In DVB-S2X, how does increasing the guard interval affect inter-symbol interference robustness and spectral efficiency?

• A. It increases robustness to ISI but reduces spectral efficiency.
• B. It increases spectral efficiency but reduces robustness.
• C. It has no effect on performance.
• D. It eliminates the need for adaptive coding.

Answer: (A)

In DVB-S2X the guard interval (the cyclic prefix in OFDM) cushions against inter-symbol interference from multipath and delayed signal components. Increasing this guard interval makes the system more robust to ISI because there’s a longer buffer that absorbs reflected and delayed copies of the previous symbol, which helps the receiver correctly separate consecutive symbols in challenging channels.

The trade-off is that a longer guard interval eats into the useful symbol duration, so for a given bandwidth you have less time to carry data. That reduces spectral efficiency (bits per second per hertz). If the guard interval is set longer than the channel’s delay spread, you don’t gain extra robustness but keep paying the efficiency penalty.

Adaptive coding and modulation (ACM) still play a role, so the option about eliminating the need for adaptive coding isn’t correct. And since there is a clear trade-off, the choice stating no effect or that you gain efficiency isn’t correct either.