Coding strategies that exploit node cooperation are developed for relay networks. Two basic schemes are studied: the relays decode-and-forward the source message to the destination, or they compress-and-forward their channel outputs to the destination. The decode-and-forward scheme is a variant of multihopping, but in addition to having the relays successively decode the message, the transmitters cooperate and each receiver uses several or all of its past channel output blocks to decode. For the compress-and-forward scheme, the relays take advantage of the statistical dependence between their channel outputs and the destination's channel output. The strategies are applied to wireless channels, and it is shown that decode-and-forward achieves the ergodic capacity with phase fading if phase information is available only locally, and if the relays are near the source node. The ergodic capacity coincides with the rate of a distributed antenna array with full cooperation even though the transmitting antennas are not colocated. The capacity results generalize broadly, including to multiantenna transmission with Rayleigh fading, single-bounce fading, certain quasi-static fading problems, cases where partial channel knowledge is available at the transmitters, and cases where local user cooperation is permitted. The results further extend to multisource and multidestination networks such as multiaccess and broadcast relay channels.

Cooperative strategies and capacity theorems for relay networks

In recent years, there has been an upsurge of interest in multihop-augmented infrastructure-based networks in both the industry and academia, such as the seed concept in 3GPP, mesh networks in IEEE 802.16, and converge extension of HiperLAN/2 through relays or user-cooperative diversity mesh networks. This article, a synopsis of numerous contributions to the working group 4 of the wireless world research forum and other research work, presents an overview of important topics and applications in the context of relaying. It covers different approaches to exploiting the benefits of multihop communications via relays, such as solutions for radio range extension in mobile and wireless broadband cellular networks (trading range for capacity), and solutions to combat shadowing at high radio frequencies. Furthermore, relaying is presented as a means to reduce infrastructure deployment costs. It is also shown that through the exploitation of spatial diversity, multihop relaying can enhance capacity in cellular networks. We wish to emphasize that while this article focuses on fixed relays, many of the concepts presented can also be applied to systems with moving relays.

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Relay-based deployment concepts for wireless and mobile broadband radio

Vehicular networks are very likely to be deployed in the coming years and thus become the most relevant form of mobile ad hoc networks. In this paper, we address the security of these networks. We provide a detailed threat analysis and devise an appropriate security architecture. We also describe some major design decisions still to be made, which in some cases have more than mere technical implications. We provide a set of security protocols, we show that they protect privacy and we analyze their robustness and efficiency.

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Securing vehicular ad hoc networks

Cognitive radio is an exciting emerging technology that has the potential of dealing with the stringent requirement and scarcity of the radio spectrum. Such revolutionary and transforming technology represents a paradigm shift in the design of wireless systems, as it will allow the agile and efficient utilization of the radio spectrum by offering distributed terminals or radio cells the ability of radio sensing, self-adaptation, and dynamic spectrum sharing. Cooperative communications and networking is another new communication technology paradigm that allows distributed terminals in a wireless network to collaborate through some distributed transmission or signal processing so as to realize a new form of space diversity to combat the detrimental effects of fading channels. In this paper, we consider the application of these technologies to spectrum sensing and spectrum sharing. One of the most important challenges for cognitive radio systems is to identify the presence of primary (licensed) users over a wide range of spectrum at a particular time and specific geographic location. We consider the use of cooperative spectrum sensing in cognitive radio systems to enhance the reliability of detecting primary users. We shall describe spectrum sensing for cognitive radios and propose robust cooperative spectrum sensing techniques for a practical framework employing cognitive radios. We also investigate cooperative communications for spectrum sharing in a cognitive wireless relay network. To exploit the maximum spectrum opportunities, we present a cognitive space-time-frequency coding technique that can opportunistically adjust its coding structure by adapting itself to the dynamic spectrum environment.

