4 ms·

Yup, there's only 1Mbit/sec for every 1Mhz of radio bandwidth, and this is shared between all users in a given cell. This is also why I don't understand the fas

Yup, there's only 1Mbit/sec for every 1Mhz of radio bandwidth, and this is shared between all users in a given cell. This is also why I don't understand the fascination with Starlink, which makes cells obnoxiously large (the size of a satellite coverage), even Musk himself says it's only for remote areas.

Ekaros 5y ago
And why I have wondered how does the math really work out. Fiber and cell tower cost only so much. And outside some mobile cases does areas where this isn't viable have enough customers? Then again, USA is always an argument where telco seems to have really failed for various reasons. Still, I see no reason why same would apply to Africa or other markets they talk about...
ac29 5y ago
> there's only 1Mbit/sec for every 1Mhz of radio bandwidth That can't be right, cellular networks have had > 1 bps/hz efficiency for decades. https://en.wikipedia.org/wiki/Spectral_efficiency https://en.wikipedia.org/wiki/Spectral_efficiency
jlokier 5y ago
> there's only 1Mbit/sec for every 1Mhz of radio bandwidth > this is shared between all users in a given cell Both statements are untrue. For example, "Wifi 7" 802.11be reaches up to 125 Mbit/s for each 1 MHz of radio bandwidth in perfect conditions, to a single user. Inside a cell area, multiple users can share the same radio frequencies at the same time using various spatial modulation techniques (typically called beamforming but it doesn't have to be a beam, and might involve 3 or more stations working together). The theoretical limits scale very differently than common understanding of bandwidth.
- H8crilA 5y ago
  > reaches up to 125 Mbit/s for each 1 MHz of radio bandwidth I see. What is the theoretical limit of bits/second per hertz of radio bandwidth, if any?
  - jlokier 5y ago
    Ignoring quantum effects, there's no upper limit in theoretically perfect radio conditions, even for a single radio channel, where there is absolutely no background noise or interference, and the antennae and electronics are literally perfect. However, nothing can be built like that, and there is always background noise. The real life limit is complicated. For each single radio channel the limit depends on other characteristics of the channel than just the (temporal) radio bandwidth, for example the antenna, background radio noise, interference, and power level. The Shannon-Hartley theorom covers capacity of a single noisy channel with fixed characteristics; it depends on the signal to noise ratio as well as the bandwidth. Radio channels used for mobile communications are typically "fast-fading", which means the conditions keep changing, and both ends have to continuously measure those conditions and adapt dynamically how they encode and decode the data. This complicates a measure of the theoretical limit, and also means the limit is changing with time, for example as the user moves the handset around. https://en.wikipedia.org/wiki/Channel_capacity#Channel_capacity_in_wireless_communications https://en.wikipedia.org/wiki/Channel_capacity#Channel_capac... The highest rate encoding used in 802.11be is "4096-QAM" which carries 24 bits/sec per Hz when noise is sufficiently low, and decreases the more error correction is required. I'm not sure if 4096-QAM is chosen because the signal-to-noise level is such that a larger number wouldn't improve thoughput enough to matter, or if it's limited by the electronics, non-linearity and calculations at those high speeds. That bit rate is for a single radio channel. Multiple channels overlapping the same frequencies are also possible, as long as the antennae are sufficiently spread out in space and/or polarisation. When something large enough like a laptop (which has multiple antenna in the display) talks to a base station, it is combined with beamforming and diversity, or in a general mathematical description, polarisation modes and spatial bandwidth. That is equivalent to having multiple radio channels using the same frequencies to the same device, but the channels are not physically distinct, they are combined and separated mathematically with linear algebra. As with MHz bandwidth, spatial and polarisation are also limited. You can't add more channels with no limit to a single device of fixed size. But if you have no limit on device size so can add more antenna spread out, and/or many base stations cooperating, you can increase the total bits/second by using those. 4G LTE does the multiple base stations thing to some extent already, to maintain channels to multiple user handsets at the same time inside a cell. It's not that each handset talks to a separate base station. It's that the signals are combined with linear algebra in such a way that each handset can decode different information from the multi-spatial-channel signal that they all receive slightly differently.

