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Comment by admax88qqq | Hacker News Reader

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

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> 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.

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.

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