.65 161.15 Isotropic path loss to the cell border 155.63 155.43 155.43 157.41 153.65 Figure 2.10 Dimensioning in the WCDMA radio network Radio Network Planning and Optimisation 2.4 Radio Wave Propagation Propagation models have been developed to be able to estimate the radio wave propagation as accurately as possible. Models have been created for different environments to predict the path loss between the transmitter and receiver. How much power needs to be transmitted using the BTS to be able to receive a certain power level from the MS? The complexity of the model affects the applicability as well as the accuracy. Two well-known models are those of Okumura-Hata and Walfish-Ikegami. The first mentioned is created for large cells, i.e. for rural and suburban areas, while the Walfish-Ikegami model is used for small cells, i.e. for urban areas. The basic electromagnetic wave propagation mechanisms are free space loss, reflection, diffraction and scattering. Free space loss describes the ideal situation, where the transmitter and receiver have line-of-sight and no obstacles are around to create reflection, diffraction or scattering. In this ideal case the attenuation of the radio wave signal is equivalent to the square of the distance from the transmitter. When the signal has been transmitted in the free space towards the receiver antenna, the power density S at the distance from the transmitter d can be written as (2.17) where Pt is the transmitted power and G is the gain of the transmission antenna. The effective area A of the receiver antenna, which affects the received power, can be expressed as (2.18) where a is the wavelength and G is the gain of the receiver (RX) antenna. The received power density can also be written as (2.19) Combining these equations, previous the format for the received power is (2.20) The free space path loss is the ratio of transmitted and received power. Here is the equation in simplified format, when the antenna gains are excluded: (2.21) and the free space loss converted in decibels 