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Comment by wakamoleguy | Hacker News Reader

Newb question: since they are both electromagnetic radiation, how does Li-Fi get up to 100x faster than Wi-Fi? Are the light transmitters and sensors that much faster? Or perhaps is it able to use a wider band of frequencies?

I'm also so curious how this ends up working in practice. Even using infrared, would it interfere with things like baby monitors in night mode? The Wikipedia article says it can be tuned to be less intense than humans can perceive, but I'm curious if that's true in practice. (Granted, babies don't typically need Internet access while they are sleeping, but maybe the monitor itself does.)

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I don't know anything about LiFi, but for EM radiation in general, the higher the frequency, the higher the theoretical maximum bandwidth.

https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore...

Wifi is 2.6 Ghz or 5Ghz, visible red light is 430 terahertz.

Or it sounds like LiFi is just pulsing the lights on an off, in which case the Nyquist Rate that was invented for telegrams is a better analogy

https://en.wikipedia.org/wiki/Nyquist_rate

Just imagine what we'll be able to do with Gamma data transmissions ;-)

I like to explain to my kids that it's all colors - we are transparent to some (such as X-rays) the same way some fish are mostly transparent to the light we can see and walls are transparent to the radios we use in Wi-Fi, and also that both snakes and bees can see light in colors we can't (snakes see IR, bees see UV).

Face melting speed.

I think you're misreading Shannon's Law. It's not that the higher the frequency the greater the max bandwidth. It's the higher the bandwidth the more data you can put through.

I am agreeing with you and providing more info:

https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore...

Maximum channel capacity depends on signal bandwidth and SNR.

> the higher the frequency, the higher the theoretical maximum bandwidth.

This is not true and is unrelated to Shannon's theorem.

Shannon's theorem shows us that wider bandwidths allow for larger bit rates. At higher frequencies our bandwidths can be bigger. For example a band from 1 to 2 terahertz is 1 terahertz wide, which is 1000 times larger than a band from 1 to 2 gigahertz (1 gigahertz wide).

The total bandwidth available (including multiple channels) for 2.4 GHz Wi-Fi is about 100 MHz. The total space available for this new standard is 800 to 1000 nm [0], which is 450 THz. That's 4.5 million times wider than Wi-Fi. That is why you get higher bit rates with this new standard, AKA more throughput, or more "bandwidth", when the term is used to mean data rate.

[0]: https://standards.ieee.org/ieee/802.11bb/10823/

I mean it sort of is true. If you're at 100THz, you can get a bandwidth of 1THz. If you're at 100KHz, you are not going to get a bandwidth of 1THz.

True, The “band” is a range of frequencies, from lower bound to upper bound. You can have a single frequency carrier that you modulate, in which case your bandwidth has more to do with your modulation scheme, and the rates implied by that

Can I ask a noob question?

Suppose we have 1Hz signal, what stopping us from sending/receiving 10 or 100 bits of info every second by modulationg amplitude of signal?

When you modulate that 1 Hz signal you're generating power at frequencies other than your 1 Hz carrier.

See https://en.wikipedia.org/wiki/Amplitude_modulation#Spectrum

Sorry, its not clear for me.

Modulated signal could be expressed as sum of signals with different frequencies, but will it be registered by receiver as signal at these frequencies?

Suppose we send 1hz signal with length = 1 hour. In the middle we change amplitude of one wave to 1/2. Does recievers recieves mix of different frequencies?

The only signal that contains only 1 Hz and no other frequencies is a perfect 1 Hz sine wave. As soon as you start modulating the amplitudes away from that sine wave, you're introducing content at higher frequencies. You can use that higher frequency content to transmit information at more than 1 bit per second, but you're not exactly using the 1 Hz signal to transmit information.

https://en.wikipedia.org/wiki/Single-sideband_modulation

No need for higher frequency content. But SNR will have to be good enough.

SSB still makes other frequencies. Viewed on a waterfall, it's just half of AM, minus the carrier. Still occupies a range of frequencies, approx half of what AM does.

No higher frequency content above the carrier.

10 bits is at least plausible, but if you want to try to transmit .1 kbps with no frequency content over 1Hz, you're going to need the mother of all SNRs.

You can use FM, AM, QAM, or other multi-bit modulation schemes to send that information, but you need to have the signal-to-noise ratio to demodulate it. WiFi actually goes up to QAM-1024 (10 bits per symbol) in the more recent specs. However, the SNR you need to decode that is perfectly is something like 35 DB, while recovering a signal that sends 1 bit at a time needs ~3 DB. A 35 DB SNR is very hard to reach unless the RF environment is quiet (basically impossible in an apartment building, for example), but 3 DB is easy.

Shannon's limit tells you about the total information capacity of a channel given its bandwidth and SNR. This is usually achieved by using deeper modulation than theoretical, and using error-correcting codes to recover the lost data.

> > the higher the frequency, the higher the theoretical maximum bandwidth.

> This is not true and is unrelated to Shannon's theorem.

You are correct that it is unrelated to Shannon, but it is still true. The higher your carrier frequency the higher your theoretical maximum bandwidth (in the correct meaning, i.e occupied spectrum), you can never have negative frequencies, so modulation the maximum bandwidth you can modulate a 1Hz to is 2 Hz (modulation bandwidth extends to positive and negative frequencies). A 10 Hz carrier can be modulated to 20 Hz...

> Shannon's theorem shows us that wider bandwidths allow for larger bit rates. At higher frequencies our bandwidths can be bigger. For example a band from 1 to 2 terahertz is 1 terahertz wide, which is 1000 times larger than a band from 1 to 2 gigahertz (1 gigahertz wide).

