What describes orthogonal frequency-division multiplexing (OFDM)?

Orthogonal frequency-division multiplexing (OFDM) is a sophisticated digital modulation technique. It works by splitting a high-rate data stream into many slower data streams, which are then transmitted simultaneously over multiple closely spaced subcarriers. The term 'orthogonal' refers to the precise mathematical spacing of these subcarrier frequencies. They are spaced such that the peak of one subcarrier aligns exactly with the zero-crossing points of the others. This unique spacing completely avoids intersymbol and intercarrier interference, even though the signals tightly overlap in the frequency domain. The other options are incorrect because OFDM is typically a high-speed, wideband digital mode, not a slow-speed, narrowband one. Furthermore, it is a digital modulation method, not an analog frequency modulation or bandwidth compression technique.