OFDM

Orthogonal Frequency-Division Multiplexing (OFDM) is a method of encoding digital data on multiple closely spaced orthogonal subcarriers to improve spectral efficiency and resilience to multipath fading. In OFDM, a high-rate data stream is divided into several lower-rate streams that are transmitted simultaneously on different subcarriers that are mathematically orthogonal, enabling simple receivers to separate data in the frequency domain.

Implementation typically uses the inverse fast Fourier transform (IFFT) at the transmitter to generate the composite time-domain signal and a fast Fourier transform (FFT) at the receiver to recover per-subcarrier data. A cyclic prefix or guard interval is added to each symbol to mitigate intersymbol interference caused by multipath. Subcarriers are modulated with QAM or PSK, and integrated with channel coding for error protection. SC-FDMA is a related variant used in some uplink systems to reduce peak-to-average power ratio.

Advantages include robustness to multipath, efficient use of spectrum, and straightforward equalization, since each subcarrier can be treated as a flat-fading channel with a one-tap equalizer. Challenges include sensitivity to carrier frequency and timing offsets, Doppler shifts, high peak-to-average power ratio, and the need for precise synchronization and channel estimation.

OFDM has been standardized in many wireless and broadcast systems, including IEEE 802.11a/g/n/ac/ax, DVB-T and DVB-T2, and mobile networks such as LTE and 5G NR. Its development began in the 1960s and 1970s and gained prominence in broadband access and wireless communications as a practical multi-carrier modulation scheme.