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Connectivity Problems with LoRa High Spreading Codes  XML
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Ken Biba



Joined: 16/12/2015 12:54:23
Messages: 6
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I am building a 900 MHz LoRa system from Miniwireless Anarduino modules and the RadioHead library.

I am looking to test for maximum range with very small packets (17 octets).

I find the system works fine when the total frame airtime is less than 1 second, begins to deteriorate (high packet error rates) when the frame airtime is about 1 second and becomes totally blocked (no packets successfully received) above that.

This seems to be correlated with high spreading codes and narrow bandwidth.

For example: 31.25 KHz @ Sf9 (.85 sec airtime) appears to work fine (as well as many other mode combinations with shorter airtimes).

I begin to see packet errors (50%+?) at 1.1 sec frame airtime (125KHz @Sf12, 62.5KHz @Sf11, 32.25KHz @Sf10). I cannot seem to get communication at 31.25 KHz@Sf12 (4600 msec), 62.5 KHz @Sf12 (2300 msec), 31.25KHz @Sf11 (2640msec). 31.25 KHz@Sf10 (1320msec)seems a bit anomalous - one software does nothing, the other gets a few packets thru.

Any thoughts?
Rick



Joined: 30/05/2014 22:08:56
Messages: 93
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Hi Ken,

Curious what library you are using for the LoRa's.
We have used the radiohead library quite successfully in several projects.
Seems to be of solid quality, and there are several working examples.
Here's the link for their LoRa class for the RF95.
http://www.airspayce.com/mikem/arduino/RadioHead/classRH__RF95.html
Here's the header file:
http://www.airspayce.com/mikem/arduino/RadioHead/RH__RF95_8h_source.html

I suggest taking a detailed look into the register settings, rate, bandwidth, spreading factor, etc.
Lots of trade-offs. It seems to me this will hold the key for improvement. It sounds like you're already doing quite a bit of this already.

Another place to look is doing some searches on the Semtech LoRa parts. For example, the RFM95 is based on the Semtech SX1276.
I would bet the documentation for the Semtech might have additional details.

Very curious to find out how things are going, and hopefully you will post more here for the benefit of others.
I've heard rumors of excellent ranges, but what range are you finding? What obstructions to you have?

Rick



Joined: 30/05/2014 22:08:56
Messages: 93
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Hi Ken,

I'm adding follow-up information here for the benefit of other customers.

According to HopeRF Engineering...

When dealing with larger spreading factor and narrow bandwidth, it may require use of a higher precision TCXO crystal.
The current crystal used is a 10ppm, so if the bandwidth is < 125Khz and spreading factor > 11, the offset frequency may not meet the application requirements.
HopeRF is now working on manufacturing a new module, with a TCXO 2ppm crystal, which will remedy this situation.

These will cost only slightly more than the normal 10ppm units (about $0.20), and we will be purchasing in volume and offering for sale as soon as possible.

Best Regards,
Rick

grandyeul



Joined: 25/04/2016 08:38:52
Messages: 3
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@Rick,

Thanks for the info. Regarding this, what would be the best modem configuration I could use for my remote nodes (in a dense woody area) to get the best range possible in the 900mhz ISM band. I don't understand all the parameters I'm playing with (CR, preamble lenght, CRC etc...) If you could explain these parameters a bit, I would be very happy Thanks !
grandyeul



Joined: 25/04/2016 08:38:52
Messages: 3
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Little update, I managed to get the modem to work with 41.7BW SF12. According to the LoRa Modem Calculator Tool, this setting has a max crystal offset of 12.1ppm so we're probably fine with the 10ppm the RFM95 has. Correct me if i'm wrong. Thanks !
Rick



Joined: 30/05/2014 22:08:56
Messages: 93
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Sounds to me like the offset is within acceptable range. Let us know how your project progresses. Thx.
DanPea


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Joined: 01/11/2015 16:41:12
Messages: 5
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I have used on of the Miniwireless modules with the RFM95 radio model installed to transmit packets over a 20km range.

I also ran into issue with loosing packets on narrow bandwidths and higher spreading factor setting.



I suspected the offset in the crystal on each board was causing the down converted IF to fall outside its bandwidth.
I also suspected that there might be some thermal drift in the crystals causing issues also.

I had a look at the 868MHz outputs of each board using a spectrum analyser to work out what offset existed between the two.

I then adjusted the reference oscillator frequency in the library file RH_RF95.h so both modules had exactly the same carrier frequency.



This took a bit of experimentation to get it right.

Im sure a more expensive crystal would allow this board to operate successfully using the lowest bandwidth and highest spreading factors.

Cheers

Dan



 
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