How often can an mDot send data?

[Mark Makarychev]

I’m looking for a platform to prototype one idea on, and multi-tech’s LoRa starter kit so far seems to perfectly fit my needs. However, I can’t seem to find a few pieces of information about this technology that would help me finally make up my mind and buy the kit.

My main question is as follows: is it possible to achieve a 3 second transmitting frequency using the LoRaWAN protocol (by making packages pretty small and decreasing range)?

So far this is what I have read and understood in regards to my question (please do correct me if I misunderstood something somewhere):
I’ve read that LoRaWAN can be optimized for 1 of 3 metrics – long range, higher throughput, or rapid transmitting. For my prototype, the first two are not as important as the last. These three traits form kind of an optimization triangle, so I’m aware that increasing one decreases the rest. For my case – range can be sacrificed a bit, and I only need to send about 13 bytes (+deviceID) at a time. I found that the mDot Box can send “LoRa Demo” data at user defined intervals, the shortest of which is 10 seconds. For my prototype to work as needed, it needs to be able to transmit data at least every 3 to 5 seconds. In addition to this, I’m going to be prototyping in the EU bands (868 MHz), so the duty cycle (as I understood) is going to be only 1%, and if the device misses its slot, it has to wait for the next. The end devices should also be able to receive commands from the gateway (server), so I would prefer to avoid getting rid of the Rx window (although if it’s the only way out, it might be an option).

And since I’ve already created this topic, I’d like to ask one more question about the mDot box: is it possible to take the box apart to add something small? In other words, does it have any accessible pins or can something be soldered to somewhere? It’s just that I’d like to have vibration or a small buzzer in my prototype as well. I know that there is the mDot EVB, but the box is a nice addition and will make the proof of technology prototype look a lot more professional!

Thanks in advance for any help!

[Jason Reiss]

The duty-cycle in implemented as time-off-air after time-on-air.
If the device transmits for 1.6 seconds than it cannot transmit in the same frequency band for 160 seconds.

Time-on-air for 13 bytes can vary from 0.03 – 1.6 seconds depending on datarate.

There is a band at the top of 863-870 that can be used with 100% duty cycle if ERP is below 7dBm.

[Mark Makarychev]

0.03 time-on-air would lead to about 3 seconds off-air, correct? That would be pretty acceptable, however I still don’t quite understand – is the throughput (datarate) controllable via code? And approximately what range would I have using the max available datarate?

And I have the same question in regards to the “band at the top of 863-870 that can be used with 100% duty cycle” – will I be able to set such a “mode” via arm-mbed code? And about what range would I get with 7dBm (if there is some range equivalent for ERP)?

[Andrew Lindsay]

Using a 100% duty cycle is a bad idea as you’ll be blocking the signal for any other device using the same frequencies.

The spreading factor/datarate is a trade off:

Low SF will allow larger payloads to be transmitted in a shorter time but over a much reduced distance.
High SF will require smaller payloads to be transmitted over a longer time but the range will be much greater.

You pick whats best for your application and use-case.

Andrew

[Leon Lindenfelser]

Hi Mark,

In regards to your second question about modifying the dot-box, it is easy to open and it has an mDot built in with the form factor seen at this link.
https://developer.mbed.org/platforms/MTS-mDot-F411
So you would have easy access to the available peripherals of the mDot. Keep in mind that you have to manage your added component with the on board sensors.

Leon

[Mark Makarychev]

Hi Andrew,

Thank you for your answer! Does a 1% duty-cycle then mean that the maximum number of devices I can have is <100? I believe I remember reading something about the mDot being able to “jump” between frequencies – is this what allows for more than 100 devices to be in a LoRaWAN network?

As for the spreading factor – I guess I’ll need to use the lowest SF possible to achieve the ~0.03 on-air-time and ~3 second off-air-time. The lowest SF seems to be SF_7 125kHz, and it has a transfer rate of 5.47 kbps – so 13 byte should send in ~0.02 seconds, right? And what range am I looking at with SF7 in a rural environment? Is 2 kilometers possible, or even less than that?

And Leon,

Thanks for your answer as well! Looking at the EVB, I noticed that is has an “8-pin connector”. Are these like GPIO pins, or are they for debug purposes? And you said I’ll have to manage my added components with the on-board sensors – do you by chance know how many ports already have something hooked-up to them, and how many are still free?

• This reply was modified 8 years ago by Mark Makarychev.

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