2016 Seminars

Date: November 18, 2016

Title: Resource Allocation in Wireless Networks Under Uncertainties: A Stochastic Optimization Framework

Speaker: Dr. Mohammad J. Abdel-Rahman

Abstract: Emerging wireless networks operate using dynamic and uncertain resources that render them susceptible to severe performance degradation. Managing stochastic resources in such networks while ensuring a certain level of network performance is challenging. In this talk, I introduce stochastic optimization as a powerful tool to handle resource allocation in uncertain networks, discuss various approaches to modeling uncertainty, and explain different feasibility and optimality approaches under uncertainty. I will talk about both static and adaptive stochastic optimization approaches. In particular, I focus on (i) chance-constrained, (ii) simple recourse, and (iii) multi-stage stochastic optimization approaches. Throughout the talk, I will illustrate how to use some of the aforementioned stochastic optimization techniques to formulate example resource allocation problems in emerging wireless networks. Specifically, I will consider: (i) Joint channel and base station allocation in opportunistic LTE networks, (ii) orchestrating a robust virtual LTE-U network from hybrid half/full-duplex Wi-Fi APs, (iii) adaptive link scheduling and beam-steering in millimeter-wave networks, (iv) adaptive controller assignment in software-defined cellular networks, and (v) stochastic resource allocation with slicing in virtualized wireless access networks.

Bio: Mohammad J. Abdel-Rahman is currently a Research Associate in the Department of Electrical and Computer Engineering at Virginia Polytechnic Institute and State University. He received his PhD degree from the Electrical and Computer Engineering Department at the University of Arizona in 2014. He is the recipient of the College of Engineering 2014 Outstanding Graduate Student Award. He received his MSc degree from the Electrical Engineering Department at Jordan University of Science and Technology, Jordan, in 2010, and his BSc degree from the Communication Engineering Department at Yarmouk University, Jordan, in 2008. Dr. Abdel-Rahman’s research is in the broad area of wireless communications and networking, with particular emphasis on resource management, adaptive protocols, and security issues. He serves as a reviewer for several international conferences and journals. He is a member of the IEEE.

Date: November 11, 2016

Title: Spectrum Sharing Test and Demonstration: LTE- DoD Coexistence

Speaker: Mike Smith

Abstract: On November 13, 2014, the Federal Communications Commission (FCC) initiated the Advanced Wireless Services (AWS) - 3 Auction (“Auction 97”), to competitive bidding. Auction 97 has provided new wireless service licenses in the 1695-1710 MHz (Uplink), 1755-1780 MHz (Uplink), and 2155-2180 MHz (paired downlink) bands for AWS-3. Auction 97 generated more than $42B in revenue. As a result, many DoD communications systems are required to relocate out of these spectrum bands in accordance with established DoD Transition Plans over the next 10 years.

As part of the DoD Transition Plan for AWS-3, the Defense Information Systems Agency (DISA) Defense Spectrum Organization (DSO) proposed the development of a Spectrum Sharing Test & Demonstration (SST&D) program. The current objectives of the SST&D Program include:

• Facilitating expedited and expanded entry of commercial wireless network deployments into the 1755-1780 MHz band.
• Identify, demonstrate, and operationalize interference mitigation techniques consistent with commercial Long Term Evolution (LTE) standards that support increased sharing between LTE and incumbent DoD systems
• Prototype and demonstrate advanced sharing concepts for long-term sharing in the 1755-1780 MHz band.
   

To meet these objectives, the SSTD program has been partitioned into activities which address all aspects of the signal interference scenario between these to-be-deployed wireless networks and to-be-displaced DoD transceivers.

This presentation provides a description of the LTE Characterization activity which is focused on understanding how LTE network operations (today and in the future) establish uplink emissions under a variety of operating conditions (e.g., traffic loading; ISD; power control algorithms; resource allocation algorithms; etc.) and in the presence of interfering signals (e.g. DoD SATOPS). To do this, the team is developing LTE network models that can enhance spectrum coexistence assessments and performing LTE network testing and measurements providing real network operation statistics to inform these modeling efforts.

Bio: Mr. Smith is a Senior Systems Engineer for the Virginia Tech Applied Research Corporation (VT-ARC). Mr. Smith is an experienced Systems Engineer with functional expertise in Wireless Networking, Communication Systems, RF Design, Semiconductor Manufacturing, and Satellite Communications. He has demonstrated experience in planning, analysis, design, and delivery of communications systems for the DoD and Commercial Service Providers.

