送受同形エンファシスのスペクトラム反転秘話への適用

Abstract

Accompanying social advances and diversification, more and more utility is being found for automobile, airplane and other types of mobile telephones. The user of these mobile communication systems finds merit in being able to place and receive calls whenever or wherever he wants. However, since the communication of these calls is carried by radio waves, it is easy to eavesdrop on them, placing their confidentiality in jeopardy. To overcome this demerit, the idea of adding an encryption function to radio comunications has been studied from various approaches. However, as is common with most radio communication systems to which an encryption function has been added, the spectrum scrambler requires sacrifice in speech quality. The increase of effective modulation deviation produced in the spectrum scrambling process is a major cause of transmission SNR degradation. The larger this effctive PM index, the wider the frequency bandwidth occupied by the radio carrier becomes. Therefore, it is necessary to avoid this increase in effective PM index when providing an encrytion function to communication systems. In order to guard against effective PM index increasing from spectrum scrambling, conventionally it has been necessary to suppress the input signal level by more than 10dB (in power). Therefore, irrespective of whether scrambling is carried out or not, or to its properties, the SNR characteristic will degrade relative to fading noise at a fixed level or in inverse proportionate to the input signal level. This study has shown the exsitence of a new type of emphasis for spectrum inversion - hereafter called "Isomorphic Emphasis" - in which effective PM index of the arbitrary signal is always maintained equal with non-encrypted effective PM index and in which the sending and receiving emphasis are also equal. The findings of this study are quantitatively discussed. Furthermore, an equal sending and receiving emphasis characteristics means that repeatability in the manufacturing of the emphasis circuit can be increased and manufacturing costs reduced. At the same time, it also means that a pre-emphasis circuit for sending and a de-emphasis circuit for receiving can be used together in simplex (press to talk) communication sytem.