Author Topic: RFM69HW RSSI readout question  (Read 4623 times)

tomaszlanger

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RFM69HW RSSI readout question
« on: February 06, 2014, 04:07:17 AM »
Hi,

I have some problem with RFM69HW module.
I wrote some code based on LOWPOWERLAB RFM69 library.
I have 4 modules that communicate without problems with FSK modulation and 57,6kbs baudrate (868Mhz, +20dbm mode).

Now I try avoid some collision when transmitting data. I`m checking RSSI before sending data like in LowPowerLab library.
When RSSI is above -90 dbm channel is busy and transmission is delayed.

LowPowerLab library code in this case is:
//
 if (_mode == RF69_MODE_RX && PAYLOADLEN == 0 && readRSSI() < CSMA_LIMIT) //if signal stronger than -100dBm is detected assume channel activity
CSMA_LIMIT is set to -90dbm
//

That means, author of this code has RSSI lower than -90dbm (-100dbm in comment ?) when there is no RF signal.

In my case I have signal between -90 and -80dbm (depends from module) when is no RF signal (my CSMA_LIMIT is -90dbm).
This indicates to code that channel is still busy.
Additional, I can still receive proper packets when RSSI is about -90dbm.

I can move my CSMA_LIMIT for example to -70dbm
That means, when signal level is LOW, I can still receive packets, but I can`t recognize channel activity


My questions are:
What are practical RSSI values for RFM69HW when nobody transmit data (channel is free)?
Maybe I`m doing something wrong. Should be my RSSI values lower than -90dbm when channel is free.

Best regards,
TL


« Last Edit: February 06, 2014, 07:29:20 AM by Felix »

Felix

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Re: RFM69HW RSSI readout question
« Reply #1 on: February 06, 2014, 07:28:29 AM »
Lower = smaller number (ie -100dBm is lower than -90).
So at -90 there is more energy than at -100. You can assume that at -100 the channel is free and at -90 there is something going on.
Something like -80, -70, -50 is going towards very strong signal.

tomaszlanger

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Re: RFM69HW RSSI readout question
« Reply #2 on: February 06, 2014, 08:28:55 AM »
My  problem is with RSSI readout.

I made test:
1. I was sending stream of packets non-stop from transmiter.
2. On receiver I was looking for RSSI of every received packet on LCD Display.
3. I started receding with receiver from transmiter and checking RSSI of received packets.
4. Last packets I was able to receive was about -90dbm. It`s looks ok.

But, When I try to read RSSI when nobody transmits I have values about -80dbm.

This mean that for distand devices I`m able to receive packets, but I`m not able to detect channel activity during data transmitting.
This confusing me, because I wanted use RSSI readout to avoid collisions,  (carrier sense, channel activity).

TL


Felix

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Re: RFM69HW RSSI readout question
« Reply #3 on: February 06, 2014, 10:19:57 AM »
The library is doing the RSSI checking for activity before sending, so it just works, not much effort needed from your part.
If you modify my library or port it, I don't know how you use it and no way of testing it with your hardware so I can't help much.
If you have faulty modules then that also will cause confusion.

john k2ox

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Re: RFM69HW RSSI readout question
« Reply #4 on: February 27, 2014, 11:00:02 PM »
Your radios are not the only ones being heard.

chalupien

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Re: RFM69HW RSSI readout question
« Reply #5 on: April 25, 2014, 08:45:09 AM »
What happens if I change the CSMA_LIMIT to -110??

Felix

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Re: RFM69HW RSSI readout question
« Reply #6 on: April 25, 2014, 09:07:07 AM »
It will be more sensitive, but might also pick up some noise.

chalupien

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Re: RFM69HW RSSI readout question
« Reply #7 on: April 25, 2014, 09:10:22 AM »
Would noise look garble in the data received?

Felix

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Re: RFM69HW RSSI readout question
« Reply #8 on: April 25, 2014, 11:55:46 AM »
Yes