% This script simulates the classical turbo encoding-decoding system.
% It simulates parallel concatenated convolutional codes.
% Two component rate 1/2 RSC (Recursive Systematic Convolutional) component encoders are assumed.
% First encoder is terminated with tails bits. (Info + tail) bits are scrambled and passed to
% the second encoder, while second encoder is left open without tail bits of itself.
%
% Random information bits are modulated into +1/-1, and transmitted through a AWGN channel.
% Interleavers are randomly generated for each frame.
%
% Log-MAP algorithm without quantization or approximation is used.
% By making use of ln(e^x+e^y) = max(x,y) + ln(1+e^(-abs(x-y))),
% the Log-MAP can be simplified with a look-up table for the correction function.
% If use approximation ln(e^x+e^y) = max(x,y), it becomes MAX-Log-MAP.
%
% Copyright Nov 1998, Yufei Wu
% MPRG lab, Virginia Tech.
% for academic use only
clear all
% Write display messages to a text file
diary turbo_logmap.txt
% Choose decoding algorithm
dec_alg = input(' Please enter the decoding algorithm. (0:Log-MAP, 1:SOVA) default 0 ');
if isempty(dec_alg)
dec_alg = 0;
end
% Frame size
L_total = input(' Please enter the frame size (= info + tail, default: 400) ');
if isempty(L_total)
L_total = 400; % infomation bits plus tail bits
end
% Code generator
g = input(' Please enter code generator: ( default: g = [1 1 1; 1 0 1 ] ) ');
if isempty(g)
g = [ 1 1 1;
1 0 1 ];
end
%g = [1 1 0 1; 1 1 1 1];
%g = [1 1 1 1 1; 1 0 0 0 1];
[n,K] = size(g);
m = K - 1;
nstates = 2^m;
%puncture = 0, puncturing into rate 1/2;
%puncture = 1, no puncturing
puncture = input(' Please choose punctured / unpunctured (0/1): default 0 ');
if isempty(puncture)
puncture = 0;
end
% Code rate
rate = 1/(2+puncture);
% Fading amplitude; a=1 in AWGN channel
a = 1;
% Number of iterations
niter = input(' Please enter number of iterations for each frame: default 5 ');
if isempty(niter)
niter = 5;
end
% Number of frame errors to count as a stop criterior
ferrlim = input(' Please enter number of frame errors to terminate: default 15 ');
if isempty(ferrlim)
ferrlim = 15;
end
EbN0db = input(' Please enter Eb/N0 in dB : default [2.0] ');
if isempty(EbN0db)
EbN0db = [2.0];
end
fprintf('\n\n----------------------------------------------------\n');
if dec_alg == 0
fprintf(' === Log-MAP decoder === \n');
else
fprintf(' === SOVA decoder === \n');
end
fprintf(' Frame size = %6d\n',L_total);
fprintf(' code generator: \n');
for i = 1:n
for j = 1:K
fprintf( '%6d', g(i,j));
end
fprintf('\n');
end
if puncture==0
fprintf(' Punctured, code rate = 1/2 \n');
else
fprintf(' Unpunctured, code rate = 1/3 \n');
end
fprintf(' iteration number = %6d\n', niter);
fprintf(' terminate frame errors = %6d\n', ferrlim);
fprintf(' Eb / N0 (dB) = ');
for i = 1:length(EbN0db)
fprintf('%10.2f',EbN0db(i));
end
fprintf('\n----------------------------------------------------\n\n');
fprintf('+ + + + Please be patient. Wait a while to get the result. + + + +\n');
for nEN = 1:length(EbN0db)
en = 10^(EbN0db(nEN)/10); % convert Eb/N0 from unit db to normal numbers
L_c = 4*a*en*rate; % reliability value of the channel
sigma = 1/sqrt(2*rate*en); % standard deviation of AWGN noise
% Clear bit error counter and frame error counter
errs(nEN,1:niter) = zeros(1,niter);
nferr(nEN,1:niter) = zeros(1,niter);
nframe = 0; % clear counter of transmitted frames
while nferr(nEN, niter)<ferrlim
nframe = nframe + 1;
x = round(rand(1, L_total-m)); % info. bits
[temp, alpha] = sort(rand(1,L_total)); % random interleaver mapping
en_output = encoderm( x, g, alpha, puncture ) ; % encoder output (+1/-1)
r = en_output+sigma*randn(1,L_total*(2+puncture)); % received bits
yk = demultiplex(r,alpha,puncture); % demultiplex to get input for decoder 1 and 2
% Scale the received bits
rec_s = 0.5*L_c*yk;
% Initialize extrinsic information
L_e(1:L_total) = zeros(1,L_total);
for iter = 1:niter
% Decoder one
L_a(alpha) = L_e; % a priori info.
if dec_alg == 0
L_all = logmapo(rec_s(1,:), g, L_a, 1); % complete info.
else
L_all = sova0(rec_s(1,:), g, L_a, 1); % complete info.
end
L_e = L_all - 2*rec_s(1,1:2:2*L_total) - L_a; % extrinsic info.
% Decoder two
L_a = L_e(alpha); % a priori info.
if dec_alg == 0
L_all = logmapo(rec_s(2,:), g, L_a, 2); % complete info.
else
L_all = sova0(rec_s(2,:), g, L_a, 2); % complete info.
end
L_e = L_all - 2*rec_s(2,1:2:2*L_total) - L_a; % extrinsic info.
% Estimate the info. bits
xhat(alpha) = (sign(L_all)+1)/2;
% Number of bit errors in current iteration
err(iter) = length(find(xhat(1:L_total-m)~=x));
% Count frame errors for the current iteration
if err(iter)>0
nferr(nEN,iter) = nferr(nEN,iter)+1;
end
end %iter
% Total number of bit errors for all iterations
errs(nEN,1:niter) = errs(nEN,1:niter) + err(1:niter);
if rem(nframe,3)==0 | nferr(nEN, niter)==ferrlim
% Bit error rate
ber(nEN,1:niter) = errs(nEN,1:niter)/nframe/(L_total-m);
% Frame error rate
fer(nEN,1:niter) = nferr(nEN,1:niter)/nframe;
% Display intermediate results in process
fprintf('************** Eb/N0 = %5.2f db **************\n', EbN0db(nEN));
fprintf('Frame size = %d, rate 1/%d. \n', L_total, 2+puncture);
fprintf('%d frames transmitted, %d frames in error.\n', nframe, nferr(nEN, niter));
fprintf('Bit Error Rate (from iteration 1 to iteration %d):\n', niter);
for i=1:niter
fprintf('%8.4e ', ber(nEN,i));
end
fprintf('\n');
fprintf('Frame Error Rate (from iteration 1 to iteration %d):\n', niter);
for i=1:niter
fprintf('%8.4e ', fer(nEN,i));
end
fprintf('\n');
fprintf('***********************************************\n\n');
% Save intermediate results
save turbo_sys_demo EbN0db ber fer
end
end %while
end %nEN
diary off
