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

[Xavier Arteaga]

Hi Leonardo,
Many thanks for your email.

The figure you sent made me understand the problem.

I wonder if the cause is that some LLR's amplitudes are too small
for high-order modulation.

Does it happen the same if the modulation is fixed to BPSK and uses Target
Code Rate instead of MCS?

Xavier


On Tue, 22 Jun 2021 at 15:04, Leonardo Lo Schiavo <
leonardo.loschiavo at imdea.org> wrote:

> Hi Xavier,
>
> yes, I have tried all the implementations and observed that those
> implementations with 8-bits LLRs (avx512, avx2, and basic 8bits) yield the
> issue that I mentioned in the last email. The other implementations (16bits
> LLRs and float LLRs) do not yield such issue, and are giving much more
> reasonable BLER performances.
>
> I am attaching some plots showing the avx512 layered implementation and
> float LLRs implementation performance in terms of number of decoding
> iterations needed to successfully decode the codeblock. White points
> indicate that for the specific SNR-MCS pair it was not possible to decode
> within 10 iterations and three different scaling factors were tested (0.5,
> 0.8 and 1.0).
>
> I would appreciate if you had further suggestion to investigate this
> issue. Many thanks in advance.
>
> Best Regards,
>
> Leonardo Lo Schiavo
> PhD Student @ IMDEA Networks Institute
> Leganes, Madrid, Spain
>
>
> On 15-06-2021 09:45, Xavier Arteaga wrote:
>
> 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
>> _______________________________________________
>> srslte-users mailing list
>> srslte-users at lists.softwareradiosystems.com
>> https://lists.softwareradiosystems.com/mailman/listinfo/srslte-users
>
>
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