Simulation of multiple carriers through non-linear satellite links

It is important to plan a satellite link before selling it. Simulation can be used as an aid to planning in order to: (1) make most use of the satellite transponder, since it is the most expensive part of the link; (2) decide the optimum size for Earth station high power amplifiers (HPAs); (3) increase the accuracy of the link budget. If the amplifiers are operated in a non-linear region, such as near saturation, then intermodulation occurs within carriers and between carriers, producing a finite carrier-to-interference (C/I) ratio. BT Exact Technologies has developed a simulation tool using Signal Processing Worksystem (SPW) that allows the C/I ratio to be determined for each of the carriers in a multi-carrier link, so that the transponders can be planned for the optimum number of carriers. The tool, which can be run for mixtures of carriers such as xPSK, QAM and analogue TV, allows the measurement of C/I beneath each carrier by subtraction. In this way, the bit-error rate can be determined from the C/I from reference tables, saving time compared with error counting methods. Interference from the transponders on the intended polarisation and on the crosspolarisation can be taken into account. The model also gives the amount by which signals are suppressed by the non-linearity, so that link budgets can be corrected.