Frequency Planning Steps for GSM
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Transcript of Frequency Planning Steps for GSM
Frequency Planning Steps for GSM
Frequency Planning is possible by manually or automatic.At initial stage of new RF Network
automatic frequency planned and then after for optimization based on KPI manual frequency
planing done.
Manual Frequency Planing Steps
The steps to be followed in manual frequency planning are:
Calculating the frequency reuse distance theoretically.
Determining the cell repeat pattern
Planning the frequency groups.
Inputting the planned frequency into the planning tool.
Generating the C/I and C/A plots and checking out the results.
Rectifying the fault areas.
Determining the cell repeat pattern
Frequencies have to be reused at different cells throughout the network to maximize
capacity.
The distance cells using the same set of frequencies is called the frequency reuse
distance.
This reuse distance depends on the number of frequency reuse groups N.
Once N has been determined every Nth cell will be assigned the same frequencies.
Also a minimum received signal level has to be provided throughout the coverage
area of the network.
The cell repeat pattern is dependent on the frequency spectrum available, the traffic
required and most important on the way the network is planned.
Generally 7/21 or 7 site repeat pattern and 4 site repeat patterns are used.
Cell reuse pattern
The distribution of the C/I ratio desired in a system determines the number of
frequency groups, F, which may be used.
If we have N carrier frequencies then
No of carriers / group = N/F
Since the number of frequency groups are fixed, a smaller number of frequency
groups (F) results in more carriers per set and per cell.
Hence a reduction in the number of frequency groups would allow each site to carry
more traffic.
However decreasing the number of frequency groups and reducing the frequency
reuse distance results in lower average C/I distribution in the system.
Generally 7/21 and 4/12 reuse patterns are used.
7/21 Cell reuse pattern
Say we have 42 frequencies and we require 2 carriers per site then we can use 7 site
repeat pattern.
Hence a cluster will be formed of 7 sites.
4/12 Cell reuse pattern
Say we have 48 frequencies and we require 4 carriers per site then we can use 4 site
repeat pattern.
Hence a cluster will be formed of 4 sites.
The frequencies for manual frequency planning for a cluster size of 7 and for a cluster size of
4 are arranged as shown below
Interference Prediction
Once the repeat pattern is determined the frequencies should be entered in the
planning tool.
Enter the C/I threshold and C/A threshold. For GSM put 12dB (GSM specifies > 9dB)
as C/I and 0dB (GSM specifies C/A > -9dB) as C/A.
Generate a C/I and C/A plot.
Analyze the plot and check for problems.
Debug and solve the interference problems.
Note that to get a correct C/I and C/A plot, all the sites prediction on the tool should
be completed.
Moreover the models should be correctly tuned and the coverage predicted by the
propagation model should match the coverage on the field.
Automatic Frequency Plan
Planning tools nowadays have automatic frequency planning options.
This tool uses predictions. Hence the models have to be accurately tuned.
Moreover Co-cell and co-site separations, allowed frequency bands, target levels for
allowed co-channel and adjacent channel interference need to be defined.
Frequency Co-Ordination
On international borders frequencies are commonly co-ordinate with neighboring
countries to avoid mutual interference.
Generally sets of reserved or preferential frequencies are negotiated between the
national authorities of the respective countries.