What is the purpose of the guard interval in OFDM-based SATCOM (e.g., DVB-S2X), and how does its length affect performance?

• A. Guard interval mitigates inter-symbol interference; longer guard increases robustness but reduces spectral efficiency.
• B. Guard interval increases data rate; longer guard improves efficiency.
• C. Guard interval is irrelevant for SATCOM.
• D. Guard interval is only used for QPSK.

Answer: (A)

In OFDM-based SATCOM, the guard interval is there to prevent inter-symbol interference from the channel’s delay spread and timing offsets. By inserting a cyclic prefix (the guard interval) that is at least as long as the channel’s longest delay, the OFDM symbol can be treated as a circular convolution, so the FFT-based equalizer can separate the subcarriers cleanly. A longer guard interval improves robustness to multipath and timing errors, but it also uses more of the symbol duration for non-data transmission, which reduces spectral efficiency and the effective data rate. So the guard interval helps with ISI protection, and its length trades off robustness against throughput. The other choices aren’t correct because the guard interval does not increase data rate, it is relevant for SATCOM, and it applies to any OFDM modulation, not just QPSK.