Dr. Dai will serve as Program Director for the Communications, Circuits, and Sensing-Systems Program within the Division of Electrical, Communications and Cyber Systems, Directorate for Engineering at the National Science Foundation. The assignee's managerial and technical background will enhance the management, operation, and evaluation of programs within the Division. The assignee will gain useful experience in the development, coordination and management of large and complex Federal programs. The National Science Foundation will benefit from the academic and scientific insights of the assignee. NC State will benefit from the knowledge and experience the assignee will gain in the program and policy-level matters and issues involving these sciences at the Federal level.

The use of Radio Controlled (RC) Unmanned Aerial Vehicles (UAVs) or Drones, for civilian and commercial purposes have been growing steadily for the past decade. Their non-military use range from responding to Hurricane Harvey to rescuing swimmers caught in 10-ft swells, and from pizza delivery to Prime Air, the Amazon drone delivery system. UAV������������������s have historically had a limited flight radius dictated by line-of-sight radio controllers; however, distance limitation from which the drone operator can control them can be bypassed by using commercial wireless networks to control the drone. It is expected that billions of 5G mobile and Internet-of-Things (IoT) wireless devices will be all connected with the new millimeter wave frequency bands. We propose to analyze and validate the hypotheses that 1) using mmWave with antennas tilted upward for RF coverage in the sky can lead to a secure and reliable wireless network for UAV/Drone operation, 2) there is a unique opportunity to use NOMA (Non Orthogonal Multiple Access) with the primary mmWave beam aimed towards a swarm of Drones to control them with high security and efficiency, 3) further increase security, reliability, and spectral efficiency by using co-operative communications among the swarm of Drones and 4) this swarm of drones can be effectively used for emergency recovery of critical infrastructure such as a cyber compromised power grid that requires a black restart.

In many existing and emerging large-scale networks, an important application is to spread the information quickly and efficiently over the network. Over the past decade, this topic has received great research interest, and is relatively well studied for static networks. In contrast, our knowledge is far from complete when the network structures change over time, which is typical due to various reasons including environment changes, device and user mobility, variation of social relationship, and growth of the networks. There have been extensive studies on protocol and algorithm development in the area of mobile wireless networks, but many of them resort to simulation and experimentation with synthetic and real-world mobility traces; a general analytical framework is lacking. In this project, built on our promising preliminary results, we intend to work towards a unified analytical framework for mobile networks that can address various types of mobility patterns and handle both connected and delay-tolerant networks. We also plan to extend our study to mobile social networks, which possess some unique features for information spreading that deserve separate and in-depth considerations. As emerging networks are complex and exhibiting unpredictable dynamics, random-walk based algorithms become an appealing architectural solution for them. A pertinent question is whether we can further improve the efficiency of these algorithms while maintain their simplicity and robustness. Our preliminary results indicate that, by exploiting some additional information which may readily be available, a speedup by an order of magnitude is potentially achievable. Underlying our efficient algorithms is a design framework based on non-reversible Markov chains. In the second research thrust, we plan to deepen our study on this design framework, and further extend its underlying principle to the study in mobile social networks. The proposed research will be assessed through a comprehensive evaluation plan.

Cognitive radio (CR) is emerging as a key enabling technology to address the ever increasing demands on the scarce spectrum for wireless communications. While wireless networks are prone to security attacks, CR networks are even more vulnerable due to improved intelligence available at attackers or compromised devices, and additional constraints imposed on CR users. This interdisciplinary proposal aims at making contributions in the general area of security of wireless signals and systems in the context of spectrum sharing, to facilitate the realization of the national spectrum objectives in the years to come. In this project, instead of adding contributions to existing literature on software and wireless network security, we will focus on the vulnerabilities and attacks unique to CR functionalities, and advocate a cross-layer viewpoint for both attacks and defenses.

Wireless security is receiving increasing attention as wireless systems become a key component in our daily life as well as critical cyber-physical systems. Recent progress in this area exploits physical layer characteristics to offer enhanced and sometimes the only available security mechanisms. The success of such security mechanisms depends crucially on the correct modeling of underlying wireless propagation. It is widely accepted that wireless channels decorrelate fast over space, and half a wavelength is the key distance metric used in existing wireless physical layer security mechanisms for security assurance. We believe that this channel correlation model is incorrect in general: it leads to wrong hypothesis about the inference capability of a passive adversary and results in false sense of security, which will expose the legitimate systems to severe threats with little awareness. In this project, we seek to understand the fundamental limits in passive inference of wireless channel characteristics, and further advance our knowledge and practice in wireless security.

