Al- Ameen Engg. College
Interference on voice channels causes cross talk, where the subscriber hears
interference in the background due to an undesired transmission.
On control channels, interference leads to missed and blocked calls due to errors in
the digital signaling.
Interference is more severe in urban areas, due to the greater RF noise floor and the
large number of base stations and mobiles.
The two major types of system-generated cellular interference are co-channel
interference and adjacent channel interference.
Co-channel Interference and System Capacity
Frequency reuse implies that in a given coverage area there are several cells that use
the same set of frequencies.
These cells are called co-channel cells, and the interference between signals from
these cells is called co-channel interference.
Unlike thermal noise which can be overcome by increasing the signal-to-noise ratio
(SNR), co-channel interference cannot be combated by simply increasing the carrier
power of a transmitter.
This is because an increase in carrier transmit power increases the interference to
neighboring co-channel cells.
To reduce co-channel interference, co-channel cells must be physically separated by
a minimum distance to provide sufficient isolation due to propagation.
When the size of each cell is approximately the same and the base stations transmit
the same power, the co-channel interference ratio is independent of the transmitted
power and becomes a function of the radius of the cell (R) and the distance between
centers of the nearest co-channel cells (D).
By increasing the ratio of D/R, the spatial separation between co-channel cells
relative to the coverage distance of a cell is increased.
The parameter Q, called the co-channel reuse ratio, is related to the cluster size For
a hexagonal Geometry
• N small, Q small, larger capacity
N large, Q large, better transmission quality due to a small level of co-channel
signal-to-interference ratio for a mobile receiver which monitors a forward channel: