 Co-channel interference with hexagonal cells same order. Parameters D and R are illustrated in Figure 2.28. The re-use pattern factor K can be calculated geometrically: (2.59) From Figure 2.27, K can be calculated as K = 12+1x2+22: = 7 (2.60) Using the re-use distance gives D=V3x7R=V21R~ = 4.58R (2.61) To enable maximum capacity, the parameter K should be optimised to be as small as possible when the system is operational and fulfilling the planning requirements. Parameter q is the co-channel inter- ference reduction factor, where the higher the value of q the smaller is the co-channel interference (see Fegure 2.28): (2.62) The co-channel interference can be calculated as the ratio of the carrier (C) to the sum of the interferers (In): (2.63) where N is the number of interferers and the received carrier is C = ad-r (2.64) Frequency Planning where d describes the distance between the transmitter and the receiver, a is a constant and y is the propagation path loss slope. Interference for the first tier is (2.65) and is simplified for the first tier, as all the interferers in the same tier are equally strong: (2.66) first tier Interference for the second tier is (2.67) Isecond tier The C/I relation can be to improve the network using the following methods: decrease transmission power, fine tuning of the antenna azimuth and antenna down-tilting. All the methods have an impact on the cell coverage area and therefore they need to be used carefully, keeping in mind the coverage targets. Also the overall situation needs to be verified, preferably with field test measurements, because the changes made in one area can have other types of impact in the adjacent area. The spectrum efficiency techniques, as indicated by their name, provide ways to utilise the frequencies more effectively. This is critical in network planning due to the limited resources, i.e. limited frequencies. More effective usage of the frequency band is possible without increasing the interference level. In this way the network capacity increases. 2.7.1