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

The study of Underwater Wireless Sensor Networks (UWSNs) as a research field has grown significantly in recent years offering a multitude of proposals for resolving the communication between nodes and protocols for information exchange networks. Acoustics has been used by nature for many millennia to communicate in underwater environments as a language; dolphins and whales for instance are able to use it for sending information between their groups. Wireless sensor networks have been proposed for their deployment in underwater environments where many applications, such as aquiculture, pollution monitoring, offshore exploration, etc., would benefit from this technology. Despite having a very similar functionality, Underwater Wireless Sensor Networks exhibit several architectural differences with respect to the terrestrial ones, which are mainly due to transmission medium characteristics (sea water) and the signal employed to transmit data (acoustic ultrasound signals). Then, the design of appropriate network architecture for UWSNs is seriously hardened by the conditions of the communication system and, as a consequence, what is valid for terrestrial WSNs is perhaps not valid for UWSNs. So, a general review of the overall network architecture is required in order to supply an appropriate network service for the demanding applications in such an unfriendly submarine communication environment. Propagation conditions in an underwater acoustic channel are known to vary in time, causing the received signal strength to deviate from the nominal value predicted by a Deterministic Propagation Model (DPM). To facilitate large-scale system design under such conditions (e.g. power allocation), we develop a Statistical Propagation Model (SPM) in which the transmission loss is treated as a random variable. By repetitive computation of an acoustic field using a ray tracing tool (DPM) for a set of varying environmental conditions (surface height, wave activity, small displacements of a transmitter and a receiver around nominal locations), an ensemble of transmission losses is compiled which is then used to infer the statistical model parameters. A reasonable agreement is found with log-normal distribution, whose mean obeys a log-distance increase, and whose variance appears to be constant for a certain range of inter-node distances in a given deployment location. A statistical prediction model is deemed useful for higher-level system planning where simulation is needed to assess the performance of candidate network protocols under various resource allocation policies, i.e. to determine the transmitting power and bandwidth allocation necessary to achieve a desired level of performance (connectivity, throughput, reliability, etc.).

Realistic acoustic prediction models to efficiently design higher layer protocols in underwater wireless sensor networks

Coherent digital communications for rapidly fading channels with applications to underwater acoustics

We consider tracking of multiple objects using a wireless sensor network where distributed nodes transmit to a fusion center using random access. During an initialization phase, targets are identified on a discrete set of locations using a sparse identification method. Tracking then proceeds to update the target locations and amplitudes explicitly, using a gradient algorithm to solve the underlying non-linear optimization problem. Updating continues at the pace dictated by the average sensing/transmission rate, which can be adjusted to suit an expected target velocity. By focusing explicitly on the target locations, as opposed to continuing with sparse identification over a quantized space whose size may be much greater than the number of targets, the goal is to reduce the computational complexity, improve the performance, and eliminate the spatial quantization effects.

https://repository.library.northeastern.edu/files/neu:1241/fulltext.pdf

Target localization and tracking in a random access sensor network

Alamouti space-time coding is investigated in conjunction with OFDM modulation for high-rate underwater acoustic communications over time varying channels. The scheme's diversity gain is exploited using a low-complexity adaptive multichannel receiver. Performance is demonstrated using experimental data transmitted in a 10 kHz bandwidth over a 1 km shallow water channel south of the Martha's Vineyard island in New England. The two-transmitter Alamouti scheme shows the expected improvement over the same-rate single-transmitter scheme.

/pdf/alamouti-space-time-coded-ofdm-for-underwater-acoustic-5elew42suf.pdf

Alamouti space time coded OFDM for underwater acoustic channels

In this paper, we investigate an automatic repeat request (ARQ) for reliable transmission over half-duplex links. We design a method based on grouped packet coding (GPC) that combines a stop-and-wait (S&W) ARQ procedure with random linear packet coding and selective acknowledgments applied to groups of coded packets. Our goal in doing so is to boost the throughput efficiency on poor-quality links with long delay. Such links are notably encountered in underwater acoustic channels, where the bit error rate may be as high as $10^{-3}$ and round-trip delays can be measured in thousands of bits. To quantify the benefits of the proposed S&W-GPC method, we evaluate its throughput efficiency analytically and compare it with the throughput efficiency of standard S&W methods, as well as the benchmark efficiency of full-duplex methods. Our results show that S&W-GPC outperforms all other techniques on half-duplex links with long delay, as well as rateless packet coding on full-duplex links with long delay. We present results for a point-to-point link, as well as for a multicast network. In addition to the performance analysis, we offer guidelines for an optimal system design, which involves a judicious choice of the packet size, packet coding, and grouping parameters.

https://par.nsf.gov/servlets/purl/10084224

Milica Stojanovic

Papers

Realistic acoustic prediction models to efficiently design higher layer protocols in underwater wireless sensor networks

Coherent digital communications for rapidly fading channels with applications to underwater acoustics

Target localization and tracking in a random access sensor network

Alamouti space time coded OFDM for underwater acoustic channels

Grouped Packet Coding: A Method for Reliable Communication Over Fading Channels With Long Delays