s are consecutive there is some neighbour channel interference, but this is less serious. The interference level cannot be high when building a functional network. The interference level increases with high transmission power in a close location. The frequency re-use rate is simplest to explain using a hexagonal model. Frequency re-use patterns are not used in practice because the cells are not hexagons, as already explained in the coverage planning section. The cell shapes are different and cells do not have equal sizes. Therefore the frequency re-use rate is not a constant throughout the network, but varies from one place to another and can also vary between BCCH and TCH layers. The available frequency band and the capacity plan give boundary conditions for the largest possible frequency re-use rate. Figure 2.27 shows two examples of the frequency re-use rate for hexagons. The first describes the re-use pattern for 7 cells and the second one the pattern for 12 cells. The following is an example of the connection between frequency and capacity planning. If the operator bandwidth is 6 MHz, the number of channels is 30. In general, frequency re-use affects the number of carriers that can be used for one sector. The number of carriers per sector can be calculated by dividing the available bandwidth by the product of the re-use rate and bandwidth for a single carrier, i.e. dividing the number of channels by the frequency re-use rate. Using a re-use rate of 12, the number of TRXs, carriers per sector, is calculated below: 6 MHz 2.5(TRX) 0.2 MHz 12 Continuing, the capacity of the network with 100 BTSs and 600 cells can be calculated as 600 X 2.5 TRX = 1500 TRX An interesting factor in frequency re-use is the distance between base stations using the same frequency. This distance is known as D and the cell (hexagon) radius as R. The re-use distance is explicit for the hexagonal model, cells with equal sizes and frequency re-use patterns repeating the frequencies in the Radio Network Planning and Optimisation Figure 2.28