Underwater acoustic sensor networks

Ocean exploration, through the development of oceanobservation systems, has been recognized as a key step toward a fuller understanding of life on Earth. With the rapid developments in technology, underwater acoustic sensor networks (UASNs) will help to fulfill the needs of these ocean-observation systems, whose applications include gathering of scientific data, early warning systems, ecosystem monitoring, navigation aids, and military surveillance. The applications depending on UASNs are remote control in offshore oil industry, pollution monitoring in environmental systems, collection of scientific data recorded at oceanbottom stations, disaster detection and early warning, national security and defense (intrusion detection and underwater surveillance), as well as for the discovery of new resources. Due to its importance, UASN has played a key enabling technique for marine technology. Although a UASN shares many features of a traditional sensor network on land, it has many unique features which require special designs and techniques to handle them. The characteristics of the underwater acoustic channel introduce a unique design complexity into almost every layer of the network protocol stack. This includes low communication bandwidth which reflects on the transmission rate and requires packet sparsing, long propagation delay which necessitates the design of specialized scheduling mechanisms, high error probability that leads to need for two efficient transport protocols, and sensor node mobility which affects packet routing. This special issue introduces state-of-the-art trends in the design and applications of UASNs. Both theoretical and practical issues related to UASNs are considered. The focus is on the physical layer, the medium access control (MAC) layer, and the network layer. Out of the many high-quality submissions, the papers chosen focus on signal detection, channel coding, transmission scheduling, power control, and routing. The contributions in this special issue present a thorough analysis, algorithmic design, numerical simulations, and results from sea experiments. The original works in physical layer include three contributions. In UASNs, multipath propagation is unavoidable. The multipath propagation decreases the transmission efficiency and distorts the source signal. In ‘‘Alternative Approach for Combination of Fingers in Underwater Acoustic Communication,’’ the authors propose a more reliable rake receiver based on bit error rate (BER) of training sequence duration. The authors conducted experiments using simulation and also lake trials to evaluate its performance. The authors show that the uncoded BER of the proposed rake receiver is lower than that of the conventional rake receiver and that of the nonrake receiver. The proposed method also shows better performance with correct path detection using BER of training sequence duration. The authors of ‘‘The Partial Power Control Algorithm of Underwater Acoustic Sensor Networks Based on Outage Probability Minimization’’ focus on reducing the energy consumption of the UASN. A solution is offered to reduce interference due to highpower transmission by modeling the channel as an auto-regression process and estimating the transmission loss, thereby minimizing the outage probability in the network. In the work titled ‘‘Optimization of LDPC Codes over the Underwater Acoustic Channel,’’ the authors propose a channel coding scheme that combats the large delay spread in the channel through feedback from the channel equalizer and the channel decoder. The result is a low-density parity check decoder optimized to the unique channel conditions of the underwater acoustic channel. Due to the non-negligible physical restrictions of the UASN communication, most MAC protocols used in existing terrestrial sensor networks become inapplicable. The MAC layer of the network is considered in ‘‘MHM: A Multiple Handshaking MAC Protocol for Underwater Acoustic Sensor Networks.’’ The main idea is to allow multiple nodes to transmit and receive data packets at the same time. The authors propose a multiple handshaking MAC protocol for three UASNs. Using the multiple handshaking and a competitive mechanism of control packets, the new protocol makes the contending nodes share the underwater acoustic channel much more fairly and efficiently. Analysis and simulation results are used to validate their claims. The authors of ‘‘Throughput and Delay Analysis of an Underwater CSMA/CA Protocol with MultiRTS and Multi-DATA Receptions’’ proposed an