A processing satellite transponder for multiple access by low-rate mobile users

A digital processing transponder is considered which performs a discrete (2 to the K) point spectral analysis over a bandwidth W for the received uplink signals and retransmits decisions on which of the (2 to the K) frequencies contain significant energy. The transponder is intended for use with low data-rate mobile users employing MFSK modulation with code division multiple access (CDMA) by frequency hopping over the (2 to the K) frequencies in the common bandwidth W. It is shown that the number of users, M, which may simultaneously transmit at data rate R, is as high as (3/8)W/R when the maximum Doppler uncertainty-to-bandwidth ratio, D/W, is small and the bit energy-to-noise density, Eb/No, is very large. Finally the technique described also has the advantage of being far more robust than coherent PN techniques for mobile users, and much easier to acquire and track through large Doppler rate motion.