function L_all = sova(rec_s, g, L_a, ind_dec) % 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) )%% Copyright: Yufei Wu, Nov. 1998% MPRG lab, Virginia Tech% for academic use only% Frame size, info. + tail bitsL_total = length(L_a);[n,K] = size(g); m = K - 1;nstates = 2^m;Infty = 1e10;% SOVA window size. Make decision after 'delta' delay. Decide bit k when received bits% for bit (k+delta) are processed. Trace back from (k+delta) to k. delta = 30;    % Set up the trellis defined by g.[next_out, next_state, last_out, last_state] = trellis(g);% Initialize path metrics to -Inftyfor t=1:L_total+1   for state=1:nstates      path_metric(state,t) = -Infty;   endend% Trace forward to compute all the path metricspath_metric(1,1) = 0;for t=1:L_total   y = rec_s(2*t-1:2*t);   for state=1:nstates      sym0 = last_out(state,1:2);      sym1 = last_out(state,3:4);      state0 = last_state(state,1);      state1 = last_state(state,2);      Mk0 = y*sym0' - L_a(t)/2 + path_metric(state0,t);      Mk1 = y*sym1' + L_a(t)/2 + path_metric(state1,t);            if Mk0>Mk1         path_metric(state,t+1)=Mk0;         Mdiff(state,t+1) = Mk0 - Mk1;         prev_bit(state, t+1) = 0;      else         path_metric(state,t+1)=Mk1;         Mdiff(state,t+1) = Mk1 - Mk0;         prev_bit(state,t+1) = 1;      end   endend      % For decoder 1, trace back from all zero state, % for decoder two, trace back from the most likely stateif ind_dec == 1   mlstate(L_total+1) = 1;else   mlstate(L_total+1) = find( path_metric(:,L_total+1)==max(path_metric(:,L_total+1)) );end% Trace back to get the estimated bits, and the most likely pathfor t=L_total:-1:1   est(t) = prev_bit(mlstate(t+1),t+1);   mlstate(t) = last_state(mlstate(t+1), est(t)+1);end% Find the minimum delta that corresponds to a compitition path with different info. bit estimation.       % Give the soft outputfor t=1:L_total   llr = Infty;   for i=0:delta      if t+i<L_total+1         bit = 1-est(t+i);         temp_state = last_state(mlstate(t+i+1), bit+1);         for j=i-1:-1:0            bit = prev_bit(temp_state,t+j+1);            temp_state = last_state(temp_state, bit+1);         end         if bit~=est(t)             llr = min( llr,Mdiff(mlstate(t+i+1), t+i+1) );         end      end   end   L_all(t) = (2*est(t) - 1) * llr;end