Prior to joining VT-ARC, Mr. Smith was a Systems Engineer at Harris Corporations Electronic Systems focused on developing and managing Software Defined Tactical Radios and Power Amplifiers for DoD and foreign military customers. Earlier in his 22-year Harris career, Mike focused on business development for Harris commercial satellite systems, serving as Lead Systems Engineer on several payload programs and proposals emphasizing the application of phased array antenna systems. Earlier in his career Mr. Smith also successfully led the development and implementation of semiconductor manufacturing yield management methodologies at Filtronic Solid State and Zeevo, Inc.

Mr. Smith received a B.S. degree in Electrical Engineering from Virginia Tech in 1985.

Date: November 4, 2016

Title: ETSI’s RRS Solution for MD Reconfiguration: Standard Architecture & Interfaces.

Speaker: Dr. Seungwon Choi from Hanyang University, South Korea

Abstract: This talk summarizes the standardization activities in TC(Technical Committee) RRS(Reconfigurable Radio System) of ETSI(European Telecommunications Standard Institute) for determining a standard architecture for reconfigurable mobile device and related interfaces. Firstly, we briefly introduce the standardization procedure within ETSI and documents that have been published within Working Group 2 of TC-RRS of ETSI. Then, MD reconfiguration class is discussed in order to summarize how the SDR technologies for MD are evolved in terms of the degree of reconfigurability. Standard architecture of reconfigurable MD is introduced showing each component of radio computer needed for the radio reconfiguration. Then we consider three cases of distributing the radio application code, i.e., in the form of executable code, source code and Intermediate Representation. Then, operational structure of unified radio application are shown for the three cases. Finally, we introduce the standard interfaces defined for the standard architecture of the reconfigurable MD, i.e., multiradio interface, reconfigurable RF interface, unified radio application interface and radio programming interface. The concept of radio virtual machine is also introduced of which the objective is to resolve the problem of portability between the radio application code and hardware platform whose granularity might vary depending upon different manufacturers.

Bio: Dr. Seungwon Choi received the BS degree from Hanyang University, Seoul, Korea, and the M.S. degree from Seoul National University, Korea, in 1980 and 1982, respectively, both in electronics engineering, the MS degree (computer engineering) in 1985, and the PhD degree (electrical engineering), in 1988, both from Syracuse University, Syracuse, NY, USA. Between 1988 and 1989, he served as an assistant professor in Syracuse University. He Joined Hanyang University as an assistant professor in 1992 and he is now a full professor in the Department of Electrical and Computer Engineering of Hanyang University. He was the director of HY-SDR Research Center between 2002 and 2010 during which he actively participated WINNF (Wireless Innovation Forum) as a board of director. Currently, he is the director of HY-MC Research Center in which 13 professors and 98 graduate students are engaged with researches on mobile communication-related fields including reconfigurable communication systems. Since 2011, Dr. Choi has been actively participating the standardization activities in ETSI (European Telecommunications Standards Ins.) and now he is serving as the chairman of WG2 of TC-RRS of ETSI. His research interests include digital communications and adaptive signal processing with a recent focus on the implementation of Software Defined Radio systems for both base station and mobile devices. He has published about 90 international journal papers and held nearly 50 patents.

Date: Friday, October 7, 2016

Title: Automatic Generation of Efficient Parallel Streaming Hardware.

Speaker: Thaddeus Koehn

Abstract: Digital signal processing systems demand higher computational performance and more operations per second than ever before, and this trend is not expected to end any time soon. Processing architectures must adapt in order to meet these demands. The two techniques most prevalent for achieving throughput constraints are parallel processing and stream processing. By combining these techniques, significant throughput improvement have been achieved. These preliminary results apply to specific applications, and general tools for automation are in their infancy. In this presentation techniques are developed to automatically generate efficient parallel streaming hardware architectures.

Bio: Thaddeus Koehn is a Ph.D candidate at Virginia Tech as well as a Senior Research Associate at the Virginia Tech Ted and Karyn Hume Center. His advisor is Prof. Peter Athanas. He received his B.S. from University of Rhode Island in 2006, and his M.S. E.E. from University of Illinois in 2009. His research interests include digital signal processing, high performance computing, computer architecture and computer-aided design.

