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There are basically two kinds of 4G masts: Frequency | Range | Bandwidth ----------|-------------|---------------------------------- 800MHz | Long (500m) | Low (can handle a bit of traffic) 1800Mhz | Short (5km) | High (can handle lots of traffic) The 1800Mhz masts are better in terms of how much traffic they can handle and how fast a connection they provide. But because they have quite short range, it’s only economical to install them in very built up areas†. In more rural areas the 800MHz masts are better because they cover a wider area, and have enough bandwidth for the lower population density. The net effect of this is that cell broadcasts in rural areas are likely to bleed further, because the masts they are being broadcast from are less precise. We can use population density as a proxy for how likely it is to be covered by 1800Mhz masts, and therefore how much bleed we should expect. So this commit varies the amount of bleed shown based on the population density. I came up with the formula based on 3 fixed points: - The most remote areas (for example the Scottish Highlands) should have the highest average bleed, estimated at 5km - An town, like Crewe, should have about the same bleed as we were estimating before (1.5km) – Pete D thinks this is about right based on his knowledge of the area around his office in Crewe - The most built up areas, like London boroughs, could have as little as 500m of bleed Based on these three figures I came up with the following formula, which roughly gives the right bleed distance (`b`) for each of their population densities (`d`): ``` b = 5900 - (log10(d) × 1_250) ``` Plotted on a curve it looks like this: This is based on averages – remember that the UI shows where is _likely_ to receive the alert, based on bleed, not where it’s _possible_ to receive the alert. Here’s what it looks like on the map: --- †There are some additional subtleties which make this not strictly true: - The 800Mhz masts are also used in built up areas to fill in the gaps between the areas covered by the 1800Mhz masts - Switching between masts is inefficient, so if you’re moving fast through a built up area (for example on a train) your phone will only use the 800MHz masts so that you have to handoff from one mast to another less often