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Data speed and the inverse square law

tim.jtq

Super Pro Member
Hi Everyone

Lets say hypothetically you are standing next to a mast and the nearest other mast is more than 100 miles away. You then go on a walk for several miles in a straight line, never losing line-of-sight with the mast. Throughout the entire journey, you are continuously doing speed tests.

Would your download speeds approximately obey the inverse square law?
 
They'd probably drop in steps as modulation profiles change, and then as you lose the higher frequencies entirely alongside lower order modulations that degradation would accelerate.
Thank you, XGS_Is_On, very much for your reply.

I have looked up "modulation profiles" on Google and it came back with a mess of results, a vast majority of which are probably completely irrelevant.

Do you know of a good web page or YouTube video which explains modulation profiles in a beginner-friendly manner?

Thank you very much.
 
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Thank you, XGS_Is_On, very much for your reply.

I have looked up "modulation profiles" on Google and it came back with a mess of results, a vast majority of which are probably completely irrelevant.

Do you know of a good web page or YouTube video which explains modulation profiles in a beginner-friendly manner?

Thank you very much.
Sure. 5G uses OFDMA _ https://www.5gtechnologyworld.com/the-basics-of-5gs-modulation-ofdm/ and each of those OFDMA carriers is using QAM: https://www.everythingrf.com/community/what-is-qam-or-quadrature-amplitude-modulation
 
Unlikely to follow exactly but won't be far off.

The biggest variable with the most determining factor is antenna size and gain. Assuming transmission power is fixed and a standard panel antenna used (with a 120 degree beam width), if you were to travel further away you will see a drop in received power because that power is spread over a much wider area the further you travel away which would contribute to reduced quality and reliability resulting in lower modulation schemes and subsequently slower speeds.

However if you were to increase the size and directivity of the antenna proportionately to the distance travelled and the power loss to maintain the same received power eventually the rule of diminishing returns would have a greater effect.

However (2) if the transmitting tower used a high gain, highly directive, highly focused antenna then in theory then it'll go a lot further whilst maintaining speeds for greater distances.
 
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Hi Everyone

Lets say hypothetically you are standing next to a mast and the nearest other mast is more than 100 miles away. You then go on a walk for several miles in a straight line, never losing line-of-sight with the mast. Throughout the entire journey, you are continuously doing speed tests.

Would your download speeds approximately obey the inverse square law?
I would speculate "no" as received power might go down as the inverse square, but when one reads into signal modulation and throughput calculations, there doesn't seem to be a linear relationship between throughput and received power such that the resultant observation should fall as the inverse square of the distance.


The data throughput which can be transmitted is ultimately determined by the channel width available in the frequency domain, and the attenuation of the modulation signal as to how much of the signalling is dedicated to error correction.

Once maximal throughput with minimal error correction is obtained, being closer gets you no more throughput. As you move further away, more bits of transmitted symbols are consumed for error correction but not with a linear dependence on received power but instead dependent on the error rate within the channel which is itself dependent on the signal to noise ratio (which again is not linearly dependent on the received power of the carrier signal).

Depending on the frequency of the channels, entire channels will fall away (from the highest frequencies first) when it is no longer possible to transmit reliably and you would need very many contiguous channels even to get received power of active channels falling as the inverse square of distance. In reality, you may continue to have received power long after signalling has ceased in a high frequency channel, and gaps between channels should give sharp fall offs in throughput.

A simple test would be with WiFi. If you double your distance from the router, it isn't usually a four times decrease in throughput. There's normally a plateau within some radius and then a rapid fall off and disconnection.

The cliché with digital signalling is that you get all or nothing, although we're all familiar with the data stream corruption in a band between the all and nothing.
 
I would speculate "no" as received power might go down as the inverse square, but when one reads into signal modulation and throughput calculations, there doesn't seem to be a linear relationship between throughput and received power such that the resultant observation should fall as the inverse square of the distance.


The data throughput which can be transmitted is ultimately determined by the channel width available in the frequency domain, and the attenuation of the modulation signal as to how much of the signalling is dedicated to error correction.

Once maximal throughput with minimal error correction is obtained, being closer gets you no more throughput. As you move further away, more bits of transmitted symbols are consumed for error correction but not with a linear dependence on received power but instead dependent on the error rate within the channel which is itself dependent on the signal to noise ratio (which again is not linearly dependent on the received power of the carrier signal).

Depending on the frequency of the channels, entire channels will fall away (from the highest frequencies first) when it is no longer possible to transmit reliably and you would need very many contiguous channels even to get received power of active channels falling as the inverse square of distance. In reality, you may continue to have received power long after signalling has ceased in a high frequency channel, and gaps between channels should give sharp fall offs in throughput.

A simple test would be with WiFi. If you double your distance from the router, it isn't usually a four times decrease in throughput. There's normally a plateau within some radius and then a rapid fall off and disconnection.

The cliché with digital signalling is that you get all or nothing, although we're all familiar with the data stream corruption in a band between the all and nothing.
Top quality stuff. I learned this working for the cable company for my sins 😂

The last paragraph is especially prescient given the increasing use of higher order modulations above simple binary on fibre optic networks.

I say increasing: the vast majority of the UK's cable network still uses RFoG, digitally modulated analogue, it just becomes coax a little bit before it reaches homes.

Always a band where BER escalates while the signal holds on and plenty of work a while back on cable went into responding to this better with dynamic modulation and frequency hopping to avoid noise, with modern fibre networks measuring maximum modulation order on a link by link basis.

OFDM/A helps with this, though I'm sure we're all familiar with issues on xDSL.

Your longer form post explaining this stuff is stellar.
 
What I was trying to emphasise was that the weather conditions, humidity, solar flares, other interference totally negate the original question "Would your download speeds approximately obey the inverse square law?"

Not in our universe they wouldn't. The very fact there are so many uncontrollable variables makes the question invalid.

There's no point speaking hypothetically in this instance because of the multitude of external factors (of which they are more negative then positive).
 
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Based on some rough and ready testing I've done recently, I think signal strength is less important than SINR. What I've seen is that SINR is worse indoors, not just because the signal strength is lower, but mostly because the noise level is greater. Loads of stuff in the house is transmitting, from obvious potential interference sources, like WiFi, to less obvious sources like LED lights (these can be truly dreadful).

My guess is that the increase in BER when the SINR gets worse may well tend to override a change in signal strength at medium to long range. This may well make the inverse square law look as if it doesn't truly apply, I think.
 
My guess is that the increase in BER when the SINR gets worse may well tend to override a change in signal strength at medium to long range. This may well make the inverse square law look as if it doesn't truly apply, I think.

Yes, totally. Whilst it'd be great if that was the case, it's wishful thinking at best.
 
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