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The research on noise-based secure communication systems is mainly carried out from an information theoretical point of view, and is focused on evaluation in different communication conditions of the secrecy capacity, which is defined as the maximum achievable perfect secrecy rate, where the perfect secret rate is the amount of information that can be sent not only reliably but also confidentially.
He noted that the channel capacity of the authorized channel is higher than the channel capacity of the eavesdropper, and one coding strategy is able to transmit information to the authorized receiver with a vanishing block-code error probability in a completely secure way, at a rate limited by the difference between the authorized and eavesdropper channel capacity.
This causes a decrease in the channel capacity of the unauthorized receiver compared to the authorized receiver, which can transmit information buried in the additional noise affecting the eavesdropper.
In this paper, it will be shown that, in the noise-based unconditionally secure communication systems, the degrees of freedom of the electromagnetic field are used in a third way.
Paralleling the analysis undertaken in, it takes advantage of the concept of the number of degrees of freedom (NDF) of the electromagnetic field, defined as the minimum number of functions required to represent the field on an observation manifold within a given representation error, fixed by the noise corrupting the observed field.
Successively, in a seminal paper, Wyner suggested that the presence of additional noise when information is tapped from a line makes the unauthorized channel noisier than the authorized channel.
Broadly speaking, any antenna uses the available degrees of freedom of the electromagnetic field mainly following two possible goals: to concentrate (and possibly maximize) energy on the receiving antenna or to maximize the amount of information available on the receiving antenna.
In this framework, an approach to obtain unconditionally secure communication based on the use of the degrees of freedom of the electromagnetic field is discussed.
Signal and noise are transmitted at the same time using a strategy that avoids any increase in noise at the authorized receiver taking advantage of the degrees of freedom that are not used to transmit information.
The physical mechanism at the basis of the secret communication is explained by means of the degrees of freedom of the field, which are able to convey information and noise in spatially orthogonal channels.
As a final observation, in order to make the channel secure, a non-negligible percentage of the available power is radiated as artificial noise.
According to some fundamental results of information theory, it is possible to hide information in the additional noise affecting the eavesdropper.
Loosely speaking, the allocation of power to noise causes a decrease in the channel capacity of the system that follows logarithmic law.
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