Prof. Tapan Sarkar of Syracuse University

Multiantenna systems are becoming exceedingly popular because they promise a different dimension, namely spatial diversity, than what was available to the communication systems engineers: The use of multiple transmit and receive antennas provides a means to perform spatial diversity, at least from a conceptual standpoint. In this way, one could increase the capacities of existing systems that already exploit time and frequency diversity. In such a scenario it could be said that the deployment of multiantenna systems is equivalent to using an overmoded waveguide, where information is simultaneously transmitted via not only the dominant mode but also through all the higher-order modes. We look into this interesting possibility and study why communication engineers advocate the use of such systems, whereas electromagnetic and microwave engineers have avoided such propagation mechanisms in their systems. Most importantly, we study the physical principles of multiantenna systems through Maxwell’s equations and utilize them to perform various numerical simulations to observe how a typical system will behave in practice. There is an important feature that is singular in electrical engineering and that many times is not treated properly in system applications: namely, superposition of power does not hold. Consider two interacting plane waves with power densities of 100 and 1 W/m2. Even though one of the waves has only 1% of the power density of the other wave, if the two waves interfere constructively or destructively, the resulting variation in the received power density is neither 101 nor 99 W/m2, but rather is 121 or 81 W/m2, respectively. [constructive: 121= destructive: 81= ]. Hence, there is a 40% variation. Using power additively leads to a significant error in the received power density. The key point here is that the fields or amplitudes can be added in the electrical engineering context and NOT the powers. This simple example based on Maxwellian physics clearly illustrates that the materials available in standard text books on Wireless Communication which claims that “an M-element array, in general, can achieve a signal-to-noise ratio improvement of 10log10(M) in the presence of Additive White Gaussian Noise with no interference or multipath over that of a single element” has to be taken with a big grain of salt. Hence, we need to be careful when comparing the performance of different systems in making valued judgments. In addition, appropriate metrics which is valid from a scientific standpoint should be selected to make this comparison. Examples will be presented to illustrate how this important principle impact certain conventional way of thinking in wireless communication. Also, we examine the phenomenon of height-gain in wireless cellular communication, and illustrate that under the current operating scenarios where the base station antennas are deployed over a tall tower, the field strength actually decreases with the height of the antenna over a realistic ground and there is no height gain in the near field. Therefore, to obtain a scientifically meaningful operational environment the vertically polarized base station antennas should be deployed closer to the ground. Also, when deploying antennas over tall towers it may be more advantageous to use horizontally polarized antennas than vertically polarized for communication in cellular environments. Numerical examples are presented to illustrate these cases. We next look at the concept of channel capacity and observe the various definitions of it that exist in the literature. The concept of channel capacity is intimately connected with the concept of entropy, and hence related to physics. We present two forms of the channel capacity, the usual Shannon capacity which is based on power; and the seldom used definition of Hartley which uses values of the voltage. These two definitions of capacities are shown to yield numerically very similar values if one is dealing with conjugately matched transmit-receive antenna systems. However, from an engineering standpoint, the voltage-based form of the channel capacity is more useful as it is related to the sensitivity of the receiver to an incoming electromagnetic wave. Furthermore, we illustrate through numerical simulations how to apply the channel capacity formulas in an electromagnetically proper way. To perform the calculations correctly in order to compare different scenarios, in all simulations the input power fed to the antennas needs to remain constant. Also conclusions should not be made using the principles of superposition of power. Second, one should deal with the gain of the antennas and not their directivities, which is an alternate way of referring to the input power fed to the antennas rather than to the radiated power. The radiated power essentially deals with the directivity of an antenna and theoretically one can get any value for the directivity of an aperture but the gain is finite. Hence, the distinction needs to be made between gain and directivity if one is willing to compare system performances in a proper way. Finally, one needs to use the Poynting’s theorem to calculate the power in the near field and not exclusively use either the voltage or the current. These restrictions apply to the power form of the Shannon channel capacity theorem. The voltage form of the capacity due to Hartley is applicable to both near and far fields. Use of realistic antenna models in place of representing antennas by point sources further illustrates the above points, as the point sources by definition generate only far field, and they o not exist in real life. The concept of a multiple-input-multiple-output (MIMO) antenna system is illustrated next and its strengths and weaknesses are outlined. Sample simulations show that only the classical phased array mode out of the various spatial modes that characterize spatial diversity is useful for that purpose and the other spatial modes are not efficient radiators. Finally, how reciprocity can be used in directing a signal to a preselected receiver when there is a two way communication between a transmitter and the receiver even in the presence of interfering objects is demonstrated. This embarrassingly simple method based on reciprocity, is much simpler in computational complexity than a traditional MIMO and can even exploit the polarization properties for effectively decorrelating multiple receivers in a multiple-input-single-output (MISO) system.

