What is Orthogonal frequency-division multiplexing?
[[File:OFDM2.png|thumb|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 [[signal]]s 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 effect|Doppler]] shift. * Synchronizing frequencies can be problematic * Sensitive to frequency synchronization problems. * High [[Crest factor|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: * [[Digital subscriber line|ADSL]] and [[VDSL]] * [[Power line communication]] * [[Wireless LAN]], namely 802.11a,g, and n, and [[HiperLAN|HiperLAN/2]] (802.11b uses [[Direct-sequence spread spectrum]]) * 4G LTE [[Mobile phone|mobile]] networks use OFDMA (OFDM multi-users variant) * Some [[Digital radio]] systems * Some [[Digital TV]] systems, most notably [[DVB-T]] [[Category:Radio technology]]