Jamming resistance is crucial for applications where reliable wireless communication is required, such as rescue missions and military applications. Spread spectrum techniques such as Frequency Hopping (FH) and Direct Sequence Spread Spectrum (DSSS) have been used as countermeasures against jamming attacks. However, these anti-jamming techniques require that senders and receivers share a secret key to communicate with each other, and thus are vulnerable to insider attacks where the adversary has access to the secret key. The objective of this project is to develop a suite of techniques to defend against insider jammers in DSSS and FH based wireless communication systems. We will develop novel and efficient insider-jamming-resistant techniques for both DSSS- and FH-based wireless communication systems. Our proposed research consists of two thrusts. The first thrust is to develop novel spreading/despreading techniques, called DSD-DSSS (which stands for DSSS based on Delayed Seed Disclosure), to enhance DSSS-based wireless communication to defend against insider jamming threats, while the second thrust is to develop a new approach, called USD-FH (which stands for FH based on Uncoordinated Seed Disclosure), to enable sender and receivers using FH to communicate without pre-establishing any common secret hopping pattern. A key property of our new approaches is that they do not depend on any secret shared by the sender and receivers. Our solution has the potential to significantly enhance the anti-jamming capability of today?s wireless communication systems.

This work intends to contribute to an automatic reasoning framework for networked systems through research in two areas: structured variational methods and their distributed implementation, and distributed clustering. Interaction and integration of these two components will also be explored, leading to a holistic cross-layer approach for automatic reasoning in networked systems.

Intellectual Merit: Quickest detection is an important technique to detect the change of probability distribution in a random process being monitored. It is widely used in problems like financial decision making, environmental monitoring and industrial quality control. With the rapid development of networking techniques, there exist pressing demands to carry out quickest detection based on observations from many nodes and make decision at more than one nodes. Motivated by this demand, we propose to study collaborative quickest detection in ad hoc networks, in which nodes exchange observation statistics and make local decisions about distribution change. In contrast to existing theory of decentralized quickest detection, our proposed scheme does not need a data processing center, thus avoiding the round-trip time overhead and possible data congestion. Moreover, collaboration can enhance the agility and robustness of the detection of change. An important application of collaborative quickest detection is spectrum sensing in cognitive radio systems. In such a system, secondary nodes need to monitor the activity of primary users, and should quit the frequency band once primary users emerge. It is essentially a problem of quickest detection since the secondary nodes need to detect the change as quickly as possible..Our proposed research can substantially reduce the response time of secondary nodes and decrease false alarms. Our proposed research on collaborative quickest detection comprises the following four thrusts: 1. Aspect of statistical signal processing: we plan to study the rules of change detection when observations from different collaborators have different delays; we also plan to use Skorokhod embedding to study the performance of quickest detection. 2. Aspect of communication and information theory: we plan to study source coding for exchanged information as well as the corresponding communication complexity. 3. Aspect of networking: we plan to study the scheduling of broadcast for collaborative quickest detection in wireless networks, as well as the topology control for information exchange. 4. Aspect of application in cognitive radio: we plant to study monitoring the change of primary radio users in single or multiple frequency bands. Broader Impact: The proposed research will contribute fundamental concepts and analytical tools to the new arena of collaborative quickest detection. It also provides new methodologies and techniques for fields like signal processing, communication theory and wireless networking. We also plan to apply the results of our proposed work in collaborative projects with Oak Ridge National Lab (ORNL). The inter-disciplinary essence of our proposed research also lends itself to cross-disciplinary education. We plan to devise a one-semester graduate level lecture introducing quickest detection, cooperative communication and cognitive radios. Besides involving graduate students working toward master and doctoral degrees, this project also expects to attract traditionally underrepresented groups, particularly through the collaboration with the UTK chapter of the National Society of Black Engineers (NSBE).

