4B Digital RF Measurements
TechnicalReport (TR)101290, formerlyknownasETR290,which describes measurement guidelines for DVB systems. One measurement, Modulation Error Ratio (MER), is designed to provide a single ‘figure of merit’ analysis of the received signal. MER is computed to include the total signal degradation likely to be present at the input of a commercial receiver’s decision circuits and so give an indication of the ability of that receiver to correctly decode the signal. The MER computation compares the actual location of a received symbol (a ‘symbol’ represents a digital value in the COFDM modulation process) to its ideal location, giving a figure of merit for system performance. As degradation occurs, and the received symbols land further from their proper locations, the MER value will decrease. Ultimately the symbols start being incorrectly interpreted; this is the threshold or ‘cliff’ point. Figure 7.4B.1 shows this BER/MER relationship for a suitable equipped receiver. The graph was obtained by connecting the MER receiver to a test modulator. Noise was then introduced in gradually increasing quantity, and the MER and pre-Viterbi BER values recorded. With no additive noise, the MER starts as 35dB with the BER near zero. Note that as noise is added the MER gradually decreases, while the BER stays constant. When the MER reaches 24dB the BER starts to climb rapidly, indicating threshold. MER has allowed us to see progressive system degradation long before reaching the ‘cliff’.