Pioneering work in the area of airborne networks and cognitive radios.

Faculty Research Statement Covering the period since tenure 2002 2008 Timothy X Brown Electrical and Computer Engineering Department Interdisciplinary Telecommunications Program October, 2008 My research is focused on wireless network communication, the key enabling technology for wireless Internet access, disaster and emergency communications, and mobile telephone communication. My work up until tenure (through 2001) was primarily theoretical. 1 In networking research, theory is important but distributed networks are notoriously tricky to model and new advances must be in the context of existing networks and their users. Arguments over the theoretically best approach bog down on subtle modeling differences and how much we are willing to change the existing network. For these reasons networking researchers also value what the Internet Engineering Task Force calls rough consensus and running code. 2 In practice, this means that successful networking research must consider a larger context and must ultimately reduce the research ideas to practice. In wireless networking these issues are especially acute since the complex wireless environment further challenges theoretical models. Further, you can build a new fully functioning radio but yet be unable to turn it on because of regulatory restrictions from the Federal Communication Commission (FCC). The tightly managed wireless spectrum is often the greatest limit to innovation. With tenure, I took the opportunity to tackle these challenges through two new research programs: one to study real wireless network implementations; and the other to tackle the regulatory fetters to new wireless technologies, specifically a technology denoted as cognitive radios. Summary of Research Accomplishments Leader of the Ad hoc Unmanned aircraft Ground Network (AUGNet) research group since its founding in 2003. This group has been instrumental in bringing recognition to the University of Colorado as a leader in unmanned aircraft systems (UAS) research. I have been PI on the majority of AUGNet funding and collectively have been PI or Co-PI (with Prof. Argrow, Prof. Frew, and Prof. Gates) on $2.1M of AUGNet related funding since 2003. The group and its activities are the foundation of the Research and Engineering Center in Unmanned Vehicles (RECUV) founded in 2004. Equal-share collaboration in major funded programs from NSF (3 projects, $1.5M), AFOSR (1 project, $450k), DARPA (1 project $580k), AF Materiel Command (3 projects through L3- Comcept, $1.3M), and FAA (2 projects, $274k). Sole investigator on 9 other projects totaling $295k. Pioneering work in the area of airborne networks and cognitive radios. Supervision of graduate students since tenure, 2002 2008: 6 Ph.D. students (3 of whom have graduated), 15 M.S. thesis students (all of whom have graduated), 36 masters capstone students (9 projects, all completed). Including participating on committees and undergraduate and graduate independent study projects, I have advised or was on the committee of 111 students. Publications since tenure, 2002 2008: 2 book chapters; 4 edited books; 10 journal papers; 34 peer-reviewed full-length conference papers, 6 of which are journal equivalent. 3 I am the leading faculty author on both book chapters, 7 out of 10 journal papers, 29 out of 34 conference papers, and 6 out of 6 of the journal equivalent papers. This research statement elaborates on these accomplishments. 1 A sample of this work which appeared in print during my post tenure period includes [1 4]. 2 http://www.ietf.org/tao.html 3 Journal equivalent I define as less than 33% acceptance rate, at least 7 pages in length, at least three reviews provided back to authors, and the conference is regarded as a key place to publish in the field. 1

