Editing Turbo code (section)

==Soft decision approach==
The decoder front-end produces an integer for each bit in the data stream. This integer is a measure of how likely it is that the bit is a 0 or 1 and is also called ''soft bit''. The integer could be drawn from the range [−127, 127], where:
* −127 means "certainly 0"
* −100 means "very likely 0"
* 0 means "it could be either 0 or 1"
* 100 means "very likely 1"
* 127 means "certainly 1"

This introduces a probabilistic aspect to the data-stream from the front end, but it conveys more information about each bit than just 0 or 1.

For example, for each bit, the front end of a traditional wireless-receiver has to decide if an internal analog voltage is above or below a given threshold voltage level. For a turbo code decoder, the front end would provide an integer measure of how far the internal voltage is from the given threshold.

To decode the {{nowrap|''m'' + ''n''}}-bit block of data, the decoder front-end creates a block of likelihood measures, with one likelihood measure for each bit in the data stream. There are two parallel decoders, one for each of the {{frac|''n''|2}}-bit parity sub-blocks. Both decoders use the sub-block of ''m'' likelihoods for the payload data. The decoder working on the second parity sub-block knows the permutation that the coder used for this sub-block.