ANANT SAHAI Electrical Engineering and Computer Sciences Associate Professor sahai@eecs.berkeley.edu Official website
Research For prospective students Bio Teaching My Students	267 Cory Hall UC Berkeley 94720-1770 Spring 2013 Office Hours: Tue 4-5 in 400 Cory And by appointment I also drop by the Thu afternoon Problem Set Party in 380 Soda

Announcements:

Our guest blog post on the Television Whitespaces has appeared at Google's "Policy by the Numbers" blog.

Congratulations to Pulkit Grover and Kristen Ann Woyach for being co-authors with me on our Leonard G. Abraham Prize winning paper.

Congratulations to Kate Harrison for winning the best student policy paper award at DySpAN 2012.

Congratulations to Pulkit Grover for winning the Eliahu Jury Prize in 2011 and the best student paper award at CDC 2010.

Welcome 2009 Communication Magazine Readers. A slight bug was introduced by the editors during the production process. Equation 1 should be .

My areas of interest span communications, control, and signal processing. Within that range, my focus is on the communications theory side, particularly in the areas of wireless and information theory. Within information theory, my main interest is in developing the conceptual tools needed to understand feedback, interaction, delay, reliability, and complexity. To that end, I am interested in decentralized control systems as they provide well understood mathematical models that do not mesh with the classical notions from information theory. They also tell us why delay is important. On the wireless communication side, I am interested in power consumption and how multi-scale heterogenous wireless systems can coexist peacefully. Cognitive radio and wireless spectrum sharing are of particular interest.

My research sits within the Wireless Foundations center, where we study the fundamental basis for the wireless technologies of the future. The following pages give more details about some of my research interests, with links to additional publications, etc.

• Representative talk slides (a bit old)
• UNI: Understanding Networks and Information (a bit old)
• RASP: Robust and Adaptive Signal Processing for cognitive radios (a bit old)
• Official publication/invited-talks list

Key contributions from my group (Details can be found using the links above)

• Decentralized Control
  1. Built a bridge between control theory and information theory by showing that anytime reliability is the right way to evaluate the quality of a noisy channel being used in a feedback loop for stabilization. Developed tight bounds for anytime reliability.
  2. Obtained the first provably positive results for the longstanding Witsenhausen counterexample in distributed control by demonstrating nonlinear control strategies that are within a constant factor of optimal for all problem parameters.
• Classical Communication
  1. Overturned prior assertions to show how feedback improves the tradeoff between delay and the probability of error. Even noisy/unreliable feedback helps. Moreover, the details of this illuminated an interesting duality between source and channel coding.
  2. Developed a new bound on how the probability of error must scale with "decoding neighborhood sizes" and used this to illuminate the implementation complexity of codes in terms of power consumption. This showed that for short-range communication, approaching Shannon capacity is probably the wrong thing to aim for.
• Dynamic Spectrum Sharing
  1. Showed that modeling uncertainties introduce fundamental limitations (SNR Walls) to spectrum sensing.
  2. Developed new metrics for evaluating spectrum sensing in the dynamic spectrum access context, introduced the key idea of the spatial overhead of sensing, and showed that diversity (from cooperative sensing) is required to reduce this overhead.
  3. Introduced a new paradigm for the light-handed regulation of cognitive radios inspired by criminal rather than civil law and explored the resulting regulatory overhead.

A few recent papers: (Descriptions and more can be found using the links above)

Before joining the faculty at Berkeley in 2002, I spent 2001 at the startup Enuvis, Inc. where I was on the theoretical/algorithmic side of a team that developed new techniques for GPS detection in very low SNR environments (such as those encountered indoors in urban areas). From 1994-2000, I was a graduate student at MIT studying Electrical Engineering and Computer Science (Course 6 in MIT-speak) and was based in the Laboratory for Information and Decision Systems under Prof. Sanjoy Mitter. My research interests there started in machine understanding but shifted toward the intersection of control and information theory. I did my undergraduate work in EECS here at the University of California at Berkeley from 1990-1994.

I served as the Treasurer for the IEEE Information Theory Society from 2007-2009 and am currently the faculty adviser for the Berkeley chapter of Eta Kappa Nu.

• EECS 70: Discrete Mathematics and Probability

Teaching History:

• Baris Nakiboglu: Came to me from MIT, where he had done a wonderful dissertation on topics near to my heart in feedback and information theory. Continuing to build on that work with me.

Current students:

• Kristen Ann Woyach: Great master's thesis on regulatory issues in cognitive radio and spectrum sharing. Continuing her explorations in this direction for her doctoral research, and also bringing in some spatial stochastic geometry. Co-author on a Leonard G. Abraham Prize winning paper.
• Se Yong Park: Exploring the intersection of control theory and information theory. So far, he has essentially solved the problem of intermittent Kalman Filtering, connected network coding to decentralized LTI control and thereby showed that implicit communication exists even there, and has greatly built upon the Witsenhausen counterexample to approximately solve the scalar infinite-horizon two-user decentralized LQG control problem. Won the Lim prize for his prelim exam performance.
• Gireeja Ranade: Joined the group after finishing her MS with Jose Carmena. Internships at IBM Research and Microsoft Research. Explored network-extensions of the Witsenhausen counterexample, deterministic models, and the nature of information flow in control systems.
• Kate Harrison: Built on Mubaraq's work to do an MS thesis on really understanding the cognitive radio opportunity in the television whitespaces, and proposed an idea for approximately universal power-control rules. Building on this towards her dissertation on understanding the true potential of the cloud-oriented spectrum management architecture only hinted at in the recent PCAST report on spectrum. We won a best policy paper award at DySpAN 2012.

