50MHz QAM256 4x4 MIMO - Uplink

[RafalArciszewski]

Hello,
I am trying to run MIMO on UL direction, but regardless of the mode 4x1 4x2 4x4 I always get the same result 50Mbps. Does srsRAN currently supports only rx_diversity?

The second question is for Downlink. Maksimum MCS received by the UE is 26. Is it possilbe to use MSC 27?

The setup

Uplink Traffic:

Downlink Traffic:

Regards,
Rafał Arciszewski

[improperaffo]

The maximum MCS order for the DL channel can either be 27 or 28 depending on which msc table you are using (28 for qam64 and 27 for qam256). In SRS you can also specify a minimum and maximum MCS within a table:

  pdsch:
    min_ue_mcs: 0                           # Optional UINT (0). Sets a minimum PDSCH MCS value to be used for all UEs. Supported: [0 - 28].
    max_ue_mcs: 28                          # Optional UINT (28). Sets a maximum PDSCH MCS value to be used for all UEs. Supported: [0 - 28].
    mcs_table: qam64                        # Optional TEXT (qam64). Sets the MCS table to use for PDSCH. Supported: [qam64, qam256].

Looking at the logs, it seems that you have some bit error rate in the DL channel. So, despite having a CQI of 15, the Outer-layer Link Adaptation (OLLA) might be lowering your MCS to 26 to reduce the coding rate in an effort to reduce retransmissions. You could try to set olla_cqi_inc_step to 0 to verify this.

  pdsch:
    olla_cqi_inc_step: 0                # Optional FLOAT (0.001). Sets the outer-loop link adaptation (OLLA) increment value. The value 0 means that OLLA is disabled. Supported: [0 - 1].

[improperaffo]

Regarding UL MIMO, which UE are you using? Do you know if it supports UL MIMO?

[RafalArciszewski]

@improperaffo thanks for the tips.
I am using UE that supports UL MIMO. I achieved on other gNB following trougputs in uplink: 4x1 -> 45.4M, 4x2 -> 90.8M, 4x4 -> 166M. I am not quite sure why UL troughput didn't double in 4x4 comparing to 4x2, but we observe it also in DL.

Regarding DL, even with perfect RF conditions and no BLER there is no MCS more than 26. What is peculiar when there is UL traffic ongoing I can see the srs allocating MCS 27 in DL direction:

  pdsch:
    mcs_table: "qam256"
    min_ue_mcs: 0           
    max_ue_mcs: 28          
    olla_cqi_inc_step: 0     
  pusch:
    mcs_table: "qam256"

DL traffic:

UL traffic:

Even if I set min_ue_mcs=27 and max_ue_mcs = 27, I still get 26.
When I set min_ue_mcs=16 and max_ue_mcs=16, I get 16 so the mechanizm is working, but not for mcs>26
I've tested also min_ue_mcs, max_ue_mcs for QAM64 and it works fine. So the problem is with setting mcs>26 with QAM256.

[dvdgrgrtt]

Hi @RafalArciszewski,
srsRAN Project doesn't support UL MIMO (you need the Enterprise version for that) - in srsRAN Project, the uplink always works as if the UE has no MIMO UL capabilities, regardless of the configuration and the actual UE.

As for the DL MCS issue, I'm not sure what's happening. Can you try after updating the code to the most recent release? From your screenshots I can see you are running a pretty old version of srsRAN.

[RafalArciszewski]

@dvdgrgrtt thank you for confirming UL MIMO support.

I've updated the code to the latest commit a041e31 from main branch and it is still the same. The console output shows mcs=26 for qam256. (qam64 is ok)

However I did some more test with manual settings in configurations.

min_ue_mcs: 26 and max_ue_mcs: 26

min_ue_mcs: 27 and max_ue_mcs: 27

The througput incresased I and see mcs=27 allocations on UE side.

So the problem is with the console not presenting correct average mcs value.

I can also see that scheduler does not allocate slot16 for PDSCH but that is a different issue do resolve:

[RafalArciszewski]

Just informational
I had csi_rs_period set to 10. When I changed it to default or 80 there is MCS =27 in the output console.

[dvdgrgrtt]

Great! Thanks for letting us know.