[Xastir-dev] LORA and interfacing

Steve stevewa206 at gmail.com
Sun Aug 1 12:34:44 PDT 2021 

You have to set the Rnode with the parameters you want to sniff. So I had
to set the baud rate, bandwidth and CR to match the other tracker.

A lot of gadgets that are doing off the grid chat are using the Lora chips.
I’ve been watching these guys.
https://www.sonnetlabs.com

They say their code is open source..




On Sun, Aug 1, 2021 at 12:08 PM Kristoff <kristoff at skypro.be> wrote:

> Hi Steve,
>
>
> I must really look into the Rnode.
> If you say that you put the rnode into sniffer mode so you could
> determine the spreading-factor, does this mean that the Rnode can do
> that automatically, or that you just tried all of different spreading
> factors  and found that SF12 worked?
>
>
> Yes, SF12 packets are long, but that is exactly one of the features of
> digital communication: the possibility to 'tune' the transmission. The
> parameters are bandwidth, bitrate (speed), transmission-power and range.
> It's like WSPR or QRSS. The transmissions are very long, but you can
> decode signals that are 20 to 30 db below the noise-floor.
>
>
> If you look at page 20 of the sx1276 datasheet (*), there are some
> examples of the RF sensitivity with different SF values.
> It's not linear, but roughly speaking, going down 1 step in the SF
> scale, you lose about 2 to 3 db of sensitivity.
>
> So if you go down 2 steps in the SF scale, your bitrate will be 4 times
> faster, but you will loose some 40 to 50 % of range.
>
>
>
> In the context of APRS, I think aprs-over-lora has a place for (say)
> portable or bicycle-mobile applications next to regular 1200 baud AFSK
> on VHF or UHF for (car) mobile use.
> So I do not think that the lower bitrate of LoRa is that necessarily
> much of an issue.
>
> I hope that aprs-over-lora will make applications like chat more popular
> as that is a feature of APRS that has deserves much more attention then
> what it gets.
>
>
>
> Note.
> I had a chat in the matrix-room of the GNU Radio community on Hamming
> FEC on the Coding-Rates.
> The conclusion at this time is that -indeed- there is no reason to use
> Coding Rates 6 or 8. In both cases, they just add error-detection
> capability to CR5 or CR7; but as aprs-over-LoRa has CRC error-detection
> anyway, this does not add any value.
>
> So the choice is this:
> CR5: no error-correction, for radio-channels with little RF interference,
> CR7: error-correction enabled, for radio-channels with more RF
> interference.
>
>
> Keep in mind that a CR7 packet is 7/5 times the size of a CR5 packet.
>
>
>
> 73
>
> kristoff - on1arf
>
>
> (*)
>
> https://www.semtech.com/products/wireless-rf/lora-core/sx1276#download-resources
>
>
> On 01.08.21 18:36, Steve wrote:
> > Interesting that you mentioned coding rates.
> >   I was sniffing  OE5BPA’s Lora transmitter because Xastir is not
> decoding
> > it. I have a feeling his packets are not quite right over the air. You
> can
> > put Rnode into a packet sniffer mode, which makes it easy to see things.
> > OE5BPA’s config file has a default CR of 12… it is very long on its
> > transmission. I changed it to CR of 5.
> >
> > Just a FYI… I’m just a person who has fun using this stuff, not a
> developer
> > with coding  experience ..
> >
> > Steve N0FPF
> >
> > On Sun, Aug 1, 2021 at 5:27 AM Kristoff <kristoff at skypro.be> wrote:
> >
> >> Hi Steve,
> >>
> >>
> >> Interesting code. I'll look into it. Perhaps it is a better option then
> >> me building something (non-standard) myself.
> >>
> >>
> >> Concerning modifying the Coding-Rate, I think one should look at this
> >> per application.
> >> APRS is mostly a 'broadcast' service the most logical thing to do is to
> >> use one single 'standard' modulation-scheme for everything.
> >>
> >> However for a service like 'aprs-messages', although the traffic is
