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.