Cognitive Wireless Networks Cognitive Wireless Networks Concepts, Methodologies and Visions Inspiring the Age of Enlightenment of Wireless Communications Edited by Frank H.P. Fitzek Aalborg University, Denmark and Marcos D. Katz VTT, Finland A C.I.P. Catalogue record for this book is available from the Library of Congress. ISBN 978-1-4020-5978-0 (HB) ISBN 978-1-4020-5979-7 (e-book) Published by Springer, P.O. Box 17, 3300 AA Dordrecht, The Netherlands. www.springer.com Printed on acid-free paper © 2007 Springer No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. To our parents Eta-Marie and Werner Fanny and Abraham (in memoriam) for their eternal support and loving. What is Cognitive Radio and Cognitive Networks? Bernhard Walke RWTH Aachen University, Aachen, Germany Cognitive Networks based on cognitive radio are addressing a revolutionary technology aiming, besides others, at remarkably improving efficiency of spec- trum usage. When introduced, it will fundamentally change the way radio spectrum is regulated and used. Before this may happen, new enabling prop- erties of radios are required such as sensing spectrum occupancy covering a wide range of spectrum and flexible spectrum access adapting to variable channel widths based on reasoning. Cognition has much to do with coex- istence management: Coexistence of radio based systems operating in the same or in partly overlapping channels using the same or even different air- interfaces is the challenge to be solved. This appears to be especially difficult to achieve when coexistent radios operate different air interfaces and apply dif- ferent transmit power levels, since the near-far and hidden terminal problems will apply then and would make coexistence management using de-central control hardly possible. The well-known pilot channel based control applied by an incumbent to control access to its licensed spectrum by third parties to manage coexistence of both appears workable only, if coexistence control is reduced to “yes or no” access permission for non-incumbents, depending on the needs of the primary user what is in fact a TDMA based resource sharing on a long time scale and not coexistence management. Much more sophis- ticated cooperation strategies appear necessary to enable small time-scale, location-specific coexistence management of radio systems, e.g. based on ex- plicit information exchange between the radios involved or just being based on observations made, recently, by a radio following game theoretic reasoning, possibly, combined with well designed back-off algorithms. Prioritization of an incumbent might be part of the rules applied. As is visible from the assembly of contributions to this book, the state-of-the-art towards this is not much progressed, although a rich set of ideas exist potentially contributing to make efficient coexistence control of radios happen. It need not be mentioned that dynamic spectrum assignment can be achieved only if mobile terminals are able to re-configure on all layers of its protocol stacks. Cognitive radio aims to promote technologies as well as changes in radio regulation to overcome some VIII Bernhard Walke existing barriers aiming to improve efficiency of spectrum utilization with- out scarifying highly reliable communication meeting high quality of service targets. Cognitive networks as a generic approach to exploit cognition in wireless networks: using cognitive principles to improve utilization of resources. In addition to spectral efficiency, for instance energy (power) efficiency can be also enhanced by exploiting cognition. Cognitive radio is just a particular instance of cognitive networks. However, Cognitive Networks / Cognitive Radio is a buzzword, too, since it umbrellas a number of more specific terms used to describe existent radio technology, namely • Mobile radio, a context-aware radio able to identify (based on cognitive capabilities like reasoning on measurement results) the best suited base station, supporting a given air interface standard, to serve a running ses- sion (called handover) or to associate to the base offering the strongest signal. • Multi-band adaptive radio to switch between distant channel groups, with- out changing the air-interface standard, like in use for GSM 900/1800/1900 MHz bands. Clearly, cognitive capabilities are required to decide on the band to use, based on measurement results. The related technique is dynamic spectrum assignment. • Multi-standard radios able to associate to one out of a number of different air interface standards like GSM/UMTS/CDMA2000/WLAN/others, or even switch air interface across standards during a running session (inter- standards handover). Nobody would call this cognitive radio operation, although a lot of cognition related functions is required to operate a radio like this. • Multi-homing radios able to support different standards air interfaces at the same time. • Reconfigurable radio, e.g., – Hardware Defined Radio (HDR) comprising a set of radios housed in one box, each radio designed to serve a given air-interface, able to decide (based on cognitive capabilities), which radio to operate at a time. – Software Defined Radio (SDR) able to adapt transmission related or even protocol related parameters so that some (or many) properties of the radio are adapted to the needs. SDR would clearly need cognitive capabilities, too, to make senseful decisions. Wireless systems operated in license-exempt (ISM) bands, coexisting and sharing the spectrum according to a standard-specific set of common rules deciding on medium access, reflecting to own observations and based on measurement results, combined with reasoning (using cognitive capabil- ities). Spectrum etiquette and policies and open spectrum are the terms related to that operation. What is Cognitive Radio and Cognitive Networks? IX Taking all of these already existent technologies into account, cognitive radios apparently need to go beyond in its aims and ambitions that currently are not well specified and expressed. There is a clear need for sorting things and differentiating known technolo- gies from new ideas. Application of game theoretic models, vertical and hori- zontal spectrum sharing, overlay sharing (like UWB), reasoning and machine- understandable regulatory rules - in general, feature detectors and cyclo- stationary detectors, spectrum opportunity identification and self-organization and cooperation in wireless networks are examples for new dimensions to be considered in the cognitive radios domain. Besides addressing cognitive networks, the book has a focus on user co- operation: The book advocates the concepts of wireless grids, which is an ad hoc cooperative cluster made of wireless terminals connected over short-range links, but at the same time, being connected to the cellular network. As be- hind each terminal there is a user who ultimately may decide to join or not a wireless grid, user decisions (individual and group) will have an impact on network operation and performance. Wireless networks enabling social net- working and social networks shaping wireless networks are also discussed in the book. Cognitive networks and user cooperation are clearly related and the contri- butions presented by experts in the respective fields are really worth reading. I have found the themes of the articles invited to form the book very interesting and representing most recent research subjects. May this prove useful to you too. Aachen, Germany, Bernhard Walke May 2007 Preface Sapere aude! Dare to know! Habe Mut, dich deines eigenen Verstandes zu bedienen! Sapere aude!, the emblematic motto associated with the Age of Enlight- enment, is perhaps a rather eccentric expression to open a book on wireless communications. Alluding to that maxim, the German philosopher Immanuel Kant encouraged people to use their own minds as the basis for reasoning instead of following dogmatic rules. However, the expression has rich con- notations, and we particularly see how dazzlingly inspires the current devel- opments and future of wireless communication networks. This book basically deals with two complementary principles, cognition and cooperation, and how they are becoming essential for future wireless networks. Implicit cognition and cooperation have always been present in any wireless network as funda- mental principles for ensuring basic network operation, as for instance the use of common protocols or signaling across the network, estimation of instan- taneous channel conditions, etc. This book focuses on techniques exploiting cooperative and cognitive principles in an explicit manner, that is, purposely implemented by design, and aiming at enhancing the most relevant link and network performance figures as well as improving the efficiency in the use of resources. In general many of these techniques have just recently emerged and cur- rently they are receiving increasing attention by the research community and industry. Cooperation in wireless networks is a well established and rather mature field, whereas cognitive principles, in their explicit way, are rapidly finding their way to the wireless world. The book presents a comprehensive cross-section of these promising fields, exploring these