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.

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Fading channels: information-theoretic and communications aspects

An extension of distance-aware collision avoidance protocol (DACAP) [1] is proposed that permits its implementation in large networks where maximal connectivity is not available. The technique proposed increases the energy efficiency by optimizing the transmission power of the nodes. The optimal power is the minimal power that still guarantees connectivity between each node and the sink. Simulation results show that this transmission power also results in throughput maximization. For a network of nodes uniformly distributed within a rectangular grid, the optimal transmission range can be determined as a function of the node density. A closed form approximation for this dependence is obtained.

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Optimizing the Transmission Range in an Underwater Acoustic Network

In this paper, we discuss the design and testing of an underwater acoustic ad hoc network using OPNET Modeler/Radio. The network is intended for the long-term monitoring of a selected ocean area. The data flow of the network is mainly towards a master node, which is responsible for collecting data generated by sensor nodes. When the network is first deployed, an initialization algorithm is executed and preliminary routes are determined. Since the network nodes are battery powered, an important measure of effectiveness is power efficiency. Therefore, in addition to the initialization algorithm, a genetic algorithm based routing optimization that maximizes the network’s lifetime is employed. Simulation results are presented.

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Initialization and Routing Optimization for Ad Hoc Underwater Acoustic Networks

Existing coherent underwater acoustic communica- tionsystems relyonsingle carrier transmission andadaptive decision feedback equalization todealwithtime-varying and highly dispersive underwater acoustic (UWA)channels. Equaliza- tioncomplexity prevents anysubstantial rateimprovement with theexisting single-carrier approach, asthechannel frequency selectivity increases considerably whenthesymbol rateincreases. Multicarrier modulation intheformoforthogonal frequency division multiplexing (OFDM),ontheotherhand,converts a frequency selective channel intoa setofparallel frequency- flat subchannels, thusgreatly simplifying receiver equalization. Motivated by thesuccess ofOFDM inradiochannels, we investigate its useforunderwater acoustic channels. Inthis paper, wedevelop apilot-tone basedreceiver design forzero-padded OFDM transmissions, andtest itinarealunderwater acoustic channel. Ourproposed receiver performs carrier frequency offset compensation, channel estimation, anddatademodulation onthe basis ofindividual OFDM block. Thisapproach isappealing to applications withshort databursts, orfast varying channels, as itdoesnotrelyonchannel dependence across OFDM blocks. I.INTRODUCTION

Underwater Acoustic Channel

In this paper we propose new bounds on the achievable information rate for discrete-time Gaussian channels with intersymbol interference (ISI) and independent and uniformly distributed (i.u.d.) channel input symbols drawn from finite-order modulation alphabets. Specifically, we are interested in developing new bounds on the achievable rates for sparse channels with long memory. We obtain a lower bound which can be achieved by practical receivers, based on MMSE channel shortening and suboptimal symbol detection for a reduced-state channel. An upper bound is given in the form of a semi-analytical solution derived using basic information theoretic inequalities, by a grouping of the channel taps into several clusters resulting in a newly defined single-input multiple-output (SIMO) channel. We show that the so obtained time-dispersive SIMO channel can be represented by an equivalent single-input single-output (SISO) channel with a significantly shorter channel memory. The reduced computational complexity allows the use of the BCJR algorithm for the newly defined channel. The proposed bounds are illustrated through several sparse channel examples and i.u.d. input symbols, showing that the upper bound significantly outperforms existing bounds. Performance of our lower bound strongly depends on the channel structure, showing best results for minimum-phase and maximum-phase systems.

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Bounds on the Information Rate for Sparse Channels with Long Memory and i.u.d. Inputs

A multihop underwater acoustic network, which consists of a series of equal-distance hops connected by relay transceivers in a tandem, is considered. Messages are originated as coded packets from a source node at one end, relayed (decoded and re-encoded) sequentially hop by hop, and finally received by a destination node at the other end of the network. Several key characteristics of underwater acoustic channels, namely, frequency-dependent signal attenuation and noise, inter-hop interference, half-duplex constraint, and large propagation delay, are taken into account in the analysis. A simple transmission protocol with spatial reuse is considered, and the transmission schedule is designed to satisfy the half-duplex constraint on relay transceivers in the presence of large propagation delay. To efficiently cope with frequency-dependent channel characteristic and inter-hop interference, the power spectral density function of signaling is analytically optimized in a way analogous to water-filling. Furthermore, the problem of determining the minimum number of hops to support a prespecified rate and reliability with and without a maximum coded packet length constraint is examined. Finally, numerical results are presented to illustrate the analysis.

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Milica Stojanovic

Papers

Optimizing the Transmission Range in an Underwater Acoustic Network

Initialization and Routing Optimization for Ad Hoc Underwater Acoustic Networks

Underwater Acoustic Channel

Bounds on the Information Rate for Sparse Channels with Long Memory and i.u.d. Inputs

Analysis of a simple multihop underwater acoustic network