Orthogonal Frequency Division Multiplexing (OFDM)

Orthogonal Frequency Division Multiplexing (OFDM) is a modulation scheme that converts a high-rate data stream into a number of low-rate streams that are transmitted on parallel subcarriers (tones). A digital implementation of this principle makes use of the inverse Fast Fourier Transform (IFFT). In delay-dispersive channels, intercarrier interference and intersymbol interference can be avoided, or at least mitigated, through a cyclic prefix at the beginning of each OFDM symbol, which converts the linear convolution of the signal with the channel impulse response into a circular convolution, and thus allows single-tap equalization. Coded OFDM allows to exploit the frequency diversity by distributing redundant information over different subcarriers; alternatively spreading information over subcarriers by a spreading sequence results in multicarrier CDMA (MC-CDMA), or (by spreading with an FFT sequence) single-carrier transmission with frequency-domain equalization (SC-FDE). Estimation of the channel transfer function, required for demodulation, is obtained from pilot symbols. By combining least-squares (LS) estimates with knowledge about the channel statistics, linear minimum mean square error (LMMSE) estimates can be obtained. Reduced-rank channel estimation can be obtained through the use of eigenvalue decompositions. A major drawback of OFDM is a high peak-to-average power ratio (PAPR), as it leads to spectral regrowth when nonlinear power amplifiers are used. Methods for its mitigation include coding for PAPR reduction, phase adjustment, and correction by multiplicative or additive functions. Adaptive modulation and coding adjusts the modulation and coding scheme on a particular subcarrier to the channel quality. Optimum power allocation to the subcarriers can be determined from the waterfilling rule. A combination of OFDM with TDMA or FDMA results in Orthogonal Frequency Division Multiple Access (OFDMA).