Wireless Networks Implementations Wireless network research is separate from standard network research because of the unique characteristics of the wireless radio environment. These include a shared media that is subject to outside noise and interference, links whose quality is difficult to predict because of node dynamics and complex radio propagation, and a surprisingly difficult to define notion of a communication link due to the broadcast nature of the communication and many physical layer parameters that can affect the likelihood any given packet will be received. Over the past several years, our research has been able to develop indoor and outdoor testbeds that enable us to test new communication models in realistic environments. Through this process we have been able to identify and solve key challenges to successful wireless networks. As our lab started on its efforts to develop expertise implementing wireless ad hoc network protocols on Linux-based laptops, it became clear that we needed a live test environment with which we could perform realistic but repeatable experiments. For the indoor environment we developed a table-top method for testing wireless networks that used RF attenuators on antennas to shrink the communication range so that meaningful multi-hop ad hoc network experiments could be performed in a room [5 7]. This worked well for static networks but was not able to generate interesting mobility. Furthermore, experimental control was compromised by the proximity of many radio sources in the engineering center. At this point we made an ambitious goal to develop a full-scale and fully-instrumented outdoor test bed that would have fixed nodes, mobile ground nodes, and, through a collaboration with the Aerospace Department (Prof. Argrow), would include radio nodes mounted in small high-speed unmanned aircraft. Such a network we denoted an Ad hoc Unmanned aircraft and Ground Network (AUGNet), 4 which we built and tested at the nearby Table Mountain National Radio Quiet Zone [8 13]. Through this test bed we learned two primary lessons. The first is that, though we can make protocols to better manage mobility [14], large outdoor networks will always require an overlay that is robust to long outages. This led us to develop one of the first delay tolerant network (DTN) implementations [15 18]. DTN is a relatively new concept (Burleigh, Hooke, and Cerf IEEE Comm. Mag. 2003) which enables end nodes to piece together partial routes over time in situations where simultaneous end-to-end connections may never exist. Our implementations solved several outstanding questions in routing and service discovery. The second lesson is that the key to good connectivity is to actively use mobility to maintain connectivity. A freely mobile node (such as a small unmanned aircraft) can move to locations to assist in network connectivity or, if necessary, physically carry data from one place to another. Others had developed notions of such controlled mobility (Li and Rus MobiCom 2000) or data ferries (Zhao and Ammar IWFTDCS 2003). Our insight is to treat the mobile node as a type of shared media with unique characteristics. We developed a theory of this media; defining optimal modes of operation and performance limits [19 26]. In October 2006 the FAA effectively shut down all UAS-based research, barring any UAS flights, even small UAS such as ours, without going through a lengthy certificate of authorization (COA) process. With Prof. Argrow and Prof. Frew, I have worked with the FAA to understand the effect of UAS in the National Airspace System (NAS). Further, Prof. Sicker and I are investigating how new networking and communication technologies can support safe UAS operations in the NAS [27 31]. Several related projects have spunoff from these activities. We are now participants in NASA s interplanetary DTN network test bed. The Aerospace department has used our networking protocols to control their UAS for the past three years [32]. We have also tested other types of radio systems [33]. This work was nationally recognized when I was invited by the National Research Council to participate as an expert on wireless networking and networked UAS for communications in a study prepared for FEMA and Congress [34 37]. AUGNet s success in synergistic and multidisciplinary research led Prof. Argrow, myself, and others to found a new center in 2004, the Research and Engineering Center in Unmanned Vehicles (RECUV). RECUV now houses AUGNet s activities as well as other related projects. I look to continue these activities. My primary research now is to understand the relationship be- 4 See augnet.colorado.edu or recuv.colorado.edu for more details. 2

