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

Multi-hop transmission is considered for large coverage areas in bandwidth-limited underwater acoustic networks. In this paper, we present a scalable routing technique based on location information, and optimized for minimum energy per bit consumption. The proposed Focused Beam Routing (FBR) protocol is suitable for networks containing both static and mobile nodes, which are not necessarily synchronized to a global clock. A source node must be aware of its own location and the location of its final destination, but not those of other nodes.The FBR protocol can be defined as a cross-layer approach, in which the routing protocol, the medium access control and the physical layer functionalities are tightly coupled by power control. It can be described as a distributed algorithm, in which a route is dynamically established as the data packet traverses the network towards its final destination. The selection of the next relay is made at each step of the path after suitable candidates have proposed themselves.The system performance is measured in terms of energy per bit consumption and average packet end-to-end delay. The results are compared to those obtained using pre-established routes, defined via Dijkstra's algorithm for minimal power consumption. It is shown that the protocol's performance is close to the ideal case, as the additional burden of dynamic route discovery is minimal.

/pdf/focused-beam-routing-protocol-for-underwater-acoustic-4p7kwnpqn9.pdf

Focused beam routing protocol for underwater acoustic networks

Underwater acoustic networks are generally formed by acoustically connected ocean bottom sensor nodes, autonomous underwater vehicles (AUVs), and surface stations that serve as gateways and provide radio communication links to on-shore stations. The QoS of such networks is limited by the low bandwidth of acoustic transmission channels, high latency resulting from the slow propagation of sound, and elevated noise levels in some environments. The long-term goal in the design of underwater acoustic networks is to provide for a self-configuring network of distributed nodes with network links that automatically adapt to the environment through selection of the optimum system parameters. This paper considers several aspects in the design of shallow water acoustic networks that maximize throughput and reliability while minimizing power consumption.


Keywords:

underwater;
acoustic;
communications;
network;
media access control (MAC);
routing;
ad hoc;
Seaweb

Shallow-Water Acoustic Networks†

This paper proposes a channel access protocol for ad-hoc underwater acoustic networks which are characterized by long propagation delays and unequal transmit/receive power requirements. The protocol saves transmission energy by avoiding collisions while maximizing throughput. It is based on minimizing the duration of a hand-shake by taking advantage of the receiver's tolerance to interference when the two nodes are closer than the maximal transmission range. Nodes do not need to be synchronized, can move, are half-duplex, and use the same transmission power. This protocol achieves a throughput several times higher than that of the slotted FAMA, while offering similar savings in energy. Although carrier sensing ALOHA offers a higher throughput, it wastes much more power on collisions.

/pdf/distance-aware-collision-avoidance-protocol-for-ad-hoc-1z3q5906hq.pdf

Distance aware collision avoidance protocol for ad-hoc underwater acoustic sensor networks

Acoustic propagation is characterized by three major factors: attenuation that depends on the signal frequency, multipath propagation, and low speed of sound (1500 m/s). The channel has a sparse impulse response, where each physical path acts as a time-varying low-pass filter, and motion introduces additional Doppler spreading and shifting. Because propagation is best supported at low frequencies, acoustic communication systems are inherently wideband. The way in which these facts influence the design of signal processing methods is considered for single-carrier and multi-carrier systems. Moreover, the facts that the available bandwidth and transmission power depend heavily on the distance, and that channel latency is high, bear important implications on the design of network architectures and related protocols.

/pdf/underwater-acoustic-communications-design-considerations-on-3om53w9u1t.pdf

Underwater Acoustic Communications: Design Considerations on the Physical Layer

We examine the problem of utilizing an autonomous underwater vehicle (AUV) to collect data from an underwater sensor network. The sensors in the network are equipped with acoustic modems that provide noisy, range-limited communication. The AUV must plan a path that maximizes the information collected while minimizing travel time or fuel expenditure. We propose AUV path planning methods that extend algorithms for variants of the Traveling Salesperson Problem (TSP). While executing a path, the AUV can improve performance by communicating with multiple nodes in the network at once. Such multi-node communication requires a scheduling protocol that is robust to channel variations and interference. To this end, we examine two multiple access protocols for the underwater data collection scenario, one based on deterministic access and another based on random access. We compare the proposed algorithms to baseline strategies through simulated experiments that utilize models derived from experimental test data. Our results demonstrate that properly designed communication models and scheduling protocols are essential for choosing the appropriate path planning algorithms for data collection.

/pdf/underwater-data-collection-using-robotic-sensor-networks-3ztwrkj75v.pdf

Milica Stojanovic

Papers

Focused beam routing protocol for underwater acoustic networks

Shallow-Water Acoustic Networks†

Distance aware collision avoidance protocol for ad-hoc underwater acoustic sensor networks

Underwater Acoustic Communications: Design Considerations on the Physical Layer

Underwater Data Collection Using Robotic Sensor Networks