LoRa Very Long Range Low Data Rate Telemetry. - RC Groups
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Feb 06, 2015, 04:47 AM
Stuart
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Discussion

LoRa Very Long Range Low Data Rate Telemetry.


In 2013 Semtech announced the SX127X series of RF transceiver ICs, principally designed for use in ISM low data rate applications. In the last year, Dorji and Hope have now released low cost transceiver modules that use the Semtech LoRa device.

LoRa (for Long Range) is a type of chirp spread spectrum modulation (although Semtech have not released the details) which is radically different to the FSK modulation used by modules such as the Hope RFM22B. The benefits of LoRa are that it can operate with signals that are below ambient noise, the net effect being a very substantial improvement in range capability over FSK devices.

Dorji kindly supplied me with some of their DRF1278F module to test last year, and latterly I have been testing the Hope RFM98, both modules use the same Semtech IC and have similar performance. For the $50SAT project I did a substantial amount of testing on the RFM22B, and was thus able to carry out a direct comparison between that device and these newer LoRa devices.

In a 40km hilltop to hilltop test, the RFM22B modules, fitted with simple wires (434 MHz) had needed 100mW to reliably cover the 40km at 1000bps. Using the same antennas and PCBs (retrofitted with LoRa devices) the same distance was covered with only 2mW\3dBm, also at 1000bps. That’s a 17dB improvement, and represents a range improvement of 7 times on a like for like power basis.

I had previously noted that for the RFM22B, there was not much of a range improvement to be gained by dropping below 1000bps. This is not the case with LoRa, by dropping the data rate to 100bps, still fast enough for control and tracking, you get a further 10dB improvement.

When fitted to a high altitude balloon tracker, and using a 5dB forward gain Omni directional vertical antenna at the receiver end I got these results for LoRa;

7dbm\5mW, 13700bps, range, 105km
10dBm\10mW, 1042bps, range 269km.
2dBm\1.6mW, 98bps, range, 242km

The Dorji DRF1278F and RFM98 LoRa devices are capable of 50mW continuous output, I was testing at the UK ISM limit of 10mW. Extrapolating the above results to the full 17dBm\50mW (if permitted) should give ranges of approx.;

17dBm\50mW 13700bps range 332km
17dBm\50mW 1042bps range 602km
17dBm\50mW 98bps range 1360km

If you were using a simple wave wires at the receiving end, expect about half the ranges above.

There is a report and working PICAXE code here;

https://www.dropbox.com/sh/zix2mt9b9...Ma?n=176594865

Dorji have some working Arduino code on their website.

Note:
The range figures for 98bps are likely an underestimate, due to a limitation of the testing method.
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Feb 06, 2015, 05:43 AM
Registered User
Does this encoding offer much advantage at higher data rates?
Feb 06, 2015, 06:35 AM
Stuart
srnet's Avatar
The test result I had at 13700bps (mentioned above) suggests there is an advantage as does the SNR spec in the data sheet. I can't make a direct comparison as I have not tested FSK devices such as RFM22B at high data rates.

Also bear in mind that high data rates are only achieved by a substantial increase in signal bandwidth, up to 500khz.

At a common bandwidth requirement to not exceed 25khz, the maxium data rate from the LoRa modem is only 1560bps, plenty for most of its intended uses.
Feb 06, 2015, 11:45 AM
Registered User
ttabbal's Avatar
Thanks for the detailed report and testing. They look great for lots of things.
Apr 18, 2015, 04:19 AM
Stuart
srnet's Avatar
An Instructable on LoRa;

http://www.instructables.com/id/Introducing-LoRa-/
May 21, 2015, 09:13 AM
Registered User
thx, Srnet. This stuff looks impresive. I will buy some of them to test.

I would like to make a lost model beacon, or just for telemetry needs, but may be later this could be fitted in the openlrs project.
May 21, 2015, 03:19 PM
Stuart
srnet's Avatar
Quote:
but may be later this could be fitted in the openlrs project
How ?

The LoRa devices are in no way compatible with the RFM22B (Si4432) used in openLRS ?
May 22, 2015, 02:23 AM
Registered User
Quote:
Originally Posted by srnet
How ?