tween mobility, networking, and control theory. Distributed control often treats the communication as an exogenous constraint, while conversely mobile networking often considers the mobility control as exogenous. Working with my collaborators in AUGNet, we seek to develop a unified theory. For instance a pair of unmanned aircraft may be given the task to quickly identify an unknown radio source. They can better localize by forming a wide baseline but must stay close enough in order to communicate and coordinate their search. Thus the mobility and control are constrained by the communication and vice versa. Cognitive Radios It is well documented that most radio spectrum goes unused at any given time even in dense urban areas (FCC Spectrum Policy Task Force, 2002). Unfortunately, accessing this unused spectrum is impossible because it is allocated to other users or requires expensive licenses under FCC regulations. A new radio technology, denoted cognitive radios, is well-understood by technical researchers to be able to dynamically access and share the spectrum and exploit this unused white space. However, this technical achievement is moot against the FCC regulations. Therefore, I participated in a multiprong approach to change the regulatory environment. The first hurdle was gaining consensus among technical, economic, and legal researchers on a path forward. I chaired or co-chaired several well received NTIA symposiums with this focus [38, 39]. The consensus was to focus on TV white space as the FCC was already developing notices of proposed rule making in this area. A second hurdle was that broadcaster incumbents argue vigorously that any new radios will cause harmful interference. A careful analysis showed that harmful interference to TVs was very unlikely [40,41]. Wireless microphones also operate in the TV band and it was argued that new radios would cause harmful interference to them. A further study showed that this interference could be bounded [42, 43]. Technical papers and conferences are not enough in this area. Researchers must also engage in the regulatory process. For several regulatory proceedings, I submitted technical reply comments on the white spaces topic to the FCC and NTIA [44 46]. It became clear through this process that the public debate suffered from a lack of definition of what harmful interference even means. I joined an IEEE standardization process to clarify this point and contributed the harmful interference taxonomy that is central to the resulting standard [47, 48]. The economics of spectrum sharing has been studied but often with abstract technical models or a narrow model of how the spectrum would be licensed. I (with Prof. Sicker) have examined economic models with more technical fidelity to provide insights into the economic benefits of sharing [49, 50] and the conditions when cognitive radios would be economically viable [51, 52]. I also (with Profs. Sicker, Hatfield, Lookabaugh, and Weiser) contributed to the DARPA next generation radio project. This project focused on military spectrum and had the potential to sidestep the commercial spectrum concerns. We advised DARPA on strategies and policies for white space usage by secondary users [53]. The radios that use white spaces have new capabilities such as sensitive detectors of existing primary users. These capabilities open up potential denial of service attacks. Based on an early paper on DoS [54] we carefully explored the vulnerabilities of cognitive radios [55 58]. A major outcome of this analysis is the relative vulnerability of different cognitive radio architectures. This analysis is being used by the IEEE 802.22 standards group concerned with cognitive radios [59]. Since the standard is a derivative of the IEEE 802.16 wireless standard we also carefully analyzed the DoS vulnerabilities of this standard [60]. Collectively, this cognitive radio work covers a broad spectrum of research areas and styles. It was recognized at CU with a Provost Faculty Achievement Award in 2007. The work has smoothed many obstacles to cognitive radio deployments. However, further work is needed. 3

