Orthogonal frequency-division multiplexing (OFDM) is a method of digital signal modulation in which a single data stream is split across several separate narrowband channels at different frequencies to reduce interference and crosstalk. The original data stream bits -- that in a conventional single-channel modulation scheme would be sent serially (one after the other) -- are transmitted in parallel (several at once on separate channels) but at lower speed in each substream (a stream within another stream) relative to the original signal. This means symbols sent in the substreams are longer and spaced farther apart. In the original stream, each bit might be represented by a 1-nanosecond (ns) segment of the signal, with 0.25 ns spacing between bits. Splitting the signal across four component streams lets each bit be represented by 4 ns of the signal with 1ns spacing between. This reduces interference among symbols and makes it easier to receive each symbol accurately while maintaining the same throughput (in this example, 4 bits every 5 ns).
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OFDM technology was first conceived of in the 1960s and 1970s during research into minimizing interference among channels near each other in frequency and to achieve clean data transmission in situations prone to interference and signal corruption when more conventional modulation schemes are used.