It is the propagation, Maurice! I haven't come back to that issue yet because it is complex for someone who doesn't have a background in microwave. (Moreover I'm not retired yet.)
Let me write under the assumption that you don't know microwaves. If you know, please, correct me.
Maurice, electromagnetic waves tend to travel straight the higher the frequency. GSM 900MHz spreads around. Light, the visible part of spectrum travels in straight path. What this has to do with the towers, foundations and soil becomes clear later.
As the frequency goes higher, you get more bandwidth so you get more information transported over the radio medium. But you pay a price. As the freq. goes higher they start behaving like visible light, they suffer absorption, refraction, reflection. Radio signals are faster attenuated as they travel in the free space.
In short a higher freq. gets useless with shorter distance to the transmitter. At lower frequency a radio signal goes further. WLL DECT, the one I implemented, operates at 1.8GHz. At this freq. you have to space your base stations with short distance between one another. So you have to construct more towers to cover a given area.
Since WLL only make economic sense in a high density users' area, this areas usually do not have -in Asia- a lot of room to construct towers. So we decided to erect masts. Not the guyed type but self-standing masts. The foundation for these masts have to be very big because of the whole weight of the mast and the base station have to be dispersed over a wide area. We needed that wider area because the soil bearing capacity is very low 0.5 Kg per square inch. More civil works means more costs. More wider area means a limitation on the sites you can choose. Further, this was the tropics the tree coverage is high. So we have to construct these masts to clear the tree coverage adding to the problem above.
WLL roots are its problems. WLL is a technology used in cordless PABX's. You give a WLL handy for the people roam around you office, your warehouse or construction site. Someone have the 'brillant' idea of using WLL as a replacement to the copper local loop, in Chechenya, Brazil and Thailand. But it dind't work. Because it requires too much engineering and too much infrastructure. I don't trust ny technology -and I don't mean telecoms only- that is too 'mass' intensive.
Now if you say that one day GSM will hit the wall, and run out of carrying capacity and there is a new technology that solves the problem I agree with you. But do not call it WLL. WLL, MMDS and LMDS have no future and will never take off.
But be careful. Engineers have this nasty -for investors- mania of always extracting more out of what they already have. Remember copper. Fiber to the home didn't take off because we can extract more bandwidth out of existing copper with clever components and algorithms. Now if they did this with copper imagine what they can do with a chunk of the spectrum!
Moreover, these GSM infrastructure guys are charging high prices for the stuff they sell that there is room for fighting a price war against new entrants -Ionica, CT2 and PHS- that is not negligible.
Now that are at it I go to the end: The soil of a region seating on a sedimentary basin is usually softer. It consists of sediments deposited by water running down the mountain slopes. You have volcanoes in Kiwi land. All that material runing down the slopes is deposited in the plains below near the sea. Ask some geological survey guys what is the soil's bearing capacity is these areas.
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