Mandar Chitre

Underwater Acoustic Communications and Networking: Recent Advances and Future Challenges

There has been a growing interest in underwater acoustic communications over the past 30 years because of its defense, offshore oil industry, marine commercial operations, oceanography, and marine research applications. As compared to initial communication systems, improved performance and robustness have resulted from continued research over the years. The authors aim to provide an overview, spanning this decade, of key underwater networking protocol and point-to-point communication technique developments. Insight into some of the open challenges and problems researchers in this field will face in the near future is also provided by the authors.

A high-frequency warm shallow water acoustic communications channel model and measurements

Underwater acoustic communication is a core enabling technology with applications in ocean monitoring using remote sensors and autonomous underwater vehicles. One of the more challenging underwater acoustic communication channels is the medium-range very shallow warm-water channel, common in tropical coastal regions. This channel exhibits two key features-extensive time-varying multipath and high levels of non-Gaussian ambient noise due to snapping shrimp-both of which limit the performance of traditional communication techniques. A good understanding of the communications channel is key to the design of communication systems. It aids in the development of signal processing techniques as well as in the testing of the techniques via simulation. In this article, a physics-based channel model for the very shallow warm-water acoustic channel at high frequencies is developed, which are of interest to medium-range communication system developers. The model is based on ray acoustics and includes time-varying statistical effects as well as non-Gaussian ambient noise statistics observed during channel studies. The model is calibrated and its accuracy validated using measurements made at sea.

Recent advances in underwater acoustic communications & networking

The past three decades have seen a growing interest in underwater acoustic communications. Continued research over the years has resulted in improved performance and robustness as compared to the initial communication systems. Research has expanded from pointtopoint communications to include underwater networks as well. A series of review papers provide an excellent history of the development of the field until the end of the last decade. In this paper, we aim to provide an overview of the key developments, both theoretical and applied, in the field in the past two decades. We also hope to provide an insight into some of the open problems and challenges facing researchers in this field in the near future.

Performance of coded OFDM in very shallow water channels and snapping shrimp noise

Although acoustic energy has been used effectively for point-to-point communications in deep-water channels, it has had limited success for horizontal transmissions in shallow water. Time-varying multipath propagation and non-Gaussian snapping shrimp noise are two of the major factors that limit acoustic communication performance in shallow water. Rapid time variation in the channel can limit the use of equalizers to compensate for frequency selective fading introduced due to multipath propagation. OFDM (orthogonal frequency division multiplexing), a communication technique widely used in wired and wireless systems, divides the available bandwidth across a number of smaller carriers, each of which experiences flat fading. This simplifies the equalizer structure and provides robustness against time-varying frequency-selective fading. Another source of signal degradation is impulsive noise from snapping shrimp, which affects several OFDM carriers at the same time. OFDM, when coupled with coding, can provide robustness against impulsive noise by distributing the energy for each bit over a longer period of time. We tested coded OFDM in a very shallow water channel in Singapore waters. The results show that it is a promising technique for use in very shallow, warm water channels

AquaOptical: A lightweight device for high-rate long-range underwater point-to-point communication

This paper describes AquaOptical, an underwater optical communication system. Three optical modems have been developed: a long range system, a short range system, and a hybrid. We describe their hardware and software architectures and highlight trade-offs. We present pool and ocean experiments with each system. In clear water AquaOptical was tested to achieve a data rate of 1.2Mbit/sec at distances up to 30m. The system was not tested beyond 30m. In water with visibility estimated at 3m AquaOptical achieved communication at data rates of 0.6Mbit/sec at distances up to 9m.

