IEEE JOURNAL OF SELECTED TOPICS IN SIGNAL PROCESSING, VOL. 2, NO. 1, FEBRUARY 2008
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Introduction to the Issue on Signal Processing and Networking for Dynamic Spectrum Access
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HE paradox between the overly crowded spectrum and the pervasiveness of idle frequency bands in both time and space highlights the drawbacks of the current static spectrum allotment policy. The perceived spectrum scarcity is largely due to poor spectrum management policies rather than due to the actual physical scarcity of bandwidth. Much of the spectrum is allocated to licensed users, but there is a growing proliferation of services leading to ever-increasing, but sporadic, demand. Due to the burstiness of demand, guard bands in space and idle frequency bands are inevitable under the current static spectrum allocation policies. Recently, we have witnessed a flurry of research activities in search of dynamic spectrum access (DSA) strategies for improved efficiency. This research is multi-disciplinary and spans the engineering, economics and regulatory communities. Ideas of cognitive radio (CR) are also integral to much of this research. DARPA was one of the early proponents of such research under its XG program. XG sought to develop spectrum management policies and technologies for DSA, based on the notions of realtime, low-power, wideband sensing, rapid channel characterization, and opportunity determination by a policy reasoner. XG concepts are being incorporated in subsequent DARPA programs such as WNAN and WAND. NSF recently launched the Programmable Wireless Networks (ProWiN) program to address basic issues in spectrum agile radios and networks. Research in Europe has been supported under the Research Framework and the DRiVE program. Standardization efforts are underway under the IEEE 802.22 umbrella. Regulatory agencies in the U.S. (e.g., the FCC), the European Union, and elsewhere are in the process of redefining spectrum policies. There have also been several conferences and workshops devoted to DSA and CR (e.g., DySpan, CrownCom, IEEE Workshop on Cognitive Radio Networks). The basic idea underlying DSA is that if spectrum usage can be dynamically monitored, and if short-term predictions of spectrum usage can be made, then resources can be dynamically allocated or accessed, resulting in more efficient usage. Approaches envisioned for DSA fall under three general models: dynamic exclusive use, open sharing (or spectrum commons), and a hybrid hierarchical access scheme. To realize the potential of DSA, advances in signal processing, communications and networking are crucial. DSA leads to new challenges in both theory and design in multiple areas: coexistence of primary and secondary users, (distributed or cooperative) spectrum sensing, spectrum management and resource allocation, appropriate PHY, MAC and routing proDigital Object Identifier 10.1109/JSTSP.2008.917511
tocols, security issues, and cooperative monitoring to enforce policy. In the last few years, the importance of the PHY layer in wireless communications has become apparent. This is also true in DSA, where novel PHY technologies are expected to play a crucial role. This issue covers diverse ideas and approaches envisioned for DSA. The underlying theme is to explore the role of signal processing in dynamic spectrum access and to illuminate the close interaction between signal processing and networking for spectrum efficiency. We have grouped the eight papers in this issue into three broad categories: spectrum sensing, spectrum management, and fundamental capacity limits. The first part on spectrum sensing consists of three papers, progressing from single sensor detection to cooperative detection. The paper by Tandra and Sahai addresses the problem of robust detection of weak primary users. It is argued that due to inherent system limitations in accurately estimating the underlying noise statistics, there exist SNR walls that limit detection performance: a primary signal below the SNR wall cannot be robustly detected by a secondary. A robustness versus capacity tradeoff is identified to capture the underlying tension. How can secondary users cooperate to find spectrum holes? How should the individual sensors set their thresholds so as to maximize a system-wide metric? How should the fusion rule take into account the correlation in the signal sensed at the different radios? These are some of the questions answered in the paper by Unnikrishnan and Veeravalli. The paper by Quan, Cui, and Sayed proposes strategies for reliable detection of primary signals by linearly fusing measurements from a network of spectrum sensors. The determination of the optimal linear combining weights is posed as a non-linear optimization problem, and iterative algorithms are proposed. In the paper by Ghasemi and Sousa, the distribution of the aggregated interference at a primary receiver from a network of Possion distributed secondary transmitters is obtained. The impact of fading, detector sensitivity, and cooperative sensing on this distribution is studied. The second part of the paper consists of two papers on spectrum management. Given a system-wide metric (e.g., weighted sum-rate), and given channel conditions, how should users be allocated power spectral densities, subject to power constraints? These are some of the questions addressed in the paper by Luo and Zhang. They show that this set of problems is generally NP-hard, and identify a subclass that has polynomial complexity. The paper by Wang Krunz, and Cui considers a related spectrum management problem. A distributed protocol based
