What is a cell?
A cell is a base transceiver service area as seen by the mobile
station (MS). A cell uses a specific set of frequencies.
There are two  types of cells:
Omni cells:
An omni cell is a cell where the
antenna transmits omni-directional. The coverage area of an omni cell is
in principle a hexagon/circle, but in reality a rough
pattern.
Sector cells:
A sector cell is a cell where the
antenna transmits directional. 
Examples of sector cell types are:
-           2-sector
cells (e.g. for highways)
-           3-sector
cells.
The
following figure shows examples of different cell types
  Sector vs. omni cells 
Advantages of sector cells are (compared to omni cells):
•           Increased coverage area per site (by
the use of higher gain antennas)
•           Possibility of
mechanical tilting antennas (to reduce unwanted interference)
•           Simpler
antenna mounting (reduced clearance to prevent interaction with other antennas).
Disadvantages of sector cells are:
•           More equipment
required at each site
•           Greater
environmental impact (more antennas)
•           Longer
frequency re-use distance for a given C/I
•           Increased cell
handovers.

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