Biography: Tapan K. Sarkar received the B.Tech. degree from the Indian Institute of Technology, Kharagpur, in 1969, the M.Sc.E. degree from the University of New Brunswick, Fredericton, NB, Canada, in 1971, and the M.S. and Ph.D. degrees from Syracuse University, Syracuse, NY, in 1975. From 1975 to 1976, he was with the TACO Division of the General Instruments Corporation. He was with the Rochester Institute of Technology, Rochester, NY, from 1976 to 1985. He was a Research Fellow at the Gordon McKay Laboratory, Harvard University, Cambridge, MA, from 1977 to 1978. He is now a Professor in the Department of Electrical and Computer Engineering, Syracuse University. His current research interests deal with numerical solutions of operator equations arising in electromagnetics and signal processing with application to system design. He obtained one of the “best solution” awards in May 1977 at the Rome Air Development Center Spectral Estimation Workshop. He received the Best Paper Award of the IEEE Transactions on Electromagnetic Compatibility in 1979 and in the 1997 National Radar Conference. He has authored or coauthored more than 300 journal articles and numerous conference papers and 32 chapters in books and fifteen books, including his most recent ones, Iterative and Self Adaptive Finite-Elements in Electromagnetic Modeling (Boston, MA: Artech House, 1998), Wavelet Applications in Electromagnetics and Signal Processing (Boston, MA: Artech House, 2002), Smart Antennas (IEEE Press and John Wiley & Sons, 2003), History of Wireless (IEEE Press and John Wiley & Sons, 2005), Physics of Multiantenna Systems and Broadband Adaptive Processing (John Wiley & Sons, 2007), Parallel Solution of Integral Equation-Based EM Problems in the Frequency Domain (IEEE Press and John Wiley & Sons, 2009), and Time and Frequency Domain Solutions of EM Problems Using Integral Equations and a Hybrid Methodology (IEEE Press and John Wiley & Sons, 2010).
Dr. Sarkar is a Registered Professional Engineer in the State of New York. He received the College of Engineering Research Award in 1996 and the Chancellor’s Citation for Excellence in Research in 1998 at Syracuse University. He was an Associate Editor for feature articles of the IEEE Antennas and Propagation Society Newsletter (1986-1988), Associate Editor for the IEEE Transactions on Electromagnetic Compatibility (1986-1989), Chairman of the Inter-commission Working Group of International URSI on Time Domain Metrology (1990–1996), distinguished lecturer for the Antennas and Propagation Society from (2000-2003), Member of Antennas and Propagation Society ADCOM (2004-2007), on the board of directors of ACES (2000-2006), vice president of the Applied Computational Electromagnetics Society (ACES), a member of the IEEE Electromagnetics Award board (2004-2007), and an associate editor for the IEEE Transactions on Antennas and Propagation (2004-2010). He is currently on the editorial board of Digital Signal Processing – A Review Journal, Journal of Electromagnetic Waves and Applications and Microwave and Optical Technology Letters. He is the chair of the International Conference Technical Committee of IEEE Microwave Theory and Techniques Society # 1 on Field Theory and Guided Waves.
He received Docteur Honoris Causa both from Universite Blaise Pascal, Clermont Ferrand, France in 1998 and from Politechnic University of Madrid, Madrid, Spain in 2004. He received the medal of the friend of the city of Clermont Ferrand, France, in 2000.

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Address:Department of Electrical Engineering and Computer Science, Syracuse University, 11 Wexford Road, Dewitt, New York, United States, 13244

Prof. Tapan Sarkar of Syracuse University

Biography:

Email:

Address:Dewitt, New York, United States