说明：   This function implememts Soft Output Viterbi Algorithm in trace back mode Input: rec_s: scaled received bits. rec_s(k) = 0.5 * L_c(k) * y(k) L_c = 4 * a * Es/No, reliability value of the channel y: received bits g: encoder generator matrix in binary form, g(1,:) for feedback, g(2,:) for feedforward L_a: a priori information about the info. bits. Extrinsic info. from the previous component decoder ind_dec: index of the component decoder. =1: component decoder 1 The trellis is terminated to all zero state =2: component decoder 2 The trellis is not perfectly terminated. Output: L_all: log ( P(x=1|y) ) / ( P(x=-1|y) ) Frame size, info. + tail bits