In GSM systems, there can be interference to cells from their neighbor cells that use
the same frequency or adjacent frequencies or both. There can even be
interference from cells in other systems. Interference can be classified as Co-Channel
Interference (CCI) and Adjacent Channel Interference (ACI) from the same system
(intrasystem interference) or between different systems (intersystem interference).
Co-channel
Interference
Because of frequency reuse in GSM systems, the reused frequencies in other cells can
interfere with the serving cell. CCI depends on the reuse plan. For example,
the 7/21 reuse plan has a CCI greater than the 9/27 reuse and less than the
4/12 reuse plan. Therefore, to reduce the CCI it is recommended to increase the
reuse pattern or, equivalently, increase the reuse distance between cells of
the same frequency groups. The impact of increased CCI is a degradation in
voice quality. The CCI is measured in terms of
Carrier-to-Interference Ratio ( C/I) . For analog systems, C/I of 17 dB is
considered appropriate for good voice quality.The
BER for a digital system with and without diversity for different C/I levels.
Relationship
between C/I and BER
Adjacent Channel Interference
The avoidance of using adjacent
channels in neighbor cells is a good practice and is possible in a theoretical
7/21 reuse pattern. In practical systems, adjacent channel assignments in
neighbor cells can occur quite often. When the carrier power is increased in a
cell, it can cause interference to an adjacent channel in the neighbor cell.
The IS-136 standard specifies acceptable levels for Carrier-to-Adjacent
Interference Ratio (C/A) that produces acceptable voice quality in terms of BER
for uplink interference as shown in figure
