What does MER indicate in a digital modulation system?

• A. Modulation error rate
• B. The guard band width
• C. The envelope of the modulated signal
• D. The distance in dB between received symbol points and their ideal locations

Answer: (D)

In digital modulation, MER measures how clean the modulation is by looking at how far the received constellation points are from where they should be. Each symbol has an ideal location in the constellation; noise, distortion, and nonlinearity push the received point away from that spot. MER captures that average separation as a ratio of signal energy to error energy, usually expressed in decibels. Conceptually, it’s about the distance between the received symbol points and their ideal locations, and the dB form reflects how much distortion is present relative to the symbol energy. The other ideas don’t describe this geometric accuracy: modulation error rate would be about how often symbols are decoded incorrectly, the guard band width concerns spectral spacing between channels, and the envelope refers to amplitude shape rather than constellation-point accuracy.