USC SIAM STUDENT CHAPTER
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Monday, November 1, KAP 249
CAMS/SIAM Chapter Colloqium
“The Polyspectral Method — A New Approach to Nonlinear System-Level Modeling
and Distortion Compensation”
Dr. Christopher P. Silva, Aerospace Corporation
The increasing demands for performance, mobility, and services in difficult physical and frequency allocation environments, in both commercial wireless and military contexts, pose formidable new challenges to communications systems designers. These challenges include the classical delicate balance between power efficiency, which is precious for both personal handsets and satellite transponders, and the performance-limiting nonlinear distortion produced in power amplifiers operated to achieve these desired efficiencies. This balance is made even more delicate by the use of non-constant envelope modulations needed to achieve high bandwidth efficiencies. Current activity in these areas has especially focused on the system-level or behavioral modeling of solid-state amplifiers (SSAs) and traveling wave tube amplifiers (TWTAs), with a concomitant effort on their distortion compensation in the form of predistorters, linearizers, and equalizers. Refined nonlinear measurement and modeling approaches will be required to successfully support these efforts, which will become more difficult in the foreseeable future due to increasingly complex modulations and broader bandwidths.
The Aerospace Corporation has concretely encountered the above challenges in its development support for advanced wideband satellite communications system using bandwidth efficient modulations (BEMs). It began to meet these challenges by first developing new and unique frequency- and time-domain measurement techniques that can be applied to accurately measure hardware components and links. The next stage, which is the subject of this talk, has been the development of a new approach to wideband communications modeling and distortion mitigation based on formal nonlinear system identification principles (termed the polyspectral method). Current system modeling approaches, which are dominated by probing signals of at most a multi-tone nature, prove to be inadequate for representing channels with operational bandwidths reaching the multi-GHz range. As a natural consequence, distortion compensation designs, which are also typically based on these models, will likewise prove to be less than optimal in their effectiveness.
This talk will provide an introduction to current system modeling approaches and the polyspectral method, covering the latter’s origins, formal basis, construction procedures, and basic modeling and compensation features. The modeling application and validation of the polyspectral method has been extensive for both representative SSAs and developmental TWTAs. These new models produced the highest predictive fidelities ever found for system-level amplifier models, essentially solving the nonlinear system identification problem for these components. Two representative amplifier modeling applications will be provided to illustrate the effectiveness of the method, involving high power amplifiers operating in the 20-GHz range with 9.6 Gbps, 16-amplitude phase-keyed (APK) modulations. The modeling activity continues both internally and with several outside organizations. Besides the continued application of the polyspectral method to amplifier modeling, the future development of the modeling and compensation aspects of this approach will also be discussed. Because of its many powerful features, wide-ranging applicability, and state-of-the-art effectiveness, it is expected that the polyspectral method will lead to a new field in communication systems modeling and compensator design.
Presenter Biography:
Dr. Christopher P. Silva received the B.S., M.S., and Ph.D. degrees, all in electrical engineering, in 1982, 1985, and 1993, respectively, from the University of California at Berkeley. Professor Leon O. Chua directed his graduate work with an emphasis on nonlinear circuit and system theory. His dissertation work was on the analytical detection of chaotic dynamics in nonlinear circuits, which included a detailed rigorous study of the qualitative dynamics of both the well-known double-scroll chaotic circuit and second-order analog phase-locked loops.
He joined the Electronics Research Laboratory of The Aerospace Corporation in 1989 and is currently a Senior Engineering Specialist in the Communication Electronics Department, Electronic Systems Division. He has been the principal investigator on several internally funded research projects addressing nonlinear microwave CAD, secure communications by means of chaos, and the modeling and compensation of nonlinear satellite communications channels, the latter of which has evolved into a program-wide support effort for advanced technology development. He has given many invited talks at conferences, society meetings, universities, industry, and laboratories on the applications of nonlinear techniques to communications and signal processing, along with corresponding publications in various venues.
Dr. Silva is a member of Eta Kappa Nu, Tau Beta Pi, Phi Beta Kappa, the Institute of Electrical and Electronics Engineers (Fellow Grade), the American Association for the Advancement of Science, the American Institute of Aeronautics and Astronautics (Senior Grade), the Society for Industrial and Applied Mathematics, and the American Mathematical Society.
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