Can a higher frequency EM signal encode more information?

Factual Questions
1

I’ve heard a couple times that a GHz signal can encode more information than a 100 MHz signal. My contention was that a photon is identical to any other except for the frequency/wavelength, and so an antenna can only notice that a photon arrived or not, and that the information is encoded essentially by the number of photons arriving in a millisecond or whatever time frame.

Just preparing to type this though, I remembered the basis of FM radio. So is a 5G cell system varying the frequency slightly so that the information is encoded also in the change of frequency? Does that allow a greater amount of information because there are more waves overall to shift? Somehow that still seems unlikely to me.

I’m willing to be wrong. Is there an advantage to these higher freqs?

2

Suppose your transmission is subject to the Shannon–Hartley law, which implies that in the presence of a certain amount of noise, the rate of information you can transmit will be proportional to the bandwidth of the channel. Then it should be plausible that much higher frequencies will enable using more bandwidth, all other things being equal.

As for discrete photons and other sources of shot noise, the lower your signal-to-noise ratio, the lower the bandwidth as you can see from Hartley’s formula

\text{Capacity} = B \log_2\left(1+\frac{S}{N}\right)

Changing frequencies, whether to use FM or AM and other modulation schemes involve various tradeoffs like using more or less bandwidth. I think 5G can, for example, switch between using 2-, 16-, 64, and 256-level QAM. Also, you would not expect a medium-length radio wave to behave the same as a microwave, would you? Different frequency bands will have different characteristics and be affected differently by weather, plus you have to use whatever spectrum you can get a license for or else your pirate cell service will be shut down pretty quickly.

3

Thank you for the effort, but I’m not a radio engineer. I understood less than half of that.

4

There are a few things to unpack here.
Saying a 1Ghz signal versus a 100MHz signal does not contain enough information to state anything about the comparative information carrying capabilities.

As above, Shannon-Hartley says that the information carrying capacity is defined by the bandwidth times and the signal-noise ratio. We have neither when just saying the frequency of what we assume is just the carrier.
Being painfully pedantic we might assume that the bandwidth is thus zero, and thus the capacity of either channel is also zero.

In actual use channels are typically allocated bandwidth to services as a fraction of the carrier frequency. So say a service is allocated a band of 0.1% of the carrier frequency. A 100MHz service might get a bandwidth of 100kHz. A 1GHz service might then be expected to get a 1MHz bandwidth. So instantly we see that we might expect, all else being equal, that 1GHz services can carry 10 times the information. But in reality it just because we are carving the bands up using fractions of the carrier, not fixed frequency ranges. In the end that is mostly the answer.

But we didn’t discuss signal to noise. Noise comes form a whole range of sources. Thermal noise in your receiver is often dominant. But our universe is intrinsically noisy. Even if you douse the entire she-bang in liquid helium, there are still remaining noise sources. This places a fundamental base to your information carrying capability. If you can push more energy into your transmission, you get a wider margin from the noise. But that takes effort, and the real life problems with things like causing interference to other services are never far away. So real life services are always power limited, even if they have limitless power available to them. Portable devices clearly have worse issues.

So eventually you get your two hard limits. Signal to noise and Bandwidth. These are the iron clad, no such thing as a free lunch, type of limits.

How you get close to the theoretical limit is a matter for technology. A simple binary on/off modulation only exploits the top 6dB of your s/n. You can only modulate your signal so quickly, as modulation of the carrier spreads energy out from the carrier frequency, hence occupying your restricted bandwidth. So if you have a 100KHz bandwidth, you might only be able to made 100,000 encoding changes to the signal per second. You need to squeeze your ability to modulate signal into your bandwidth aggressively.
If you realise that you can encode both amplitude and phase information you swiftly realise that you are effectively able to encode locations on the x-y plane (or the complex plane depending on how you want to disect the mathematics.) minimally you have four quadrants, and thus one signal modulation change can encode not two but four states - so two bits per transition. two amplitude levels and you can put four points into each quadrant, and you have 16 values for each transition - and thus 4 bits per transition. This is the basis of QAM. The better your signal to noise the more levels you can cram in, and the more information can be encoded. As above, if you can dynamically switch encoding you can ride the signal to noise to get the best utilisation of the channel. If you divide your allocated bandwidth into lots of sub-bands you can perform sub-band relative adaptive modulation, and thus squeeze your way around little narrow band lumps of interference, pushing the envelope at every corner.

As the the energy in the photons bit, yes this is part of the intrinsic noise floor. But it is more complex than just defining this is a fundamental quantisation limit below which information does not exist. The arrival of photons is stochastic, and this means that there is a distribution of arrivals over time, and without going too deep into the weeds, this allows for information below the level of quantisation. It ends up becoming folded into the noise floor of the Shannon-Hartley theorem and doesn’t change things.

5

The reverse is certainly true: extremely low frequencies can’t transmit information very quickly at all.

The tradeoff is that ELF penetrates solid/liquid

objects much better than high frequencies. This includes seawater, so we can (and do) use ELF to communicate with submarines. But with the terribly low bandwidth, they can only transmit a few characters per minute. It’s basically used to tell submarines “hey, come up to the surface so we can tell you something important.”

6

Thanks for the effort. I got most of that.

People keep thinking that the reader will have an education in the topic, and use terms like QAM with no explanation. I don’t discuss the effect of Q on non-standard state electrolytic cells in terms of voltage outside of my AP Chem class. I assume no one will understand.

My basic question was whether 5G can carry so much information partly because of the frequency of the signals. You seem to be saying that it can because there is more room to make noticeable changes in frequency. Is that right?

7

This (to my non-radio-expert ears) sounds like an issue. By calling it a “photon” you are thinking of the “information carrying thing” as a particle but of course frequency/wavelength describes it as a wave.

