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Coded OFDM, or COFDM, is a term used for a system in which the error
control coding and OFDM modulation processes work closely together. An
important step in a COFDM system is to interleave and code the bits prior to
the IFFT. This step serves the purpose of taking adjacent bits in the source
data and spreading them out across multiple subcarriers. One or more subcarriers
may be lost or impaired due to a frequency null, and this loss would
cause a contiguous stream of bit errors. Such a burst of errors would typically
be hard to correct. The interleaving at the transmitter spreads out the contiguous
bits such that the bit errors become spaced far apart in time. This spacing
makes it easier for the decoder to correct the errors. Another important step
in a COFDM system is to use channel information from the equalizer to determine
the reliability of the received bits. The values of the equalizer response
are used to infer the strength of the received subcarriers. For example, if the
equalizer response had a large value at a certain frequency, it would correspond
to a frequency null at that point in the channel. The equalizer response would
have a large value at that point because it is trying to compensate for the weak
received signal. This reliability information is passed on to the decoding blocks
so that they can properly weight the bits when making decoding decisions.
In the case of a frequency null, the bits would be marked as “low confidence”
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