(Orthogonal Frequency Division Multiplexing) A digital transmission technique that uses a large number of carriers spaced apart at slightly different frequencies. Although frequency division multiplexing (FDM) implies multiple data streams, orthogonal FDM carries only one data stream broken up into multiple signals. Hundreds or thousands of carriers, known as "subcarriers," are used for a single data channel.
Lower Speeds - Easier Detection
The multiple subcarriers enable the receiver to more easily detect the signals in environments with multipath and other interference. In addition, each subcarrier can transmit a lower-speed signal, all of which are aggregated at the receiving side into the original high-speed signal. Lower speed signals are also more easily deciphered at the receiving end.
OFDM subcarriers can be modulated by any method, although QAM and QPSK are typically used (see
QAM and
PSK). Coded OFDM (COFDM) adds forward error correction.
First promoted in the early 1990s for wireless LANs, OFDM is used in many wireless applications including Wi-Fi, digital radio and TV broadcasting in Europe and Japan and ultra-wideband (UWB). It is also used in land-based ADSL (see
OFDMA). See
CCK/OFDM and
8-VSB.