But even as particles you can imagine a simple model: imagine the photons being packed end to end, with information being passed along in the color of the photon. With two streams moving at the same speed, you can pass along more information with photons the size of ping pong balls than with photons the size of bowling balls.

8

Without getting needlessly quantum-mechanical, it’s not very useful to thing of “Radio” antennas as receiving photons. It’s better to think of them turning E/M fields into voltage. But in your example, receiving more photons/mS would result in a greater voltage at the antenna, but no other difference in the information received. So you could do something simple like AM (Amplitude Modulation), or even simpler with OOK (On-Off keying), which is how early wireless Morse Code was transmitted.

9

To perhaps put it in even simpler terms: You can’t transmit any information at all with just a single frequency. You need a range of frequencies, such as from 1.000 GHz to 1.001 GHz. That range is called a “band”, and the difference between the top frequency and the bottom frequency is thus, literally, the “bandwidth”. So if your band is from 1.000 GHz to 1.001 GHz, then your bandwidth is 0.001 GHz, or 1 MHz. Which means that, all else being equal, you can transmit the same amount of information using that band as you can using any other 1 MHz band, such as the band from 100 MHz to 101 MHz, or the band from 1 MHz to 2 MHz.

10

In AM, amplitude modulation, you are restricted in the frequency of the information you can use by the carrier wave frequency. Say you had transmitter and receiver pair that worked only at 2000 Hz. You could not transmit an audio tone properly above that frequency. There is a lot of information available for us to use above that frequency with just our ears.

You could also consider a motion picture capture device, shutter speed. Things happening at faster times may not be seen by the camera. Information is lost. Faster shutter speed captures more information. This is often visible by rotating things not being shown correctly during playback.

FM, frequency modulation, though operating differently is still within the limitations of such things.

11

Sorry put mine up while you did.

12

Another example is amateur radio. In the low HF frequency bands (180 and 80 meters, or ~1.8 and ~3.5 MHz), voice and Morse code are the highest bandwidth modes appropriate because of the narrow range of the bands. When you move up to 20 meters (14.000 to 14.350 MHz), digital modes like slow scan images, FT8, and PSK31 are possible. In the VHF and UHF bands (144 - 148 and ~445 MHz) packet, fast scan video, and digital modes for internet gateways (text only, I believe) are possible. In the GHz bands, even more broadband modes are possible. So, as the frequency increases, there is more “room” for encoding more and more information because the bandwidth increases.

This is an example of digital mode bandwidth vs. frequency of use:

https://images.app.goo.gl/mi2rnca2nEtR59LW9

13

Thank you for the bandwidth description and the point about the size of it. It makes sense to me that a 1MHz band is as useful as any other. Why then do I see mentions that the 5G frequencies are more useful? The contention was that the more waves per second was an inherent advantage that was a reason to put up with the more line-of-sight nature of the signal.

14

Doesn’t AM work by simply varying the strength of the signal, creating a “graph” of the sound wave intended? I was pretty sure that this was the first method created because of its simplicity. The voltage when amplified simply creates the magnetic field in the speaker coil and shakes the speaker cone, without computers or any kind of complex circuit, right?

I’ve even had an ambition to make a foxhole AM receiver with HS kids out of aluminum foil, a paper clip, and a razor blade. The problem was that I can’t find someone to explain how it works even in simple terms, which makes it pretty much just a crafts project instead of a demonstration of a HS level science principle.

15

There is no such thing as the 5G frequencies. 5G is a modulation technology. You can use it in any frequency band. G stands for generation.
5G makes better use of the frequency band it is given than earlier generations. It can and does use some of the same frequency bands that were used by earlier less efficient technologies.

Which frequency bands 5G is used in isn’t a technological question. It is a matter of the various carriers and regulators finding bits of available bandwidth to use. Since bandwidth allocation is across all services it is a precious and now highly valued thing, and access is sold for billions. Use by 5G services is in competition with other services- hence the argument with aviation and radio altimeter operation. Across the world there is some loose commonality in allocation of spectrum, but it is far from universal. Phones need to work on a large range of frequencies and in any given locale the exact ones allocated to 5G services can be different to another.

16

I know 5G is a protocol. However, it was argued that the higher frequencies it’s been shoved into also help the data transmission rate. That’s what I’m asking about.

17

You are pretty close to how AM works. The name says it all. Amplitude Modulation. You have a carrier signal, the radio frequency signal at the nominal frequency of the radio station and you use the audio signal to vary the strength of the carrier. Very simple. Not very efficient but as simple as it gets.
In your receiver you need to recover the modulating audio from the revived signal. That is very simple to do. The part of the circuit that does this is known as a detector. A simple diode will act as a detector, and this forms the basis of the simplest possible receiver. Usually these go by the name of Crystal Set. A crystal of galena with a point contact wire will operate as a diode and forms the detector and gave it the name. A point contact with a razor blade likely works similarly.
A simple resonant circuit tuned to the carrier frequency of the transmission selects the wanted signal from the antenna feed and feeds that signal to the detector. Then a small bypass capacitor acts to low pass the output, leaving you with audio. There is enough energy to drive an efficient pair of high impedance headphones directly.
Everyone should build a crystal set.
The steady creep of digital radio services is taking over from AM radio stations. I fear that in the not too distant future there will be nothing to listen to with a crystal set. But until then, they are a fabulous device to inspire and educate.

18

The point is, being at higher frequencies means that there’s room for wider bands.

19

But a 100MHz band is identical in any spot, as you said. That much is the same percentage of the usable spectrum no matter where it sits. Why is there more room at one end?

20

Because, how many 100MHz bands can you fit in the frequency range of 100MHz - 200MHz, vs 1GHz - 2GHz? Both are one “octave” apart.