Hello and welcome to the official website of R. Michael Buehrer's research group.
This website serves as the information gateway to the latest and greatest research being performed by his M.Sc. and Ph.D. students in the Mobile and Portable Radio Research Group (MPRG) within the Wireless@VT umbrella.
Please feel free to browse around at your leisure. Should you desire to contact Dr. Buehrer, please find his contact information here.
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The Office of the Vice President for Research recognizes R. Michael Buehrer as a Virginia Tech Scholar of the Week. His research includes wireless communications, ultra-wideband communication and sensing systems, cellular and personal communications, multiuser detection, "intelligent" antennas, and cognitive radio. The director of Wireless@Virginia Tech, Buehrer advances world-changing technologies in wireless communications, ultra-wideband communication and sensing systems, cellular and personal communications, multiuser detection, “intelligent” antennas, and cognitive radio. More details are available here.
Abstract: With the rapid development of wireless technologies, the demand for positioning services has grown dramatically over the past three decades. The Global Positioning System (GPS) is widely used in wireless devices for positioning purposes. However, in addition to having bulky and expensive equipment, GPS receivers do not operate properly in dense and indoor environments. Difficulties in using GPS lead us to use sensor localization in which the position information is obtained from the measurements collected within the network without the aid of external resources. Sensor localization has been a great topic of interest during past decades. Although many positioning algorithms have been developed previously in the literature, positioning is still a challenging task. There are many factors that can affect the positioning performance if they are neglected or not treated properly. These factors introduce many nuisance parameters which need to be either estimated or considered when the location is estimated. In this work, we exploit cooperative localization as a recent and trending technology and semidefinite programming (SDP) as a powerful tool in our research. Cooperative localization has several advantages over the traditional noncooperative localization in terms of positioning accuracy and localizability. Cooperation is also highly beneficial for networks with few anchor nodes and low communication range. On the other hand, SDP provides an alternative solution to the optimal maximum-likelihood (ML) estimation. Unlike in the ML estimator, convergence to the global minimum is guaranteed in SDP. It also has significantly lower complexity especially for cooperative networks in exchange for small performance degradation. Using these two concepts, four open problems within the area of cooperative localization and tracking in the presence of nuisance parameters are addressed. In particular, we focus on cooperative received signal strength-based localization when the propagation parameters including path-loss exponent and transmit powers are unknown. Cooperative time-of-arrival-based localization in harsh environments in the presence of severe non-line-of-sight (NLOS) propagation is also investigated. Cooperative localization in asynchronous networks is studied where the clock parameters are considered as nuisance parameters and the focus is on a joint synchronization and localization approach. Lastly, source tracking in NLOS environments is studied where source nodes are mobile and their status changes rapidly from LOS to NLOS and vice versa.
Michael Buehrer received the Dean's Award for Teaching Excellence from the College of Engineering. The award was presented during the annual College of Engineering reception and awards ceremony on May 19, 2014. Buehrer has developed several new ECE courses, including Spread Spectrum Communications and Multi-Channel Communications (doctoral-level course). Multi-Channel Communications which is only taught in a few top universities covers the fundamentals of the most recent communications technology such as LTE and WiMAX. He consistently earns positive student evaluations. More details are available here.
On September 11-12, 2013, VT CogRad represented the Wireless@VT research group in the DARPA Spectrum Challenge's Preliminary Tournament. Along with 17 other teams, VT CogRad designed a software-defined radio to compete in the competitive and cooperative tournaments. VT CogRad team including SaiDhiraj Amuru, Daniel Jakubisin, Jeffrey Poston, and R. Michael Buehrer placed fourth in the competitive challenge. In the competitive match VT CogRad's design successfully created interference to the opposing team while rapidly transmitting packets of its own which allowed the team to win four rounds before being eliminated by the eventual second place team. VT CogRad successfully qualified for the tournament in April by passing the Hurdles with a 11th place score. Teams now have the opportunity to improve their strategy before competing in the DARPA Spectrum Challenge's Final Tournament which will be held in March 2014. In the Final Tournament, the DARPA Spectrum Challenge plans to award $50,000 to the winners of the competitive and cooperative matches. Full Preliminary Tournament results are available here.
Abstract: The Cross Ambiguity Function (CAF) is often used for passive geolocation of an emitter based on the time difference of arrival (TDOA) and frequency difference of arrival (FDOA) of the received signals. CAF performance has been thoroughly investigated in regards to traditional single-input single-output (SISO) signals. Little is known about how the CAF will respond to signals from multiple-input multiple-output (MIMO) systems which utilize multiple antennas. This thesis focuses on characterizing the CAF's magnitude distribution in order to determine the probability of correctly determining the correct TDOA/FDOA bin, and the resulting impact on geolocation. The received signals are studied in the presence of additive white Gaussian noise (AWGN) as well as multi-channel propagation effects such as phase ambiguities and offsets due to multi-antenna transmission.
Two and four transmit antennas using either a form of spatial multiplexing or space-time block coding are the focus of this work because they are mostly commonly found in currently deployed communication systems. The effects of these transmit schemes are studied with respect to TDOA/FDOA error and the resulting position error. The analysis is performed using a detection theory framework as opposed to estimation theory in order to emphasize the impact of MIMO transmission on determining the correct TDOA/FDOA bin. A simple method using the CAF magnitude as a decision statistic is also presented so that TDOA/FDOA errors can be detected and altered in an attempt to improve positioning estimates.
Abstract: The localization of wireless devices, i.e. mobile phones, laptops, and handheld GPS receivers, has gained much interest due to the benefits it provides, including quicker emergency personnel dispatch, location-aided routing, as well as commercial revenue opportunities through location based services. GPS is the dominant position location system in operation, with 31 operational satellites producing eight line of sight satellites available to users at all times making it very favorable for system implementation in all wireless networks. Unfortunately when a GPS receiver is in a challenging environment, such as an urban or indoor scenario, the signal quality often degrades causing poor accuracy in the position estimate or failure to localize altogether due to satellite availability.
Our goal is to introduce a new solution that has the ability to overcome this limitation by improving the accuracy and availability of a GPS receiver when in a challenging environment. To test this theory we created a simulated GPS receiver using a MATLAB simulation to mimic a standard GPS receiver with all 31 operational satellites. Here we are able to alter the environment of the user and examine the errors that occur due to noise and limited satellite availability. Then we introduce additional user(s) to the GPS solution with the knowledge (or estimate) of the distances between the users. The new solutions use inter-receiver distances along with pseudoranges to cooperatively determine all receiver locations simultaneously, resulting in improvement in both the accuracy of the position estimate and availability.
Our research team including Reza Monir Vaghefi, Javier Schloemann, and R. Michael Buehrer won the 1st Contest on Localization Algorithms in Dresden on 19th of March. The contest was hosted by the 10th Workshop on Positioning, Navigation and Communication 2013 (WPNC'13) in Dresden, Germany, March 20-21, 2013 in cooperation with BUTLER FP7 EU project. The participants compared their localization algorithms based on given distance datasets and evaluation metrics.
SaiDhiraj Amuru, Daniel Jakubisin, Jeffrey Poston, and R. Michael Buehrer, the members of the team VT CogRad, were qualified for the DARPA Spectrum Challenge tournaments. 90 teams registered as Challenge entrants, with participants from around the world. However, only 15 teams were selected as contestants for the Challenge tournaments where VT CogRad ranked 11th. The DARPA Spectrum Challenge is a competition to demonstrate a radio protocol that can best use a given communication channel in the presence of other dynamic users and interfering signals.