currently I am involved in a project were a data link is needed to communicate different devices. This is in one to many configuration and the devices are stand alone. For this project there is need 9.2Kbps, even less, and the expected coverage is not so huge, is just about 500 mts in a rural environment (trees, hills) and the worst part... this device should work around the globe. The problem here is with the approvals.

LoRa could be a solution, but there is some differences in the regions (868/915/923) and the FCC rules and EU rules regarding duty cycle and dwell time are quite different. I thought to use a NarrowBand radio, at 403-470MHz, using licensed channels (no problem to request a license), but these modules are quite expensive. Do you have any idea? do you know any radio module transceiver for world wide?

Your problem is that there's really but one globally usable unlicensed band, and that's the 2.4 GHz band.

But that doesn't sound so bad. People think "high frequency = short reach", stemming from the well-known Free-Space Path loss formula

$$P_r = P_t \cdot G_t G_r \left( \frac{c_0}{4 \pi fd} \right)^2\text,$$

where the received power $$P_r$$ falls with the square of the frequency $$f$$ (for a fixed distance $$d$$), assuming you keep the transmit power constant and use antennas that have the same gain $$G$$ on transmit- and receive-side.

However, what people tend to forget: your antenna directivity at constant antenna area also grows quadratically with frequency, so that it's typically a zero-sum game, if, and that is the great if here, you can have a directive link:

If you want to keep the size of your system the same, this works out, because you can point the antennas at each other, and make a directive link.

If either side needs to be able to move, you usually can't just realign the antennas all the time, and this doesn't work.

However, is this really a problem?

Assuming a gain for one antenna of 6 dB (which is somewhat logical, you don't build an antenna that illuminates the sky if you want to talk to things on the ground, nor do you let it illuminate the ground directly below it), and the other with 0 dB gain, and stay within the world-wide limit of 100 mW (= 20 dBm) for the 2.4 GHz band, your received power at 500 m becomes is 83 dB lower than your transmit power, so -63 dBm.

That's not at all bad! Say, you're using a cheap 1 MS/s device, so you can only do 1 MHz of bandwidth at once, you get a noise power of $$N_{[\text{dBm}]}=-174+B_{[\text{dBHz}]}=-114$$ dBm (1 MHz = 60 dBHz). That gives you a very comfortable SNR! You should pretty trivially be able to communicate at your rates over that.

• – Phil Frost - W8II Apr 28 '20 at 15:32
• Hi Marcus, Really nice answer, and really deep arguments, congratulations. You are right in your explanation, but in my case, a directive antennas are not suitable for a target as it is moving, and auto-traking system is not reliable in this project. The problem here is with the environment. I did many wireless COFDM links with 20 dBm for several Kms and hight data throughput, but with LoS and proper antennas. With this frequencies, any object size is huge if you compare with λ, so the attenuation for objects like trees is so hight. Thanks for your explanation, really appreciated. – Jordi NC May 15 '20 at 7:05
• @JordiNC matter of fact, friend of mine did his PhD on Low Power Wide Area Network feasibility in 2.4 GHz ISM bands; among the things he did was a 13 dBm proof of concept over > 4 km including definitely shadowing high-rise buildings; you really have to depend on the multipath nature of your urban and suburban channels there. – Marcus Müller May 15 '20 at 8:09
• @JordiNC He's done a couple presentations, might be of interest to you. Also, not part of that presentation, but important result of his dissertation: Mathematically clear that LoRa and typical sub-GHz competitors simultaneously tell their investors about billions of IoT devices they'll enable in cities, and have no technical way of making them work in that number, since their transmission schemese aren't orthogonal or at least well successive-cancellable. At the point where Aloha breaks down, the whole network has no path forward... – Marcus Müller May 15 '20 at 8:17
• ... and that is not a great perspective. Hence, he looked into where there's more spectrum than on the sub-GHz ISM bands, and found it at 2.4 GHz. You're right about the path loss exponent problem for urban scenarios, though: the difference between ca 900 MHz and ca 2.4 GHz is roundabout 9 dB. With a nearly idiotically simple diversity scheme (the IoT device has two small chip antennas,mounted $\lambda/2$ apart, and a switch, and which antenna is used is part of the data),he could compensate about 9 dB – so, you get the same reliability as on 900 MHz ISM at 2.4 GHz,but with way more bandwidth. – Marcus Müller May 15 '20 at 8:20

I found a LoRa device which covers all the frequencies and I prepared a matching network to cover all range, and after some tests the results were so good, so the problem is solved. Many thanks

• Well, you can configure LoRa devices to work in 433 MHz, 868, 915 or 923 MHz, depending where on earth they're supposed to work. But you'll need an antenna for the band you're using, which usually isn't part of the module! Another thing: LoRa is fine for now, but I'm 100% certain they're not telling us what their plans are, once these very narrow sub-1 GHz bands get congested with IoT devices. – Marcus Müller May 15 '20 at 8:12
• Hi @MarcusMüller, You are right. I am sure the IoT bands will be congested. If I am not wrong, the ISM bands will be allocated in new frequencies too,.I was looking for a module that covers all HI- bands for all regions, this device is expected to work around the world. The control over dwell time will be done by software in the main MCU. This device covers all this bands (868,915,923). Of course, I will need to change the antenna, but not the device which will be integrated to electronics. I will use LoRa modulation, but not the Infrastructure, neither LoRaWan. Thanks! – Jordi NC May 15 '20 at 15:29