Orthogonal Frequency-Division Multiplexing

Orthogonal Frequency-Division Multiplexing (OFDM) is a digital multi-carrier modulation scheme that partitions a high-rate data stream into multiple lower-rate streams, each transmitted on a separate subcarrier frequency. The subcarriers are chosen to be mathematically orthogonal — their spectra overlap intentionally, yet they can be separated without inter-carrier interference because the peak of each subcarrier coincides with the zero-crossings of all others. This spectral overlap makes OFDM radically more bandwidth-efficient than traditional frequency-division schemes that guard each carrier with dead spectrum.

The technique is the physical-layer foundation of 4G LTE, Wi-Fi (802.11a/g/n/ac/ax), and digital broadcasting standards. Its resilience to multipath fading — the same signal arriving at a receiver through multiple delayed paths — comes from a guard interval (cyclic prefix) that absorbs inter-symbol interference. In effect, OFDM turns a hostile frequency-selective channel into a collection of benign flat-fading channels, each carrying a thin slice of the original data.

OFDM's computational efficiency depends on the Fast Fourier Transform, which makes modulation and demodulation practical in silicon. Without the FFT, OFDM would be a theoretical curiosity; with it, OFDM is the dominant air-interface technology of the broadband era. The choice of subcarrier spacing, guard interval length, and windowing function is a trade-off between spectral efficiency, latency, and robustness — a trade-off that can be analyzed through signal-processing theory but is ultimately settled by standardization committees with proprietary simulations.