Date: Friday, September 23, 2016

Title: Cache Aided Networks: From Practice to Theory?

Speaker: Avik Sengupta

Abstract: In this talk we will focus on recent advances in cache-aided networks which has been the topic of a vast body of recent research both in academia and industry. Cache-aided systems aim to exploit available network resources to increase efficiency in content distribution by effectively converting "memory into bandwidth". We will focus on the applications of reinforcement learning techniques and the role of optimization in caching. A number of practical system settings will be studied, which gives rise to the need for deeper understanding of theoretical limits of caching and delivery methods. To this end, we will reflect on an information theoretic perspective of caching and cache-aided interference management, highlighting recent interesting results in this paradigm.

Bio: Avik Sengupta is a Ph.D. candidate in ECE at Virginia Tech, working with Wireless@VT and the Hume Center. He received the B.Tech degree in Electronics and Communication Engineering from West Bengal University of Technology, India, in 2010 and the M.S. degree in Electrical and Computer Engineering from Kansas State University in 2012. His research interests include wireless communication systems with an emphasis on caching and content distribution in wireless networks. He has interned with Blackberry, Huawei Research and Qualcomm working on LTE PHY-layer systems design. Avik's advisor is Dr. Charles Clancy.

Date: Friday, September 16, 2016

Title: An OFDM Scheme for 3.5 GHz LTE

Speaker: Seungmo Kim

Abstract: In April 2015, the FCC voted to approve rules for the new Citizens Broadband Radio Service (CBRS) service, incorporating smaller exclusion zones and setting the stage for commercial deployment in 3,550–3,700 MHz (the 3.5 GHz band). Primary radio systems currently operating in this band are pulse radars. When sharing spectrum between a radar and a digital communications system, a problem arises: it is more likely that the communications system experiences higher interference, due to very high transmit power of a radar compared to a digital communications system. In this talk, we discuss an OFDM scheme that is more robust against interference from pulsed radar. Moreover, applying the 3.5 GHz LTE rule that has recently been finalized, we also discuss the impact of imperfect sensing by Environment Sensing Capability (ESC), the spectrum sensing capability in the 3.5 GHz LTE spectrum sharing architecture.

Bio: Seungmo Kim received his B.S. and M.S. from the Korea Advanced Institute of Science and Technology (KAIST) in 2006 and 2008, respectively. He worked for Samsung in 2008 through 2012 on development of embedded software of MAC and NWK protocol stack for IEEE 802.15.4 and ZigBee. He is currently a Ph.D. candidate in the Dept. of Electrical and Computer Engineering at Virginia Tech. His research interests include spectrum sharing, 3GPP, 5G, and millimeter wave bands. Mo's advisor is Dr. Carl Dietrich.

Date: Friday, April 29, 2016

Title: Information theoretic and control theoretic problems motivated by energy harvesting communications

Speaker: Venkat Anantharam, EECS, University of California, Berkeley

Abstract: The Internet of Things is expected to lead to an increase by orders of magnitude in the number of networked devices. Many of these devices, such as sensors, are expected to be energy limited.Partly to avoid constant maintenance of these devices and also because of its increasing technological feasibility, there is a growing interest in the development of energy harvesting technologies, where the energy to power a device or to augment its existing capabilites is derived from the environment from solar, mechanical, or other means. An energy harvesting device typically will have a battery of limited capacity, which can store harvested energy to smooth out the fluctutations in the energy harvesting process.

The introduction of energy harvesting aspects in devices poses many interesting new challenges for the analysis and design of communication and control systems. Depending on the time scale of variations in energy harvesting relative to the time scale of the dynamics of the systems being monitored, both information theoretic and control theoretic models are of interest.

As an example, the study of the Shannon capacity of communication with an energy harvesting transmitter requires an understanding of how to design transmission codebooks subject to the requirement that each transmitted codeword meets the available energy budget at every moment during the course of its transmission. We will discuss our recent results on problems of this flavor.

From a mathematical point of view, the problem is one of high dimensional geometry, where the high dimensional unit balls of traditional Shannon theory are replaced by certain high dimensional polytopes defined by the real time nature of the constraints on the available energy. A remarkable parallel between convex sets in high dimensions and large deviations theory emerges in this regime. This is joint work with Varun Jog.