References [1] T. X Brown, Switch Packet Arbitration via Queue Learning, Advances in Neural Information Processing Systems 14, ed. T.G. Dietterich et al., MIT Press, 2002. [2] H. Tong 5, T. X Brown, Reinforcement Learning for Call Admission Control and Routing under Quality of Service Constraints in Multimedia Networks, Machine Learning, v. 49, n. 2/3, Nov./Dec. 2002, pp. 111 140. [3] T. X Brown, H. Gabow, Future Information in Input Queueing, Computer Networks, v. 42, n. 4, 15 July 2003, pp. 441 460. [4] T. X Brown, How can anyone afford mobile wireless mass media content?, in J. Groebel, E. Noam, V. Feldmann ed. Mass media content for mobile wireless communication, pp. 3 19. Erlbaum, 2006. [5] S. Sanghani, T. X Brown, S. Bhandare, S. Doshi, Tabletop Emulation of Mobile Ad Hoc Wireless Networks, in Proc. Wireless Communication Networking Conference, IEEE, 2003. [6] S. Doshi, S. Bhandare, T. X Brown, An On-demand Minimum Energy Routing Protocol for a Wireless Ad Hoc Network, Mobile Computing and Communications Review, v. 6, n. 3, July 2002, pp. 50 66. [7] S. Bhandare, S. Doshi, S. Sanghani, T. X Brown, Comparison of two wireless ad hoc routing protocols on a hardware test-bed, in Proc. Wireless Communication Networking Conference, IEEE, 2003. [8] T. X Brown, S. Doshi, S. Jadhav, J. Himmelstein, Test Bed for a Wireless Network on Small UAVs, in Proc. AIAA 3rd Unmanned Unlimited Technical Conference, Chicago, IL, 20 23 Sep 2004. [9] T. X Brown, B. Argrow, S. Doshi, R.-G. Thekkekunnel, D. Henkel, Ad Hoc UAV Ground Network (AUGNet), in Proc. AIAA 3rd Unmanned Unlimited Technical Conference, Chicago, IL, 20 23 Sep 2004. [10] T. X Brown, G. Argrow, H. Gates, Wireless Communication Test Bed Final Report, to L3-Comcept, Nov. 29, 2004. 192 p. [11] S. Jadhav, T. X Brown, S. Doshi, D. Henkel, R.-G. Thekkekunnel, Lessons Learned Constructing A Wireless Ad Hoc Network Test Bed, in Proc. of the Wireless Network Measurement Workshop, Trentino, Italy, 3 Apr. 2005. [12] T. X Brown, S. Doshi, S. Jadhav, D. Henkel, R.-G. Thekkekunnel, A Full-Scale Wireless Ad Hoc Network Test Bed, in Proc. of International Symposium on Advanced Radio Technologies, Boulder, CO, March 1 3, 2005. [13] T. Brown, B. Argrow, E. Frew, C. Dixon, D. Henkel, J. Elston, H. Gates Experiments Using Small Unmanned Aircraft to Augment a Mobile Ad Hoc Network, in B. Bing ed. Emerging Technologies in Wireless LANs: Theory, Design, and Deployment, pp. 695 718, 2008 [14] Y. Zhang, T. X Brown, Aisle Routing for Mobile Ad Hoc Networks, in Proc. IEEE Wireless Communications and Networking Conference (WCNC), Las Vegas, Mar. 31 Apr. 3 2008 6pp. [15] D. Henkel, C. Dixon, J. Elston, T. X Brown, A Reliable Sensor Data Collection Network Using Unmanned Aircraft, in Proc. Second International Workshop on Multi-hop Ad Hoc Networks: from theory to reality (REALMAN), Florence, May 26, 2006. [16] A. Jenkins, D. Henkel, T. X Brown, Sensor Data Collection Through Gateways in a Highly Mobile Mesh Network, in Proc. IEEE Wireless Communications and Networking Conference (WCNC), Hong Kong, March 2007. [17] A. Jenkins, D. Henkel, T. X Brown, Sensor Data Collection through Unmanned Aircraft Gateways, Proc. AIAA Infotech Aerospace Conference, May 7 10, 2007. 8pp. [18] A. Jenkins, D. Henkel, T. X Brown, Reliable Data Collection in Challenged Networks using Unmanned Aircraft, in Proc. ACM MobiCom workshop on Challenged Networks (CHANTS), Montreal, Aug. 17 2007. 3pp. [19] E. W. Frew, C. Dixon, B. M. Argrow, T. X Brown, Radio Source Localization by a Cooperating UAV Team, Proc. AIAA Infotech@Aerospace Technical Conference, Arlington, VA, Se6t. 26 29, 2005. [20] T. X Brown, D. Henkel, On Controlled Node Mobility in Delay-Tolerant Networks of Unmanned Aerial Vehicles, in Proc. of International Symposium on Advanced Radio Technologies (ISART), Boulder, CO, March 7 9, 2006. [21] E. W. Frew, T. X Brown, C. Dixon, D. Henkel, Establishment and Maintenance of a Delay Tolerant Network through Decentralized Mobility Control, in Proc. IEEE International Conference On Networking, Sensing and Control, Ft Lauderdale, FL, Apr. 23 25, 2006. p. 584 9. [22] C. Dixon, D. Henkel, E. W. Frew, T. X Brown, Phase Transitions for Controlled Mobility in Wireless Ad Hoc Networks, in Proc. AIAA Conference on Guidance Navigation and Control, Keystone, CO, Aug., 2006. p. 3346 3356. [23] D. Henkel, T. X Brown, Route Design for UA-based Data Ferries in Delay Tolerant Wireless Networks, Proc. AIAA Infotech Aerospace Conference, May 7 10, 2007. 9pp. 5 The names of student authors are underlined. 4

