ctc_rscdec.m

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%  Property of Freescale
%  Freescale Confidential Proprietary
%  Freescale Copyright (C) 2005 All rights reserved
%  ----------------------------------------------------------------------------
%  $RCSfile: CTC_RSCDec.m.rca $
%  $Revision: 1.1 $
%  $Date: Mon Jan 29 18:11:35 2007 $
%  Target: Matlab
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%
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% Double binary circular Turbo decoder   FIXED POINT VERSION
%
% msg_dec = CTC_RSCDec(yk, Nep, niter, g) implements the decoder
% of double binary circular Turbo encoder defined in 8.4.9.2.3
% Convolutional turbo codes IEEE Std 802.16 2004 P.594
%
% Output:
% msg_dec => Decoded bits stream
%
% Inputs:
% yk => Rearranged signal
% Nep => Size of one block.
% niter => No. of iterations
% g => Generator polynomial.


function msg_dec = CTC_RSCDec(yk, Nep, niter, g)

% Turbo code generator polynomials
% 8.4.9.2.3.1 CTC encoder IEEE Std 802.16-2004 P.594
g = [1 1 0 1;1 0 1 1;1 0 0 1];

% Temporary
L_c = 2;

% Calculation No. of couples in transmitted block
L_total = Nep/2;

% Interleaved address for CTC interleaver
alpha = mTx_add_inter (L_total);

% Initialize extrinsic information
%Infty = 1.4;
Infty = 0;

ln_p00(1:L_total) = -Infty*ones(1,L_total);
ln_p01(1:L_total) = -Infty*ones(1,L_total);
ln_p10(1:L_total) = -Infty*ones(1,L_total);
ln_p11(1:L_total) = -Infty*ones(1,L_total);

% Initialize Alpha and Beta
Alpha_in1(1:8) = zeros(1,8);
Beta_in1(1:8)  = zeros(1,8);
Alpha_in2(1:8) = zeros(1,8);
Beta_in2(1:8)  = zeros(1,8);

for iter = 1:niter

    % First decoder (need to be deinterleaved)
    % Deinterleaving the priori info. from 2nd decoder
    for j = 1:L_total   % updated for std compliancy
        L_a_p00(alpha(j)+1) = ln_p00(j)*0.75;  % a priori info. for couple 00.
        L_a_p01(alpha(j)+1) = ln_p01(j)*0.75;  % a priori info. for couple 01.
        L_a_p10(alpha(j)+1) = ln_p10(j)*0.75;  % a priori info. for couple 10.
        L_a_p11(alpha(j)+1) = ln_p11(j)*0.75;  % a priori info. for couple 11.
    end

  
    for j = 1:L_total
        if (mod(j,2) == 0) % updated for std compliancy
            tmp_L = L_a_p10(j);
            L_a_p10(j) = L_a_p01(j);
            L_a_p01(j) = tmp_L;
        end
    end


    %-- CTC_MAPDec
    [L_00, L_01, L_10, L_11, ln_p00, ln_p01, ln_p10, ln_p11, Alpha_out1, Beta_out1] = ...
        CTC_MAPDec (yk(1,:), g, L_a_p00, L_a_p01, L_a_p10, L_a_p11, Alpha_in1, Beta_in1);


    %Update Alpha_in and Beta_in
    Alpha_in1(1:8) = Alpha_out1(1:8);
    Beta_in1(1:8)  = Beta_out1(1:8);


    % Second Decoder
    % Interleaving the priori info. from 1st decoder
    for j = 1:L_total
        if (mod(j,2) == 0)  % updated for std compliancy
            tmp_L = ln_p10(j);
            ln_p10(j) = ln_p01(j);
            ln_p01(j) = tmp_L;
        end
    end

    for j = 1:L_total        % updated for std compliancy
        L_a_p00(j) = ln_p00(alpha(j)+1);  % a priori info. for couple 00.
        L_a_p01(j) = ln_p01(alpha(j)+1);  % a priori info. for couple 01.
        L_a_p10(j) = ln_p10(alpha(j)+1);  % a priori info. for couple 10.
        L_a_p11(j) = ln_p11(alpha(j)+1);  % a priori info. for couple 11.
    end

    %-- mCTC_MAPDec
    [L_00, L_01, L_10, L_11, ln_p00, ln_p01, ln_p10, ln_p11, Alpha_out2, Beta_out2] = ...
        CTC_MAPDec (yk(2,:), g, L_a_p00, L_a_p01, L_a_p10, L_a_p11, Alpha_in2, Beta_in2);

    Alpha_in2 = Alpha_out2;
    Beta_in2  = Beta_out2;

end %iter

% Deinterleaving posteriori info.
for j = 1:L_total  % updated for std compliancy
    L_final_00(alpha(j)+1) = L_00(j);
    L_final_01(alpha(j)+1) = L_01(j);
    L_final_10(alpha(j)+1) = L_10(j);
    L_final_11(alpha(j)+1) = L_11(j);
end

for j = 1:L_total
    if (mod(j,2) == 0) % updated for std compliancy
        tmp_L = L_final_10(j);
        L_final_10(j) = L_final_01(j);
        L_final_01(j) = tmp_L;
    end
end

% Hard Decision (pick the maximum one as the decoded pair)
for i = 1:L_total
    tmp = max(max(L_final_00(i),L_final_01(i)), max(L_final_10(i),L_final_11(i)));
    if tmp == L_final_00(i)
        y(1,i) = 0;
        y(2,i) = 0;
    elseif tmp == L_final_01(i)
        y(1,i) = 0;
        y(2,i) = 1;
    elseif tmp == L_final_10(i)
        y(1,i) = 1;
        y(2,i) = 0;
    elseif tmp == L_final_11(i)
        y(1,i) = 1;
        y(2,i) = 1;
    end
end

% Parallel to Serial Convert
% for i = 1:L_total
%     msg_dec(1,2*i-1) = y(1,i);
%     msg_dec(1,2*i) = y(2,i);
% end
msg_dec = reshape(y,1,L_total*2);