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

Orthogonal frequency division multiplexing (OFDM) offers simplicity of FFT‐based implementation with frequency‐domain channel equalization, and has emerged as a standard in many terrestrial systems. Its application to underwater systems, however, is challenged by the motion‐induced Doppler distortion, which creates nonuniform frequency offset in a wideband acoustic signal. To counteract this problem, recent research has focused on synchronization methods, demonstrating the feasibility of wideband OFDM in initial sea trials. We consider the design of a complete OFDM receiver based on three key techniques: adaptive carrier tracking, spatial diversity combining, and sparse channel estimation. In particular, we focus on time‐domain channel estimation, which provides a natural platform for channel sparsing. Sparsing of the impulse response, or discarding of its insignificant coefficients, is shown to offer performance gains on acoustic channels, since they are often naturally sparse. Decision‐directed operation, which is made possible by adaptive carrier tracking, yields a low overhead, and further improves the performance of channel estimation beyond that of nonadaptive pilot‐based schemes. The technique proposed is applied to experimental data recorded in a shallow water channel over 1 km. Results show excellent performance of coded OFDM signals, transmitted in the 19‐31 kHz band with a varying number of subcarriers.

High rate acoustic communications based on orthogonal frequency division multiplexing

Peer-Reviewed Technical Communication Space-Frequency Block Coding for Underwater Acoustic Communications

We propose computationally efficient carrier frequency offset estimators for multicarrier underwater acoustic communication using identical pilot tones equi-spaced in the frequency domain. The first estimator uses the phase of the maximal eigenvector of a channel-dependent correlation matrix. Next, the phase of the minimal eigenvector of a channel-independent correlation matrix is combined with the first estimation using a weighted linear least squares principle. The third estimator solves a generalized eigenvalue decomposition problem by jointly considering the two correlation matrices, and then performs a similar second step as the previous estimator. Simulations and pool trials show that the proposed estimators achieve similar performance as common estimation techniques while surpassing them in severe environments.

Eigenvalue decomposition based estimators of carrier frequency offset in multicarrier underwater acoustic communication

Latest achievements in underwater acoustic communications are leading the way to modern instrumentation for acoustical oceanography by eliminating the need for tethering or diver’s contact with submerged sensors. To meet the various telemetry needs, an acoustic link must provide both reliability and a high rate of data transfer, requirements that are often conflicting. The first approximation of an acoustic channel is that of a time‐invariant multipath communication channel. It is well known that by increasing the transmission rate on such a channel, multipath propagation causes longer intersymbol interference, ultimately limiting system performance. Recent experimental results obtained with a newly developed receiver algorithm, however, showed a seemingly surprising result: an increase in transmission rate resulted in improved system performance. An explanation for this phenomenon is found in the time variation of the ocean multipath. Notably emphasized in the fully saturated shallow water channels, higher transmission rates allow finer sampling of the rapidly varying channel, and therefore better tracking. Experimental results obtained in the Woods Hole harbor are presented showing a dramatic improvement in performance of QPSK coherent detection over a one‐mile range as the data rate is increased from 5 to 30 kilobits‐per‐second. A theoretical analysis based on stochastic channel modeling supports these observations.

Performance of a high‐rate communication link on a shallow water acoustic channel

Multi-input multi-output (MIMO) communication techniques are considered with the goal of increasing the information throughput via spatial multiplexing over an underwater acoustic link. We focus on a MIMO system based on multicarrier modulation in the form of orthogonal frequency division multiplexing (OFDM). The system relies on the simultaneous estimation of parallel transmit channels and multichannel receive-side data detection. Polar codes are implemented for channel coding. We report on the experimental results obtained using off-line processing of the signals recorded over a 10 km acoustic link in the Suruga Bay, Japan.

Milica Stojanovic

Papers

High rate acoustic communications based on orthogonal frequency division multiplexing

Peer-Reviewed Technical Communication Space-Frequency Block Coding for Underwater Acoustic Communications

Eigenvalue decomposition based estimators of carrier frequency offset in multicarrier underwater acoustic communication

Performance of a high‐rate communication link on a shallow water acoustic channel

MIMO-OFDM Over a 10km Acoustic Link: An Experimental Study at the Suruga Bay, Japan