[srslte-users] srsRAN BER Problem with LDPC Decoding for version avx512 (layered)

[Leonardo Lo Schiavo]

Dear srsRAN community, 

I am writing this email to report an issue with BER performance in LDPC
Decoding tests for the 8-bit version avx512 (.exe file in
srsRAN/build/lib/src/phy/fec/ldpc/test/ldpc_dec_avx512_test) 

Basically I am generating the input LLRs for the LDPC Decoder with a
MATLAB pipeline by using different parameters, namely PRB allocation,
SNR and MCS.
The problem is that for a given SNR value, I am getting BER=0 for higher
MCS and BER > 0 for lower MCS with 10 decoding iterations, while the
behaviour should be the opposite. 

I did the tests for both the 8-bits and 16-bits version by using the
same input LLRs (just changing the quantization) and I am not getting
the above issue for the 16-bits LLRs version (.exe file in
srsRAN/build/lib/src/phy/fec/ldpc/test/ldpc_dec_s_test).
Considering the above, I am assuming there is some kind of systematic
error when decoding using the 8-bit version for the input LLRs. 

In order to replicate the tests, I am attaching:
1) The modified source files for 8-bit and 16-bit version 
2) The input folders for 8-bit and 16-bit version (each one containing
input LLRs for each codeblock and source bits per codeblock) 

And this is an example of a test reporting the above issue:
- In srsRAN/build/lib/src/phy/fec/ldpc/test/ , run:

1) ./ldpc_dec_avx512_test -f
LDPC_srsRANq_Files/Simulation_1/ldpc_BG1_K7744_PRB50_SNR20.00_mcs15_4_/
-i 10
2) ./ldpc_dec_avx512_test -f
LDPC_srsRANq_Files/Simulation_1/ldpc_BG1_K8448_PRB50_SNR20.00_mcs16_4_/
-i 2
3) ./ldpc_dec_s_test -f
LDPC_srsRANq16_Files/Simulation_1/ldpc_BG1_K7744_PRB50_SNR20.00_mcs15_4_/
-i 2
4) ./ldpc_dec_s_test -f
LDPC_srsRANq16_Files/Simulation_1/ldpc_BG1_K8448_PRB50_SNR20.00_mcs16_4_/
-i 2 

>From the above tests, only the test 1) will give a BER > 0 (even with 10
iterations), which is not expected considering that with an higher MCS
(16) I am getting BER = 0. 

Please be aware that the same issue appears also with other
configurations of PRB, SNR and MCS. 

Could you please investigate on the above issue? Many thanks in advance.


Best Regards, 

Leonardo Lo Schiavo
PhD Student @ IMDEA Networks Institute
Leganes, Madrid, Spain
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