# My LDPC BER curve is bad

4 views (last 30 days)
Rishi Balasubramanian on 22 Jan 2021
Hello,
For my Masters Thesis, I am trying to simulate Quasi Cyclic Low-density Parity-Check Codes (QC - LDPC) codes on Matlab. Here are the details regarding it.
Parity Matrix - Quasi Cyclic LDPC (93 x 155)
Encoder - Approximate Lower Triangular Encoding (rearrange matrix to ALT form)
Decoders - Sum-Product and Min Sum Decoders.
Message Size - 62 bits
Modulation - BPSK
Channel - AWGN
I believe that I have properly coded. And I have been unable to track where the mistake is happening. But for some unidentifiable reason, by error rates are not what they're expected to be. I have attached the BER curve for reference. Any sort of help would be really appreciated. I have been plucking out all my hair for the past 20+ days failing to create a proper simulation. Thanks in advance.
clear
clc
[bb, Ha] = qcldpc(3, 5, 2, 5, 31); %Creates PCM.
H = alt(Ha); %Rearrange PCM to ALT form for Encoding
[N1, N2] = size(H);
k = N2-N1;
pcs =1;
while (pcs ~= 0)
y = round(rand(1, N2-N1));
encode = enc(H,y);
msg = [y encode];
cs = mod(msg * H', 2); %to detect a proper codeword
pcs = sum(cs);
end
[~, bits] = size(msg);
dB = 1:5; % range of SNR values in dB
R = k/N2; % code rate
iter = 10; %Iterations
N0 = (10.^(-dB/10)); %Variance values
for i = 1 : length(dB)
ber1(i) = 0; ber2(i) = 0;
for j = 1:bits
mod = bpsk(msg); %bpsk modulation
bp = mod';
tx = bp + sqrt(N0(i)/2) * randn(size(bp)); %Transmitted with AWGN
dp(i,:) = dbpsk(tx); %Demodulated
Li = -dp(i,:);
vhat1(i,:) = SPA(tx, Ha, N0(i), iter); %Decoding Using Sum Product Algorithm
vhat2(i,:) = MSA(Li, Ha, iter); %Decoding Using Min Sum Algorithm
[n1, r1] = biterr(vhat1(i,:), msg);
[n2, r2] = biterr(vhat2(i,:), msg);
ber1(i) = ber1(i) + r1;
ber2(i) = ber2(i) + r2;
end
BER1(i) = ber1(i) / (bits * length(dB)); %BER Calculation
BER2(i) = ber2(i) / (bits * length(dB));
end
figure('Name','BER vs SNR','NumberTitle','off');
semilogy(dB, BER1, 'o-');
hold;
semilogy(dB, BER2, 'o--');
legend('SPA','MSA')
grid on;
hold off;

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