[24] D. Henkel, T. X Brown, Optimizing the Use of Relays for Link Establishment in Wireless Networks, in Proc. IEEE Wireless Communications and Networking Conference (WCNC), Hong Kong, March 2007. [25] D. Henkel, T. X Brown, Delay-Tolerant Communication using Mobile Robotic Helper Nodes, Proc. The First Workshop on Wireless Multihop Communications in Networked Robotics, Berlin, Germany, Apr. 4, 2008. [26] D. Henkel, T. X Brown, Towards Autonomous Data Ferry Route Design through Reinforcement Learning, Proc. Autonomic and Opportunistic Communications Workshop, Newport Beach, Jun. 23, 2008, 6pp. [27] T. X Brown, The Role of Communication in UAS in the Next 10 Years, First Community Symposium dedicated to Civilian Applications of Unmanned Aircraft Systems (CAUAS), Boulder, CO, Oct. 2, 2007. [28] T. X Brown, Radio Technologies for UAS Communication, NSF/AUVS/FAA/DHS Workshop on UAS Research Directions for the National Air Space, June 10, 2008. [29] E. W. Frew, T. X Brown, Airborne Communication Networks for Small Unmanned Aircraft Systems, Proc. International Symposium on Unmanned Aerial Vehicles (UAV O8), Orlando, Jun. 23 25, 2008. [30] E. W. Frew, T. X Brown, Networking Issues for Small Unmanned Aircraft Systems, Journal of Intelligent and Robotic Systems, to appear, published online June 2008, 17 p. [31] E. Frew, T. X Brown, Airborne Communication Networks for Small Unmanned Aircraft Systems, IEEE Proceedings, to appear. [32] E. W. Frew, C. Dixon, J. Elston, B. Argrow, and T. X Brown. Networked Communication, Command, and Control of an Unmanned Aircraft System, AIAA Journal of Aerospace Computing, Information, and Communication, v. 5, n. 4, April 2008, pp. 84 107. [33] B. Hartzog, T. X Brown, WiMax - Potential Commercial-Off-The-Shelf (COTS) Solution for Tactical Mobile Mesh Communications, in Proc. Military Communications Conference (MILCOM), Washington, Oct. 23 25, 2006. [34] R. R. Rao, Y. Arens, A. Botterell, T. X Brown, J. R. Harrald, R. Howard, N. Jesuale, D. Kehrlein, W. Maheu, R. R. Murphy, R. Neches, M. Shinozuka, P. Steenkiste, G. Wiederhold, ed. J. Eisenberg, T. Schmitt, Summary of a Workshop on Using Information Technology to Enhance Disaster Management, National Academies Press, 2005. 39 p. [35] R. R. Rao, Y. Arens, A. Botterell, T. X Brown, J. R. Harrald, R. Howard, N. Jesuale, D. Kehrlein, W. Maheu, R. R. Murphy, R. Neches, M. Shinozuka, P. Steenkiste, G. Wiederhold, ed. J. Eisenberg, T. Schmitt, Improving Disaster Management: The Role of IT in Mitigation, Preparedness, Response, and Recovery, National Academies Press. 2007. 176p. [36] T. X Brown, Wireless Networking Performance Axis for Disaster Management, Emergency Communications and Disaster Management Workshop,, Honolulu, HI, Jan. 13, 2008. [37] T. X Brown, Wireless Networking for Disaster Management, GEOSS Workshop on Advanced Communications and Networking Technologies for Disaster Management, Honolulu, HI, Jan. 12, 2008. [38] J. W. Allen, T. X Brown, eds. Proc. of the International Symposium on Advanced Radio Technologies, NTIA Special Publication SP-03-401, Springfield, VA, 2003. 125 p. [39] J. W. Allen, T. X Brown, D. C. Sicker, and J. Ratzloff, eds. Proc. of the International Symposium on Advanced Radio Technologies, NTIA Special Publication SP-04-409, Springfield, VA, 2004. 