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

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.

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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

Location-aware technologies will revolutionize many aspects of commercial, public service, and military sectors, and are expected to spawn numerous unforeseen applications. A new era of highly accurate ubiquitous location-awareness is on the horizon, enabled by a paradigm of cooperation between nodes. In this paper, we give an overview of cooperative localization approaches and apply them to ultrawide bandwidth (UWB) wireless networks. UWB transmission technology is particularly attractive for short- to medium-range localization, especially in GPS-denied environments: wide transmission bandwidths enable robust communication in dense multipath scenarios, and the ability to resolve subnanosecond delays results in centimeter-level distance resolution. We will describe several cooperative localization algorithms and quantify their performance, based on realistic UWB ranging models developed through an extensive measurement campaign using FCC-compliant UWB radios. We will also present a powerful localization algorithm by mapping a graphical model for statistical inference onto the network topology, which results in a net-factor graph, and by developing a suitable net-message passing schedule. The resulting algorithm (SPAWN) is fully distributed, can cope with a wide variety of scenarios, and requires little communication overhead to achieve accurate and robust localization.

Cooperative Localization in Wireless Networks

A new radio access network architecture for a mobile broadband system is introduced that uses fixed wireless routers to provide radio coverage to otherwise shadowed areas. Highest spectrum capacity and lowest possible transmit power as main targets will be reached by using low power (1 W) pico base stations using a wireless or mobile broadband air interface at access points (AP) to the core network and at FWRs to trade the high capacity available at APs against radio coverage range. The so-called wireless media system (WMS) will provide broadband access to terminals with medium velocity of movement and is embedded into a cellular radio network to support a high velocity of terminals with medium transmission rate. The low power used at the base stations leads to a pico-cellular concept relying essentially on multi-hop communication across fixed wireless bridges or routers and to some extent also on ad hoc networking. The new concept to achieve broadband radio coverage in densely populated areas is described and first analysis results of some crucial elements are presented.

A mobile broadband system based on fixed wireless routers

The paper provides a qualitative performance assessment of relay based deployment concepts in comparison to single-hop deployment. The results in this paper prove the general assumption that the deployment of relay nodes (RNs) provide a strong opportunity to reduce the network deployment cost substantially. It is shown how low power RNs have to be applied in cellular scenarios with high power base stations (BSs) and what cost benefits can be expected. The qualitative results show that the low power relays do have the potential to decrease the deployment cost. It is further shown that a parallelization of the relay link between the RNs and the serving BS will help to improve the cost efficiency of the relay based deployment concept.

Fixed Relays for Cost Efficient 4G Network Deployments: An Evaluation

This paper discusses fixed layer-2 relays for infrastructure based radio network deployment concepts. Relay enhanced cells (RECs) for different optimizations goals like area size optimization, cell capacity optimizations in a given area and the application of relays to cover otherwise shadowed areas are introduced. As a basis for the proof of concept by means of simulation, a relaying capable MAC protocol as well as the used REC concept is briefly explained. The new REC concepts for both, area optimizations and capacity optimizations are validated by analytic and simulation results. Results are shown for the C/I in a multi-cellular environment and the throughput for an REC cluster with four relay nodes per REC using different cluster orders. The results are compared to that of a comparable single-hop op cell cluster.

Layer-2 Relays in Cellular Mobile Radio Networks

The huge growth in wireless communication has led to a wide range of technologies each addressing a particular scenario or need. The goal of the WINNER project is to develop a single new ubiquitous radio access system concept to address the whole spectrum of mobile communications scenarios. This paper surveys the challenge of providing ubiquitous operation and introduces the key technologies investigated in WINNER to meet these needs.

D.C. Schultz

Papers

Relay-based deployment concepts for wireless and mobile broadband radio

A mobile broadband system based on fixed wireless routers

Fixed Relays for Cost Efficient 4G Network Deployments: An Evaluation

Layer-2 Relays in Cellular Mobile Radio Networks

WINNER - Towards Ubiquitous Wireless Access