At the other extreme of time scales, packet drop models are more appropriate to study the overall system design. We will discuss how the theory of estimation of state over packet drop channels needs to be modified when the sensing infrastructure that communicates its observations to a remote estimator is of an energy harvesting type. This is joint work with Omur Ozel.

Bio: Venkat Anantharam is on the faculty of the EECS Department at the University of California, Berkeley. His research interests include coding theory, communication networks, information theory,and stochastic control.

He received the Philips India Medal and the President of India Gold Medal from IIT Madras in 1980 and an NSF Presidential Young Investigator award in 1988.He is a corecipient of the 1998 Prize Paper Award of the IEEE Information Theory Society, and a corecipient of the 2000 Stephen O. Rice Prize Paper Award of the IEEE Communications Theory Society. He received the Distinguished Alumnus Award from IIT Madras in 2008.

Student Panel on 5G

Date: Friday, April 22, 2016

Time: 2:45 pm - 3:45 pm

About the Student Seminar: 5G wireless is a wide research area, as some of the goals of 5G are to simultaneously increase network speeds, connection density and spectral efficiency while also significantly reducing latency and energy consumption of wireless devices. 5G wireless communications are also expected to occur throughout the spectrum at frequencies from 1 to 100 GHz.

At the panel, you’ll have the opportunity to hear your fellow student’s thoughts on the directions for 5G technology and contribute your questions and thoughts to the discussion, too.

Student Panelists

Avik Sengupta is a Ph.D. candidate in the Department of Electrical and Computer Engineering at Virginia Tech, working with Wireless@VT and the Hume Center for National Security and Technology. He received the B.Tech degree in Electronics and Communication Engineering from West Bengal University of Technology, Kolkata, India, in 2010 and the M.S. degree in Electrical and Computer Engineering from Kansas State University, Manhattan KS, USA in 2012. His research interests include design and information theoretic modeling of wireless communication systems with an emphasis on content distribution and caching in 5G wireless networks. He was a research intern with the Advanced Technology Group at Blackberry and with the Wireless Access Lab at Futurewei Technologies (Huawei R&D).

Daniel Jakubisin received his BSEE from UNC Charlotte in 2006 and his MSEE from Virginia Tech in 2012. He is currently a PhD candidate under the supervision of Dr. Michael Buehrer within the Wireless @ Virginia Tech research group. His research is in applying iterative probabilistic processing to the design of communications receivers with a focus on multi-user detection and co-channel interference. His interests in 5G include the PHY/MAC developments taking place and their impact on receiver design.

Priyabrata Parida joined Wireless@VT as a PhD student during Fall 2015 after graduating with MS degree in Telecommunication from Indian Institute of Technology, Kharagpur, India. He conducts his research activities under the supervision of Dr. Harpreet Singh Dhillon. His research interest includes MIMO, stochastic geometry, and optimization of wireless communication systems. In the past, he has worked as an Assistant Manager as a part of 3G network operation and maintenance team for Ideal Cellular, a leading telecom provider in India.

Abid Ullah is currently working toward the Ph.D. degree in electrical engineering at Virginia Polytechnic Institute and State University. His research interests include multi-antenna resource allocation for spectrum management, and he is advised by Dr. Jeffrey H. Reed.

Date: April 8, 2016

Speaker: Dr. Aylin Yener

Title: Towards Design Principles of Next Generation Networked Systems: Information Security and Energy Sustainability 

Abstract: Next generation networked systems are expected to have several salient features that require new paradigms in related systems research areas. These systems are expected to contain a large number of connected nodes of various heterogeneous capabilities. They are expected to be dynamic, self-organizing networks that operate in the wireless medium. Operating in a vulnerable medium with many others of various trust levels, they call for new security measures, that are low overhead and scalable, yet powerful in their guarantees. Similarly, they are expected to be energy self-sufficient with extended life times. In this talk, we will focus on two system-level directions that are promising in each of these concerns: physical-layer security and energy harvesting communication networks.

