14 2. Background· Time Of Arrival: TOA based methods deduce the distance between transmit-ter and receiver by comparing the timestamp of a packet, originating at thetransmitter with the local timestamp of the receiver. Prior knowledge of thespeed of the transmitted signal combined with the timestamp difference can beused for estimating the covered distance of the signal. A source of error is in-troduced by asynchronous clocks and NLOS conditions that lead to multipatheffects.· Time Difference Of Arrival: Methods based on TDOA use the difference oftwo TOA measurements emitted by signals at exactly the same time at dif-ferent APs. By using only this difference the requirement of a synchronizedclock between the different transmitting APs and the receiving device can bedropped. But errors induced by timestamp affecting NLOS conditions remain.· Angle Of Arrival: AOA based methods rely on measuring the angle of theincoming signal at the receiver with directional antennas. A source of erroris induced by NLOS conditions leading to receiving signals of the same APfrom different directions. And another error source in the probable incompleteknowledge of the orientation of the receiver. The requirement of directedantennas at the receiver excludes the use of the commodity WLAN hardwarethat is currently available.Using the RSSI value as the primary source of information for the positioning systemmakes a good radio propagation model mandatory. Two large sources of errors areexpected. At first, there is the error originating in the unpredictable measurementbehaviour or other noisy effects of the Wi-Fi capable devices. And the other classof errors originates in an inadequate modelling of the radio propagation. By usinga raytracing generated radio propagation model, the reduction of such errors was amajor focus in the presented approach. Especially the inherent modelling of NLOSconditions makes a raytracing approach promising.Without a raytracer, one has to resort to approximate the dampening effects ofwalls by introducing an attenuation parameter that determines the magnitude ofdampening at a material intersection. Such an attenuation parameter would behighly material dependent. In the raytracer approach, the corresponding modellingis represented by the interaction of the and material parameters of the employedbasic BRDF.Another source of noise with a high impact on the RSSI values is given by the shadoweffect of the human body. Radio waves with 2.4GHZ are easily absorbed by materialswith a high proportion of water. Furthermore, it is expected that location awaredevices are attached or very close to the owner of the device, therefore boosting thisdampening effect.2.2.1 TechniquesThere are different positioning techniques that have been developed by using thementioned information sources. One of the simplest techniques is called ProximitySensing that uses only the identity of the transmitter instead of any distance or angle