Circuit Theory as Basis for Physically Consistent Modeling of Communication Systems

Technische Universitaet Muenchen, Institute for Circuit Theory and Signal Processing, Muenchen, Germany

Communication Systems are nowadays modeled in a very abstract manner thereby sometimes loosing physical consistency. Two key issues to be discussed are as follows: the interconnection of subsystems has to be implemented via ports and the noise within a subsystem has to be introduced into the system with the help of Kirchhoffs current and voltage law (KCL and KVL). This is quite in contrast to the usual block diagram description of communication systems, where subsystems are connected with only a single variable per input/output.
To correctly describe power/energy flow always pairs of conjugated variables are needed (voltage and current, incident and reflected wave, electric field and magnetic field). With a single variable per port it is not always possible to compute power flow correctly.
Noise is usually introduced ad hoc at some point with an adder, which adds noise with a given variance (see e.g. the socalled Additive White Gaussian Noise AWGN). In this ad hoc manner any dependency between the variance of the noise and some deterministic systems parameters is not taken into account.
With very simple examples it will be shown how to model noisy subsystems correctly and how this reveals dependencies , which are otherwise are ignored. This opens up new insight for the design of matching networks for highly sensitive receivers.