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As we saw in a previous chapter.
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LoRa is a spread spectrum modulation.
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There are may ways,
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to produce spread spectrum modulation.
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And a specific one we use is called chirp spread spectrum.
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That's because we use a special symbol to create the radio frequency message.
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It's a chirp.
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So what is it exactly?
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I've simulated a chirp here in MATLAB.
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This is simply a sinusoidal that switched from 0 to 125 kilohertz.
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125 kilohertz is the most common situation used in LoRaWAN.
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But in reality, it can also go up to 500 kilohertz.
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That is what we call the bandwidth and the higher the bandwidth, the more information we can transmit.
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So that increases the bitrate.
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Producing a higher bandwidth also has its counterpart, because if you use a larger band for one transmission,
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then we have less room for the others.
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This symbol, as it's represented here, is not very convenient.
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Actually, we are not really interested in the amplitude, but we would rather know the frequency.
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We therefore use another representation.
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This representation draws the chirp frequency instead of its amplitude.
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The 125 kilohertz band is centered of what we called the channel, and I remind you that we are in the 868 MHz band
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.
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So there I've choosen a random channel of 868.1 megahertz. We'll see later that this specific channel must
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be known by all LoRaWAN devices in Europe.
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So how a frequency sweep starts at 868.1 megahertz minus half of 125 kHz, so -62.5 kilohertz.
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That gives this frequency to 868.1 megahertz plus 62.5 kilohertz.
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And that gives this frequency.
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So the chip starts from the low frequency and sweeps until reaching this high frequency.
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Soon we'll see that this chirp, even if it has the same shape, it doesn't always start at the same frequency.
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So the basic shape of the chirp will be renamed a symbol when we come to talk about all the different
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form it can have. And a time to send a symbol,
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so a specific chirp, is represented here on the graph.
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Now, what we want to know is what does this symbol represent exactly?
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How can we use these symbols to represent the binary data of our messages?
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That's what we will see in the next video.