Coverage in Network Planning
The coverage plan is mostly dependent on geographical and environmental factors: the type of the terrain or if the area is urban, suburban or rural.
The "location probability" is a concept that needs to be determined in order to obtain the best coverage quality in the respective area. Location probability refers to the probability of having a field of strength at a higher level then the sensitivity of the target area. This prediction is typically performed through radio propagation models.
There are standard models used depending on general characteristics of the designated network area, or if need be, network planners can design their own propagation models. Using standard propagation models leads to lowering costs and saving time, yet they are also less accurate then the personalized propagation models.
To obtain coverage areas predictions, network planners need to have used propagation models, performed drive tests and established correction factors.
For a very efficient coverage of the GSM signal, planners can use both omnidirectional and sector antennas. Ensuring the coverage for a particular area implies mounting more antennas, and each antenna creates a radio cell.
The distance between the antennas must be thoroughly thought out. If the antennas are too close to one another, they will overlap, and the network deployment will be very costly. At the same time, if they are installed too far away from one another, there will be areas without GSM coverage.
- omni antennas
If the planner chooses to use omni antennas, the following formula needs to be used for calculating the coverage area:
2.6km × r2
For calculating the distance between the antennas, the planner will use the following formula:
1.73 × r
- two sector antennas
If the planner opts for sector antennas, the coverage area will be calculated with the formula:
1.59km × r2
The distance between the antennas will be calculated with the formula:
1.5 × r
In the above formulas r is the range of the base station.
Keep in mind that these formulas are only representative for two sector antennas.
Note: To obtain the total number of base stations to be deployed, divide the total coverage to the base station coverage area.
After finishing the design, a network planner needs to calculate the link budget. This ultimately leads to a network design that functions correctly according to all requirements at a reasonable cost. Link budget takes into account all the gains and losses from the transmitter to the receiver from: path losses, antennas, antenna feeders, power levels and receiver sensitivity.
The link budget can be calculated with the below formula:
RP = TP + G − L
where: RP – received power (dBm)
TP – transmitted power (dBm)
G – gains (db)
L – losses (db)
The factors that determine link budget can vary in time, as path loss does, and, in such cases,the worst case scenario will be taken into account.
The link budget calculation essentially leads to determining the cell size by accounting for the maximum allowed path loss without affecting the communication quality.
Radio frequency propagation models
The Hata model is probably the most efficient radio frequency propagation model for predicting mobile communications in suburban areas. These suburban areas have distances between buildings that are larger than the buildings themselves. It predicts the total path loss experienced along a link of terrestrial cellular radio transmissions.
However, this model is not recommended for scenarios where the antennas are below the rooftop levels.
The carrier frequency for this model must be from 150 to 1500 MHz. The base station antenna height must be from 30 to 200 meters. The distance between the base station and the mobile station must be from 1 to 20 km.
COST231 is a radio propagation model that extends the Hata model to cover more complex frequencies. It is mostly suited for urban areas.
This model works best if the antenna is at the rooftop level or just below it.
The carrier frequency for this model must be from 1500 to 2000 MHz. The base station antenna height must be from 30 to 200 meters. The distance between the base station and the mobile station must be from 1 to 20 km.
The Walfisch-Ikegami radio propagation model works best in urban environments with buildings in the vertical plane between the transmitter and the receiver.
This propagation model is most efficient when the antenna is above the roof height. It takes into account various factors, such as the density and height and width of the buildings, the width of the streets or the direction of the streets in relation to the direct trajectory of the antenna and the mobile station.
The carrier frequency for this model must be from 800 to 2000 MHz The base station antenna height must be from 4 to 30 meters. The distance between the base station and the mobile station must be from 0.02 to 5 km.