GSM is a cellular network, which means that the mobile phone is connected to the nearest cell area that it can search. The GSM network operates on a number of different radio frequencies.
The GSM network has a total of four different cell sizes: giant cells, micro cells, pico cells and umbrella cells. Coverage varies by environment. A giant cell can be thought of as the kind of base station antenna that is mounted on an antenna mast or on top of a building. Microcells are those whose antenna height is lower than the average building height and are generally used in urban areas. The picocell is a small cell that covers only a few tens of meters and is mainly used indoors. The umbrella cell is a dead zone for covering a smaller cellular network, filling the signal blank area between cells.
The radius of the cell can range from more than 100 meters to tens of kilometers depending on the antenna height, gain and propagation conditions. The longest distance GSM specification actually used supports up to 35 kilometers. There is also the concept of an extended cell, where the cell radius can be doubled or more.
GSM also supports indoor coverage, and the power of the outdoor antenna can be distributed to the indoor antenna distribution system through the power splitter. This is a typical configuration for indoor high-density calling requirements, which is common in shopping centers and airports. However, this is not necessary, as indoor coverage can also be achieved by wireless signals crossing the building, but this can improve signal quality and reduce interference and echo.
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