Electronics > Projects, Designs, and Technical Stuff

IR communication protocol/design for low latency/fast response - ideas?

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ogden:

--- Quote from: pwlps on May 10, 2019, 08:41:14 pm ---I don't want to go further in this kind of discussion

--- End quote ---

Good idea indeed.

max_torque:
It is possible you are missing the point of my (rather flippant) post!

My point was that when you get "stuck" with some problem, it makes a lot of sense to take a step back and think about the best route forwards.  Now that may be to "fix" the problem come-what-may, but it also very often is to realise that your problem is in fact a symptom of some artifact that is entirely due to the way you originally approached the problem!

 :-+

pwlps:
So is this thread dead?  I would be interested in the OP development based on this discussion.
Basically I would repeat my thoughts: 1)QPSK is preferred over BPSK because carrier alignement is not necessary, 2) LO-based detection is preferred over synchronous detection because there is no need for heterodyning or passband.  All this is indeed very short and pithy and rereading my own posts I admit my writing is quite behind my thoughts ! (and has earned me some critics from Professor Ogden :) ), but I can explain it further on demand.

ogden:

--- Quote from: pwlps on May 13, 2019, 04:52:30 pm ---Basically I would repeat my thoughts: 1)QPSK is preferred over BPSK because carrier alignement is not necessary,

--- End quote ---

BS. Both requires carrier frequency alignment. QPSK needs better alignment than BPSK. - Compare BPSK/QPSK constellations and consider which one needs better phase angle precision.


--- Quote ---2) LO-based detection is preferred over synchronous detection because there is no need for heterodyning or passband.

--- End quote ---

LO-based detector is nothing more than demodulator with built-in digital downconverter. Heterodyne is digital, but still there.

In fact phase modulation is bad choice for IR transmission. None of IRDA modes uses used phase modulation. Even fastest 0.5Gbps / 1Gbps mode developed used 2ASK/4ASK. Slower modes used just various pulse amplitude modulations (mostly RZ/NRZ/RZI/NRZI).

pwlps:

--- Quote from: ogden on May 13, 2019, 06:21:42 pm ---BS. Both requires carrier frequency alignment. QPSK needs better alignment than BPSK. - Compare BPSK/QPSK constellations and consider which one needs better phase angle precision.

--- End quote ---
Again my loose wording. Talking QPSK I had in mind the demodulation technique whatever the actual number of points in the constellation.  The main point is here we need low-latency therefore a non-coherent or differentially coherent demodulation will be preferred over the standard carrier recovery techniques. The quadrature demodulator (called qpsk demodulator in the literature but it will do as well with a two-point constellation) is interesting because the decision rules can be implemented in software making it a natural choice for differentially coded signals, this is precisely the technique used in the article I cited.  This is just simpler than a bpsk synchronous demodulator (which is more delicate to implement), even though in theory  both should have a similar SNR (actually both loose about 3dB compared to demodulation with PLL-based carrier-recovery).  I'm not just talking general principles here, I try to figure out the practical implementation of the whole thing to see which scheme would be the simplest and most robust at comparable SNR/BER, and from this point of view a QPSK demodulator has some advantages. If you were thinking this way instead of just searching where to put your 'BS' marks in my text (but I don't mind) you would probably come to similar conclusions.


--- Quote ---LO-based detector is nothing more than demodulator with built-in digital downconverter. Heterodyne is digital, but still there.

--- End quote ---
Nothing to do with digital vs analog issues, no need for any digital parts in heterodyne.


--- Quote ---In fact phase modulation is bad choice for IR transmission. None of IRDA modes uses used phase modulation. Even fastest 0.5Gbps / 1Gbps mode developed used 2ASK/4ASK. Slower modes used just various pulse amplitude modulations (mostly RZ/NRZ/RZI/NRZI).

--- End quote ---
There is no ASK vs PSK issue in fast IrDA since it uses a baseband PPM coding without any carrier. This is sufficient for a point-and-shoot communication, here however the initial OP requirement was to be able to define independent frequency channels and the whole discussion begun from that point.  Actually in IrDA multiple channels can be defined too, but only at the IRLAP/IrLMP protocol level (sort of CDMA but I don't know the details).

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