Which statement about modulation effects on telemetry is true?

• A. Higher order modulation increases data rate and always improves robustness to noise.
• B. Spectral efficiency is independent of modulation type.
• C. Robust modulation improves reliability but may reduce data rate.
• D. Modulation type has no impact on the required bandwidth.

Answer: (C)

The key idea here is the trade-off between reliability and data rate when choosing a modulation scheme for a telemetry link. Using a robust, lower‑order modulation (for example, schemes that carry fewer bits per symbol) makes the signals less sensitive to noise and interference, so the likelihood that each symbol is received correctly goes up. That improves reliability of the link. But because each symbol conveys fewer bits, the overall data rate falls unless you increase the symbol rate, which is not always possible within a given bandwidth. So robust modulation enhances reliability and may reduce data rate, which is exactly what this statement describes.

Higher order modulation can boost data rate, but it also makes the system more susceptible to noise, so it does not inherently provide better robustness. Spectral efficiency depends on what modulation is used and how many bits per symbol you transmit, so it’s not independent of modulation type. And the modulation choice does affect bandwidth requirements because it changes how many bits you carry per Hz of spectrum.