说明：  基于维特比译码算法的BPSK解调方式，survivor_state是一个矩阵，它显示了通过网格的最优路径，这个矩阵通过一个单独的函数metric(x,y)给出，这个算法既能用于硬判决译码也能用于软判决译码。
(BPSK demodulation method based on viterbi decoding algorithm, survivor_state is a matrix, the optimal path of it shows through the grid, the metric matrix through a separate function (x, y), the algorithm can not only used for decoding hard decision can also be used to the soft decision decoding. )