OFDM

OFDM - Orthogonal frequency-division multiplexing. OFDM transceiver implemented using IFFT / FFT.

The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions (for example, attenuation of high frequencies in a long copper wire, narrowband interference and frequency-selective fading due to multipath) without complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to handle time-spreading and eliminate intersymbol interference (ISI).

Advantages:

• • Can easily adapt to severe channel conditions without complex equalization.

  • Robust against narrow-band co-channel interference.

  • Robust against intersymbol interference (ISI) and fading caused by multipath propagation.

  • High spectral efficiency as compared to conventional modulation schemes, spread spectrum, etc.

  • Efficient implementation using Fast Fourier Transform (FFT).

  • Low sensitivity to time synchronization errors.

  • Tuned sub-channel receiver filters are not required (unlike conventional FDM).

  • Facilitates single frequency networks (SFNs); i.e., transmitter macrodiversity.

Disadvantages:

• • Sensitive to Doppler shift.

  • Sensitive to frequency synchronization problems.

  • High peak-to-average-power ratio (PAPR), requiring linear transmitter circuitry, which suffers from poor power efficiency.

  • Loss of efficiency caused by cyclic prefix/guard interval.