There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

It is shown that, particularly for terrestrial cellular telephony, the interference-suppression feature of CDMA (code division multiple access) can result in a many-fold increase in capacity over analog and even over competing digital techniques. A single-cell system, such as a hubbed satellite network, is addressed, and the basic expression for capacity is developed. The corresponding expressions for a multiple-cell system are derived. and the distribution on the number of users supportable per cell is determined. It is concluded that properly augmented and power-controlled multiple-cell CDMA promises a quantum increase in current cellular capacity. >

On the capacity of a cellular CDMA system

Underwater sensor nodes will find applications in oceanographic data collection, pollution monitoring, offshore exploration, disaster prevention, assisted navigation and tactical surveillance applications. Moreover, unmanned or autonomous underwater vehicles (UUVs, AUVs), equipped with sensors, will enable the exploration of natural undersea resources and gathering of scientific data in collaborative monitoring missions. Underwater acoustic networking is the enabling technology for these applications. Underwater networks consist of a variable number of sensors and vehicles that are deployed to perform collaborative monitoring tasks over a given area. In this paper, several fundamental key aspects of underwater acoustic communications are investigated. Different architectures for two-dimensional and three-dimensional underwater sensor networks are discussed, and the characteristics of the underwater channel are detailed. The main challenges for the development of efficient networking solutions posed by the underwater environment are detailed and a cross-layer approach to the integration of all communication functionalities is suggested. Furthermore, open research issues are discussed and possible solution approaches are outlined. � 2005 Published by Elsevier B.V.

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Underwater acoustic sensor networks: research challenges

Information Theory and Reliable Communication

In this paper we review the most peculiar and interesting information-theoretic and communications features of fading channels. We first describe the statistical models of fading channels which are frequently used in the analysis and design of communication systems. Next, we focus on the information theory of fading channels, by emphasizing capacity as the most important performance measure. Both single-user and multiuser transmission are examined. Further, we describe how the structure of fading channels impacts code design, and finally overview equalization of fading multipath channels.

/pdf/fading-channels-information-theoretic-and-communications-2ipqbail97.pdf

Fading channels: information-theoretic and communications aspects

In this paper we study the relationship between effective use of the available bandwidth, energy consumption, and transmission delay in a Underwater Wireless Acoustic Sensor Network (UWASN). We compare different solutions to transport data to a sink node, namely multihop transmissions through multiple relays and the use of direct relay-to-sink links that require more power but reach the sink in one hop. We also address the effects of different error control policies. Our analysis shows that the energy consumption can be traded off for delay by choosing different policies and by varying some key parameters in each policy.

/pdf/exploiting-the-bandwidth-distance-relationship-in-underwater-22m8e3eg45.pdf

Exploiting the Bandwidth-Distance Relationship in Underwater Acoustic Networks

This paper addresses the feasibility of underwater wireless video transmission using an acoustic system, which consists of a video compressor/decompressor and an OFDM modulator/demodulator. This work includes two main parts: the design of an algorithm aimed to compensate for the Doppler effect and the assessment of a set of error resilience tools offered by the MPEG-4 standard. The results of the simulations and in-air experiments demonstrate the performance improvement in terms of visual quality of the received video in cases with motion, as well as the system robustness against channel induced errors.

/pdf/towards-underwater-video-transmission-l64qzivxd9.pdf

Towards underwater video transmission

Peer-Reviewed Technical Communication CAPTURE: A Communications Architecture for Progressive Transmission via Underwater Relays With Eavesdropping

Advantageous OFDM-based underwater acoustic (UWA) apparatus, systems and methods are provided according to the present disclosure. In general, OFDM transmissions over UWA channels encounter frequency-dependent Doppler drifts that destroy the orthogonality among OFDM subcarriers. The disclosed apparatus, systems, and methods use a two-step approach to mitigate frequency-dependent Doppler drifts for zero-padded OFDM transmissions over fast-varying channels: (1) non-uniform Doppler compensation via resampling to convert a “wideband” problem into a “narrowband” problem; and (2) high-resolution uniform compensation on the residual Doppler. The disclosed apparatus, systems and methods are based on block-by-block processing and do not rely on channel dependence across OFDM blocks. Thus, the disclosed apparatus, systems and methods are advantageously applicable for fast-varying UWA channels.

Apparatus, systems and methods for enhanced multi-carrier based underwater acoustic communications

Performance of multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) is analyzed on an experimental shallow water acoustic channel. Different modulation levels, numbers of subcarriers and transmitters were tested over a period of two weeks. The objectives in doing so were (a) to assess the effect of environmental conditions on the system performance, (b) to determine the performance limits and the data rate supported by the existing detection methods, and (c) to investigate the possibility to push these limits by employing methods for inter-carrier interference (ICI) compensation.

/pdf/underwater-acoustic-mimo-ofdm-an-experimental-analysis-2nedjbp9m1.pdf

Milica Stojanovic

Papers

Exploiting the Bandwidth-Distance Relationship in Underwater Acoustic Networks

Towards underwater video transmission

Peer-Reviewed Technical Communication CAPTURE: A Communications Architecture for Progressive Transmission via Underwater Relays With Eavesdropping

Apparatus, systems and methods for enhanced multi-carrier based underwater acoustic communications

Underwater acoustic MIMO OFDM: An experimental analysis