> >> transmitted inside a 'broadcast' packet, it is still directed at one
> >> specific destination.
> >> In that case, the igate or digipeater can operate on that. If it keeps
> >> track of the coding-rate of the last packet it received from that
> >> station, it can use that information in its transmissions towards that
> >> station.
> >> (you can find the coding-rate of the last packet received in register
> >> 0x18 of the chip).
> >>
> >> Concerning the coding-rate, yesterday I found this video from ccc 33c3
> >> (2016), where Matt Knight explained how he reverse-engineered most of
> >> the LoRa PHY (*)
> >>
> >> There is also this document from him. (**)
> >>
> >>
> >> It had an interesting part in it where he explains the 4 transformations
> >> done on the LoRa datastream, one of them is Hamming-coding Forward Error
> >> correction.
> >> His document has this on it:
> >> --- cut here --- cut here ---
> >>
> >> A larger codeword size improves the robustness of the FEC: Hamming(5,4)
> >> and (6,4) provide error detection as a parity bit would, whereas (7,4)
> >> and (8,4) provide single bit correction with (8,4) additionally
> >> providing dual error detection.
> >> --- cut here --- cut here ---
> >>
> >>
> >>
> >> As I see it, going from FEC 4/5 to 4/7 is interesting as it adds the
> >> possibility to do error-correction of a single bit.
> >> Going from 4/7 to 4/8 does add the ability to detect more errors, but it
> >> cannot correct it.
> >>
> >> Now, as APRS is a datacommunication-protocol (i.e. the content of the
> >> data is unknown, so the whole packet must be correct or it will not be
> >> processed) and the LoRa frame also has a CRC field, I do not see the
> >> additional advantage of that 8th bit (i.e. 4/8 vs. 4/7). It may provide
> >> more error-detection, but CRC does that anyway.
> >>
> >> So, as I see it, it looks that if you are in a area of bad coverage, it
> >> is useful to use the 4/7 coding-rate instead of the standard 4/5, but
> >> not 4/8 (as I previously mentioned). In fact, using 4/8 creates larger
> >> packets then when using 4/7, which -I think- might be a disadvantage as
> >> a bigger packets simply has more bits that can get corrupted.
> >>
> >>
> >> But, as I not an expert on hammings FEC, I might be wrong.
> >>
> >> If somebody knows more about this, feel free to reply.
> >>
> >>
> >>
> >>
> >> (*) https://media.ccc.de/v/33c3-7945-decoding_the_lora_phy#t=2382
> >> (**) https://pubs.gnuradio.org/index.php/grcon/article/view/8
> >>
> >>
> >> 73
> >> kristoff - on1arf
> >>
> >>
> >> On 31.07.21 22:38, Steve wrote:
> >>> I’ve noticed that the KISS implementation on a lot of these devices is
> a
> >>> forked over version of the Rnode. He used the Sx 1276 chip as well
> >> because
> >>> it has really good RX sensitivity at the lower rates. He also has a
> newer
> >>> and easier form of KiSSattach.
> >>> https://github.com/markqvist/tncattach
> >>>
> >>>
> >>>
> >>> I like your thoughts about changing modulation on the fly. I’ve seen it
> >>> done with higher level stuff , Ubiquiti and others.  Just wonder how
> much
> >>> it would slow things down. Cause I think you would do that per packet.
> >>> Maybe per connection.
> >>>
> >>> Right now I’m just interested in tying a few of the different versions
> >> and
> >>> seeing what works. No real goals in mind, just checking it out and
> >>> experimenting.
> >>>
> >>> I’ve thought about a repeater, but that would kill the great RX
> >> sensitivity
> >>> , but a digi would be good. A old style repeater separated by distance
> >> and
> >>> a link would work.
> >>>
> >>>
> >>> Might even try Winlink on it and see what what happens when I stir that
> >> pot!
> >>> So many ideas.
> >>>
> >>> Steve N0FPF
> >>>
> >>> On Sat, Jul 31, 2021 at 12:58 PM Kristoff <kristoff at skypro.be> wrote:
> >>>
> >>>> Steve,
> >>>>