/pdf/cooperative-communications-for-cognitive-radio-networks-48j13lw4r8.pdf

Cooperative Communications for Cognitive Radio Networks

Energy efficiency in cellular networks is a growing concern for cellular operators to not only maintain profitability, but also to reduce the overall environment effects. This emerging trend of achieving energy efficiency in cellular networks is motivating the standardization authorities and network operators to continuously explore future technologies in order to bring improvements in the entire network infrastructure. In this article, we present a brief survey of methods to improve the power efficiency of cellular networks, explore some research issues and challenges and suggest some techniques to enable an energy efficient or "green" cellular network. Since base stations consume a maximum portion of the total energy used in a cellular system, we will first provide a comprehensive survey on techniques to obtain energy savings in base stations. Next, we discuss how heterogenous network deployment based on micro, pico and femtocells can be used to achieve this goal. Since cognitive radio and cooperative relaying are undisputed future technologies in this regard, we propose a research vision to make these technologies more energy efficient. Lastly, we explore some broader perspectives in realizing a "green" cellular network technology.

Green Cellular Networks: A Survey, Some Research Issues and Challenges

Traffic and travel information (TTI) systems in use today are based on a centralized structure. Sensors along the roadside monitor traffic density and transmit the results to a central unit where a situation analysis of the global traffic scenario is performed. The resulting traffic information is made available to drivers via broadcast service or alternatively on demand via cellular phone. Within the FleetNet project, a completely different proposal to establish a powerful traffic information system is developed and analyzed, which is based in inter-vehicle communications (IVC). All vehicles are part of a self-organizing traffic information system (SOTIS): each vehicle monitors the locally observed traffic situation by recurrently receiving data packets with detailed information from other vehicles. A traffic situation analysis if performed in each individual vehicle and the result is transferred via wireless data-link to all surrounding vehicles in the local neighborhood. No sensors along the highways, no central units and no local broadcast stations or cellular networks are needed, which minimizes infrastructure costs. The complete system can be easily avoided completely. Simulation results demonstrate that the proposed SOTIS technique can easily provide detailed information for each vehicle with an individual information range of more than 50 km from the current position with high accuracy and low delay. Even in situations where only a small fraction of all vehicles (e.g. 2%) is equipped with the SOTIS technology, the full functionality of a traffic information system is available.

SOTIS - a self-organizing traffic information system

The Fleetnet project aims at the development of a wireless ad hoc network for inter-vehicle communications. We present the rationale behind the choice of an appropriate radio hardware and the use of a position-based routing approach and outline applications to exploit the Fleetnet platform. In addition, we discuss simulation of vehicle movements as a basis for protocol evaluation as well as aspects of Internet integration of Fleetnet. We state the basic problems together with the intended approach of tackling these challenges, thereby providing an overview of the Fleetnet project

Position-aware ad hoc wireless networks for inter-vehicle communications: the Fleetnet project

A requirement for the performance evaluation of wireless systems by means of simulations is the knowledge of the path loss between transmitters and receivers. A new indoor path loss model for large-scale attenuation is proposed The multi-wall-and-floor (MWF) model considers the nonlinear relationship between the cumulative penetration loss and the number of penetrated floors and walls. The model has been derived from ray tracing simulations in different scenarios and the respective parameters are provided at 5.2 GHz. For validation purposes, measurements and extensive comparisons with results found in literature have been performed. The simple structure of the MWF model makes it an interesting alternative to ray tracing for system simulations in indoor environments.

A multi-wall-and-floor model for indoor radio propagation

The goal of the FleetNet project is to develop an air-interface for inter-car communication and road telematics. For the raido interface, the framework of the UMTS Terrrestrial Radio Access Time Division Duplex (UTRA TDD) air-interface shall be used with modifications. This paper introduces a concept for the air-interface, highlights the challenges for its use in an ad hoc network with rapidly changing topology and offers a description as well as an evaluation of the required changes within the medium access control and radio resource management layer

Matthias Lott

Papers

Relay-based deployment concepts for wireless and mobile broadband radio

SOTIS - a self-organizing traffic information system

Position-aware ad hoc wireless networks for inter-vehicle communications: the Fleetnet project

A multi-wall-and-floor model for indoor radio propagation

Medium access and radio resource management for ad hoc networks based on UTRA TDD