Physical layer security aims to provide secure communication guarantees built in to the network design at its foundation. Having its roots in existence results in information theory and incubated in the information theory community for close to a decade, wireless physical layer security research has grown to be a vibrant area in communications, information theory, coding theory and signal processing for communications. In this talk, we will provide an overview of our results in physical layer security in the last decade, and the resulting design insights. These include insights related to inducing judicious interference for confidentiality, i.e., cooperative jamming, structured signaling, multi-antenna signaling strategies and merits of bidirectional transmission. We will then overview some of the more recent advancements towards relaxing some of the idealized assumptions in earlier problem constructs, notably those related to the channel state information, as well as research efforts towards getting close to practical system design.

Energy harvesting communications offers the possibility of perpetual network operation without adverse effects on the environment and will lead to the green future of wireless. Energy harvesting brings new considerations, most notably, intermittency of available energy and its temporary storage. Communication and network theoretic design of such networks is an emerging area. In this talk, we will present an overview of our results in this emerging area, and the ensuing algorithmic design insights. The focus will be on transmission scheduling policies that amortize the energy expenditure for optimum system operation, highlighting the trade-offs arising from practical considerations in these systems, notably the role of the size and the storage efficiency of the battery. We will also briefly mention the new information theoretic capacity problems that arise in this setting.

Time permitting, we will present an additional important direction relevant in these cyber-physical networked system, namely the role of humans and their social interactions.

Bio: Aylin Yener is a professor of Electrical Engineering at The Pennsylvania State University, University Park, PA since 2010, where she joined the faculty as an assistant professor in 2002. During the academic year 2008-2009, she was a Visiting Associate Professor with the Department of Electrical Engineering, Stanford University, CA. Her research interests are in information theory, communication theory and network science with recent emphasis on green communications, information security and networked systems.  She received the NSF CAREER award in 2003, the best paper award in Communication Theory in the IEEE International Conference on Communications in 2010, the Penn State Engineering Alumni Society (PSEAS) Outstanding Research Award in 2010, the IEEE Marconi Prize paper award in 2014, the PSEAS Premier Research Award in 2014, and the Leonard A. Doggett Award for Outstanding Writing in Electrical Engineering at Penn State in 2014.  She is a fellow of the IEEE.

Dr. Yener is an elected member of the board of governors of the IEEE Information Theory Society for the term 2015-2017. Previously, she served on the same board of governors as the treasurer (2012-2014). She served as the student committee chair for the IEEE Information Theory Society 2007-2011, and was the co-founder of the Annual School of Information Theory in North America co-organizing the school in 2008, 2009 and 2010.  For the IEEE Communications society, she was a technical (co)-chair for various symposia/tracks at IEEE ICC, PIMRC, VTC, WCNC and Asilomar (2005-2014). She served as an editor for IEEE Transactions on Communications (2009-2012), an editor and an editorial advisory board member for IEEE Transactions on Wireless Communications (2001-2012), a guest editor for IEEE Transactions on Information Forensics and Security (2011) and a guest editor for IEEE Journal on Selected Areas in Communications (2015).  Presently, she serves as an editor for the IEEE Transactions on Mobile Computing. Additionally, she is a senior editor for the IEEE Journal on Selected Areas in Communications.

Date: February 26, 2016

Title: Moon Bounce: Hokies go to the Moon Without Leaving Blacksburg

Speaker: Dr. Dennis Sweeney

Abstract: Moon Bounce or Earth-Moon-Earth (EME) communications uses the moon as a passive reflector. A radio signal goes out 400,000 km, bounces off the moon, and returns 400,000 km. The round trip takes about 2.5 seconds. EME is an interesting footnote in the history of satellite communications and it has been taken up as a challenge by the amateur radio community. It has has been done on every amateur band from 28 MHz through 47 GHz. There is a current effort to extend this to 76 GHz.

EME is a major engineering challenge requiring high power transmitters, large antennas, and ultra low noise receivers. It has been described as the amateur radio equivalent of climbing Mt. Everest. Every 0.1 dB counts! Advanced DSP is being used to permit EME with smaller antennas and lower powered transmitters and to mitigate the effects of libration fading and Doppler shift.

This presentation will look at some of the history of EME and the engineering challenges that accompany it. It will look at the hardware currently under development to do EME at 10 GHz.

