Modulator quality figures

EVM (Error Vector Magnitude) and BER (Bit Error Rate) are compared using simulation for some realistic modulators, in order to understand whether EVM, which is easily measured through a VSA (Vector Signal Analyzer), can be considered a sufficient parameter to assess the quality of a modulator. The system loss at a given BER is actually the most important figure for the system design, and it is shown that in general the loss cannot be predicted from EVM alone. However, VSA outputs can be easily used to measure BER through the same semi-analytic techniques commonly adopted in simulation, and thus it is possible to directly estimate the loss. The semi-analytic techniques however cannot be used in presence of nonlinear blocks after the receiver such as channel decoders. However, at least in the considered cases, the loss measured with semi-analytic technique at the output of the demodulator is equal to that found at the output of the decoder. Hence, even if BER at the decoder output cannot be estimated, the loss can be predicted.