What describes orthogonal frequency-division multiplexing (OFDM)?
A frequency modulation technique that uses non-harmonically related frequencies
A bandwidth compression technique using Fourier transforms
A digital mode for narrow-band, slow-speed transmissions
A digital modulation technique using subcarriers at frequencies chosen to avoid intersymbol interference✓Правильный ответ
Объяснение
OFDM spaces its subcarriers by the reciprocal of the symbol period, so each one sits on the spectral nulls of all the others and they can overlap without interfering. That orthogonality is what prevents intercarrier interference; intersymbol interference is held off separately, by the long symbol period that the low per-subcarrier rate produces plus a guard interval — so the official wording conflates the two mechanisms, though it remains the only choice describing a digital multi-subcarrier scheme.