(Educated guess) More than likely because the antennas used have a radiation pattern of 60 degrees. Thus 6 are required for full 360 degree coverage. Clarification If you look at the top of a cell phone tower, you will see the vertical arrays that constitute the antennas, usually with the form of long vertical rectangles or boxes, sometimes arranged in parallel groups. A more detailed inspection would reveal that the vertical arrays are separated 120º from each other. This means that each array is pointing to the center of its hexagonal cell. Hence, each cell is covered by six arrays from six separate towers, one at the apex of each of its six angles. Each provider can use a fixed set of frequencies, which forces them to assign a limited number of frequencies to each adjacent cell; the frequencies start repeating on the non-adjacent cells. Similarly, each antenna array can serve a limited number of subscribers at any given time. All this means that when traffic increases within a cell (more users), the only way of coping with the extra traffic is to reduce the area of the cell so it serves less subscribers. This is done by adding more towers to form smaller cells, hence the constant demand for cell tower sites. When a cell is fractioned in smaller cells, the antenna arrays that serve the new cells are lowered and their power is reduced so their emitted energy does not go beyond the adjacent cells to avoid interfering with those other cells, farther cells that use the same sets of frequencies. Having the cell phone towers closer to each other means that the signal, although purposely weakened, can now reach inside elevators and concrete buildings. This also means that cell phones need to transmit in their high power setting much less time while in use, which allows for smaller handsets whose batteries last longer.
In the US the Bell System had done some experimentation with mobile phones mounted in car trunks before WW2 and implemented the first mobile phone exchange in June 1946 in St. Louis, Missouri. By 1950 most major cities and about 4000 miles of highway were connected to mobile exchanges, and more than 7500 cars, trucks, and other vehicles had been equipped with leased mobile phones. All mobile calls were handled via operators on switchboards (even in major cities where all standard landline calls were already handled by fully automated dial exchanges). However these mobile phones were not like modern cellular phones and due to limited radio bandwidth assignment by the FCC the initial systems saturated at about 3 active calls per exchange, although by the 1960s these systems had been improved and could support more than 25 active calls per exchange before saturating. In some situations with this early system it could take as long as half an hour after a mobile phone customer indicated they wished to make a call before they were connected to an operator. In December 1947, Douglas H. Ring and W. Rae Young, Bell Labs engineers, proposed hexagonal cells for mobile phones in vehicles however they recognized that the vacuum tube and relay technology of the phone system of the time could not handle all of the demands of such a cellular mobile phone system. In 1969 with the newly available miniaturized solid state electronics technology A. T. & T. began serious work on a vehicle trunk mounted cellular mobile phone and contacted the FCC about licensing of radio bandwidth assignments for this new cellular mobile phone system that would replace the older system. In 1971 Martin Cooper, a Motorola researcher and executive began working on a handheld portable cellular mobile phone and on April 3, 1973 he made the first call using it. For several years however cellular mobile phone calls still frequently had to be made through an operator (see the "Cannon" detective series of 1971-1976 where he had a Motorola car cellular mobile phone), but eventually direct customer dialed automated calling became universal.
Mobile phones work through radio frequencies. These frequencies have to be planned very carefully because if two people are using the same frequency in the same place it interferes (like when you get a fuzzy noise on the radio because of interference). Interference causes the signal to get corrupted and can lead to your call suddenly being dropped. So mobile operators spend a great deal of time planning frequencies to give the best performance and the optimum way to do that is in a hexagonal grid system (like the pattern of honeycomb). The mobile phone masts are ideally placed at the intersections of the hexagon shapes. So each mast should have three hexagonal areas surrounding it. Obviously these aren't exact because the real world isn't perfectly flat and you can't put masts just anywhere! But these areas are called 'cells', hence the name cellular network and they cover our world like a patchwork quilt.
Difference between GPS and GSM:- ------------------------------------------- GPS:- Global Positioning System that user Satellite signals to triangulate one's location.It is not related to any cellular technology, but phones can be designed to include GPS as a feature.... GSM:- Global System for mobile communications(originally group special mobile), the internationally used standard for cellular phones...
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