like sonars and other UCANs. In addition, to solve the unique challenge of deciding when receivers start to retrieve data from their neighbors, we propose to use a traffic predictor on each receiver to forecast the traffic loads on surrounding nodes. This allows each receiver to dynamically adjust its polling frequency according to the variation of a network traffic. Simulation results show that the performance of RISM with smart polling scheme outperforms the conventional sender-initiated approach in terms of throughput, hop-by-hop delay, and energy efficiency.

In recent years, the underwater cognitive acoustic network (UCAN) has been advocated as an efficient technique to enhance the utilization of acoustic channel, while not interrupting the activity of marine mammals, sonars and other acoustic users. In cognitive radios, the common control channel (CCC) based media access control (MAC) protocols are very popular for their high reliability, easy implementation and low overhead. However, due to the severe frequency-dependent attenuation of acoustic waves, a UCAN may not have enough bandwidth for CCC. How to prevent the control channel from congesting in a UCAN with heavy traffic should be investigated carefully. With this in mind, we propose a dynamic control channel MAC (DCC-MAC) for distributed UCANs. Nodes in DCC-MAC could adjust the bandwidth of their control channel adaptively based on the situation of network traffic. Whenever acoustic nodes detected the congestion of CCC, they could flexibly select proper data channels to extend the bandwidth of their control channel, and return excessive frequency bands back when the control channel becomes idle. Simulation results show that DCC-MAC could reduce the collision probability among control messages significantly, thereby providing a better network performance in terms of throughput and energy efficiency than conventional cognitive MAC protocols.

Abstract Recent surges in the development of underwater acoustic networks (UANs) have lead to a rapid acceptance of this technology in scientific, commercial, and military applications. However, limited work has been performed on developing secure communication mechanisms and techniques to protect these networks. Security mechanisms are wildly studied in terrestrial networks, and various defense mechanisms have been developed as safeguards. Because of the difference in communication mediums and physical environments, the existing solutions for terrestrial networks cannot be directly applied for UANs. In this paper, we study the effects of denial‐of‐service jamming attacks on UANs using real‐world field tests. We develop our own jammer hardware and signals in order to analyze the characteristics of different jamming attack models on a network. Our tests are performed on multiple commercial brand acoustic modems and an orthogonal frequency division multiplexing modem prototype. We show that UANs can be easily jammed using carefully timed attacks, which are energy efficient. Copyright © 2012 John Wiley & Sons, Ltd.

Recently, various medium access control (MAC) protocols have been proposed for underwater acoustic networks (UANs). These protocols have significantly improved the performance of MAC layer in theory. However, two critical characteristics, low transmission rates and long preambles, found in the commercial modem-based real systems, severely degrade the performance of existing MAC protocols in the real world. Thus, a new practical MAC design is demanded. Toward an efficient approach, this paper analyzes the impact of these two modem characteristics on the random access-based MAC and handshake-based MAC, which are two major categories of MAC protocols for UANs. We further develop the nodal throughput and collision probability models for representative solutions of these two MAC protocol categories. Based on the analyses, we believe time sharing-based MAC is very promising. Along this line, we propose a time sharing-based MAC and analyze its nodal throughput. Both analytical and simulation results show that the time sharing-based solution can achieve significantly better performance.

Reliable broadcast is a critical service for Underwater Acoustic Networks (UANs). In this paper, we propose a Two-phase Broadcast Scheme (TBS) for UANs. TBS includes two phases: Fast Spreading phase and Data Recovery phase. It does not require topology or neighbor information. In the Fast Spreading phase, which is a best effort phase, opportunistic overhearing and network coding are combined to accumulate encoded packets. A probability based forwarder selection scheme is employed to alleviate the broadcast storm problem. A rebroadcast scheduling algorithm is also proposed to reduce collisions. The Data Recovery phase is to guarantee reliability if a node fails to complete data decoding in the Fast Spreading phase. Through delayed request sending, the Data Recovery phase at a node will not interfere with the Fast Spreading phase at other nodes. Through simulations, we demonstrate the advantages of TBS in terms of efficiency and reliability.

In underwater acoustic networks (UANs), due to the unique characteristics of acoustic modems such as long preambles and extremely low transmission rates, contentions are usually costly. As a result, both random access and handshake based MAC protocols do not perform as well as expected. A collision-free approach is therefore considered to more likely achieve a better performance. Following this principle, we propose a collision-free scheduling-based MAC protocol, called DOS, for UANs. DOS is a cluster-based protocol, where each cluster head independently generates on-demand and collision-free schedule for its cluster members using local information. Through pure scheduling and cooperation among neighboring clusters, DOS guarantees collision-free transmissions of both control and data packets. Compared with existing collision-free scheduling based MAC protocols, DOS is distributed and on-demand, i.e., it schedules according to nodes' dynamic transmission requests. Further, DOS does not require CDMA or power adjustment for collision resolution. Extensive simulation results show that DOS far outperforms random access and handshake based MAC protocols. It also achieves a comparable throughput to a centralized scheduling algorithm, and this is a big accomplishment for DOS, a distributed solution.

