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

[Xavier Arteaga]

Hi Leonardo,
First of all, many thanks for sharing your findings with us.

Have you tried if the AVX2 and generic implementations have the same BLER?

What scaling factor are you using? I observed that for 64QAM and 256QAM the
scaling factor had to be 0.8, see
https://github.com/srsran/srsRAN/blob/master/lib/src/phy/phch/sch_nr.c#L275.
Otherwise, code blocks could systematically fail.

Regards,
Xavier

On Mon, 14 Jun 2021 at 18:52, Leonardo Lo Schiavo <
leonardo.loschiavo at imdea.org> wrote:

> 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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