When hooked to a transmitter, a directional antenna will have apprent gain--in other words it will look like you are transmitting with a higher power than you actually are. It will increase the link range.
When hooked to a receiver, a directional antenna will reject signals that are off-axis, reducing the amount of interference.
A BTS ormally has 3 antennas for 3 sectors. Theoretically, Cell coverage is circular; thus it requires 3 antennas of beamwidth 120 deg. each to cover one cell.
The problem is if we use the 120 deg antenna forall the sectors , practically the signal is also transmitted in sidelobes which overlaps the neighbouring sector/antenna's signal. In CDMA this creates an Co- Channel interfernce since CDMA has only one frequency i.e. universal freq reuse.
If we use 65 deg beamwidth antenna , then the actual signal will be transmitted in ≈ 90-110°. This serves the purpose of covering 120° sector with minimum interference.
Not only CDMA but every cellular system uses 65° beamwidth antenna to cover 120° sector. 120° beamwidth antennas are also available and used to cover 180 ° sector.
in some areas the coverage is not necessary 360
degrees as in the case of unpopulated areas but
requiring highways coverage, so 60 degrees antennas
pointing in opposite direction will cover the highway.