Possible RF collisions of gateway downlinks in RXC window and Class C

[MarekNovakACRIOS]

• [ x ] I have searched the issues of this repository and believe that this is not a duplicate.

Summary

When managing an installation of several hundreds of end-nodes acting as street light controllers with 3 gateways on a relatively small area (1.5x1.5km), I found out, by sniffing the spectrum with and SDR, that there are collisions between transmissions of the gateways. I implemented LBT on the end-node side and I plan to enable LBT on the gateways, however, there is one thing, which could help me a lot as well or anybody using Class C and a mid-intensive traffic:

• When planning the transmission, the mutual geographic location of the gateways should be taken in consideration - if there are 2 downlink frames (for RXC window, not RX2 or RX1 !) to be planned and transmitted on 2 gateways, which are very close (this could be configurable as distance in meters/km), one should be transmitted first and the second after the first, otherwise there is a collision on the air / in the band.

What is the use-case?

• class C end nodes
• Street Light Controllers - ~300-400 nodes on 1.5x1.5km area, 3 gateways
• EU868 band

Implementation description

• if the location of gateways is available, and if a new gateways is inserted, a matrix of distances should be re-calculated
• when a downlink (for class C using RXC window / e.g. 869.525MHz for EU868 band) is planned, the matrix should be checked and if the frame to be queued is to be sent to a gateway, which is too close "in time and space", then the transmission / queueing should be posponed.

Can you implement this by yourself and make a pull request?

• I am more in the Firmware side and have some contribution to https://github.com/Lora-net/LoRaMac-node project, but theretically yes
• I have however little experience with Go, more with JS/Python/C... But I can definitely make a review of associated pull/merge request!

[sp193]

If you have control over the network server: are you already using the highest-possible datarate for RX2? By using a higher datarate, you can reduce the chances of collisions, since the air time for each packet will decrease.