Viterbi 解码。此解码用递推的方法来减少计算量。对于记忆长度为L的信道，当前的理想信道输出值由当前的信道输入与过去L个信道输入决定。-Viterbi decoding. This decoding recursive way to reduce the amount of calculation. For the memory of length L of the channel, the current ideal channel output value the current an