Effect of Satellite Transponder Nonlinearity on Uplink Thermal Noise*

The satellite transponder nonlinearity causes the uplink thermal noise to be suppressed relative to the carriers. It also causes the uplink thermal noise to interact with the carriers to generate intermodulation products. For the single carrier access where-one FM-TV carrier, one FM-FDM carrier or one TDMA-PSK carrier operating at or very close to saturation, the net effect is an improvement in the carrier to uplink thermal noise ratio (C/N)u of, about 2.2 to 3.5 dB. When there are two or more carriers accessing the transponder at the same time, the net effect is reversed. For a typical dual TV operation, it results in a degradation in (C/N)u of 0.5 to 0.7 dB when the total carrier input power level is 1 dB IPBO and is negligible when the total carrier input power level is 3 dB IPBO or lower; and for a typical multiple SCPC carrier access, due to the backoff operation, it is negligible.