Guest Editorial for the Special Issue on Radio Wave Propagation

In his seminal article [1], Gabor observed that Communication theory has up to now been developed mainly on mathematical lines, taking for granted the physical significance of the quantities which figure in its formalism. But communication is the transmission of physical effects from one system to another, hence communication theory should be considered as a branch of physics. Thus it is necessary to embody in its foundations such fundamental physical data as the quantum of action, and the discreteness of electric charges $\ldots$ We observe first that all electric signals are conveyed by radiation. Even if lines or cables are used in the transmission, by the Maxwell-Poynting theory the energy can be located in empty space. Hence we can apply to our problem the well known results of the theory of radiation. The point here is that in the design of the physical layer one has to apply other concepts besides the principles of communication theory. More recently, this point was further investigated by Ivrlac and Nossek [2], [3], where they state that Electromagnetic field theory provides the physics of radio communications, while information theory approaches the problem from a purely mathematical point of view. While there is a law of conservation of energy in physics, there is no such law in information theory. Consequently, when, in information theory, reference is made (as it frequently is) to terms like energy, power, noise, or antennas, it is by no means guaranteed that their use is consistent with the physics of the communication system. 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. Ivrlac and Nossek further elaborate [3] that there exist a number of fundamental principles in physics which can be stated as conservation laws, meaning that there are quantities which can be calculated for a physical system at one time, and when recalculated at a later time come out the same [4]. They further illustrate that an example is the law of conservation of energy $\ldots$ The movement of the planets around the sun can be obtained solely by following the implications of the laws of conservation of energy and angular momentum [5]. Moreover, the conservation laws are also deep principles for they relate to symmetry in physics [6]. For instance, conservation of energy implies that the laws of Nature are time-invariant, and vice versa. Also, in signal processing and information theory, the concept of energy is a prominent one. It appears as the energy required to transfer one bit of information, or one symbol of the signal alphabet, or sometimes in form of transmit power, i.e., the rate at which energy must be supplied per unit of time to maintain communication. Yet, interestingly, the fact that energy is conserved, which is of such fundamental importance in physics, apparently plays no role in standard textbooks on information theory [7], signal processing [8], communication theory [9] or signal theory [10]. The authors are also not aware of any research work in these areas where the remarkable fact that energy is conserved is explored or discussed. The reason for this strange absence of conservation laws in signal processing, information theory and related disciplines seems to be related to inputs and outputs which are each described by single variables, instead of by a pair of conjugated variables, like position and momentum in Hamiltonian mechanics [6], or voltage and current in circuit theory [11]. Therefore, to address the development of the physical layer adequately and to ensure that systems perform according to design criteria, it is necessary to merge the principles of electromagnetics, which are primarily related to antennas and maximum power transfer, to the issues of channel capacity and how it can be quantified using the principles of physics and the radiation efficiency of antennas rather than the use of the maximum power transfer theorem [12].