0 metres. 2.4.3 Ray Tracing Model The accuracy of empirical modes is limited in the urban microcellular environment, because of multipath propagation. The previously introduced Walfish-Ikegami model does not take into account reflections and diffractions and therefore the propagation estimation using this model is complicated in an environment where this is typical. Ray tracing meets this claim. Radio Wave Propagation network planning tool measurement results post-processing model tuning test transmitter test receiver Figure 2.13 Model tuning process The principle of ray tracing is that a selected number of rays is used to make the propagation estimation. How many reflections and diffractions are taken into account is dependent on the network planning tool's algorithm. When talking about a complicated algorithm it is clear that the more accurate the propagation estimation that is required the more capabilities from the planning tool and especially time are needed. 2.4.4 Model Tuning The aim of model tuning is to investigate the propagation conditions in the planning area, in a way to customise the used propagation model for the area (Figure 2.13). A carefully tuned model is especially important in the coverage planning phase, because the coverage estimation is based on the propagation. Model tuning basically consists of three phases: preparations, model tuning field measurements and measurement analysis with a planning tool. Before the field measurements can be started some prepa- rations are needed. Firstly, a proper and as accurate a digital map as possible is required. The model tuning is based on the map information and therefore the validity and resolution are critical; the model cannot be more accurate than the map. One part of the preparations, obviously when talking about field measurements, is to collect and test the measurement equipment. A test transmitter is needed as well as a receiver for the measurement car. The measurement locations for the test transmitter have to be considered carefully and ther