Collision between LoRa packets with different signal strengths

Gateways
5

Strongest packet is demodulated if power difference >6dB.

6

Hi Weffel,
I don’'t kmow any beter document That explain LoRa spread spectrum.

I will try to oversimpliphy thinks. Later on you can re-read the simtech document
spread spectrum wiki

  • Resistance to jamming (interference
  • Resistance to eavesdropping
  • Resistance to fading
  • Multiple access capability - CDMA, WIFI and GPS are the most popular spread spectrum techniques.

Or over over simplified - Your and your neighbour WIFI can work at the same frequency channel. Yes both will work sub-optimal but none will be jammed. Even more the signal strenght of neighbour channel can be higher that your signal and you wil still receive data

@Hobo - Your statement is not valid for SpreadSpectrum. 6db is only valid for Narrow Band FSK or OOK modulation

7

That’s what Semtech says.

8

Hi,

This link is not working. Could you send me the document to ..@gmail.com

9

(took me 2 seconds to find)

10

Another nice resource as mentioned in another topic:

11

Hi
So if two modules start to send different massage with same length at same time exactly,Which massage is received in receiver module?

12

From @arjanvanb post just above;

‘strongest packet is demodulated if power difference >6dB’

13

Thanks
So if they have same strong…?
Another question
If two modules with same Sf,send two packet in small different time.what be happen(packet’s strong is same)?

14

The answer is here (0<6):

15

I am not sure if there has been an analysis by Semtech as to what happens when the time differences are ‘small’. It depends on how small is small, and the relation to the inevitable small difference in absolute frequency between nodes and drift during packet transmission.

Some collisions and conflict are inevitable, so why would it be important to know exaclty what happens in the range of possibilities ?

16

Where did you find this presentation?

17

Here: https://privatevideos.hubs.vidyard.com/watch/iXBL8d2mjyjubK8DhuGcKq

And for what happens when there is a collision with same SF, same frequency, and relatively same strength (difference of less than 6dB) : every symbol in collision will basically have a 50% chance of being demodulated correctly, so the more symbol in collision, the more likely the packet will be wrong in the end.

18

I am looking to collect corrupted packets at gateway to process further. can any one help with the configurations needed for the gateway ?

19

How do you want to do this? TTN won’t process corrupted packets so what will you be using? And what brand and model gateway are you using?

20

imho the first thing you have to do is to switch off CRC-check in the gateway. But I wouldn’t send the messages with negative CRC-check to TTS. You should handle them locally.

21

Why ?

Received packets at the Gatetway already have been ‘processed’ with forward error correction, so what is there to be achieved with further ‘processing’ ?

22

corrupted packets can be processed to find the error bits after that can use error correcting codes.

23

yes forward error correction is present but even due to single bit errors the packets are getting lost so by analyzing the bit error positions and by using additional processing we can make successful packet transmission without re transmission. it may reduce the interference due to retransmissions.

24

thank you for the answer , the negative CRC packets will be received by gateway?