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Thinking about modulation

In our presentation, Alexis Polti suggested that we should only listen to raw pulses and not emit frames, using a blank pulse for sync (to detect which is the first  emitter). The problem with this solution is that if you don’t receive the signal from one or several emitters, but you have 4 emitters in range, you will misinterpret the blank due to the out of range emitters and you will have syncing problems.

That means, we might have to send at least the number of the emitter by ultrasound. So I studied a little bit which modulation we could use for sending this data.

I warn you : I’m not really good at modulations, and it’s probable I’m wrong, please correct me.

First, we don’t want a huge data bandwidth, we only want reliable communication, so we want to reduce as much as possible the bit error probability.

A little computation shows that BPSK and ASK have the same bit error probability for the case 1 bit per symbol.

But, QPSK has the interesting property in which you can transmit 2 bits per symbol for the same error-bit probability than BPSK : the only drawback is you need twice the power for emitting the symbol (the energy per bit is constant), so it could be more interesting for us to use 2 bits per symbol. A plot with 4-QAM shows that you have a little bit more bit error probability.

peeb

As I didn’t manage to find data for MSK and FSK, I couldn’t draw them on this plot. Although, I managed to find some informations on how to boost communication range and they seems to go against MSK/FSK for this kind of use-case.

Also, we must consider that maybe, tweaking the transmission by using a more suitable modulation might be pushing the boundaries of our project a little bit too far.

And, as I said, I also can be completely wrong.

5 comments to Thinking about modulation

  • Helen

    For which kind of channel did you simulated it? AWGN? Maybe you should consider a multi-path channel, no?

  • In our presentation, Alexis Polti suggested that we should only listen to raw pulses and not emit frames, using a blank pulse for sync (to detect which is the first emitter). The problem with this solution is that if you don’t receive the signal from one or several emitters, but you have 4 emitters in range, you will misinterpret the blank due to the out of range emitters and you will have syncing problems.

    Here you’re assuming that all the emitters are using the same frequency, right? Because if you use two frequencies A and B, you can work with up to two out of range emitters by using a ABBA pattern.

    Also, how many different frequencies (or frequency bands if you use frequency modulation) can you use at the same time (think of several exhibition halls equipped with your system)?

  • Do you plan to experimentally experiment with those schemes? It would be interesting to see how resistant those schemes are in the presence of reflections on walls, floors, ceilings, people and objects in the room.

  • ale6

    @Helen: AWGN yes. I don’t know how to do it with multi-path channels, do you have some pointers so I can search ?

    @Samuel: With ABBA we can work up to two out of range emitters, but for exhibition halls it can be not sufficient enough. We haven’t planned to test this, but it could be a great idea.