Intellectual Merit: Wireless sensor network is taking an increasingly important role in our life, for which collaboration among sensor nodes is crucial for its success. In anticipated applications, a centralized solution is either not available or infeasible due to resource constraint and application demand. Therefore, cooperative schemes that are distributed, self-organized, scalable, and energy-efficient, are much desired for sensor networks. This project proposes to employ belief propagation (BP) in wireless sensor networks, to provide a systematic and yet flexible framework to facilitate in-network cooperative processing. Belief propagation is a computing algorithm operating on graphical models, while in sensor networks there is a communication graph reflecting connectivity topology. We are interested in the scenario when the computing graph meets the communication graph. On the one hand, belief propagation facilitates distributed computing and inference in sensor networks. On the other hand, the application of belief propagation in wireless sensor networks is subject to severe communication constraints. On addressing this interaction, the fact that sensor networks are application driven brings a new angle into research. Our proposed research comprises the following three main thrusts. 1) Convergence and correctness of the BP algorithm on general graphs, a challenging problem of high impact on its own, will be studied in the context of specific applications. The connection between BP fixed points and stationary points of some constrained minimization problems will also be pursued, and protocol designs will be jointly considered with theoretical study. 2) The influence of communication constraints will be explored with respect to message representation, message error and message scheduling, culminating in a comprehensive study on the tradeoffs among energy efficiency, accuracy, computational complexity, and delay. 3) The synergy of generalized belief propagation (GBP) with sensor networks, an almost brand-new area, will be explored. We will particularly study efficient methods of region partitioning for GBP, which is still more an art than a science. We also propose to study hybrid structures which can combine the advantages of in-network processing and data fusion. Broader Impacts: Though this proposal targets wireless sensor networks, the proposed framework and fundamental research apply largely to general ad hoc networks as well. They can even be extended to virtual scenarios where a set of ?sensors? distributed over the Internet cooperate on a joint task through information exchange. If we think of wireless networks as a new kind of computer systems, belief propagation can serve as an effective programming language for them. The proposed work lies in the interface of networking, communications, and computing, heavily replying on the knowledge in information theory, communication theory, Bayesian inference, graph theory and models, and communication/computation/information complexity. It has the potential to advance the theory and practice of these areas, and contribute to the evolvement of next generation wireless networks. The PI will seek to incorporate material inspired by this work (at an appropriate level) into the undergraduate and graduate curricula at North Carolina State University. Various channels will be utilized to disseminate research findings to industry and the broader public.

The demand for broadband wireless access has developed to such a point that people are talking about Gigabit transmission rate in non-line-of-sight fading environments for next generation wireless systems, with Wireless Internet and home Audio/Video networks being important anticipated applications. MIMO wireless is viewed as one of the key technologies for this ambitious goal. Substantial efforts have been devoted to incorporate MIMO technologies into emerging standards, including the third generation cellular UMTS/HSDPA standard under the International Telecommunications Union working group, IEEE 802.11n for next generation wireless local area network (WLAN), and IEEE 802.16 for outdoor fixed/nomadic wireless wide area network (WWAN). MIMO technology was first investigated for isolated single-user scenarios, and much has been known owing to extensive study in the past few years. In contrast, multiuser MIMO communications involves much more challenges and is much less well understood. In this proposal, we will differentiate two different scenarios, point-to-multipoint (equivalently multipoint-to-point) and general multiuser network. The former includes single-cell multiuser MIMO downlink, the reach back from sensor fields to the data collector, and multicell systems with cooperative base stations or access points. The latter includes non-cooperative multicell communications and multi-pair communications in ad hoc networks. However, our focus is on fundamental concepts and general scientific tools, some of which may also be applicable to other similar systems including DSL downlink and chip interconnections in high speed circuits. The field of multiuser MIMO communications is in general still in its infancy, as is evidenced by many open problems in multiuser information theory. Some of the critical challenges in multiuser MIMO communications are described as follows. First, multiuser interference or co-channel inference significantly limits the overall system performance. While orthogonal channelization solves the problem by converting it back to the single user scenario, scare resource and ever increasing demand calls for aggressive frequency reuse and multiple access. Indeed, recent breakthrough in the information theoretic capacity of vector multiple-access (MAC) and broadcast (BC) channels indicates the sub-optimality of TDMA-type schemes in terms of sum capacity, which was shown to be optimal for single-antenna systems. Second, multiuser communications naturally brings in network issues such as Quality of Service (QoS), delay constraint, and fairness concerns. In literature, user scheduling in the MAC layer is exploited for this purpose. However, current separate research in PHY and MAC layer hinders the effectiveness of efforts in each individual layer. Third, unlike single user MIMO, certain channel state information (CSI) at the transmit side is vital in differentiating users and realizing anticipated gains, thus raising issues on judicious feedback designs. Other major challenges include non-cooperativity of mobile users, joint power constraint at base stations or access points for point-to-multipoint communications, and larger dimension and higher complexity in computations. We propose to tackle some key research problems relevant to these challenges. Our first research thrust targets on fundamental tradeoffs in multiuser multi-antenna systems, which will provide insights and guidelines for efficient designs. We then propose to approach an efficient design for a cross-layer view point. By jointly considering the design goals and approaches in both the physical (PHY) and medium access control (MAC) layers, we intend to have both layers to cooperate rather than to compete, to jointly contribute to the ultimate system performance rather than to hinder each other. Realizing the importance of CSI at the transmit side for multiuser MIMO, we further propose to investigate our designs with realistic feedback, which might be insufficient or outdated, moving a substantial step toward the real world applications. Whi