The typical schematic for mobile coverage shows perfect hexagons with an omnidirectional antenna at the centre. If demand were that uniform then this would be the usual cell architecture. However various parts of the network have higher demands than others. The simplest and cheapest way to increase the capacity within a cell is to sectorise. This involves replacing the omnidirectional antenna with 3 or 6 directional antenna covering 120deg or 60deg of the cell. This effectively triples (or a six fold increase in) the capacity of the cell at a cost of replacing an antenna; the same mast is used.
In urban environments many of the masts have 3 directional antennas on them. This technique has been extensively used to increase the network capacity.
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