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Talk by Prof. Josef Nossek

Telecommunications / News / Talk by Prof. Josef Nossek
21. April 2026
Reconfigurable Intelligent Surfaces and Circuit Theory

Prof. Josef A. Nossek
Technische Universität München

Tuesday, April 21, at 2:30 pm in seminar room 402

information theory serves well as the mathematical theory of communication. However, it contains no provision that makes sure its theorems are consistent with the physical laws that govern any existing realization of a communication system. Therefore, it may not be surprising that applications of information theory or signal processing, as currently practiced, easily turn out to be inconsistent with fundamental principles of physics, such as the law of conservation of energy. It is the purpose of multiport communication theory to provide the necessary framework ensuring that applications of signal processing and information theory actually do comply with physical law.

RIS are intended to engineer the propagation environment to improve the performance of wireless communications, especially in situations where the direct link between the transmit side (Tx) and receive side (Rx) is weak or even blocked. While most research has been performed on system-level optimization, where over-simplistic models have been used, only a few publications are incorporating basic physical laws in the modeling process. Surprisingly, in these publications, different approaches to modeling have been adopted, either based on impedance or scattering parameters. In the impedance parameter approach, the variables are voltages and currents, while in the scattering parameter approach the variables are incident and reflected waves. Since these two pairs of variables are simply related to each other by a linear transformation, the impedance matrices and the corresponding scattering matrices can easily be converted from one to the other. But interestingly enough, even in the simplest end-to-end single-input single-output (SISO) link with a blocked direct channel between Tx and Rx, the published derivations lead to different results. It is impossible that such results are true at the same time. We have to resolve this dilemma.

But in addition, we can not only take mutual impedances between RIS elements into account, but we can try to decouple them with the aid of decoupling multi-ports and thereby open a new domain for RIS architectures. With decoupling networks, it is possible to optimize the channel gain of a RIS-aided SISO system in closed form which allows to analytically analyze the array gain of a RIS array under mutual coupling. From this analysis, we can draw connections to the conventional transmit array gain. In particular, we prove that a super-quadratic array gain which scales with N^4 is possible in a RIS-aided scenario. In fact, this result provides novel insights in RIS-aided communication since in previous work, it was assumed that only quadratic gains (i.e., N^2) are possible

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