Frequency-Hopped Multilevel FSK for Mobile Radio

Abstract

A multiple-access modulation technique that uses multilevel frequency shift keying (FSK) to modulate frequency-hopped, spread-spectrum carriers is examined for possible application to digital mobile radiotelephony. This technique, in which all users employ the full system bandwidth simultaneously, would be resistant to the frequency-selective fading so troublesome in mobile radio. We have studied base-to-mobile communication of 32 kb/s per user in the 20-MHz (one-way) bandwidth of the 850-MHz mobile radio band. The number of users that can be served within a given bit error rate criterion depends on the quality of the radio channel. For perfect transmission, where the only degradation is mutual interference, an error rate less than 10−3 can be maintained with up to 209 simultaneous users. Transmission impairments, consisting of white Gaussian noise and frequency-selective Rayleigh fading with an average rf signal-to-noise ratio of 25 dB, reduce the number of simultaneous users to about 170. This capacity is roughly three times that of a phase-shift-keying spread-spectrum system recently proposed for mobile radio. For mobile-to-base transmission of FH-fsk, we have yet to study impairments resulting from delay spread in a synchronous system or, alternatively, the penalty for operating asynchronously. These effects would reduce the number of possible users from the estimates we have given for base-to-mobile transmission.