The transponder is a central element in the end-to-end communications link, illustrated in Figure 1. This drawing provides a simplified system block diagram that shows the impairments that affect the system performance. Thus, the transmitting earth station on the uplink side will cause its share of distortion as will the receiving earth station on the downlink side. Some of this distortion is uncorrelated, which means that its contribution can be added more or less algebraically. However, for this to be correct, one must know the individual contributions. Other types of distortion, notably group delay, AM/AM and AM/PM, interact with one another and independence is no longer assured. Simple link budgeting techniques are available for evaluating links with additive noise; however a communications simulation tool like SystemView by Elanix is necessary for analyzing related impairments and their interaction. Another benefit of this approach is that both theoretical and measured data can be included in the simulation models.
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