Measuring Digital TV Signals in the Broadband Cable - Digital Video and Audio Broadcasting Technology - page 303

It is defined as:

CS = -10 log( P RC /P SIG ); [dB]

Resultant vector

Q

Error vector

Center of
decision field

Ideal vector

I

Fig. 18.17. Error vector for determining the modulation error ratio (MER)

18.2.5 Modulation Error Ratio (MER)

All the interference effects on a digital TV signal in broadband cable net-
works previously explained cause the constellation points to exhibit devia-
tions from their nominal position in the center of the decision fields. If the
deviations are too great, the decision thresholds will be exceeded and bit
errors are produced. However, the deviations from the decision field center
can also be considered to be measurement parameters for the size of any
interference quantity. Which is precisely the object of an artificial meas-
urement parameter like the modulation error ratio (MER). The MER
measurement assumes that the actual hits in the constellation fields have
been pushed out of the center of the respective field by interference quanti-
ties (Fig. 18.17.). The interference quantities are given error vectors and
the error vector points from the center of the constellation field to the point
of the actual hit in the constellation field. Then the lengths of all these er-
ror vectors are measured against time in each constellation field and the
quadratic mean is formed or the maximum peak value is acquired in a time
window. The exact definition of MER can be found in DVB Measurement
Guidelines [ETR290].