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IEEE JOURNAL OF SELECTED TOPICS IN SIGNAL PROCESSING, VOL. 2, NO. 1, FEBRUARY 2008
on ideas from economics (pricing and game theory) is given and shown to offer improved performance relative to existing algorithms. The third part of this issue consists of two papers that provide information-theoretic perspectives. The paper by Zhang and Liang considers the problem of allocating power across multiple antennas by a secondary system that must satisfy certain constraints on the interference power at the primary users. The paper by Sridharan and Vishwanath establishes an achievable region and outer bounds for a system for a system where both the primary and secondary user may have multiple antennas. The call for papers for this issue elicited many submissions. Each paper was carefully reviewed by at least three reviewers. In the end, many quality papers could not be included, based on relevance to this issue, and balance of topics covered. We would like to thank all the authors who submitted to this issue and we also thank the reviewers for their constructive and critical comments and insights. We hope that this special issue has captured some of the excitement at the frontiers of this growing and evolving area. ANANTHRAM SWAMI, Guest Editor U.S. Army Research Laboratory Adelphi, 20783 USA
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RANDALL A. BERRY, Guest Editor Northwestern University Department of Electrical Engineering and Computer Science Evanston, IL 60201 USA
[email protected] AKBAR M. SAYEED, Guest Editor University of Wisconsin-Madison Department of Electrical and Computer Engineering Madison, WI 53706 USA
[email protected] VAHID TAROKH, Guest Editor Harvard University Division of Engineering and Applied Sciences Cambridge, MA 02138 USA
[email protected] QING ZHAO, Guest Editor University of California-Davis Department of Electrical and Computer Engineering Davis, CA 95616 USA
[email protected]
Ananthram Swami (S’79–M’79–SM’96–F’08) received the B.Tech. degree from the Indian Institute of Technology, Bombay, the M.S. degree from Rice University, Houston, TX, and the Ph.D. degree from the University of Southern California, Losa Angeles. He is currently with the U.S. Army Research Laboratory, Adelphi, MD, where his work is in the broad area of signal processing, wireless communications, and networking.
Randall A. Berry (S’93–M’00) received the B.S. degree in electrical engineering from the University of Missouri-Rolla in 1993 and the M.S. and Ph.D. degrees in electrical engineering and computer science from the Massachusetts Institute of Technology, Cambridge, in 1996 and 2000, respectively. In 1998, he was on the Technical Staff at MIT Lincoln Laboratory in the Advanced Networks Group. Since 2000, he has been with Northwestern University, Evanston, IL, where he is currently an associate professor in the Department of Electrical Engineering and Computer Science. Dr. Berry is currently an Associate Editor for IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS and is the recipient of a 2003 NSF CAREER award.
IEEE JOURNAL OF SELECTED TOPICS IN SIGNAL PROCESSING, VOL. 2, NO. 1, FEBRUARY 2008
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Akbar M. Sayeed (S’88–M’97–SM’02) received the B.S. degree in 1991 from the University of Wisconsin-Madison and the M.S. and Ph.D. degrees in 1993 and 1996, respectively, from the University of Illinois at Urbana-Champaign, all in electrical engineering. He was a Postdoctoral Fellow at Rice University, Houston, TX, in 1996–1997 and is currently Associate Professor of electrical and computer engineering at the University of Wisconsin-Madison. His research interests include wireless communications, statistical signal processing, multidimensional communication theory, information theory, time-frequency analysis, and applications in networks. Dr. Sayeed is a recipient of the Robert T. Chien Memorial Award (1996) for his doctoral work at Illinois, the NSF CAREER Award (1999), the ONR Young Investigator Award (2001), and the UW Grainger Junior Faculty Fellowship (2003). He is a currently serving on the signal processing for communications technical committee of the IEEE Signal Processing Society.
Vahid Tarokh (M’06) is a Perkins Professor of Applied Mathematics and Hammond Vinton Hayes Senior Fellow of Electrical Engineering at Harvard University, Cambridge, MA. At Harvard, he teaches courses and supervises research in communications, networking and signal processing. Dr. Tarokh has received a number of major awards and holds two honorary degrees.
Qing Zhao (S’97–M’01) received the Ph.D. degree in electrical engineering in 2001 from Cornell University, Ithaca, NY. In 2004, she joined the University of California, Davis, where she is currently an Assistant Professor. From 2001 to 2004, she was a communication system engineer with Aware, Inc., Bedford, MA, and a postdoctoral research associate with the School of Electrical and Computer Engineering at Cornell University. Dr. Zhao received the 2000 IEEE Signal Processing Society Young Author Best Paper Award. She is an associate editor of IEEE Transactions on Signal Processing and an elected member of the Signal Processing for Communications committee of the IEEE Signal Processing Society.
