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
- 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.
- 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.