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

Motivated by the advances in acoustic modem technology and the growing number of applications that call for ad hoc deployable autonomous underwater systems (floating sensors, crawlers, vehicles), we address the problem of network initialization upon deployment. A neighbor discovery protocol is proposed, whose goal is to establish communication links over a large area, with a finite power budget that mandates multi-hopping to provide full coverage. The protocol uses random access to eliminate the need for scheduling (i.e., enable system operation without a global clock reference) and power control to ensure that full connectivity is provided using shortest links (i.e., to conserve batteries and prolong the system's lifetime). Transmit power allocation takes into account the acoustic propagation loss, while additional large-scale variation in the average received power is modeled via log-normal fading which is confirmed by experimental observations. System performance is assessed through simulation, by measuring the energy consumption, time to completion, and reliability in the presence of fading. Fading is shown to have a degrading effect on the system reliability, and protocol adjustments are proposed to recover the performance under the constraint on maximum power. The key features of the protocol are simplicity of implementation and efficient use of power.Copyright © 2012 John Wiley & Sons, Ltd.

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A node discovery protocol for ad hoc underwater acoustic networks

Modulation and detection techniques used for underwater acoustic communications include phase coherent (PSK and QAM) and noncoherent (FSK) techniques. The objective of this work is the development of algorithms that can select the best technique for a given channel. The system of interest operates in shallow water over several kilometers in the 10-15 kHz band. A wideband probe is used at the beginning of each data packet to measure the multipath and the Doppler spread, as well as the SNR. The choice of modulation is based on these measurements. The spread factor of the channel determines whether phase coherent communications are possible. If so, an equalizer is employed to combat any intersymbol interference. If the channel varies too rapidly, noncoherent signaling is chosen. Guard intervals are used to eliminate the effect of multipath. Computer simulations and theoretical analysis are presented to quantify the performance of candidate modulation methods for varying channel parameters.

Towards robust adaptive acoustic communications

Frequency and time correlation of the underwater channel are exploited to obtain an adaptive channel estimation algorithm for MIMO spatial multiplexing with low complexity and low overhead Non-uniform Doppler compensation is performed by extending the adaptive synchronization method to multiple transmitters Algorithm performance is demonstrated on experimental data recorded in several shallow water channels over distances on the order of 1 km Nearly error-free performance is observed for two and four transmitters with BCH(64,10) encoded QPSK signals We report in detail on an experiment where an information rate of 23 kbps (4 transmitters times 075 sps/Hz/transmitter times 2 bits/symbol times 10/64 times 24 kHz) is achieved using 2048 carriers in a 24 kHz bandwidth

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Adaptive MIMO detection of OFDM signals in an underwater acoustic channel

Multichannel spatial signal processing of high-speed underwater acoustic communication signals is associated with computationally intensive receiver algorithms. Besides requirements in computational time, large adaptive filters operating under computationally efficient algorithms imply increased sensitivity to numerical errors and large noise enhancement, which limit their performance at high signalling rates. To overcome these difficulties, a receiver is proposed which consists of a multi-input multi-output combiner in which many input channels are coherently combined into a smaller number, suitable for subsequent multichannel decision-feedback equalization. Receiver operations are optimized jointly to ensure minimum mean-squared error detector performance. Experimental results obtained on a long-range shallow water channel with data rates up to 2000 bits per second demonstrate the receiver capability to fully exploit the spatial diversity of underwater multipath while keeping the receiver complexity at a minimum, thus allowing both fast convergence and little noise enhancement. >

Reduced-complexity simultaneous beamforming and equalization for underwater acoustic communications

Underwater acoustic (UWA) channels are typically characterized by a multipath structure with large delay spread, where only a few propagation paths carry significant energy Each path exhibits time variability, which, together with the transmitter and receiver motion, induces Doppler spreading and shifting of the signal In this paper, we analyze the limits on the information rate achievable through multiple-input multiple-output (MIMO) communications over UWA channels Assuming full channel state information (CSI) at the receiver, we evaluate the ergodic capacity in two scenarios: one with partial CSI at the transmitter, and another with no CSI Also, we consider the constrained capacity for practical modulations, eg, BPSK and QPSK, and, exploiting the sparseness of the multipath structure, we provide new lower bounds on the achievable information rate Statistical characterization and numerical examples are given based on the data collected in a recent experiment, conducted off the coast of Kauai, Hawaii, in June 2008

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Milica Stojanovic

Papers

A node discovery protocol for ad hoc underwater acoustic networks

Towards robust adaptive acoustic communications

Adaptive MIMO detection of OFDM signals in an underwater acoustic channel

Reduced-complexity simultaneous beamforming and equalization for underwater acoustic communications

Capacity of MIMO systems in shallow water acoustic channels