An ideal Viterbi decoder would work with infinite precision, or at least with floating-point numbers. In practical systems, we quantize the received channel symbols with one or a few bits of precision in order to reduce the complexity of the Viterbi decoder, not to mention the circuits that precede it. If the received channel symbols are quantized with more than one bit of precision, the result is called soft-decision data. A Viterbi decoder with soft decision data inputs quantized to three or four bits of precision can perform about 2 dB better than one working with hard- decision inputs. The usual quantization precision is three bits.
More bits provide little additional improvement.
The selection of the quantizing levels is an important design decision because it can have a significant effect on the performance of the link. The following is a very brief explanation of one way to set those levels. 1 and standard deviation .