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.

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