Class B Downlinks not scheduled for right time in chirpstack

john 1
Viewed 90

Hi Everyone, @brocaar I have been trying to work with lorawan network for class B, So basically when i am scheduling downlink for class B using chirpstack so it is sent to the gateway with tmms (consider 1447146208.530) and packet forwarder calculates concentrator_time(consider 788596175) as per this tmms value and consider it to transmit at that concentrator time but calculated concentrator_time for transmission is around 0.2 ms to 0.9ms / 1sec behind the current concentrator time(consider 78950066) as i tried printing these in packet forwarder.
So can you pls help me in figuring out the solution for this?
Hoping best for the response.

Thanks

chirpstack waveshare class-b

Which gateway and packet-forwarder are you using? I assume it has a GNSS fix?

Orne Brocaar•
2 Answers

hi,
I was able to fix the problem by some slight modfication in semtech packet forwarder source code like calculating GPS time from UTC time and class B works fine for my case with the quectel module itself.

Thanks

john1

Hello, can you please elaborate a little more on how you fixed this situation?

faruk•

Please use the Add comment next time, as the Post your answer for answering your original question, not for providing more context.

To hopefully answer your question:

The Waveshare concentrator module does not have a uBlox GNSS module, it has a Qualtec GNSS module. The Semtech HAL expects a uBlox module, as it uses a proprietary / binary payload for time synchronization. This might be the cause of your issue.

The warning in your logs might be related to this:

WARNING: [gps] GPS out of sync, keeping previous time reference

Orne Brocaar591

No, i am able to synchronize the time using quectel l76 module as i can receive the class B downlinks when i increase the transmit time by 0.8 sec by modifying the packet forwarder but that shouldnt be the solution and semtech packet forwarder works well with Quectel L76 module.

john•

I have heard before that Quectel should work with the Semtech HAL, but I have not been able to test this successfully myself (my module does not send the proprietary time payloads and fails to timesync). I would suggest trying a concentrator module with uBlox GNSS module to see if the issue remains. As you can see here, the Semtech HAL was only tested against the uBlox module: https://github.com/Lora-net/sx1302_hal/blob/master/libloragw/src/loragw_gps.c#L282.

Orne Brocaar•
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