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

Error probability of an adaptive multiuser diversity receiver is evaluated in terms of channel fading rate and the number of code-division multiple access users. Fading-induced performance loss, which leads to the error probability floor, is established for the proposed coherent combining scheme and compared to that of the differentially coherent receiver with equal-gain combining.

/pdf/performance-of-multiuser-diversity-reception-in-rayleigh-n15fx3jt88.pdf

Performance of multiuser diversity reception in Rayleigh fading CDMA channels

The optimal multiuser receiver for syn- chronous CDMA fading channels consists of an adaptive es- timator of the fading distortion and the detector that utilizes so-obtained estimates. High complexity of the optimal de- tector motivates the use of a decorrelating detector, whose performance is significantly improved through diversity. The estimates of the complex fading processes are utilized both for maximal-ratio combining and carrier recovery. To ana- lyze the impact of channel estimation errors on the receiver performance, error probability of coherent decorrelating re- ceiver is assessed directly in terms of the fading rate and the number of active users, showing the penalty imposed by imperfect channel estimat,ion as well as fading-induced error probability floor. The coherent receiver performance is com- pared to that of a differentially coherent receiver with equal gain combining. 1. Introductioii While multiuser detection has been analyzed thoroughly for both time-invariant and fading AWGN channels, little work has been reported on the underlying channel estimation is- sues in multiple-access fading channels. The impact of im- perfect channel estimates on multiuser receiver performance in rapidly fading channels is the subject of this paper. Recent results for the single-user case (l), (a) provide meth- ods for optimal receiver design based on statistical modeling of a fading process with memory. It is shown in (a) that the optimal receiver for a nonselective channel consists of an adaptive estimator of the fading distortion and a detec- tor which utilizes the entire received signal in detecting each symbol. While the optimal receiver is difficult to implement due to its complexity which is exponential in the symbol se- quence length, a suboptimal decision-directed receiver can be implemented instead. This receiver employs linear mini- mum mean-squared error (MMSE) estimation of the complex channel distortion, and uses the obtained estimates to per- form decision-directed carrier recovery via a planar filtering method (I). In multiple-access channels, the accuracy of fading chan- nel estimates is degraded due to the presence of multiple- access interference (MAI). A vector generalization of the op- timal receiver for nonselect,ive Rayleigh fading synchronous CDMA channels and its suboptimal, decision-direct,ed real- ization were presented in (3). Due to the quadratic detect,ion rule, even the decision-directed receiver has computational complexity exponential in the number of active users. Chan- nel estimation methods, designed especially for burst CDMA transmissions, were presented in (4). In this paper we propose an adaptive decision-directed coherent receiver with diversity reception for nonselective Rayleigh fading synchronous CDMA channels. The detection part of the receiver employs a low-complexity near-far re- sistant decorrelating detector whose complexity depends lin- early only on the number of users (5), (6). The detector utilizes the MMSE estimates of the fading processes as channel refer- ences for carrier recovery and diversity combining. For such a receiver, we quantify the effects of noisy estimates, caused by fading dynamics and MAI, on the multiuser receiver per- formance. The average error probability is determined by the values of the channel estimation error covariance matrix. In contrast to the approach taken in (3), where a diagonal struc- ture is imposed on the estimation error covariance matrix by assuming noiselessly observed past fading, no such constraints are required in the analysis presented. Gauss-Markov model- ing of a Rayleigh fading channel provides a method for calcu- lating the quasi-stationary error covariance of the Kalman fil- ter which is used as an optimal channel estimator. A differen- tially coherent receiver uses the same decorrelating technique and a simple, one-shot channel estimation. We compare the two receivers showing regions where each may be preferable, depending on the fading dynamics and MAI.

https://ieeexplore.ieee.org/iel2/3879/11303/00512599.pdf

Performance of multiuser diversity reception in nonselective rayleigh fading cdma channel

This paper investigates the signal-to-interference ratio and the achievable rates of underwater acoustic (UA) OFDM systems over channels where time and frequency dispersion are high enough that (i) neither the transmitter nor the receiver can have a priori knowledge of the channel state information and (ii) intersymbol/intercarrier interference (ISI/ICI) cannot be neglected in the information-theoretic treatment The goal of this study is to obtain a better understanding of the interplay between interference and the achievable transmission rates Expressions for these rates take into account the "cross-channels" established by the ISI/ICI and are based on lower bounds on mutual information that assume independent and identically distributed input data symbols In agreement with recent statistical analyses of experimental shallow-water data, the channel is modeled as a multivariate Rician fading process with a slowly time-varying mean and with potentially correlated scatterers, which ismore general than the common wide-sense stationary uncorrelated scattering model Numerical assessments on real UA channels with spread factors around 10-1 show that reliable OFDM transmissions at 2 to 4 bits/sec/Hz are achievable provided an average signal-to-noise ratio of 15 to 20 dB

/pdf/information-theoretic-analysis-of-underwater-acoustic-ofdm-3dn8bg9pcf.pdf

Information-theoretic analysis of underwater acoustic OFDM systems in highly dispersive channels

http://senseslab.di.uniroma1.it/administrator/components/com_jresearch/files/publications/Optimized_Packet_Size_Selectio.pdf

Optimized Packet Size Selection in Underwater WSN Communications

The design of a cellular underwater network is addressed from the viewpoint of determining the cell size and the frequency reuse pattern needed to support a desired number of users operating over a given area within a given system bandwidth. By taking into account the basic laws of underwater acoustic propagation, it is shown that unlike in the terrestrial radio systems, both the cell radius R and the frequency reuse number N must satisfy a set of constraints in order to constitute an admissible solution (which sometimes may not exist). The region of admissible (R, N) defines the possible network topologies. It is determined by the user density and the system bandwidth (rho, B), and by the required signal-to-interference ratio and per-user bandwidth (SIRo, Wo). The range of admissible solutions also depends on the choice of operating frequency region. Moving to a higher frequency region than that dictated by SNR maximization, improves the SIR and yields a greater system capacity.

/pdf/on-the-design-of-underwater-acoustic-cellular-systems-1rdt35safg.pdf

Milica Stojanovic

Papers

Performance of multiuser diversity reception in Rayleigh fading CDMA channels

Performance of multiuser diversity reception in nonselective rayleigh fading cdma channel

Information-theoretic analysis of underwater acoustic OFDM systems in highly dispersive channels

Optimized Packet Size Selection in Underwater WSN Communications

On the Design of Underwater Acoustic Cellular Systems