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We know that Lorawan targets long range transmissions.
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At our university we set up three gateways on rooftops. And we've achieved a successful 57 kilometres LoRa
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transmission.
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This is a huge distance if you consider that the end device transmits with a maximum power of 14 dBm.
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Obviously, your gateway coverage depends on many parameters.
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It depends on your environment on the weather, on your gateway location and so on.
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When we look at our university campus, we are pretty sure that any end device deployed in this area will
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be able to reach one of the three gateways.
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But let's take an example.
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If we consider only one gateway, let's say this one. We now imagine that we want to deploy a device
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a little bit further than the campus area, let's say in the city on the other side of the lake, it's
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about ten kilometers from the campus.
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And of course, I would like to connect this end device to my LoRaWAN network.
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What are my options?
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The first idea is to increase the power transmission.
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That's something obvious.
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You can do it in your own device configuration, but you can only do it up to 14 dBm .
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That's because the European regulation limits the transmission to 14 dBm in the 868 megahertz band.
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Obviously, if you increase the power, you will also reduce your device battery life.
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But this option works.
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There is also another possibility you can increase the spreading factor.
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So I've designed some circles on the picture and each circle represents the coverage area for a specific
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spreading factor.
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You can notice that if I increase the spreading factor, then I increase my gateway coverage.
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Actually, when I increase the spreading factor during transmission, it becomes easier for the gateway
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to demodulate the signal.
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So we can say that the gateway can receive a LoRa frame with a lower power transmission if the spreading
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factor is higher.
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Let's take a very simple example.
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If you transmit a message from the other side of the lake, then the gateway can demodulate it.
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If you use an SF12 transmission but with the same power, it doesn't work if you use an SF11, 10 or
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lower.
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Let's use again the analogy we've previously used with one speaker and one listener.
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We imagine that the speaker uses his usual voice, so with a certain level.
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Then if you are a listener, it will be always easier for you to understand the speaker if he speaks
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clearly.
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Taking time to articulate each syllables of the sentence and on the contrary, with the same voice,
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it would be harder for you to understand.
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The Speaker If he talks very quickly.
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The way the speaker talks is exactly what the spreading factor is.
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So to make things easier to understand.
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You can just consider that sending a message to a higher spreading factor is just taking time to send
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clear message bit after bit.
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That's exactly what happened on our gateway.
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If the signal is received with a very low power, that's the case.
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If the device is far away on the other side of the lake, then the gateway can understand it only if
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it was clearly and slowly modulated, which is the case with the spreading factor.
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12.
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So one could say, why don't we always use spreading factor 12 so we can reach further devices?
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Yes, but if you think that, then you forget that an end device using a spreading factor 12 will take
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longer to transmit.
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Because if you speak clearly, then it takes longer to say your message.
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And obviously the longer your transmission time, what we call the time on air, the more energy the
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device consumes.