Interleaving and Deinterleaving
Definition: The shuffling of coded symbols or message bits out of their naturally occurring order prior to transmission for the purpose of making the distribution of symbol errors in the deinterleaved stream of symbols in their natural order as reconstructed at the receiver more uniform than may occur without the interleaving and deinterleaving steps.
Interleaving and Deinterleaving
Application: Fading and noise impulses on terrestrial links typically produce errors in bunches. (Error Detection and Correction EDAC) coding, which is also known as Forward Error Correction (FEC), methods at a given level of complexity or code rate, however, tend to perform better when errors occur uniformly in the coded symbol stream at the receiver. Since the short intervals of fades and impulsive noise that produce the errors during transmission cannot be controlled by the communicators, the order of the transmitted symbols is shuffled so that errors that occur in bunches during transmission appear to be uniformly distributed in the deinterleaved symbol stream produced at the receiver. In this way, the FEC methods perform more efficiently than they would without the interleaving and deinterleaving steps.
Interleaving and Deinterleaving
Example: Suppose three four-symbol binary words are a1a2a3a4b1b2b3b4c1c2c3c4. Suppose that severe fading occurs just as the three symbols b2b3b4 arrive at the receiver. The receiver FEC would have to correct or compensate for up to 3 out of the 4 coded symbols of word "b" in order to overcome the 3 errors caused by the fade. This may require a rather high redundancy code.
Alternately suppose the original symbols are written into a matrix by rows of the form:
a1a2a3a4
b1b2b3b4
c1c2c3c4
and then read out of the interleaving matrix by columns yielding the sequence a1b1c1a2b2c2a3b3c3a4b4c4 which is the order transmitted. Then at the time the original message had symbols b2b3b4 fade, this interleaved version of the coded symbols would have symbols c2a3b3 fade. Reconstructing the original symbol order at the receiver yields a1a2a3a4b1b2b3b4c1c2c3c4, where the symbols in error using interleaving are shown in bold for clarity. They symbol errors are now more uniformly distributed, and therefore, easier to correct. Each error is surrounded by a number of correct symbols.
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