R. M. Buehrer

Cognitive Radio and Networking Research at Virginia Tech

More than a dozen Wireless @ Virginia Tech faculty are working to address the broad research agenda of cognitive radio and cognitive networks. Our core research team spans the protocol stack from radio and reconfigurable hardware to communications theory to the networking layer. Our work includes new analysis methods and the development of new software architectures and applications, in addition to work on the core concepts and architectures underlying cognitive radios and cognitive networks. This paper describes these contributions and points towards critical future work that remains to fulfill the promise of cognitive radio. We briefly describe the history of work on cognitive radios and networks at Virginia Tech and then discuss our contributions to the core cognitive processing underlying these systems, focusing on our cognitive engine. We also describe developments that support the cognitive engine and advances in radio technology that provide the flexibility desired in a cognitive radio node. We consider securing and verifying cognitive systems and examine the challenges of expanding the cognitive paradigm up the protocol stack to optimize end-to-end network performance. Lastly, we consider the analysis of cognitive systems using game theory and the application of cognitive techniques to problems in dynamic spectrum sharing and control of multiple-input multiple-output radios.

Toward a Highly Accurate Ambulatory System for Clinical Gait Analysis via UWB Radios

In this paper, we propose and investigate a low-cost and low-complexity wireless ambulatory human locomotion tracking system that provides a high ranging accuracy (intersensor distance) suitable for the assessment of clinical gait analysis using wearable ultra wideband (UWB) transceivers. The system design and transceiver performance are presented in additive-white-Gaussian noise and realistic channels, using industry accepted channel models for body area networks. The proposed system is theoretically capable of providing a ranging accuracy of 0.11 cm error at distances equivalent to interarker distances, at an 18 dB SNR in realistic on-body UWB channels. Based on real measurements, it provides the target ranging accuracy at an SNR = 20 dB. The achievable accuracy is ten times better than the accuracy reported in the literature for the intermarker-distance measurement. This makes it suitable for use in clinical gait analysis, and for the characterization and assessment of unstable mobility diseases, such as Parkinsons disease.

A game-theoretic framework for interference avoidance

Various iterative algorithms for interference avoidance (IA) in networks with co-located receivers, suitable for distributed implementation, have been proposed in the literature. In this paper, the IA problem is cast in a game-theoretic framework and is formulated as a potential game. This formulation accommodates previously proposed algorithms and, in addition, gives us a framework that enables the design of new distributed and convergent algorithms for IA including algorithms with nonidentical utility functions for the users. Two new convergence results for potential games are then derived. The first result establishes the convergence of a class of potential games to the global solution while following best response iterations and when noise is added. The second result establishes the convergence of potential games to the Nash equilibria of the game while following random better response iterations. The first result combined with the potential game formulation allows us to show that for a large class of network scenarios, arbitrarily small noise assures the convergence of best response IA algorithms, including the eigeniterations, to an arbitrarily small neighborhood of the globally optimal signature sequence set. The second result enables the design of reduced feedback mechanisms for IA that converge to desirable solutions.

WSN15-4: A Game-Theoretic Framework for Interference Avoidance in Ad hoc Networks

A framework to construct convergent interference avoidance (IA) algorithms in networks with multiple distributed receivers (as in ad hoc networks) based on potential game theory is developed in this paper. This is motivated by the fact that direct extensions of distributed greedy IA techniques for centralized networks to these de-centralized networks do not always lead to convergence. Some channel conditions that lead to non-convergence are also identified in the paper. A waveform adaptation algorithm for IA, designed on the basis of the framework, is then proposed. It is shown that this algorithm leads to a reduction of the interference in the network and also incorporates fairness in the allocation of resources.

Analysis and Implementation of a Time-Interleaved ADC Array for a Software-Defined UWB Receiver

A software-defined radio (SDR) for ultrawideband (UWB) communication systems places several stringent requirements on the analog-to-digital converter (ADC). One alternative to using a single ADC is to sample the received signal with an array of lower speed ADCs that were driven by interleaved sampling clocks; however, mismatches among the ADCs will result in signal distortion. This paper makes three important contributions to overcoming this problem: 1) analytical quantification of the impact of ADC gain, offset, and timing mismatches on the performance of a time-interleaved sampling ADC array for UWB signals; 2) demonstration of the efficacy of using a pilot-based matched-filter architecture to mitigate the impact of timing mismatches in the presence of multipath; and 3) implementation of an 8-ADC time-interleaved UWB SDR testbed that operates at an effective sampling frequency of 6.4 GHz. In addition, our findings allow for the design specification of the number of pilots required to obtain a desired system performance. The simulation and measured performance results from this paper demonstrate that ADC mismatches can be controlled to within plusmn10%, yielding acceptable levels of distortion and bit-error-rate (BER) performance on the UWB SDR testbed. Both analytical and simulation results also demonstrate the efficacy of a pilot-based matched filter in mitigating the impact of timing mismatch errors, even in the presence of multipath.

