GSM is an interference restricted system. Carrier-to-interference ratio (C/I), also called interference protection ratio, refers to the ratio of all useful signals to all useless signals.
In GSM system, this ratio is relevant to the instantaneous location and time of MS due to irregular landform, different shape, type and number of surrounding scatterer, different type, direction and height of antenna, as well as different number and intensity of interference source, etc.
Usually, the interference signal comes from the following three kinds of sources:
1. Multi-path signal interference of useful signal itself which falls outside the system delay equalizer
2. Co-channel and adjacent frequency interference generated from frequency reuse of useful frequency itself
3. Other signal interferences from outside (radar station, illegal co-channel equipment, noise from environment, etc.)
According to the signal demodulation requirement of air interface, GSM specifies that the co-channel and adjacent frequency protection ratio must comply with the following requirements:
Co-channel carrier-to-interference rate: C/I≥9dB; add 3dB allowance in engineering, that is, C/I≥12dB;
C/I refers to the interference from other cells to service cell when different cells use the same frequency. In a board sense, certainly the electromagnetic wave energy of all useless signals falling into this frequency carrier should also be considered.
Adjacent frequency suppression rate: C/A≥-9dB; add 3dB allowance in engineering, that is, C/A≥-6dB;
C/A refers to the interference from all adjacent signals around the service cell (carrier offset 200KHz) to service cell channel under frequency reuse condition.
The carrier-to-interference ratio requirement under carrier offset 400KHz is: C/2A≥-41dB.