How to Identify Interference in GSM Network (1)

One major challenge when optimizing GSM network is the detection of cells suffering from interference as interference is limiting the capacity and harming the quality of the

network. This problem occurred due to limited bandwidth and limited number of carrier frequency.

There are two categories of interference source that are:- Internal Interference: Co-channel or Adjacent channel interference causing internal

interference.- External Interference: Exists when there is another transmitter or something else acting as a transmitter outside the network such as TV transmission, Railway Station frequency,

and microwave links.

Interference Identification using Radio Network Statistic

Radio network statistic (RNS) can be used to analyze if specific cells suffer from interference and which frequency channels are involved. A cell in GSM can suffer from interference either in the uplink or downlink or in both directions. As the interference is

related to the frequency or a set of frequencies in case of frequency hopping, it is interesting to see which TRX suffers from interference if interference happened in BCCH TRX or Hopping TRX. The interesting parameter in this case is the assigned frequency to

the individual TRX. After the disturbed cell and the related frequencies are identified, potential interferer cell can be identified. For this purpose a geographical analysis can be

performed. Frequency retune becomes the solution if BCCH frequency suffers from interference. In case TCH frequency further identification shall be performed if hopping

system is assigned to TRX’s.

There are several indicators from Radio Network Statistics that can be used to identify the interference in the network.

1. The correlation between RxLevel to RxQual.An indication for interference will be that the receive level is good but at the same time

the receive quality is poor. This can be analysed for both directions (uplink and downlink) separately. It is better to have this figure for BCCH TRX and hopping TRX so we can

identify whether interference exists at BCCH TRX, Individual TRX (non-hopping case) or Hopping TRX.

2. Drop Call Performance. Usually when the cell has interference problem, the drop call will be higher than usual,

but it depends on severity of interference level

3. Handover Performance.Intra Cell Handover indicates that cell suffers from bad quality at good level. The HO

performance to neighbor relation that has interference will look bad. The other indicator is a lot of reversion to old channel activity due to target frequency might be interfered.

How to Identify Interference in GSM Network (2)

Another ways to identify internal interference in GSM network is by scanning frequency at idle mode. At this case, GSM recommendation introduces reference value of interference

level (GSM Recommendation. 05.05).

The actual interference ratio is defined as the interference ratio for which this performance is met. The actual interference ratio shall be less than a specified limit,

called the reference interference ratio. The reference value of interference will show co-channel interference(C/Ic) and adjacent channel interference (C/Ia).

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The following figure shows sample of result of scanning frequency using Tems Investigation.

C/Ia calculation shows that carrier to interference level of first adjacent (200KHz spacing) and second adjacent (400 KHz spacing) exceeds minimum value of interference level (-9 dB for first adjacent and -41 dB for second adjacent). So, we can conclude that there is

no interference to channel 512.

Distribution C/I from drive test with dedicated mode also can be used as figure to describe level interference on drive test route. We can do analysis separately between C/I

distribution on BCCH carrier and C/I on Hopping list carrier.

External interference can’t be detected using drive test tool. As instead, scanning frequency using spectrum analyzer is used to observe the source of external frequency.