Orthogonal frequency-division multiplexing

Orthogonal Frequency Division Multiplexing is a technology related to Frequency Division Multiplexing. With it, many different signals can be sent over the same medium, at the same time. Each signal uses a different basis function. By using the basis function given, the sender and recipient will then see their signal better, the other signals will be clearly separated.

Advantages and disadvantages

Advantages

  • Can adapt easily to bad channels (noisy,etc.)
  • Robust against crosstalk between channels that are close together.
  • Robust against Intersymbol interference (ISI) and fading caused by multipath propagation
  • High spectral efficiency
  • There are good implementations available
  • Few problems with errors that come from time synchornization
  • Tuned sub-channel receiver filters are not required (unlike conventional FDM)
  • Facilitates Single Frequency Networks, i.e. transmitter macrodiversity.

Disadvantages

  • Problems with Doppler shift.
  • Synchronizing frequencies can be problematic
  • Sensitive to frequency synchronization problems.
  • High peak-to-average-power ratio (PAPR). This needs linear transmission circuits; they need a lot of power.
  • Loss of efficiency caused by Cyclic prefix/Guard interval

Uses

Many technologies use OFDM. Among them are:

  • ADSL and VDSL
  • Power line communication
  • Wireless LAN, namely 802.11a,g, and n, and HiperLAN/2 (802.11b uses Direct-sequence spread spectrum)
  • 4G LTE mobile networks use OFDMA (OFDM multi-users variant)
  • Some Digital radio systems
  • Some Digital TV systems, most notably DVB-T
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