Bio: Dr. Sweeney is currently Professor of Practice and the Director of Instructional Labs (DIL) for the Electrical and Computer Engineering (ECE) Department. He is responsible for maintaining the teaching labs and managing the undergraduate lab program. Dr. Sweeney is no stranger to Virginia Tech. He got his undergraduate EE degree at Virginia Tech in 1971 and returned in the 1980's to earn his PhD. He worked for the Virginia Tech Satellite Communications Group managed by Dr. Pratt, Bostian, and Stutzman. Upon completing his PhD, Dr. Sweeney held an instructor appointment in the ECE department. He then went to the Jet Propulsion Laboratory where he worked on GPS applications. Returning in 1994, he joined Dr. Bostian in the Center for Wireless Telecommunications (CWT). After 11 years with CWT, he went to the Aerospace Corporation in northern Virginia. There he worked on a number of space related projects. The last project was a flight mission launched in January of 2011 just before he returned (again!) to Virginia Tech as the DIL.

Dr. Sweeney is an avid builder of microwave hardware and he is an active amateur radio operator. His interests are in microwave radio technology and teaching students how to build things. He and his wife Betsy, who is an equally avid quilter, live here in Blacksburg.

Date: February 19, 2016

Title: Second EARS Workshop: An Overview and Grand Challenge Forum

Speaker: Thaddeus Czauski

Abstract: Wireless connectivity is fueling the growth of Internet of Things (IoT) devices, applications and services. By one estimate there will be 40 billion active wireless devices in the world by 2020. Supporting all of these wireless devices presents an important challenge: How to ensure that ample spectrum resources are available for each of these wireless devices?

To address this paramount issue academic researchers, industry experts, and policy makers met at the Second Enhancing Access to the Radio Spectrum (EARS) workshop to discuss open challenges and promising research that will facilitate spectrum sharing between wireless devices. The EARS workshop resulted in the formation of several grand challenges that will guide and shape the future of wireless devices and applications. This week’s Wireless Seminar will be a forum discussion about spectrum sharing including a brief overview of findings from the Second EARS workshop, and an opportunity for the audience to choose one or two grand challenges to discuss.

Bio: Thaddeus Czauski received the B.S. degree in Computer Engineering and Certificate in Nuclear Engineering from the University of Pitsburgh, and the M.S. degree in Computer Engineering from Virginia Tech. He has previously held positions at Northrop Grumman, General Electric, and the Sandia National Laboratories. He is currently pursuing his Ph.D. degree under the mentorship of Dr. Jeffrey Reed in the Wireless @ VT research center. His current research includes spectrum sharing architectures, low-latency communications. cyber-physical systems, and mobile cloud computing. Additionally, his interests and background include software engineering; user-experience assessment and design; privacy, security, and trust in computing; context aware computing; software patterns and architectures; mobile and wireless healthcare; and mobile security.

Date: February 12, 2016

Speaker: Dr. Jeffrey Reed

Title: Prospective of Current and Future Wireless Research: Technical Needs and Policy Challenges

This presentation will overview a few of the current research initiatives from Prof. Reed’s students and anticipated future research directions. The justification for current and future research directions, both from the prospective of technological needs and policy directions, will be addressed in the context of anticipated directions of the wireless industry.

Bio: Dr. Jeffrey H. Reed is the Willis G. Worcester Professor in the Bradley Department of Electrical and Computer Engineering at Virginia Tech. He currently serves as Founding Director of Wireless@Virginia Tech, one of the largest and most comprehensive university wireless research groups in the US which he founding in 2006 and served as its first director. In 2010, he founded the Ted and Karyn Hume Center for National Security and Technology and served as its Interim Director. Dr. Reed is co-founder of Cognitive Radio Technologies (CRT), a company that is commercializing of the cognitive radio technologies produced for military applications, Federated Wireless, a company that is developing spectrum sharing technologies; and for PFP Cybersecurity, a company that specializes in security for embedded systems, including Android platforms. He has also served as a consultant for approximately thirty organizations, covering topics such as merger evaluation, network neutrality, and band planning. Dr. Reed served on the President’s Council of Advisor in Science and Technology (PCAST) Advisory Group on how to transition federal spectrum for commercial economic benefits. In 2014, Dr. Reed was selected to be a member of CSMAC, the advisory group on spectrum issues for the US Department of Commerce. In 2014, Dr. Reed served as co-general chair for the IEEE Dynamic Spectrum Access Network (DySPAN) conference. In the fall of 2015, Dr. Reed testified before Congress on Improving Federal Spectrum Systems. You can read his testimony here.