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.
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.