Orthogonal frequency-division multiplexing

An example of OFDM, with 4 different signals, shown in different colors

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