ultiplexing) A digital transmission technique that uses a large number of carriers spaced apart at slightly different frequencies. First promoted in the early 1990s for wireless LANs, OFDM is used in many wireless applications including Wi-Fi, WiMAX, LTE, ultra-wideband (UMB), as well as digital radio and TV broadcasting in Europe and Japan. OFDM is also used in land-based ADSL Internet service (see DSL
OFDM Uses Subcarriers
Although frequency division multiplexing (FDM) by itself implies multiple data streams, OFDM carries only one data stream broken up into multiple signals. Hundreds of carriers are used; each one a "subcarrier" 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
). Coded OFDM (COFDM) adds forward error correction. See OFDMA