Hello there,
I am planning to build a rover which will use the Arducam to take photos of its surroundings. Originally, I wanted to store the photos that were taken on an SD card, but then I have had the idea of using the Lora antennas I will use to send commands to the rover and data back to my PC to transmit the photos stored on the SD card. First of all, would that be possible? If its a no, then what would be a better approach? And, assuming that that is possible, how exactly would I do it?
What sort of image size do you expect, and what sort of data rate do you expect from the LoRa?
I already plan with images of not really a high resolution. I need photos just to navigate and see the surroundings, so the ones that will be transmitted will be 320x240, or 640x480, and thus I rarely expect the photo size to exceed 200kb (I estimate 50kb for a 320x240 photo, 150kb for 640x480). I will take higher resolution images but these will not be transmitted, but instead saved on a SD to be seen after the rover is done with its mission.
For the data rate, I dont really want transmission to take too long, but I plan with a rather low data rate to get the best out of Lora. I havent decided yet but it would be somewhere between 500b/s and 1kbit/s,
Its possible, but far from easy. You need to send the file in a very large number of small packets. Plus you need a mechanism for ensuring your reliably receiving the correct bits of the file, byte by byte. A single wrong byte can lead to a completly corruption of the transfer.
However sending a file at 1kbit\sec, a typical rate for a LOS range for UHF LoRa of maybe 400km+, would take a very long time indeed and thus I dont see how it would be useful for 'navigation'.
In most places in the World there are legal limts on duty cycle, how much time you can transmit, typically limited to 1% or maybe 10%. So the scope for transmitting images can be limited.
Perhaps provide some more details, how far away will the rover be, what is the environment etc.
A LoRa frame can contain a payload between 1 and 255 bytes, so you will have to transmit the image in chunks of 255 bytes.
With settings optimized for the maximum possible bandwidth a chunk would take 56,22ms to transmit. You will need to transmit 200000 / 255 = ~785 chunks to complete the image, that's 56,22 * 785 = ~44133ms or around 44 seconds of air time. If you're in the EU transmitting on the 868MHz band, you're legally allowed to transmit with a duty cycle of 1%. That means that you can transmit for 36 seconds every hour. So transmitting for 44 seconds "should" take 44 / 36 = 1,22 hours. Now the rules differ, but you can be absolutely sure that some rules like it apply at your location as well. Also remember this is the best case scenario (no data loss) and with settings that favor bandwidth over range and reliability.
If you're planning to use LoRaWAN it'll be even longer.
nicolajna:
If you're planning to use LoRaWAN it'll be even longer.
For LoRaWAN the fair access limit is 30 seconds of air time and 10 downlink messages (acknowledges) per day.
Quite a small image ...................
@srnet I dont mean live nagivation, but instead just an image of the environment to find spots to drive to. The rover wont be that far away from base, I plan with a radius of 20 kilometers around, in a pretty open environment mostly consisting of fields or meadows, with occasional forests in between. No hills, just a flat area.
@nicolajna I dont plan to use LoraWAN mostly because its not really present where I live, but rather wanted to try a direct communication. I am in the EU so I am familiar with the restrictions of all that, but I assumed they only were in place for LoraWAN, that was wrong, was it? Would it be better to instead use a different frequency that I would be allowed to transmit on?
projectcolumbus:
I plan with a radius of 20 kilometers around, in a pretty open environment mostly consisting of fields or meadows, with occasional forests in between. No hills, just a flat area.
You would need a base station antenna that is at least 25m-30m+ AGL ground then, preferably more.
projectcolumbus:
@nicolajna I dont plan to use LoraWAN mostly because its not really present where I live, but rather wanted to try a direct communication. I am in the EU so I am familiar with the restrictions of all that, but I assumed they only were in place for LoraWAN, that was wrong, was it? Would it be better to instead use a different frequency that I would be allowed to transmit on?
Yes the restrictions are regardless of the type of data you transmit. I'm pretty sure that there's duty cycle restrictions on pretty much all of the unlicensed sub-GHz frequencies.
Semtech does manufacturer SX1281 which is aa 2.4GHz LoRa transceiver. The 2.4GHz band doesn't have any duty cycle restrictions (though other restrictions may apply), but really I don't think LoRa is the appropriate technology here.
The main issue with moving pictures and files around like this is quite simple, there really is not the spectrum allocations available to allow it at reasonable speed at UHF (434Mhz/868Mhz). The duty cycle limits are there because a very limited amount of spectrum needs to be shared amongst a lot of users.
However you can, in the UK at least, so maybe in the EU too, transmit on UHF at 100% duty cycle, if you keep the bandwidth\channel use below 25khz. That is only just about possible with the SX127x LoRa devices, but the SX126x series (different libraries) can handle it quite well. Unfortunatly at these low bandwidths the LoRa data rate is low, circa 1560bps, so moving large images around is going to take a while.
If you have a very high base station antenna, top of a hill maybe, then the possibilities for 2.4Ghz LoRa, where there are no duty cycle limitations, are significant. Links tests I have done with 2.4Ghz LoRa devices from a small hill obove the city where I live, suggest that I could run a link with cheap Wifi yagi antennas at 20km LOS @ 200kbps and maybe 650kbps.
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