Orthogonal frequency division multiplexing
Orthogonal frequency division multiplexing (OFDM) has been around for some time under the name discrete multi-tone modulation (DMT). It is based on traditional frequency division multiplexing (FDM), but it is used as a digital modulation scheme.
OFDM operates on a bit stream that has been divided into several lower-bit rate streams. The available spectrum is divided into a number of sub-channels and these low-bit rate data streams are on modulated onto sub-carriers in each of these channels. The modulated carriers are combined into a single signal using an inverse Fast Fourier Transform. The signal is sent onto the receiver, which repeats the process in reverse to recover the original stream. Since all of the low-bit rate channels are transmitted simultaneously, the aggregate data rate is the sum of the low-bit rate channels.
The bit rate of the channel is determined by the channel characteristics and the modulation scheme. Typical OFDM systems use BPSK, QPSK, or QAM procedures to modulate the data. If you look at a 256 sub-channel system and assume that each channel carries a 50 kbps data stream, the aggregate data rate could be as high as 12.8 Mbps. Since OFDM comes in flavors from 128–2048 sub-carriers, impressive aggregate data speeds can be attained.
At the core of OFDM is choice of sub-carrier frequencies so that the modulated data streams are orthogonal to each other. This means that there is no crosstalk between the sub-channels and guard bands between the sub-channels are not required. The orthogonality of the modulated carriers also results in operation of the sub-channel close to the Nyquist rate. Orthogonality also simplifies the equipment design.
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