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Comment by PaulHoule | Hacker News Reader

The irony is that the main case for 5G is reduced costs for operators.

4G blew the top off what people thought the Shannon Limit was but at the expense of extreme and expensive coordination between base stations. (If your digital front-ends are sampling at a rate of N Hz and a bit depth of d you need to bring all that data to one place!)

5G gets better spectral efficiency (serve more customers with expensive spectrum) with a simpler coordination model. (lower capital cost)

Of course since 5G is the new shiny carriers want to charge you more for it but they'd save money if they got you off 4G and onto 5G.

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It most certainly did not do that, neither has 5G for that matter.

The Shannon's Law refers to https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore... to see the actual formula

the basic principle is that the Capacity C of a comm channel is proportional to the amount of spectrum you have B and the Signal to Interference and Noise ratio (SINR).

So basically, if you want more throughput, you need either more spectrum or more cell sites in order to improve the SINR, you cannot simply continue to crank up the power because then you are increasing the co-channel interference (interfering with other terminals on the wireless network). This is why Spectrum is so expensive, the alternative is to build more cell sites, which is expensive.

4G LTE did come a lot closer by using OFDMA but they most certainly did not exceed the limit. 5G using mm-wave also did not, it uses phased array antennas to implement beam forming which created space separation, that is, each user gets their own beam and do not have to share it with others, but within that beam, they still are confined to the law C ~ B * SINR

The operators did add a ton of spectrum through various auctions and that plus the new tech is what is driving the tputs up.

You need more SNR (power), more BW (spectrum) , or more 'channels'. 5G solves this using the last dimension, channels. Beamforming allows the same spectrum to be reused by allowing for spacial separation, i.e. use the same spectrum 5 times by sending 5 beams to 5 physically sperare locations. MIMO is layered on top of this ans uses multipath effects to offer more bandwidth to a single user by taking advantage of the fact each path is its own channel. No violation of the Shannon limit for sure, but very smart and computationally intensive techniques to skirt around the issues of the single isotropic or sector antenna of old.

3G and prior "cellular" systems were inefficient because they can't reuse the same frequency in adjacent cells. See the diagram here

https://www.geeksforgeeks.org/frequency-reuse/

Here you have 7 different frequencies used in different groups of cells.

In 4G all frequencies can be used in all the cells. If you are between two or three cells you are probably receiving a signal from (and being received by) multiple cells and they are sharing the RF baseband to make it possible. That plus a big bag of tricks let 4G achieve radically better spectral efficiency through "spatial diversity"

> Here you have 7 different frequencies used in different groups of cells.

You should only need 4 different frequencies: https://en.wikipedia.org/wiki/Four_color_theorem

For hexagons you can get by with 3, https://en.wikipedia.org/wiki/Hexagonal_tiling#Uniform_color...

You'd think so but practically the signal bleeds across to second-adjacent cells and a real channel plan needs more than that.

Look at how the 3 WiFi channels in 2.4GHz land is nowhere near sufficient for good spatial diversity and it is even a struggle with the 5GHz channels that aren't gated to avoid interference with radar.

Indeed. In fact, the link I responded to shows a pattern matching the 7 color 3-uniform distribution I linked to

That only works if you have exact edges and that each BTW uses only one base channel (iirc, they mix and match as much as possible)

Yes, each cell interferes with the next, that is call co-channel interference. That is a major factor that limits the capacity of a cellular network. There are a number of approaches used to manage that, carriers spend a lot of money and employ many engineers designing around that.

SON (Self Optimizing Network) is the latest tech that is being deployed on 4G and 5G to help combat that.

BTW, this is one of the reasons you can get much higher tputs with mm-wave because the beams do not propagate very far which makes keeping co-chan down, that's the good news, the bad news is it doesn't propagate well, which means you need way more nodes.

It's not a hard stop between cells. You don't have radio signals and then suddenly nothing because of reflections, absorbtions and the very nature of log scale of signal distance drop off..

4G also reuses the frequencies in the exact same way, the primary difference between 3G and 4G is that 3G used CDMA and did not have the higher modulation desities, (capped at QPSK) while 4G LTE went up to 64QAM, so more bits per symbol.

4G did not use spatial separation, that requires beamforming, that is most certainly not deployed in 4G, 4G LTE uses MIMO. Spatial separation is deployed in 5G but only at the mm-wave bands due to the 1/2 wavelength separation required between antenna elements, hence, impractical with longer wavelengths.

3G (UMTS) was reuse 1, not reuse 7 !

This is not the only mistake from this website you mention : e.g. in the page "difference-between-gsm-and-cdma" I read "GSM Data speed rate: 42Mbps in HSPA" (but HSPA is 3G and an evolution of UMTS and is using WCDMA, compared to GSM which is 2G and is using TDMA). I guess when they mention "CDMA" or "3G" (which is are generic terms that also apply to UMTS) they mean "CDMA from 3GPP2", which is not used in Europe. I didn't read everything but from the little I have seen I don't find this website very reliable...

I am out of my element here but I was under the impression that the capacity of an arbitrary wireless network was still an unsolved problem. (Though I presume what you are referring to is a case that is solvable)

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

hmmm, i'm paying exactly the same rate I have been paying for years and even took them up on a second business line for free. Your theory isn't panning out for me. Also my 5g experience with t-mobile is that it's between 3-8x as fast as I was getting before on LTE in the areas that I frequent. So I'm not complaining. I mean I WANT my gigabit 5G but I'm okay right now at this pricepoint.

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