Increase uplink slots

[jasonfarmer0992]

Does anybody know if it is possible to increase the number of slots that are used for uplink and decrease the number of slots used for downlink in a 5G TDD setup? Can this be set in the configuration file, or is a code change required? It appears that the default slot allocation is 80% downlink and 20% uplink. I would like to flip that to favor uplink.

[brendan-mcauliffe]

Hi @jasonfarmer0992 if you take a look at the Configuration Reference page of the docs, here, you will see there are parameters related to the TDD slot configuration under the sub-section tdd_ul_dl_cfg within the cell-cfg section.

[frankist]

As @brendan-mcauliffe said, you can change the TDD pattern in the config file.

We never tested a setup with more UL slots than DL slots, so let me know if you run into issues.

[frankist]

Would it be possible to use the TDD-UL-DL-ConfigCommon to set a static configuration? This is discussed in section 11 of TS 38.213. I think it uses the RRC instead of PDCCH DCI. Can srsRAN already do this?

Yes, we only use the static configuration via TDD-UL-DL-ConfigCommon. When you are changing the TDD pattern in the config file, you are effectively altering this field in the RRC message. You can confirm this change via the mac or f1ap pcaps generated by the srsgnb.

Also, I've seen where it is possible to use different slot formats where each slot has some uplink and downlink symbols in it. Could slot formats be set in srsRAN to adjust UL and DL allocation? For example, table 11.1.1-1 in TS 38.213 shows slot format 36 with 3 DL symbols and 10 UL symbols.

Unfortunately, we do not fully support this at the moment. We support PDCCH, PDSCH, and PUCCH in special slots (i.e. slots with some DL and some UL symbols), but not PUSCH. To support it, we would need to support changing the DMRS positioning. Not sure how hard it is to do it.

If you do not mind me asking, is there anything in specific that you are trying to do that requires more esoteric TDD patterns?

[jasonfarmer0992]

Thank you for the information @frankist.

I want to upload multiple high resolution video data streams in real time to a server.

If I set the PDSCH to contain some UL symbols, would that increase my UL throughput?

[herlesupreeth]

@jasonfarmer0992

Also, I've seen where it is possible to use different slot formats where each slot has some uplink and downlink symbols in it. Could slot formats be set in srsRAN to adjust UL and DL allocation? For example, table 11.1.1-1 in TS 38.213 shows slot format 36 with 3 DL symbols and 10 UL symbols.

In order to specify those slot formats DCI 2_0 is needed, which is not yet supported. Also, as @frankist mentioned, currently we support scheduling of PDSCH in slots where some of the symbols are DL but we dont support scheduling of PUSCH in slots where not all symbols are UL.

[herlesupreeth]

For now, I would suggest to change the TDD configuration as follows in your gnb.yaml file and give it a try

  tdd_ul_dl_cfg:
    dl_ul_tx_period: 10
    nof_dl_slots: 4
    nof_dl_symbols: 9
    nof_ul_slots: 5
    nof_ul_symbols: 0

[jasonfarmer0992]

@herlesupreeth thank you so much! That increased my uplink speed by about 20 mbps.

[HaoxinSEU]

Hi @jasonfarmer0992,

May I ask one question about your results under this configuration? Now the numbers of slots and symbols allocated to DL and UL are quite close, so I assume their throughput should also be similar.

But what I got is that DL throughput is higher than UL. Did you get the same results? Or in your tests these two throughput are the same?

Thanks!

[herlesupreeth]

btw, you can also use 256QAM MCS table for PUSCH and may get even higher uplink throughput (you could also do the same for PDSCH as well)

  pusch:
    mcs_table: qam256

[jasonfarmer0992]

@HaoxinSEU Yes, my DL throughput is still a little higher than my UL throughput.

My previous throughput was around UL 50 Mbps, DL 110 Mbps.
My new throughput is around UL 70 Mbps, DL 90 Mbps.

Based on the slot configuration, I would have expected them to be the same. My best guess is either a lower quality receive channel on my setup or my EM9191 UE is using a lower modulation scheme for some reason. It could also be some other reason from the 5G spec that I am completely unaware of.

@herlesupreeth I will try the 256QAM table again. I used it before but my SNR was poor due to low gain antennas, so I switched back to 64QAM. I just upgraded to a sector antenna that is way better, so it might work now.

[jasonfarmer0992]

Thank you to everyone for your replies! I really appreciate your help.

I tried changing the number of DL and UL slots in the cell-cfg section, but I still can't get the uplink data rate to increase. However, the downlink data rate will decrease. I seem to be having the same problem as @HaoxinSEU. With my original setup of 3 UL slots, 6 DL slots, SISO antenna setup, and 50 MHz of bandwidth, I can achieve approximately 110 mbps download and 45 mbps upload. This seems to be in line with other users that I have seen on srsRAN forums for the maximum performance of the USRP B210 with 64QAM modulation. No matter what I change in the config file, I cannot increase the uplink speed. I can only decrease the downlink speed.

I have attached two zip archives. Each zip archive has a config and log file. I set the log all_level to info to get everything. Each log was generated by running srsRAN with the associated config file and doing a few iperf3 uplink and downlink speed tests using a Sierra Wireless EM9191 modem as the UE. The speed testing results are as follows:

• 3 uplink slots, 6 downlink slots: UL 45 mbps, DL 110 mbps
• 6 uplink slots, 3 downlink slots: UL 45 mbps, DL 45 mbps

I have tinkered with many other settings in the config file that I thought might be overriding the slot configuration. You can see in the config file where I commented out various settings I have tried. All the other settings I tried did nothing or caused worse performance.
srsran_config_and_log_3upslots_6downslots.zip
srsran_config_and_log_6upslots_3downslots.zip

[jasonfarmer0992]

I appreciate everyone's help on this. Thanks a lot @herlesupreeth and @frankist! The current UL rate can handle my setup now. I will probably have to upgrade my SDR to get more bandwidth for UL and DL in the future. I am going to go ahead and close this for now. If you ever add more slot/symbol configuration options and need help testing, let me know.

[fllay]

Hi
how does this configuration look like?

  tdd_ul_dl_cfg:
    dl_ul_tx_period: 10
    nof_dl_slots: 4
    nof_dl_symbols: 9
    nof_ul_slots: 5
    nof_ul_symbols: 0

something like

DSUUUDSUUD ?

and what nof_dl_symbols: 9 mean? Does it mean 9 out of 14 symbols in sub-frame are downlink symbols? if so, which sub-frame this 9 downlink symbols are in?

[herlesupreeth]

No, the pattern is as follows DDDDSUUUUU . And those DL symbols are for the S slot, so S slot first 9 symbols are DL and remaining are not assigned

[fllay]

Thank you very much. May I ask why do you choose nof_dl_symbols: 9 for this use case?

[herlesupreeth]

You can choose any number of symbols in the range 5-10. The reason for max of 10 DL symbols is to allow sufficient time for switching to UL

[herlesupreeth]

Of course the config doesn’t restrict you to 10 DL symbols but it is advised to allow 2-4 symbols for switching from DL to UL

[fllay]

thank you. It seems that we can control switching time using S slot.