Former Visitors:

• Hidekazu Miyoshi: visited from SEI Innovation Core in Japan. Explored cognitive radio issues.
• Mukul Agarwal: A student of Prof. Sanjoy Mitter in LIDS. Spent Summer '06 visiting me here in Wireless Foundations. Exploring the nature of information flows in systems.

Alums:

• Hari Palaiyanur: Master's thesis on applications of sequential decoding to anytime problems. Did summer internship at Nokia Research in '07. In his dissertation, he explored the information theory of active models of perception and how to understand channels with feedback --- discovering a very curious fact about the non-additivity of the classic Haroutunian bound along the way and proving that the sphere-packing bound must hold even for asymmetric DMCs with very delayed feedback. Now works at McKinsey and Company.
• Pulkit Grover: Explored model uncertainty issues from an information theoretic perspective. Summer internship at Wionics Research with Realtek in '07. And PostDoc at Stanford during 2012. Worked with me on understanding the computational complexity of iterative decoding from a power-consumption perspective. Pioneering contributions on the Witsenhausen's counterexample for his doctoral dissertation. We won a best student paper award at IEEE CDC in 2010. Pulkit won the Eliahu Jury award in our department for his dissertation. Co-author on a Leonard G. Abraham Prize winning paper. Now Assistant Professor at CMU.
• S. Mubaraq Mishra (Joint with Brodersen in BWRC): Pioneering doctoral research on cooperation and architectural issues for cognitive radios, including the first real analysis of the real-world white-space opportunity. Summer internship at Wionics Research with Realtek in '06. Now works for the startup Spidercloud in the Bay Area.
• Rahul Tandra: Master's thesis on limits to low SNR detection due to uncertainty. His pioneering doctoral thesis built upon this in the context of opportunistic spectrum use and also explored the fundamental metrics/limits for spectrum sensing. We won a best paper award for IEEE DySpAN in 2007. Now working at Qualcomm R&D in San Diego.
• Cheng Chang: Did a Master's thesis on UWB-based sensors: localization and positioning. Doctoral dissertation on multiuser anytime information theory with a focus on multiterminal source coding. Joined HP Labs as postdoc in Jan 08 and is now at the hedge fund D. E. Shaw.
• Danijela Cabric: A student of Bob Brodersen whom I coadvised on spectrum sensing issues. Great thesis and is now at UCLA as an Assistant Professor.
• Niels Hoven: Master's thesis on power control for cognitive radios. Did summer internship at Philips Research in 06. Currently taking an indefinite break from graduate school and is now working at Pocket Gems.
• Stark Draper: Postdoc with myself, Kannan Ramchandran, and David Tse. Worked on feedback and delay issues, especially in the soft deadline context, as well as streaming Slepian Wolf problems. Spent a year at MERL in Cambridge, and is now at U. Wisconsin Madison as an Assistant Professor.
• Qing Xu: Master's thesis on anytime capacity of erasure type channels. Joined a Bay Area startup and is now at Qualcomm.
• Tunc Simsek: Brilliant doctoral thesis on anytime coding with feedback. Now at Mathworks. (Joint with Varaiya)
• Padmini Pyapali: Undergrad researcher with me (and mostly my student Kristen Woyach) where she did some work on enforcing sensing on cognitive radios in a light-handed way. Now at Zynga.
• Hansen Bow: Undergrad researcher with me who went on for a PhD at MIT working on BioMEMS. With me, he did some of our earliest work on spectrum sharing as well as work on positioning in UWB systems.
• Moshe Malkin: Undergrad researcher with me who went on to get his PhD at Stanford working with Prof. Cioffi. With me, he did some work on positioning in UWB systems.
• Amirali Zohrenejad: Undergrad researcher with me who is now at Microsoft working in wireless networking. With me, he did some early work on randomized encoding strategies for tracking random walks.

General-audience Coverage

• Profile piece in the MIT LIDS ALL Magazine: mostly featuring the work with Kristen on Spectrum Jails.
• Article in Technology Review, June 2010: focusing on the work that Mubaraq and Kate had done in estimating the value of the TV whitespaces.
• Article in Technology Review, November 2008: featuring my thoughts on the FCC's historic election day ruling.

Special thanks to our past and present research sponsors:

• United States National Science Foundation under grants: ANI-0230963, ANI-0326503, CNS-0403427, CCF-0729122, CCF-0917212, CNS-0932410 as well as NSF Fellowships for my students.
• Unrestricted Gifts from Sumitomo Electric and Samsung Corporation
• Vodafone Foundation Fellowships for my students
• C2S2 and others supporting work done in the Berkeley Wireless Research Center.