Swarms of autonomous underwater vehicles (AUVs) forming mobile underwater networks often operate in moving currents, which introduce severe turbulence that interferes with coordinated and stealthy navigation of fleet. Therefore, individual AUV must adjust their heading whenever needed to ensure it can reach a pre-determined destination. To achieve accurate navigation, AUVs must maintain precise knowledge of their locations. This paper develops the “Suave” (Swarm underwater autonomous vehicle localization) algorithm to localize swarms of AUVs operating in rough waters. The purpose of Suave is to ensure that all AUVs arrive at their destinations by preserving localization throughout the entire mission. Suave lowers the probability that an AUV swarm is detected by reducing the number of occasions that vehicles must surface to obtain accurate location information from external sources such as satellites. The Suave algorithm also achieves better energy conservation through improved control of localization reference messages. Simulations show Suave significantly improves localization accuracy, lowers energy consumption, and the probability of swarm detection.

Acoustic communications are the dominant method for underwater information transmission in underwater environments where radio is heavily absorbed underwater. Due to the vast discrepancy in propagation delay between acoustic and optical communication, short range low cost optical systems have the potential to compliment the capabilities of underwater acoustic networks.In this paper we evaluate several applications in which ultralow cost, short range optical transmitter-receiver pairs can be applied to improve the performance and utility of underwater acoustic sensor networks. Applications evaluated include time synchronization between nodes and TCP communication.Next, Broadcast MAC, a prototypical media access control protocol which performs well underwater is simulated in Aquasim. Results of this simulation show that schemes which focus on using optical links to reduce the total amount of data sent over the acoustic network are more worthwhile those which focus on reducing the overhead associated with adding additional nodes.Finally a prototype, low cost IR transmitter/receiver pair is constructed from parts which can be purchased for a few dollars. A parts list, circuit diagram, and details on setting up the linux time synchronization server and establishing a TCP/IP link over the serial connection are included. While a blue-green LED would have provided much greater range underwater ( 100 ft vs 1 ft), the inherent simplicity of this device should help to save a great deal of time in the implementation of other more advanced underwater optical communication systems.

Cognitive acoustic (CA) is emerging as a promising technique for environment-friendly and spectrum-efficient underwater acoustic networks (UANs). Due to the unique features of UANs, traditional spectrum management systems used for radio networks need an overhaul to work efficiently in underwater environments. In this paper, we propose a receiver-initiated spectrum management (RISM) system for underwater cognitive acoustic networks (UCANs). RISM seeks to significantly improve the performance of UANs through a collaboration of the physical layer and medium access control (MAC) layer. This system features collaborative spectrum sensing, efficient spectrum sharing and advanced spectrum decision algorithms. It aims to provide collision-free data transmissions and efficient spectrum utilization for CA users, while avoiding harmful interference with both “natural acoustic systems”, such as marine mammals, and “artificial acoustic systems”, like sonar users and other UANs. Simulation results show that, RISM can effectively operate in both tree topology and partially connected mesh topology networks and achieve collision-free data transmissions.

Time synchronization plays a critical role in distributed network systems. In this paper, we investigate the time synchronization problem in the context of underwater sensor networks (UWSNs). Although many time-synchronization protocols have been proposed for terrestrial wireless sensor networks, none of them can be directly applied to UWSNs. This is because most of these protocols do not consider long propagation delays and sensor node mobility, which are important attributes in UWSNs. In addition, UWSNs usually have high requirements in energy efficiency. To solve these new challenges, innovative time synchronization solutions are demanded. In this paper, we propose a pairwise, cross-layer, time-synchronization scheme for mobile underwater sensor networks, called DA-Sync. The scheme proposes a framework to estimate the doppler shift caused by mobility, more precisely through accounting the impact of the skew. To refine the relative velocity estimation, and consequently to enhance the synchronization accuracy, the Kalman filter is employed. Further, the clock skew and offset are calibrated by two runs of linear regression. Simulation results show that DA-Sync outperforms the existing synchronization schemes in both accuracy and energy efficiency.