The LoRa devices are in no way compatible with the RFM22B (Si4432) used in openLRS ?
Is it a question? I dont know if these devices are compatible. If so, I would be easy to make the DIY openlrs, but LoRa (RFM98 + AVR328p + two leds...).

Has anybody do that thing already?
May 22, 2015, 02:38 AM
Stuart
srnet's Avatar
Quote:
Originally Posted by ardufriki
Is it a question? I dont know if these devices are compatible. If so, I would be easy to make the DIY openlrs, but LoRa (RFM98 + AVR328p + two leds...).

Has anybody do that thing already?
They are not at all compatible, LoRa is a completly different modulation method.

The RFM98\DRF1278F will operate as a RFM22B style FSK device as well, so potentially you could switch between FSK mode for RC, and LoRa for telemetry, but the FSK side is completly different to the operation of the RFM22B, so a great deal of work is required to get it working I suspect.

As a LoRa device they cannot operate reliably at the bandwidths used for RC control due to limitations in the crystals used, these devices are very picky about frequency differances and drift between RX and TX. Even if that problem were solved, with a better module fitted with a TCXO, the data rate would only be around 1500bps, which is no where near enough for RC control.

What the LoRa device are stunningly good at is what they were designed for, very long distance low data rate telemetry.
Last edited by srnet; May 22, 2015 at 02:45 AM.
May 22, 2015, 02:47 AM
Registered User
Quote:
Originally Posted by srnet
They are not at all compatible, completly different modulation method.

The LoRa devices are designed as low data rate devices.

They cannot operate at the bandwidths used for RC control due to limitations in the crystals used, the devices are very picky about frequency differances between RX and TX. Even if that problem were solved, with a better module fitted with a TCXO, the data rate would only be around 1500bps, which is no where near enough for RC control.

But I have seen modules in ebay.com for 433 MHz, and the pinout looks similar to rfm22. Doesnt it fit in the openlrs project?

1500bps is way low, but not for a semi autonomous aircraft/multicopter (mission & GPS guided). The range records would fall soon.
May 22, 2015, 03:42 AM
Stuart
srnet's Avatar
Quote:
But I have seen modules in ebay.com for 433 MHz, and the pinout looks similar to rfm22. Doesnt it fit in the openlrs project?
It is pin compatible, but due to differences in the pads may not fit the openLRS PCBs.

Quote:
1500bps is way low, but not for a semi autonomous aircraft/multicopter (mission & GPS guided).
But as I said you need a module with a TCXO to make it work, as yet no-one has made one.

And I cant think of a country where it would be legal to use such a device for RC model control ................
May 22, 2015, 07:39 AM
Registered User
Thx Srnet. Crystal clear understood now !!

So, for now, this has more to do with slow data / long range telemetry projects.
May 22, 2015, 11:43 AM
Stuart
srnet's Avatar
Quote:
Originally Posted by ardufriki
So, for now, this has more to do with slow data / long range telemetry projects.
Thats what its designed for.

You can use higher than than 1500bps, but that takes the bandwidth above 25khz and your restricted to 10% duty cycle, meaning you can only transmit 10% of the time. This is not a problem for data telemetry, but a show stopper for responsive RC control.

One thing that surprises me is that no-one seems to have adressed the obvious weak point of the RFM22B\openLRS setup, namely the receiver part. The RFM22B RX is really quite deaf, put a low noise amplifier in front of it and you would get a range improvement of about 4 times without having to increase the TX power.
May 22, 2015, 03:23 PM
Registered User
speedyb_tx's Avatar
"When fitted to a high altitude balloon tracker, and using a 5dB forward gain Omni directional vertical antenna at the receiver end"

would this be the correct antenna to use as it's directionality would be not be in the upwards direction of the ballon ??
May 22, 2015, 04:24 PM
Stuart
srnet's Avatar
Quote:
Originally Posted by speedyb_tx
"When fitted to a high altitude balloon tracker, and using a 5dB forward gain Omni directional vertical antenna at the receiver end"

would this be the correct antenna to use as it's directionality would be not be in the upwards direction of the ballon ??
If the balloon you are tracking is at 8km altitude, and 270km away, it that 'up' ?

A high gain vertical omni is the perfect antenna for the job, a reasonable amount of gain level with the horizon and you dont need to point it depending on where the balloon goes.
Last edited by srnet; May 22, 2015 at 04:29 PM.