Author Topic: Changing Frequencies on Transmit from Gateway  (Read 47 times)

ecliptic

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Changing Frequencies on Transmit from Gateway
« on: August 22, 2019, 12:24:48 PM »
I am running a RFM95W sensor with a custom gateway that also uses the RFM95W.  I've set both the node and gateway to communicate over 915.0 frequency.

I have several nodes on the same frequency, and I'd like to try and keep any specific frequency from getting too clogged.  So, I'm considering having the node receive on a different frequency than the transmit.  Of course the gateway would do the same (just the opposite direction).

Is this a plausible approach to reduce some possible collisions?  Can I use a frequency that is down to a decimal point?  I.e. Listen on 915.0 and transmit on 915.1?

Felix

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Re: Changing Frequencies on Transmit from Gateway
« Reply #1 on: August 22, 2019, 02:41:16 PM »
One observation is - I would not fix something that is not broken.
In real cases, congestion becomes an issue with a lot more nodes than just a few.

Also - if you have [1 listener] and [1 transmitter at 1 time] on 1 "frequency" (if you use LoRa then its already spread spectrum), then I don't see the real gain of this scheme. It's not like the gateway can listen on 1 frequency, and transmit on another, at the same time. It cannot be in 2 places at 1 time.

Whoever transmits, normally needs to:
1) Switch to the transmitting channel
2) Sense energy in the channel first to detect an ongoing transmission - rather than just shoot in the dark
3) Transmit

What that transmission channel is makes no difference, the congestion happens in that channel, and the gateway is always tied up in it. A node transmitting in the TX channel, while the gateway is gone to RX channel ... not sure if it can be called congestion reduction, but it sure doesn't help the node get its message to the gateway.
Switching frequencies and transmission modes involves delays so.

I see this as merely an extra effort of switching frequencies, rather than keeping the same frequency and managing transmissions properly and implement a retry mechanism.

I would keep the channels at least a few hundred khz apart for LoRa modulation, 500khz or even 1mhz won't hurt.

ecliptic

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Re: Changing Frequencies on Transmit from Gateway
« Reply #2 on: August 22, 2019, 03:06:05 PM »
That makes a lot of sense.  I've seen a very expensive Semtec transceiver for LoRaWAN, and my assumption is that it can handle signal processing across multiple channels simultaneously.  Like you said, even then the MCU itself needs to be taking advantage of that too (multi-core).

I was informed that the FCC may have an issue with "bad" nodes on a network that are sending too much data too often on any 1 frequency.  Is there a document that can be referenced for that kind of stuff for LoRa?

Thank you so much for your detailed explanation!