Listen Before Talk (LBT) CAM in NR-U; Negative delay error

[George]

Hello! 

I am tryng to setup a NR-U WiFi coexistance simulation. I did manage to get running the LBT CAM when having one gNB-UE and AP-STA pair. However, when I add more UEs or gNB I get the following error:

• assert failed. cond="delay.IsPositive ()", msg="DefaultSimulatorImpl::Schedule(): Negative delay", +0.499999992s 3 file=../src/core/model/default-simulator-impl.cc, line=23

Did anyone encounter this error? How should I approach debugging this error? Also, is there documentation on how the LBT CAM works in terms of detecting the energy? I am interested to do some changes in order to do directional energy-sensing through the beamformer used. Does it use SpectrumPHY energy detection? 

Thank you!

[Soulimane Mammar]

Hi,

It seems you have some kind of overflow 

Regards

[George]

Thank you for your reply!

[George]

I have tried couple of sets of parameters but the overflow always happens. As soon as I introduce Listen before talk as a channel access method for the NR-U side this  Negative delay error it is always there. If anyone could point me at least towards some things that i could start looking at to solve this problem would be great! Thank you!

[Soulimane Mammar]

Hi,

Is the simulation taking too long to finish ?

[George]

Hello!

No, the simulation runs as normal. For simulation I am using a modified version of the N-WiGig-coexistance code from NR-U examples. I have changed it so it uses sub 7GHz gNB and WiFi. I can send it to you if you want.
The negative delay problem disappears when I assign a very low transmitting power to the STA (user of the WiFi). I think this fixes the problem because the gNBs LBT does not sense the channel idle and it can transmit.

However this does not simulate a real scenario right? In a real network the packets should be dropped if they cannot be transmitted.So I am not sure from where exactly this Negative Delay error comes from...