An Improved Decision Technique for Frequency-Shift Communications systems

This paper describes a new circuit technique called a decision threshold computer, which enables a frequency-shift-keyed receiver system to use information in mark and space channels. independently, resulting in an improvement in circuit quality where fading exists between the mark and space frequencies. Frequency selective fading observed on ionospheric-scatter circuits and on high-frequency communication circuits is discussed. The results of a correlation study are presented which show such fading to be relatively uncorrelated a large portion of the time. Typical FSK demnodulators, with a fixed decision level set halfway between the long-term average amplitudes of the received mark and space frequencies, are shown to result in a high error liability when deep fades occur on either the mark or space frequency. It is further shown that such errors are unnecessary since the complete message is available on either frequency. The device discussed in this paper has been designed to make use of the normally deleterious effects of frequency selective fading, to provide up to a 30-fold reduction in teleprinter error rates over that which is theoretically possible when flat fading is assumed. Additional orders of diversity are shown to result where low correlation exists between the mark and space frequencies. Theoretical error rate equations and resulting error rate curves for the new technique are compared with those resulting from a fixed threshold device of conventional design.