Ekaros 5y ago

And why I have wondered how does the math really work out. Fiber and cell tower cost only so much. And outside some mobile cases does areas where this isn't viable have enough customers? Then again, USA is always an argument where telco seems to have really failed for various reasons. Still, I see no reason why same would apply to Africa or other markets they talk about...

ac29 5y ago

> there's only 1Mbit/sec for every 1Mhz of radio bandwidth That can't be right, cellular networks have had > 1 bps/hz efficiency for decades. https://en.wikipedia.org/wiki/Spectral_efficiency https://en.wikipedia.org/wiki/Spectral_efficiency

jlokier 5y ago

> there's only 1Mbit/sec for every 1Mhz of radio bandwidth > this is shared between all users in a given cell Both statements are untrue. For example, "Wifi 7" 802.11be reaches up to 125 Mbit/s for each 1 MHz of radio bandwidth in perfect conditions, to a single user. Inside a cell area, multiple users can share the same radio frequencies at the same time using various spatial modulation techniques (typically called beamforming but it doesn't have to be a beam, and might involve 3 or more stations working together). The theoretical limits scale very differently than common understanding of bandwidth.

H8crilA 5y ago
> reaches up to 125 Mbit/s for each 1 MHz of radio bandwidth I see. What is the theoretical limit of bits/second per hertz of radio bandwidth, if any?
- jlokier 5y ago
  Ignoring quantum effects, there's no upper limit in theoretically perfect radio conditions, even for a single radio channel, where there is absolutely no background noise or interference, and the antennae and electronics are literally perfect. However, nothing can be built like that, and there is always background noise. The real life limit is complicated. For each single radio channel the limit depends on other characteristics of the channel than just the (temporal) radio bandwidth, for example the antenna, background radio noise, interference, and power level. The Shannon-Hartley theorom covers capacity of a single noisy channel with fixed characteristics; it depends on the signal to noise ratio as well as the bandwidth. Radio channels used for mobile communications are typically "fast-fading", which means the conditions keep changing, and both ends have to continuously measure those conditions and adapt dynamically how they encode and decode the data. This complicates a measure of the theoretical limit, and also means the limit is changing with time, for example as the user moves the handset around. https://en.wikipedia.org/wiki/Channel_capacity#Channel_capacity_in_wireless_communications https://en.wikipedia.org/wiki/Channel_capacity#Channel_capac... The highest rate encoding used in 802.11be is "4096-QAM" which carries 24 bits/sec per Hz when noise is sufficiently low, and decreases the more error correction is required. I'm not sure if 4096-QAM is chosen because the signal-to-noise level is such that a larger number wouldn't improve thoughput enough to matter, or if it's limited by the electronics, non-linearity and calculations at those high speeds. That bit rate is for a single radio channel. Multiple channels overlapping the same frequencies are also possible, as long as the antennae are sufficiently spread out in space and/or polarisation. When something large enough like a laptop (which has multiple antenna in the display) talks to a base station, it is combined with beamforming and diversity, or in a general mathematical description, polarisation modes and spatial bandwidth. That is equivalent to having multiple radio channels using the same frequencies to the same device, but the channels are not physically distinct, they are combined and separated mathematically with linear algebra. As with MHz bandwidth, spatial and polarisation are also limited. You can't add more channels with no limit to a single device of fixed size. But if you have no limit on device size so can add more antenna spread out, and/or many base stations cooperating, you can increase the total bits/second by using those. 4G LTE does the multiple base stations thing to some extent already, to maintain channels to multiple user handsets at the same time inside a cell. It's not that each handset talks to a separate base station. It's that the signals are combined with linear algebra in such a way that each handset can decode different information from the multi-spatial-channel signal that they all receive slightly differently.