Interference avoidance in networks with distributed receivers

Direct extensions of distributed greedy interference avoidance (IA) techniques developed for centralized networks to networks with multiple distributed receivers (as in ad hoc networks) are not guaranteed to converge. Motivated by this fact, we develop a waveform adaptation (WA) algorithm framework for IA based on potential game theory. The potential game model ensures the convergence of the designed algorithms in distributed networks and leads to desirable network solutions. Properties of the game model are then exploited to design distributed implementations of the algorithm that involve limited feedback in the network. Finally, variations of IA algorithms including IA with respect to legacy systems and IA with combined transmit-power and WA adaptations are investigated.

Multiple-source localization using line-of-bearing measurements: Approaches to the data association problem

In this paper, we consider the data association problem of multiple-source localization using line-of-bearing measurements. More specifically, we propose two approaches that circumvent the very high complexity of a brute-force solution to this problem. The first approach relies on statistical clustering of the intersections of line-of-bearing measurements and the second approach exploits cyclostationary features of the received signals to allow for line-of-bearing measurement separation based on each signalpsilas transmission modes. These two proposed approaches have reduced time and computational complexity, while maintaining a reasonable amount of accuracy, and are of great value for dense environments with large numbers of sensors and/or targets.

Performance of Ultralow-Power IR-UWB Correlator Receivers for Highly Accurate Wearable Human Locomotion Tracking and Gait Analysis Systems

In this paper we study low-power impulse radio ultra-wideband (IR-UWB) correlation receivers with suboptimal templates, as a promising candidate for a highly accurate wearable human locomotion tracking system. Such a system is theoretically capable of providing a ranging accuracy of 1mm in practical multipath fading channels at a SNR of 18dB. This ranging accuracy is ten times better than the ranging accuracy provided by currently available systems. Furthermore, we study the theoretical BER and the improved Ziv-Zakai lower-bound on ranging accuracy in AWGN and dense multipath fading channels. We show that low-power is traded for a minimal performance loss for both BER and TOA accuracy.

A framework for the power consumption and ber performance of ultra-low power wireles swearable healthcare and human locomotion tracking systems via UWB radios

In this paper, we propose a framework for the study of power consumption and bit error rate (BER) performance of non-coherent impulse radio ultra wideband (IR-UWB) correlation receivers in the IEEE 802.15.3a channel. Using this framework, transmitted reference (TR) and energy detection (ED) correlation receivers are studied and compared. The receivers are assumed to operate in the 3.1-5GHz band targeting low-power consumption, where the correlation is performed in the analog domain. The BER performance is based on the channel averaged signal-to-noise ratio (SNR). Moreover, the framework addresses and compares different power consumption and performance parameters, namely the signal bandwidth, integration window, number of pulses per bit, and analog delay-lines. Then, we use the proposed framework for studying the fundamental design components of a wireless wearable human locomotion tracking and health-monitoring system based on UWB sensors. Ultimately, this system should provide high accuracy while consuming ultralow power. The study includes the link and power budgets of the system under investigation in addition to simulation results for the knee velocity. The accuracy provided by this system outperforms the accuracy of the current commercially available systems while preserving ultra-low power consumption.

Preliminary UWB Propagation Measurements in an Underground Limestone Mine

In an underground mine, wireless systems based on ultra wideband (UWB) signals have the potential to improve the response time to mine emergencies by providing fast, reliable communications as well as precision position location. Much of the UWB research to date has focused on the in-building and (to a lesser degree) outdoor propagation environments. As a result, little information is available on the nature of UWB propagation in an underground mining environment. This paper presents preliminary path loss and power delay profile measurement results for a typical room-and-pillar underground mine located in southwest Virginia. Transmitters and receivers were separated by distances ranging from 20 m to over 300 m in both line-of- sight and non-line-of-sight configurations. These results indicate that for long corridor environments, similar to indoor hallways, a waveguide effect results in better than free space propagation. Additionally, UWB signals were found to experience less fading over a local area than CW signals. UWB signals also provide the opportunity to aid in position location by achieving very accurate time-of-arrival measurements, or via RF fingerprinting techniques. We show that initial attempts to determine position based on such an approach were also very promising. The results presented here are insufficient in number to make some definitive statements about UWB propagation in underground mines but are only a first step towards characterizing that propagation. Additionally, the initial results are similar to the trends seen in indoor environments thus providing optimism that UWB may be a viable physical layer for wireless systems in underground mines.