What is the MER parameter in RF systems?

Well, if we assume that SNR is a baseband measurement, there really is no practical way to measure baseband data SNR. A better parameter is modulation error ratio (MER), which is analogous to baseband SNR. MER takes into account the combined effects of CNR; transmitter, upconverter or cable modem termination system (CMTS) phase noise; and impairments such as second and third order distortions. MER is, by definition, 10log(average symbol power/average error power).

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Best regards,
Dejan