Investigation of multiple f.m./f.d.m. carriers through a satellite t.w.t. operating near to saturation

One method of providing multiple-access operation in a satellite communication system involves the transmission of many frequency-modulated carriers through a common broadband travelling-wave-tube (t.w.t.) amplifier in the satellite. Owing to the necessity to operate near the saturated output power of the t.w.t., amplitude nonlinearity and a.m./p.m. conversion will be significant factors in determining the overall system performance. Some results of multiple-carrier measurements using a t.w.t. of the type used in a satellite communication transponder are given in the paper. It has been shown that the performance is essentially determined by the third-order f1+f2?f3 type products for drives up to saturation. The agreement between theory and measurement is such that the carrier-to-total intermodulation-power ratio as a function of t.w.t. output power backoff can be derived from the measurement of a typical f1+f2?f3 type product when four carriers are transmitted through the t.w.t. Applying these results to the proposed global satellite Intelsat III, it is shown that at the optimum backoff of about ?2dB the satellite telephone capacity, when operating to earth stations having a figure of merit of 39.3dB and a rain margin of 4dB, will be about 1300 telephone circuits. It is also shown that the threshold point of a frequency demodulator due to intermodulation products occurs at a carrier/intermodulation power ratio of +10dB, similar to that obtained when the noise is of thermal origin.