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I've got an unusual use case (optical communications) that would benefit from access to the wifi baseband signal before it is upconverted/modulated at 2.5Ghz. However most modern wifi chips combine baseband encoding and transmission encoding/transceiver functions on a single chip. Are there chips, or chipsets, that split these functions and would allow for custom final encoding - in my case OOK laser diode modulation at << GHz frequencies?

rossmcbee
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2 Answers2

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Your request makes no sense - WiFi baseband is OFDM, and can't be converted to OOK.

None of the things that WiFi does to synchronize would even work with OOK.

It seems to me that you just want the bits going into the modulator. If that's the case: you don't need any hardware - that is exactly what your operating system sends to your WiFi card, and you need just a tap device (Linux networking terminology) to give you access to raw L2 packets.

Based on these, write software that controls youe light source. Later, port that to become a proper network card device driver.

Marcus Müller
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  • Ah ok thank you, I think I was misunderstanding how WiFi works. Doing a bit more reading, I think any of the PSK/QAM schemes that WiFi OFDM subcarriers use would also work - the main limitation seems to be accessing them pre upconversion them to 2.4GHz (what I'm reading suggests that this encoding is done pre-upconversion). Does this one make sense or I am still misunderstanding? – rossmcbee Jul 15 '18 at 21:53
  • The baseband signal is still OFDM -that means, it consists of 52 (of 64 potentially used) carriers, and they never exist in isolation. Also, I have my doubts regarding you being able to modulate the phase of your light source! – Marcus Müller Jul 16 '18 at 08:07
  • Ok, I think I'm getting there. So is the raw bitstream that hits the WiFi card already ODFM modulated, or is that always and forever done in line with the transceiver and is therefore not recoverable? Wouldn't be trying for a light phase modulation (though a https://en.wikipedia.org/wiki/Electro-optic_modulator can do that) - would just be modulating a sufficient bandwidth but <2.4GHz carrier, say ~100MHz if the 22MHz bandwidth/4us symbol duration listed on wikipedia is accurate. Thanks so much for your help and patience! – rossmcbee Jul 16 '18 at 16:05
  • *is the raw bitstream that hits the WiFi card already ODFM modulated, or is that always and forever done in line with the transceiver and is therefore not recoverable?* OFDM is something that happens in the transceiver hardware, and you won't be able to get the baseband samples out of that. – Marcus Müller Jul 16 '18 at 16:15
  • Ok, that's what I was afraid of. Would have been nice to not have to rebuild all of that well-thought-out encoding and error correction if I could skim it before upconversion, but c'est la vie. – rossmcbee Jul 16 '18 at 17:47
  • You can use one of many systems that can generate the same signals in software. The real problem is not generating and analysing these signals, the problem is latency. Look up gr-ieee802-11 – Marcus Müller Jul 16 '18 at 20:09
  • Yeah, I was considering an SDR solution as well, just hoping to there was maybe a hardware solution/hack. Welp, time to learn GNU radio! Thank you so much for your help! – rossmcbee Jul 16 '18 at 21:53
  • Why? Software is easy, hardware is hard ;) no, really, cherish every signal that you can see in software. But, from a digital communications point of view, WiFi was optimized for the ISM band RF channel, not for the optical channel, so just taking what works good for the former is not a good choice for the latter! – Marcus Müller Jul 16 '18 at 22:57
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This sounds like a potential XY problem.

Would a Passive Optical Network(PON) work? This would give you something like an AP+stations over a single strand per station. There are readily available optical PON and GPON SFP modules.

rsaxvc
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