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.