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.

/pdf/fading-channels-information-theoretic-and-communications-2ipqbail97.pdf

Fading channels: information-theoretic and communications aspects

An adaptive decorrelating multiuser detector is considered for application in Rayleigh fading multipath channels with significant fading rate (Doppler spread). A suboptimal receiver structure, in which decorrelation is decoupled from multipath recombining, allows efficient implementation of adaptive channel estimation, e.g. via LMS algorithm. The channel estimates are used to perform maximum ratio combining. Bit error rate is evaluated analytically as a function of fading statistics and receiver tracking parameters. Results demonstrate performance sensitivity to the choice of the LMS step size when the channel is modeled as a second-order AR process. Performance degradation increases with fading rate, leading to the saturation of the error probability at a level determined by the fading rate of the channel and the deviation of the step size from the optimal value. The receiver is near-far resistant regardless of the fading rate of the channel.

/pdf/performance-of-multiuser-detection-with-adaptive-channel-4vgthqq97d.pdf

Performance of multiuser detection with adaptive channel estimation

In this paper we analyze the performance of adaptive orthogonal frequency division multiplexing (OFDM) for underwater acoustic (UWA) communications. Our goal is to maximize the system throughput under a target average bit error rate (BER). We consider two different schemes based on the level of adaptivity: in the first scheme only the modulation levels are adjusted while the power is allocated uniformly across the sub-carriers, whereas in the second scheme, both the modulation levels and the power are adjusted adaptively. We exploit a feedback from the receiver in two forms: one that conveys the modulation alphabet and quantized power levels to be used for each sub-carrier, and another that conveys a quantized estimate of the sparse channel impulse response. We compare these two strategies based on the average number of bits per OFDM block needed to achieve a pre-specified target average BER. In both cases we predict the channel one travel time ahead so as to improve the performance in the presence of a long propagation delay. The second approach is shown to be advantageous, as it requires significantly fewer feedback bits for the same system throughput. The effectiveness of the proposed schemes is demonstrated using real channel measurements recorded in shallow water off the western coast of Kauai, Hawaii, in June 2008.

Adaptive OFDM for underwater acoustic channels with limited feedback

To obtain full benefits of the Alamouti space-time code design on a time-varying acoustic channel, the channel variation is decoupled into two parts: the gain, which is assumed to be constant over an Alamouti pair (two consecutive OFDM blocks), and the phase, which is not restricted in this manner, but allowed to vary from one block to another in a piecewise linear fashion. Under this model, the receiver assumes a simplified structure in which conventional Alamouti data detection and channel estimation are aided by Doppler tracking. System performance is demonstrated using experimental data transmitted over a 1 km shallow water channel in the 8–18 kHz acoustic band, using QPSK modulation and a varying number of carriers (up to 1024). The proposed design shows a gain of up to 2 dB as compared to the reference single-transmitter case.

/pdf/a-simple-design-for-joint-channel-estimation-and-data-wzuoml28fg.pdf

A simple design for joint channel estimation and data detection in an Alamouti OFDM system

The recent surge of implantable and wearable medical devices have paved the way for realizing intra-body networks (IBNs). Traditional RF-based techniques fall short in wirelessly connecting such devices owing to absorption within body tissues. A different approach is known as galvanic coupling, which employs weak electrical current within naturally conducting tissues to enable intra-body communication. This work is focused on channel characterization of the human body tissues considering the propagation of such electrical signals through it that carry data. Experiments were conducted using porcine tissue (in lieu of actual human tissue) with skin, fat and muscle layers in the frequency range of 100 kHz to 1 MHz. By utilizing single-carrier BPSK modulated Pseudorandom Noise Sequences, a correlative channel sounding system was implemented, leading to the following contributions: (1) measurements of the channel impulse and frequency response, (2) a noise analysis and capacity estimation, and (3) the comparison of results with existing models.

Experimental assessment of human-body-like tissue as a communication channel for galvanic coupling

Random linear packet coding is considered for use over underwater acoustic channels where long delays challenge the efficiency of standard ARQ. Adaptive power control and rate control are explored to overcome the effect of fading, while minimizing the average energy per successfully transmitted bit of information. Two extreme scenarios are considered: independent fading where the channel varies independently from one data packet to the next, and block fading where the channel stays fixed over a block of packets. For the case of block fading, we design a power (rate) adaptation strategy under an outage criterion. We show that there exists an optimal transmission power (number of coded packets) for which the average energy per bit is minimized. Finally, we quantify the benefits of adaptive power (rate) control using real channel data recorded over a mobile link in the 10kHz acoustic band. These results show that energy savings are available from adaptive power (rate) control strategies.

http://ieeexplore.ieee.org/iel7/6732179/6740936/06741181.pdf

Milica Stojanovic

Papers

Performance of multiuser detection with adaptive channel estimation

Adaptive OFDM for underwater acoustic channels with limited feedback

A simple design for joint channel estimation and data detection in an Alamouti OFDM system

Experimental assessment of human-body-like tissue as a communication channel for galvanic coupling

Random linear packet coding for fading channels