10 2. BackgroundFigure 2.2 The two optical effects reflection and absorption occur at the transi-tions between materials with different optical densities. They can be simulated withraytracing algorithms. The employed PHOTON raytracer, described in 2.1.1, isconfigured to model only these two effects.Furthermore, an initial signal strength for a simulated ray is needed. A simple modelof these optical effects, as primarily used in this thesis, is defined by a numericalvalue for rate of reflection and the rate of absorption for each material and oneadditional parameter for an initial signal strength of each AP class. The valuesof these parameters are assumed to be initially unknown and have to be trained.Therefore, the number of trainable parameters nfreefor the employed model is:nfree= 2 nmat+ napclswith nmatas the number of different materials and napclsas the number of differentAP classes.A strategy for finding these free parameters of the model consists of comparing realworld measurements at different locations with the corresponding simulation resultsof the raytracer and use the minimum averaged error over all locations as the targetfor an optimization algorithm. Of the different parameter optimization methods,that were evaluated, Genetic Algorithms , described in 2.1.2, have shown to be mosteffective.2.1.1 PHOTON RaytracerThe PHOTON raytracer[23] that is used1for the radio propagation model representsa deterministic approach to model radio propagation with concepts from geometricaloptics. The signal of a radio wave is modelled as a single particle, called PHOTON,that travels on a straight line until intersecting with another physical medium. Atthese intersections the physical effects reflection and absorption are simulated. Eventhough it is possible to simulate behaviour of radio propagation that is more complexthan diffraction, this has not been used in the presented setup. Instead a simplerform of Bidirectional Reflectance Distribution Function(BRDF) is configured bythe reflection parameter[0, 1] and an absorption parameter[0, 1]. The1During the preparation phase of the thesis a fast 2D raytracer has been implemented that wasable to simulate 60 raytracing frames per second with basic transmission and reflections rules. Itwould probable be possible to use this implementation to simulate dynamic effects as opening ofdoors or the body shadow of moving people.