Recently, various Medium Access Control (MAC) protocols have been proposed for underwater acoustic networks. These protocols have significantly improved the performance of MAC layer in theory. However, two critical characteristics, low transmission rates and long preambles, found in the commercial modem-based real systems, drastically degrade the performance of existing MAC protocols in the real world. A new practical MAC design is demanded. Toward a proper approach, this paper analyzes the impact of the two newly found modem characteristics on the random access-based MAC and handshakebased MAC, which are two major types of MAC protocols for underwater acoustic networks. We further develop the nodal throughput and collision probability models for representative solutions of these two MAC protocol types. Based on the analysis, we believe time sharing-based MAC is very promising. Along this line, we propose a time sharing-based MAC and analyze its nodal throughput. Both analytical and simulation results show that the time sharing-based solution can achieve significantly better performance.

Cooperative communication has been studied extensively as a promising technique for improving the performance of terrestrial wireless networks. However, in underwater cooperative acoustic networks, long propagation delays and complex acoustic channels make the conventional relay selection schemes designed for terrestrial wireless networks inefficient. In this paper, we develop a new best relay selection criterion, called COoperative Best Relay Assessment (COBRA), for underwater cooperative acoustic networks to minimize the one-way packet transmission time. The new criterion takes into account both the spectral efficiency and the underwater long propagation delay to improve the overall throughput performance of the network with energy constraint. A best relay selection algorithm is also proposed based on COBRA criterion. This algorithm only requires the channel statistical information instead of the instantaneous channel state. Our simulation results show a significant decrease on one-way packet transmission time with COBRA. The throughput and delivery ratio performance improvement further verifies the advantages of our proposed criterion over the conventional channel state based algorithms.

Time synchronization is an important requirement for many services provided by distributed networks. A lot of time synchronization protocols have been proposed for terrestrial Wireless Sensor Networks (WSNs). However, none of them can be directly applied to Underwater Sensor Networks (UWSNs). A synchronization algorithm for UWSNs must consider additional factors such as long propagation delays from the use of acoustic communication and sensor node mobility. These unique challenges make the accuracy of synchronization procedures for UWSNs even more critical. Time synchronization solutions specifically designed for UWSNs are needed to satisfy these new requirements. This paper proposes Mobi-Sync, a novel time synchronization scheme for mobile underwater sensor networks. Mobi-Sync distinguishes itself from previous approaches for terrestrial WSN by considering spatial correlation among the mobility patterns of neighboring UWSNs nodes. This enables Mobi-Sync to accurately estimate the long dynamic propagation delays. Simulation results show that Mobi-Sync outperforms existing schemes in both accuracy and energy efficiency.

A mobile underwater wireless sensor network is a system of mobile underwater sensors which are acoustically networked. Existing ocean studies using independent mobile sensors have led to many interesting discoveries. Therefore, the networking capability will extend the capability of existing approaches and enable novel applications because it provides more control and coordination of the sensors. However, network mobility and environment uncertainty create numerous challenging issues which need to be addressed before such a paradigm becomes reality. Network deployment is among the most important problems because it has a significant impact on other research areas. In this paper, we consider the reuse-based deployment scheme in which a mobile underwater sensor network operates in a bounded area and a sensor, when reaching beyond the area, is deployed back into it. We propose a deployment framework to study the connectivity and coverage of such networks and show its effectiveness through two case studies. Our framework is based on the idea that the reuse of network nodes stabilizes these network metrics. By modeling them, we can estimate the number of nodes needed to achieve a requirement on network coverage or network connectivity only by solving algebraic equations rather than running more simulations.

In this paper, we investigate reliable data transfer for multi-hop underwater acoustic networks. Motivated by experiences from real-world field tests, we propose a Practical Coding-based Multi-hop Reliable Data Transfer (PCMRDT) protocol. For the per-hop reliable data transfer, PCMRDT combines random linear coding and selective repeat to achieve high reliability and efficiency. We analyze the data recovery capability for random linear coding so that we can set an appropriate coding rate. In addition, PCMRDT utilizes a multi-hop coordination mechanism to eliminate collisions and decreases average end-to-end delay over multiple hops. Simulation results show that PCMRDT can significantly reduce the network delay with high energy efficiency.

In this paper, we investigate multi-hop reliable data transfer for underwater acoustic networks. We propose a new protocol, called FOuntain Code based Adaptive multi-hop Reliable data transfer (FOCAR). FOCAR is essentially a hybrid ARQ scheme which integrates Fountain codes with hop-by-hop retransmission-upon-failure. It considers the half duplex nature of the underwater acoustic modems and adapts the block size of each hop to optimize the end-to-end delay for the multi-hop network scenario. Extensive simulation results show that FOCAR can achieve high reliability with low end-to-end delay and high energy efficiency.

