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I choose here random binary message. I divide this message into a certain number of bits.
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And here I choose two divided it in packets of ten bits.
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This means that each symbol represents ten bits with ten bits half 1024 possible combinations.
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So there would be 1024 different symbols.
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Here I have a representation of a LoRA modulation in SF10.
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And if I had to use an SF12, I would have divided it by group of 12 bits.
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And you can see in the first transmission representing the first packet that it has the specific symbol
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.
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On this first symbol, I start from a certain frequency, which is not the bottom one.
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This is one symbol among the 1024 existing ones.
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So when I take another packet of ten beats, I have a different symbols.
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The second sweep started the higher frequency and the third one to an even higher one.
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But they all have the same duration.
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So the LoRa frame is simply a succession of symbols all centered around the channel frequency.
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Let's see how it's represented when we received this message on a spectrum analyzer.
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We obtained the following diagram.
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Once again, we have the frequency on the y-axis and a time on the x-axis.
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We already used this presentation earlier.
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We can see here two different symbols.
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First, there is a symbol which starts as a certain frequency which is not the lowest one since the
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low frequency is at the bottom.
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And then a new symbol starts.
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And again, this new symbol starts at a different frequency.
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Anyway, whatever the symbol, it will always sweep the other bandwidth, which is usually 125 kilohertz.
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What are the frequencies at which these symbols could have started?
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We use here spreading factor of 10.
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So it could have been one of the 1024 frequency available along the axis.
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But there is one thing you have to be very careful about when I remind you that the SF10 representation was
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used.
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We said that there were 10 bits in each symbol.
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We must absolutely not imagine that the fact that they are more bits in the symbol will increase the
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bitrate because it's exactly the opposite in reality.
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Obviously, if I say that there are more bits in the symbol, we can state that more information will
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be transmitted.
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But in fact, there is one thing that I haven't mentioned so far.
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This is the symbol time.
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Of course.
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Here I said that we were working in an SF10.
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So we have the feeling that there are more binary elements than if we were working in SF8.
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But in reality, there is a big difference between the symbol transmission time in SF10 and
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the symbol transmission time in SF8.
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So we cannot consider and it's completely false to say that having a packet of 10 bits will increase
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the bitrate.
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So we'll speak about that in the next video and once we'll have understood the time of a symbol and the
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number of bit of which symbol, then we'll easily calculate the bitrate of a LoRa transmission.