So you are contradicting yourself? Not sure why you said the earlier statement is not treu?

My point was that Shannon's theorem is defined in terms of bandwidth. I think speaking about frequency is misleading, even though it's true when discussing carrier/central frequencies. I shouldn't have said OP's statement was untrue, since it's strictly true as you say, that higher frequencies allow for wider bandwidth. They just don't have to have wider bandwidths, which I was trying to make clear. Thanks for the correction though!

I think this all boils down to the confusion because people use bandwidth and capacity to really mean throughput. Talk about the capacity of a your internet connection to a communication theorist if you want to start a rant.

I think Li-Fi is more comparable to Ethernet instead of Wi-Fi, not because the fundamentals are very different but because of the link budgets available. If you think of an ethernet line as a pipe carrying data, then a collimated laser can be thought of in a similar way - most of the energy that you transmit is going to make it to the destination, and that is not the case with Wi-Fi, even with very high gain antennas. This allows for different modulation schemes and thus higher throughput. Copper ethernet is now capable of 1.6 Tbps [1] and Li-Fi doesn't seem so very fast compared to that; however keep in mind this is comparing only physical layers. Demonstrations of optical laser links of hundreds of Gbps over hundreds of kilometers have taken place [2] using COTS optical ethernet transceivers and special output stages providing precise collimation and pointing.

[1] https://en.wikipedia.org/wiki/Ethernet_physical_layer#1.6_Tb...

[2] https://ntrs.nasa.gov/api/citations/20210026855/downloads/sp...

For your second question, I'm not sure how baby monitors work but the proposed wavelengths are unlicenced and there are little if any rules for how to deal with interference. There are rules for eye safety of laser which limits the maximum energy that can be delivered to the output. Generally as Li-Fi gets more common we will have to learn to deal with interference as it arrives. For example, Lidar systems (older, noncoherent ones) interfere with one another and are even susceptible to interferece from IR motion detectors and such, but these aspects have to be considered during design.

My understanding is they went with 800 to 1000 nm (infrared), or ~ 375 to 300 THz. I'm not sure as to the total combined bandwidth of all of WiFi 6 or 7, but 75,000 GHz band gives them a lot to play with.

> Newb question: since they are both electromagnetic radiation, how does Li-Fi get up to 100x faster than Wi-Fi?

They specified that they wanted to be 100x faster, and they added parallel channels, until they reached that goal. No, really, this is the real reason.

It is entirely nonsensical to ask for a physical reason, because different channels are just different.

well the physical reason is that the band available at the visible light spectrum allows you to add that much channels in parallel. You can't do that at 2.4GHz

> Even using infrared, would it interfere with things like baby monitors in night mode?

If you're referring to the infrared LEDs that illuminate the baby, their light is not polarized, while light used for communication is, so a polarizing filter in the receiver can filter out such noise.

Baby monitors with night vision usually have a small set of tiny infrared lights so that they can see. Adding additional infrared to the room will help the monitor to see better.

Given how they are calibrated I don't think so.... More likely the image would be getting constantly washed out, right?

Depending on how fast the extra IR light is pulsing, it might end up looking like your baby is sleeping through a rave - which would be entertaining enough to be passed off as a feature rather than a bug.

"Between 3-30 hertz (flashes per second) are the common rates to trigger seizures but this varies from person to person. While some people are sensitive at frequencies up to 60 hertz, sensitivity under 3 hertz is not common"

https://epilepsysociety.org.uk/about-epilepsy/epileptic-seiz...

This came immediately to mind which when comorbid with infrared sensitivity is likely to trigger people without any apparent cause to third party observers....

People forget that the average human barely see jack sht compared to the remarkable exceptions of our species, let alone that such exceptions often have disabling/uncommon conditions comorbid with thier remarkable capabilities.

It's likely going to be super fast.

You can encode a lot more information in higher frequencies.

But each frequency has different things that are opaque to it, and travel different distances before dropping off.

Wi-Fi is already “really low frequency, really low intensity infrared light”, so I suspect the article is correct when it says we won’t notice it.

Correct. The reason why microwaves can cook food isn't the fact that it is a microwave frequency. Microwaves ovens cook by flipping the polarity back and forth. The frequency emitted is the same resonant frequency as water molecules, so the water molecules attempt to align constantly to the ever changing polarity. Movement is heat; thus, the water heats the food.

I've heard the resonance with water explanation is a common misconception: https://physics.stackexchange.com/questions/150128/how-do-mi...

more of an oversimplification. polar bonds interact with EM field of microwave oven.

water is one of many thousands of compounds having polar bonds

That doesn't sound like resonance to me, which is about constructive interference and natural frequencies. So more of an inaccuracy I think.

To set the record straight for the above comment -

- it's true that there isn't a precise frequency needed for microwave ovens to heat food

- however, "polarity flipping" is just a description of electromagnetic radiation itself, and shooting enough EM radiation at food in a frequency range it absorbs will heat it up via dielectric heating

- microwaves have no relationship to any specific resonant frequency of water - the vibration frequencies are orders of magnitude higher https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_... while rotation response inherently does not have a peak

- otherwise, yes, the motion of (polar) molecules induced by an electric field is indeed the mechanism of dielectric heating

> Wi-Fi is already “really low frequency, really low intensity infrared light”

No, infrared by definition starts around 300 GHz and goes up from there.

Sorry. I was just going by the literal reading of the infra (under) red.

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