Time synchronization and localization are basic services in a sensor network system. Although they often depend on each other, they are usually tackled independently. In this work, we investigate time synchronization and localization problems in underwater sensor networks. We propose a joint solution for localization and time synchronization, in which the stratification effect of underwater medium is considered, so that the bias in the range estimates caused by assuming sound waves travel in straight lines in water environments is compensated. By combining time synchronization and localization, the accuracy of both are improved jointly. Additionally, an advanced tracking algorithm IMM (interactive multiple model) is adopted to improve the accuracy of localization in the mobile case. Furthermore, by combining both services, the number of required exchanged messages is significantly reduced, which saves on energy consumption. Simulation results show that both services are improved and benefit from this scheme.

In this paper, we investigate the multichannel MAC problem in underwater acoustic sensor networks. To reduce hardware cost, only one acoustic transceiver is often preferred on every node. In a single-transceiver multichannel long-delay underwater network, new hidden terminal problems, namely, multichannel hidden terminal and long-delay hidden terminal (together with the traditional multihop hidden terminal problem, we refer to them as "triple hidden terminal problems”), are identified and studied in this paper. Based on our findings, we propose a new MAC protocol, called CUMAC, for long-delay multichannel underwater sensor networks. CUMAC utilizes the cooperation of neighboring nodes for collision detection, and a simple tone device is designed for distributed collision notification, providing better system efficiency while keeping overall cost low. Analytical and simulation results show that CUMAC can greatly improve the system throughput and energy efficiency by effectively solving the complicated triple hidden terminal problems.

Time synchronization plays a critical role in distributed network systems. In this paper, we investigate the time synchronization problem in the context of underwater sensor networks (UWSNs). We propose a pairwise, cross-layer, time synchronization scheme for mobile underwater sensor networks, called TSMU. Facilitated by the Kalman Filter, the proposed method greatly improves the dynamic propagation delay estimation by exploring the Doppler effect. Simulation results show that TSMU outperforms existing synchronization schemes in both accuracy and energy efficiency.

Due to harsh aqueous environments, non-negligible node mobility and large network scale, localization for large-scale mobile underwater sensor networks is very challenging. In this paper, by utilizing the predictable mobility patterns of underwater objects, we propose a scheme, called Scalable Localization scheme with Mobility Prediction (SLMP), for underwater sensor networks. In SLMP, localization is performed in a hierarchical way, and the whole localization process is divided into two parts: anchor node localization and ordinary node localization. During the localization process, every node predicts its future mobility pattern according to its past known location information, and it can estimate its future location based on the predicted mobility pattern. Anchor nodes with known locations in the network will control the localization process in order to balance the trade-off between localization accuracy, localization coverage, and communication cost. We conduct extensive simulations, and our results show that SLMP can greatly reduce localization communication cost while maintaining relatively high localization coverage and localization accuracy.

Due to the long propagation delay and high error rate of acoustic channels, it is very challenging to provide reliable data transfer for time-critical applications in an energy-efficient way. On the one hand, traditional retransmission upon failure usually introduces very large end-to-end delay and is thus not proper for time-critical services. On the other hand, common approaches without retransmission consume lots of energy. In this paper, we propose a new multipath power-control transmission (MPT) scheme, which can guarantee certain end-to-end packet error rate while achieving a good balance between the overall energy efficiency and the end-to-end packet delay. MPT smartly combines power control with multipath routing and packet combining at the destination. With carefully designed power-control strategies, MPT consumes much less energy than the conventional one-path transmission scheme without retransmission. Besides, since no hop-by-hop retransmission is allowed, MPT introduces much shorter delays than the traditional one-path scheme with retransmission. We conduct extensive simulations to evaluate the performance of MPT. Our results show that MPT is highly energy-efficient with low end-to-end packet delays.

Time synchronization is a critical service for distributed networks. In this paper, we investigate this problem in underwater sensor networks (UWSNs). We propose a novel time synchronization scheme, called ``Mobi-Sync''. Mobi-Sync effectively utilizes the spatial correlation of underwater mobile sensor nodes to estimate the long and dynamic propagation delays. Simulation results show that Mobi-Sync outperforms existing schemes in both accuracy and energy efficiency.

In this paper, we investigate the multi-channel MAC problem in underwater acoustic sensor networks. To reduce hardware cost, only one acoustic transceiver is often preferred on every node. In a single-transceiver multi-channel long-delay underwater network, new hidden terminal problems, namely multi-channel hidden terminal and long-delay hidden terminal (together with the traditional multi-hop hidden terminal problem, we refer to them as "triple hidden terminal problems"), are identified and studied in this paper. Based on our findings, we propose a new MAC protocol, called CUMAC, for long delay multi-channel underwater sensor networks. CUMAC utilizes the cooperation of neighboring nodes for collision detection, and a simple tone device is designed for distributed collision notification, providing better system efficiency while keeping overall cost low. Analytical and simulation results show that CUMAC can greatly improve the system throughput and energy efficiency via effectively solving the complicated triple hidden terminal problems.