> >>>>
> >>>> I also have this model:
> >>>>
> >>>>
> >>
> https://www.amazon.nl/DollaTek-LoRa32-SD-kaart-draadloze-module/dp/B07RXSKPBX/
> >>>> The disadvantage is that it lacks the 8 Mbit PSRAM chip (which is on
> the
> >>>> T-beam) so the ESP32 is a bit short on memory for embedded python.
> (less
> >>>> then 1 MByte of RAM available).
> >>>>
> >>>>
> >>>> Software development-wize, perhaps it makes more sense to turn the
> >>>> radio-device into just a dumb 'packet-modem', connect it to a
> >>>> single-board computer (say, a raspberry pi) and do all development
> >>>> overthere. That way you can run higher-level code, easily interface
> with
> >>>> the internet, connect other devices (e.g. a VHF/UHF packet mode), etc.
> >>>>
> >>>> Last week, I found this python module for processing aprs message (*)
> >>>> which I think would be a good basis to do some experimental
> development
> >>>> for aprs-code.
> >>>>
> >>>>
> >>>>
> >>>>
> >>>> I do like the sx1276 chip. It has a couple of nice features.
> >>>>
> >>>> - as it also has a FSK modem, It might also be able to transmit POCSAG
> >>>> (paging) messages. If this works, we should be able to build a
> >>>> APRS-to-POCSAG gateway with just one piece of hardware: send a aprs
> >>>> text-message to a certain APRS node and it transmit that message as a
> >>>> POCSAG paging message.
> >>>>
> >>>> I once wrote some code to create pocsag-message. (**)
> >>>>
> >>>>
> >>>> - The automatic header-detection in lora is nice,
> >>>> This means that if you send a packet with (say) a coding-rate of 4/8
> to
> >>>> a lora-node configured with 4/5, the chip will still receive and
> decode
> >> it.
> >>>> So, if you know that you are in area with bad APRS-over-lora coverage,
> >>>> you can switch to a coding-rate that provides better error-correction
> >>>> (like 4/8). Your packets will become larger (by 37.5 % in this case)
> but
> >>>> your packet might still arrive, which it might not have done with a
> >>>> coding-rate of 4/5.
> >>>>
> >>>> Note that that bandwidth and Spread-Factor do need to match. If not,
> >>>> your packet will not be demodulated/decoded.
> >>>>
> >>>> - Another nice thing is that, although the chip does have
> CRC-generation
> >>>> and CRC validation in silicon, it will still pass packets to the
> >>>> receiver, even if they fail the CRC check.
> >>>> It will flag it as 'CRC error', but the code on application-level will
> >>>> still be able to retrieve it.
> >>>> Most other radio-chips I have looked at so far do have do not do this
> >>>> and will silently drop that message.
> >>>>
> >>>> This would allow for a nice lora "DX-ing" mode by implementing
> >>>> additional error-correction code on application-level.
> >>>>
> >>>>
> >>>>
> >>>>
> >>>> Ah yes, there is also a GNU Radio implementation of the Lora PHY.
> (***)
> >>>> I haven't tried it yet, so I do not know if it works but It would be
> >>>> interesting to experiment with this, e.g. tweaking the parameters even
> >>>> further to do even further DX.
> >>>>
> >>>>
> >>>>
> >>>> I am not on Facebook.
> >>>>
> >>>> I prefer mailing-lists for in-depth discussions. To get real-time
> help,
> >>>> ...  I am on matrix / element.io which is an open-source and
> >>>> decentralized IM/voice/video service.
> >>>> There are quite some open-source groups reachable via matrix, either
> >>>> directly on matrix itself or via bridges to IRC or some other IM
> >> protocol.
> >>>> It's used the GNU Radio community.
> >>>>
> >>>>
> >>>> (*) https://github.com/ampledata/aprs
> >>>> (**) https://github.com/on1arf/pocsag
> >>>> (***) https://www.epfl.ch/labs/tcl/resources-and-sw/lora-phy/
> >>>>
> >>>>
> >>>> 73
> >>>> kristoff - on1arf
> >>>>
> >>>>
> >>>> On 31.07.21 20:31, Steve wrote:
> >>>>> I gave two of the TTGO units including a couple of Rnodes.
> >>>>>
