Ethem M. Sozer

Underwater acoustic networks

With the advances in acoustic modem technology that enabled high-rate reliable communications, current research focuses on communication between various remote instruments within a network environment. Underwater acoustic (UWA) networks are generally formed by acoustically connected ocean-bottom sensors, autonomous underwater vehicles, and a surface station, which provides a link to an on-shore control center. While many applications require long-term monitoring of the deployment area, the battery-powered network nodes limit the lifetime of UWA networks. In addition, shallow-water acoustic channel characteristics, such as low available bandwidth, highly varying multipath, and large propagation delays, restrict the efficiency of UWA networks. Within such an environment, designing an UWA network that maximizes throughput and reliability while minimizing the power consumption becomes a very difficult task. The goal of this paper is to survey the existing network technology and its applicability to underwater acoustic channels. In addition, we present a shallow-water acoustic network example and outline some future research directions.

Shallow water acoustic networks

Underwater acoustic networks are generally formed by acoustically connected ocean bottom sensor nodes, autonomous underwater vehicles (AUVs), and surface stations that serve as gateways and provide radio communication links to on-shore stations. The quality of service of such networks is limited by the low bandwidth of acoustic transmission channels, high latency resulting from the slow propagation of sound, and elevated noise levels in some environments. The long-term goal in the design of underwater acoustic networks is to provide for a self-configuring network of distributed nodes with network links that automatically adapt to the environment through selection of the optimum system parameters. This article considers several aspects in the design of shallow water acoustic networks that maximize throughput and reliability while minimizing power consumption.

Reconfigurable acoustic modem for underwater sensor networks

There is a growing interest for underwater sensor networks where long term monitoring of water masses around the world for scientific, environmental, commercial, and military reasons is desired. In this paper we will present the concept of a highly flexible acoustic modem called the Reconfigurable Modem (rModem) that can be used for rapid testing and development of such networks.

Iterative equalization and decoding techniques for shallow water acoustic channels

We investigate the application of iterative equalization and decoding techniques to the shallow water acoustic channel. The first receiver is a joint decision feedback equalizer (DFE) and turbo decoder. The second receiver is a turbo equalizer, which jointly estimates the channel, performs MAP equalization, and decodes received symbols. Although the MAP equalizer is optimum for a known channel, channel estimation errors degrade the performance of the turbo equalizer. We compared the performance of both receivers under real channel conditions.

Performance comparison of RAKE and hypothesis feedback direct sequence spread spectrum techniques for underwater communication applications

Direct sequence spread spectrum (DSSS) techniques for low probability of intercept (LPI) and multi-user communications applications for underwater acoustic communications are investigated. Two promising receivers, a RAKE based receiver and a hypothesis-feedback equalization based architecture are presented. The performance of the proposed receiver structures are compared based on simulations and also actual field test data.

An Architecture for Underwater Networks

As electromagnetic waves do not propagate well underwater, acoustics plays a key role in underwater communication. Due to significant differences in the characteristics of electromagnetic and acoustic channels, networking protocols for underwater systems differ from those developed for wired and wireless radio networks. Various schemes have been proposed for one or many aspects of underwater networks. However, no widely accepted common framework exists for underwater acoustics to unify these proposed schemes into a functional underwater network. The availability of such a framework will enable easy integration of independently developed techniques and thus accelerate the pace of research in underwater acoustic networking. In order for a common framework to be successful, it needs to have a wide acceptance. To gain such an acceptance, a framework needs to take into account a wide variety of different constraints and requirements for various underwater applications. This requires inputs from various research groups and end users. To help define the use cases and a common framework for underwater networking, a joint effort has been initiated between acoustic communication research groups at the Acoustic Research Laboratory (National University of Singapore), Woods Hole Oceanographic Institution and the Massachusetts Institute of Technology. In this paper, we discuss the first draft of the framework specifications from this effort. We welcome feedback from the underwater acoustic research community and potential end users of underwater networking systems.

Iterative equalization, decoding, and soft diversity combining for underwater acoustic channels

The performance of iterative equalization, decoding, and soft diversity combining in underwater acoustic channels is studied. A decision feedback equalizer (DFE) is employed instead of a MAP equalizer to reduce the complexity of the iterative algorithm. The performance of a soft diversity combining scheme, where multiple DFEs are employed for spatial diversity or time diversity as well as a DFE with multiple feedforward filters is presented. The proposed receiver structures are tested and compared using experimental data.

Performance comparison of iterative/integral equalizer/decoder structures for underwater acoustic channels

The use of an adaptive decision feedback equalizer (DFE) in the demodulation of high speed data transmitted through underwater acoustic channels has been well established. In many channels, however, the performance obtained with the conventional DFE and decoder is not adequate for particular applications. This paper considers four different iterative equalization/decoder techniques for improving the performance of the receiver. One technique uses the hard decisions from the decoder output to feed back to the DFE for making additional passes through the data. The second technique uses the soft outputs from the decoder output to feed back to the DFE. The third technique, termed an integral iterative equalization scheme, is designed to mitigate and correct the errors being fed back to the DFE in a block fashion within the data packet. Finally, the fourth technique, called a turbo equalizer, is an iterative scheme which employs a MAP equalizer and a MAP decoder. These iterative/integral equalization/decoding techniques are applied to convolutionally encoded BPSK and QPSK data received during several field tests. The performance of the iterative equalizer/decoder algorithms is compared on the basis of bit error rate and packet statistics.

Simulation and rapid prototyping environment for underwater acoustic communications: reconfigurable modem

Acoustic communications and networking research depends heavily on validation of the proposed algorithms through experiments. These experiments are usually carried out by off-line processing of data recorded in real channels. In this paper, we present a new simulation and rapid prototyping environment called the reconfigurable modem (rModem). The rModem can model acoustic communication and networking systems, simulate the system behavior, and generate C code based on the system model that can be run on a DSP board for real-time experimental studies.