135 p. [40] T. X Brown, An Analysis of Licensed Channel Avoidance Strategies for Unlicensed Devices, in Proc. First IEEE Int. Symposium on New Frontiers in Dynamic Spectrum Access Networks (DySPAN), Baltimore, MD, Nov. 8 11, 2005. [41] T. X Brown, A Harmful Interference Model for Unlicensed Device Operation in Licensed Service Bands, J. of Communications, v. 1, n. 1, April 2006. pp. 13 25. [42] R. Dhillon, T. X Brown, Modeling and Analysis of the Interference Potential of Cognitive Radio Devices to Wireless Microphones Operating in TV bands, International Symposium on Advanced Radio Technologies (ISART), Boulder, Jun. 2 4, 2008. [43] R. Dhillon, T. X Brown, Models for Analyzing Cognitive Radio Interference to Wireless Microphones in TV Bands, IEEE Dynamic Spectrum Access Networks (DySpAN), Chicago, Oct. 14 17, 2008. [44] T. X Brown, Enabling and Evaluating Unlicensed Operation in the TV Broadcast Bands, reply comments In the matter of Unlicensed Operation in the TV Broadcast Bands, Federal Communications Commission Docket No. 04-186, Jan. 29, 2005. 17 p. [45] P. Kolodzy, A. Lippman, R. Broderson, T. X. Brown, M. A. Sirbu, D. G. Sweeney, J.H. Snider, M. Calabrese, H. Feld, A. J. Schwartzman, Technical Reply Comments, In the Matter of Unlicensed Operation in the TV Broadcast Bands, Federal Communications Commission Docket No. 04-186 Jan. 29, 2005. 20 p. [46] T. X Brown, A Model for Analyzing Unlicensed Device Operation in Licensed Broadcast Service Bands, In the Matter of Implementation and Administration of a Coupon Program for Digital-to-Analog Converter Boxes, National Telecommunications and Information Administration, Docket No. 06051 21 29-61 29-01, Oct. 31, 2006. 7 p. 5

[47] T. X Brown, Harmful Interference Models for Unlicensed Devices in Licensed Bands, Presented at the IEEE P1900 Standards Meeting Boulder, CO Jan. 25, 2006. [48] H. S. Berger, K. R. Kontson, A. E. Leu, T. X Brown, F. Frantz, D. M. Parker, V. Prasad, D. Stewart, IEEE Recommended Practice for the Analysis of In-Band and Adjacent Band Interference and Coexistence Between Radio Systems, IEEE Standard, P1900.2-2008, 29 July, 2008. 106 p. [49] Q. Wang, T. X Brown, Pricing versus admission control in multi-class loss networks, in Proc. 40th annual Conference on Information on Sciences and Systems, Princeton, NJ, March, 2006. [50] Q. Wang, T. X Brown, Public Safety and Commercial Spectrum Sharing via Network Pricing and Admission Control, IEEE J. on Selected Areas in Communication, v. 25, n. 25. April 2007, pp. 622 632. [51] T. X Brown, D. Sicker, Can Cognitive Radio Support Broadband Wireless Access?, IEEE Dynamic Spectrum Access Networks (DySpAN), Dublin, April 17 20, 2007 [52] T. X Brown, D. C. Sicker, Examining the Viability of Broadband Wireless Access under Alternative Licensing Models in the TV Broadcast Bands, EURASIP Journal on Wireless Communications and Networking, vol. 2008, Article ID 470571, Jan. 2008. 12 p. [53] J. B. Bernthal, T. X Brown, D. N. Hatfield, D. C. Sicker, P. A. Tenhula, P. J. Weiser, Trends and Precedents favoring Regulatory Embrace of Smart Radio Technologies, IEEE Dynamic Spectrum Access Networks (DySpAN), Dublin, April 17 20, 2007 [54] T. X Brown, J. E. James, A. Sethi, Jamming and Sensing of Encrypted Wireless Ad Hoc Networks, in Proc. Seventh ACM International Symposium on Mobile Ad Hoc Networking and Computing (MobiHoc), Florence, 22 25 May 2006. [55] T. X Brown, A. Sethi, Potential Cognitive Radio Denial-of-Service Vulnerabilities and Countermeasures, in Proc. Int. Symposium on Advanced Radio Technologies (ISART), Boulder, Feb. 2007. [56] T. X Brown, A. Sethi, Potential Cognitive Radio Denial-of-Service Vulnerabilities And Protection Countermeasures: A Multi-dimensional Analysis/Assessment, Proc. Second Int. Conf. on Cognitive Radio Oriented Wireless Networks and Communications (CrownCom), Orlando, FL, Aug 1 3, 2007. 10pp. [57] T. X Brown, A. Sethi, Potential Cognitive Radio Denial-of-Service Vulnerabilities and Protection Countermeasures: a Multi-dimensional Analysis and Assessment, Journal of Mobile Networks and Applications, to appear, published online July 2008, 17 p. [58] A. Sethi, T. X Brown, Hammer Model Threat Assessment of Cognitive Radio Denial of Service Attacks, IEEE Dynamic Spectrum Access Networks (DySpAN), Chicago, Oct. 14 17, 2008. [59] T. X Brown, Threat Assessment to Primary and Secondary Users in a Centralized Cognitive Radio Network, IEEE P802.22 Wireless RANs Plenary Meeting, Denver, July 17, 2008. [60] S. Maru, T. X Brown, Denial of Service Vulnerabilities in the 802.16 Protocol, Proc. of the Wireless Internet Conference (WICON), Maui, HI, Nov. 17 19, 2008. 6