> >>>>> I’m going to try a couple of the existing firmware options out there.
> >>>> Some
> >>>>> are KISS based some are not. I like the possibility of using the wifi
> >> or
> >>>>> Bluetooth with the TTGO with some of the APRS programs fir the
> phones.
> >>>>>
> >>>>> Seems for a mobile device, not have a extra board such as a PI might
> be
> >>>>> nice… we’ll see!
> >>>>>
> >>>>> The TTGO can also be used for this:
> >>>>> https://tinygs.com
> >>>>>
> >>>>> The Rnode guy is also implementing a MESH system but is only in the
> >> alpha
> >>>>> stage. Beyond my ability.
> >>>>>
> >>>>> I’ll have to get back to you on Freq, I only got my TTGO going last
> >> night
> >>>>> and didn’t bother to look!
> >>>>>
> >>>>> There is a LORA APRS group on Facebook, grin, that is mostly from
> >> Europe
> >>>>> who have a lot of experience with this .
> >>>>>
> >>>>> Steve N0FPF
> >>>>>
> >>>>>
> >>>>>
> >>>>> On Sat, Jul 31, 2021 at 11:20 AM Kristoff <kristoff at skypro.be>
> wrote:
> >>>>>
> >>>>>> Steve,
> >>>>>>
> >>>>>>
> >>>>>> Your project interests me too.
> >>>>>>
> >>>>>> We are using this hardware:
> >>>>>> https://github.com/LilyGO/TTGO-T-Beam
> >>>>>>
> >>>>>> We are on 433.775 MHz, 150 KHz, SF12, 4/5
> >>>>>> What frequency and what mode do you use?
> >>>>>>
> >>>>>>
> >>>>>> The software comes from this project:
> >>>>>> https://github.com/lora-aprs
> >>>>>> But if the code of your project is standard arduino, it shouldn't be
> >> to
> >>>>>> difficult to port it to ESP32.
> >>>>>>
> >>>>>> Apparently, there are also some high-altitude balloon projects that
> >> use
> >>>>>> lora for their telemetry download.
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> My personal interest is to use this for doing some software
> >>>>>> experimentation, so I use micropython on the ESP32 (fast
> development)
> >>>>>> and not the arduino code.
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> 73
> >>>>>> Kristoff - on1arf
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> On 31.07.21 20:05, Steve wrote:
> >>>>>>> I connected it to my 10 db Omni antenna at home right at the base
> to
> >>>>>>> minimize the loss.
> >>>>>>>
> >>>>>>> I was impressed that it was heard around some of the hills in west
> >>>>>> seattle .
> >>>>>>> I have plans to put a IGate at a high point around the end of the
> >>>> summer.
> >>>>>>> I also found a small bi- directional amp too.  Boost it to a watt
> or
> >>>> two.
> >>>>>>> Just don’t want to kill the receive sensitivity. Around $60.
> >>>>>>>
> >>>>>>> It’s all in the antenna.
> >>>>>>>
> >>>>>>> Steve N0FPF
> >>>>>>>
> >>>>>>>
> >>>>>>> On Sat, Jul 31, 2021 at 10:59 AM Tom Russo <russo at bogodyn.org>
> >> wrote:
> >>>>>>>> On Sat, Jul 31, 2021 at 10:35:44AM -0700, we recorded a
> >>>> bogon-computron
> >>>>>>>> collision of the <stevewa206 at gmail.com> flavor, containing:
> >>>>>>>>> I have interfaced the Rnode Lora device. It does Kiss and used a
> PI
> >>>> 3.
> >>>>>>>>> Pretty straight forward.
> >>>>>>>>>
> >>>>>>>>> https://unsigned.io/projects/rnode/
> >>>>>>>>>
> >>>>>>>>> I was able to do a client and a IGAte.
> >>>>>>>> That is pretty cool!
> >>>>>>>>
> >>>>>>>> --
> >>>>>>>> Tom Russo    KM5VY
> >>>>>>>> Tijeras, NM
> >>>>>>>>
> >>>>>>>>      echo "prpv_a'rfg_cnf_har_cvcr" | sed -e 's/_/ /g' | tr
> >> [a-m][n-z]
> >>>>>>>> [n-z][a-m]
> >>>>>>>>
> >>>>>>>> _______________________________________________
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> >>>>>>>> http://xastir.org/mailman/listinfo/xastir-dev
> >>>>>>>>
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> >>>>>>
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> >>>>
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> >>
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-- 
Pardon the brevity, sent from a mobile device. So there.

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