Towards a circuit theory of communication

Electro-magnetics provides the ground for a physical theory of communications, while information- and signal theory approach the problem from a purely mathematical point of view. Nevertheless, the latter theories do refer to physical terms, such as: energy, power, noise, antennas, or waves. Strangely enough, at present time there is no provision being made such that the usage of such terms in information theory is at least consistent with the governing physics. More often than not, this results in less than optimum signal processing solutions and does not contribute to a complete understanding of the communication system. Circuit theoretic channel models can help here to bridge the gap between the physics of electro-magnetic fields, and the mathematical world of information theory. Using the multi-port concept one can ensure that such important physical notions as energy, power and noise are captured correctly, and terms such as antennas and waves are applied consistently with their physical meanings in information theory and signal processing. In this paper, we demonstrate how to make circuit theoretic channel models and apply them to a wireless communication system which uses multiple antennas at both ends of the link. We thereby show that, in contrast to common belief, arrays of closely spaced antennas actually do support bandwidth- and power-efficient multi-stream transmission.