Underwater acoustic channel characterisation for medium-range shallow water communications

The ability to effectively communicate underwater has numerous applications for researchers, marine commercial operators and defence organizations. As electromagnetic waves cannot propagate over long distances in seawater, acoustics provides the most obvious choice of channel. Although acoustics has been used effectively for point-to-point communications in deep-water channels, acoustics has had limited success for horizontal transmissions in shallow water. Time-varying multipath propagation and nonGaussian noise are two of the major factors that limit acoustic communication performance in shallow water. Although it is known that medium-range shallow water propagation is dominated by time-varying multipath arrivals, very few measurements of the variability of the multipath structure are available. In this paper, we present channel measurements made in a shallow water channel (depth 15-20 m) up to a range of 1 km. An analysis of the temporal variability of the arrival structure is presented. Most communication systems make the assumption that the noise is additive and Gaussian. Snapping shrimp dominate the ambient noise spectrum above a few kHz in warm shallow waters. It is known that snapping shrimp noise is impulsive and highly nonGaussian. These noise characteristics need to be taken into account when designing communication systems if robust and near-optimal performance is desired. An analysis of the ambient noise characteristics from some warm shallow water channels is also presented.

Cellware---a multi-algorithmic software for computational systems biology

UNLABELLED The intracellular environment of a cell hosts a wide variety of enzymatic reactions, diffusion events, molecular binding, polymerization and metabolic channeling. To transform these biological events into a computational framework, distinct modeling strategies are required. While currently no tool is capable of capturing all these events, progress is being made to create an integrated environment for the modeling community. To address this niche requirement, Cellware has been developed to offer a multi-algorithmic environment for modeling and simulating both deterministic and stochastic events in the cell. AVAILABILITY The software is available for free and can be downloaded from http://www.bii.a-star.edu.sg/sbg/cellware

Cooperative positioning using range-only measurements between two AUVs

This paper presents a cooperative positioning system between two autonomous underwater vehicles (AUVs). Each AUV is equipped with some navigational sensors. However, AUVs with different tasks have different navigational capabilities. By introducing acoustic communication between AUVs, information from multiple AUVs can be fused to give a more accurate position estimates to AUVs with poorer navigational capabilities. We present results from a field trial where a lawnmower mission is executed by a survey AUV with poor navigational sensors while another AUV with higher positioning accuracy plays the role of a beacon AUV. The beacon AUVs task is to help improve the survey AUVs position accuracy by providing it regular range updates from various locations. An Extended Kalman Filter (EKF) was implemented to fuse the range information updates with the navigational sensor data on the survey AUV. We were able to avoid unbounded error growth in the position estimate of the survey AUV in our experiments through cooperative positioning between two AUVs using range-only measurements.

Throughput of Networks With Large Propagation Delays

Propagation delays in underwater acoustic networks can be large as compared to the packet size. Conventional medium-access control (MAC) protocol design for such networks focuses on mitigation of the impact of propagation delay. Most proposed protocols to date achieve, at best, a throughput similar to that of the zero propagation delay scenario. In this paper, we systematically explore the possibility that propagation delays can be exploited to make throughput far exceed that of networks without propagation delay. Under the assumptions of the protocol model in a single collision domain for a half-duplex unicast network, we show that the upper bound of throughput in an N-node wireless network with propagation delay is N/2. We illustrate network geometries where this bound can be achieved and study transmission schedules that help achieve it. We show that for any network, the optimal schedule is periodic and present a computationally efficient algorithm to find good schedules. Finally, we show that N-node network geometries that achieve throughput close to the N/2 bound exist for any N and present a lower bound on achievable maximum throughput for bounded geometries. This paper chiefly endeavors to explore the impact and potential of nonzero propagation delays on network throughput. We believe that the novel observations in this paper could motivate further research into this area, especially random access networks with large propagation delay, with a fundamentally changed outlook on maximum achievable throughput. This could lead to novel scheduling and network configuration approaches with applications in underwater and satellite networks.

Grid Cellware: the first grid-enabled tool for modelling and simulating cellular processes

Modelling and simulation of complex cellular transactions involve development of platforms that understand diverse mathematical representations and are capable of handling large backend computations. Grid Cellware, an integrated modelling and simulation tool, has been developed to precisely address these niche requirements of the modelling community. Grid Cellware implements various pathway simulation algorithms along with adaptive Swarm algorithm for parameter estimation. For enchanced computational productivity Grid Cellware uses grid technology with Globus as the middleware.