In what scenario is it possible that collision happens between two packets. And if a collision happens, is it possible to demodulate? How? Regarding here it says there are two types of interference. Intra and internetwork for unlicensed networks. I suspect when a node from outside network interferes with signals inside network, it is recoverable. But how? Thanks.
I am trying to answer all your questions one after the other below.
In what scenario is it possible that collision happens between two packets.
If at the location of the receiver the signal of two or more LoRaWAN radio packets interfere so that the receiving device is not able to demodulate that packet that was addressed to it, we say that the packet was lost due to collision.
And if a collision happens, is it possible to demodulate?
If the signal of two LoRaWAN radio packets interfere at the location of the receiver, it may still demodulate the one that was addressed to it.
A radio packet that is facing collision with another radio packet at the receiver can be demodulated with high likelihood under the following conditions:
- The two radio packets are sent on different LoRaWAN channels.
(LoRa is using several channels with 125 or 500 kHz channel bandwidth.)
- The two radio packets are sent on the same LoRaWAN channel but they are modulated with a different spreading factor.
(LoRa applies 6 possible spreading factors for modulation. These are referred as SF7..SF12 where SF7 corresponds to the highest, SF12 to the lowest data rate)
There is only a little interference between neighbouring radio channels and LoRaWAN modulations of different spreading factors are orthogonal.
Regarding here it says there are two types of interference. Intra and internetwork for unlicensed networks. I suspect when a node from outside network interferes with signals inside network, it is recoverable. But how? Thanks.
Your referred link (here) is talking about inter-system/intra-system interferences and not inter-network/intra-network interferences that you mentioned in your question. It is important to make a difference between the two scenarios.
In case of LoRaWAN technology, inter-network interference means collisions with LoRaWAN radio packets that are sent by such devices that belong to other, (foreign) LoRaWAN networks. In these cases the conditions of successful demodulation of a radio packet are the same as listed above (different channel, different spreading factor).
Inter-system interference means interference with radio signals produced by non-LoRa devices (e.g.: a garage opener) that operate in the same ISM band. In these cases the signal of foreign systems is considered as noise of the LoRa signal to be demodulated. Whether or not demodulation will be successful depends on the SNR threshold of the actual spreading factor and receiver.
Lora’s spreading and frequency chirps provide some level of de-confliction, but it’s not perfect. Lora devices transmit randomly on their own schedule, so there’s always the possibility of collision. If the Lora gateway cannot de-conflict both packets, then they’re dropped. The transmitting device won’t know that the packet was dropped, unless the device was requesting information or acknowledgement. You can implement a retry feature where the gateway requests packets that were missed based on sequence numbers. If the gateway receives packets 1, 2, 3, and 5, it can request packet 4 from the device. It’s up to your device to implement that feature.
LoRa modulation can demodulate packets transmitted on same center frequency, given their spreading factors are different.
But, packets [simultaneously] sent on same center-freq and SF will collide and will not be demodulated.
However, some regions such as KR920 offer LBT (Listen-Before-Talk) that takes a reading on given center-freq for N samples to measure against a preset THRESHOLD in order to avoid possible collision.