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.

/pdf/underwater-acoustic-sensor-networks-research-challenges-15fjl1gvhc.pdf

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

We study the energy performance of random linear network coding for time division duplexing channels. We assume a packet erasure channel with nodes that cannot transmit and receive information simultaneously. The sender transmits coded data packets back-to-back before stopping to wait for the receiver to acknowledge the number of degrees of freedom, if any, that are required to decode correctly the information. Our analysis shows that, in terms of mean energy consumed, there is an optimal number of coded data packets to send before stopping to listen. This number depends on the energy needed to transmit each coded packet and the acknowledgment (ACK), probabilities of packet and ACK erasure, and the number of degrees of freedom that the receiver requires to decode the data. We show that its energy performance is superior to that of a full-duplex system. We also study the performance of our scheme when the number of coded packets is chosen to minimize the mean time to complete transmission as in [1]. Energy performance under this optimization criterion is found to be close to optimal, thus providing a good trade-off between energy and time required to complete transmissions.

/pdf/random-linear-network-coding-for-time-division-duplexing-3lmkfrkjla.pdf

Random Linear Network Coding for Time Division Duplexing: Energy Analysis

Multiple-input multiple-output (MIMO) communications based on orthogonal frequency division multiplexing (OFDM) are considered for improving the performance and bandwidth utilization of underwater acoustic systems in which cooperation is possible between distributed transmitters The major challenge in such a framework - and the principal difference from the traditional case where multiple transmitters are co-located - is the fact that distributed transmitter-receiver pairs may experience significantly different Doppler distortion (eg two vehicles moving in different directions with respect to the receiver) The conventional approach of front-end resampling that corrects for a common Doppler scaling will then fail, rendering a post-FFT signal that is contaminated by transmitter-specific inter-carrier interference To counteract this problem, we propose a front-end receiver structure that utilizes multiple resampling branches, each followed by FFT demodulation As a result, a set of sufficient statistics are acquired, which are subsequently processed using custom-designed, linear or nonlinear detection schemes Numerical results illustrate significant performance improvements as compared to the conventional, single-resampling schemes

/pdf/cooperative-mimo-ofdm-communications-receiver-design-for-45mnoetbuf.pdf

Cooperative MIMO-OFDM communications: Receiver design for Doppler-distorted underwater acoustic channels

Underwater acoustic communications are a rapidly growing field of research and engineering, driven by the expansion of applications which require underwater data transmission without wired connections. In this chapter, we explore the problems of underwater acoustic communications in three parts. The first part presents an overview of modern applications in underwater data transmission and today’s achievements in this area. System requirements are reviewed, and propagation characteristics of underwater acoustic channels are given. It is shown that the majority of underwater acoustic channels are severely band-limited, with signal distortions depending on the link configuration, and ranging from benign to extreme ones caused by time-varying multipath propagation and signal phase variations. Examples of existing systems are given, with emphasis on the methods used for intersymbol interference mitigation. Most of these systems use noncoherent or a differentially coherent signal modulation and detection methods. Phase-coherent detection, which offers better efficiency in bandwidth utilization, is the subject of the second part of this chapter. In this part, the design of high-speed digital communication systems, which rely on powerful equalization and multiple sensor signal processing methods is treated.

High-Speed Underwater Acoustic Communications

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.

/pdf/performance-comparison-of-rake-and-hypothesis-feedback-44w1o7mayj.pdf

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

This paper explores the properties of short-range broadband wireless communications for underwater operations using electric conduction Electric field in the water is generated by a pair of electrodes with opposite current and detected by two receiving electrodes Ranges of operation can be shorter than 1 m, suitable for contactless data collection by remotely operated vehicles (ROVs), and even as short as 1-10 cm, suitable for contactless riser health monitoring for deep sea drilling sensors Experiments were conducted at frequencies between 100 kHz and 65 MHz, using orthogonal frequency division multiplexing (OFDM) Our lab tests were performed in a plastic tank filled with salt water, and our sea test at the ocean surface and 5 m depth (boundary free) Magnitude and phase-delay of the channel transfer function were modeled based on inference from dipole radiation theory in a conducting medium An exponential attenuation model fitted to the lab measurements indicated inverse cubic range dependence (near-field compliant) A rational-polynomial model provided the best match for the recorded magnitude, especially at low frequencies The phase characteristic obtained from the ocean measurements exhibited a minimum around 2 MHz, which agrees with theory

http://wuwnet.acm.org/2012/files/Papers/05_paper20.pdf

Milica Stojanovic

Papers

Random Linear Network Coding for Time Division Duplexing: Energy Analysis

Cooperative MIMO-OFDM communications: Receiver design for Doppler-distorted underwater acoustic channels

High-Speed Underwater Acoustic Communications

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

Underwater electromagnetic communications using conduction: channel characterization