ation esti- longer intervals. For NLoS streets, the median values of mates despite similar power delay profiles [SaGo01]. the lengths of these stationarity intervals ranged from 2.6 Figure 4.7-31 shows these differences between up- and m to 3.3 m, depending on street orientation [Bult03]. downlink for frequency correlation estimates by direct Another technique developed by Bultitude for testing on correlation as opposed to those derived from Fourier WSS is inspection of the variation around the mean in transforming power delay profiles. spectral lines [Bult02b]. In the process of determining frequency correlation functions, it is common practice to use a Fourier 4.7.5.3 Path loss and building transform on the APDP. However, this assumes Un- penetration correlated Scattering (US). One test on US is to de- termine frequency correlation functions directly from A vast number of different path-loss models have been the spectrum and to test whether the resulting corre- proposed in the past two decades, ranging from simple lation functions are symmetric with respect to arbitrary empirical to more complex physical ones [Corr01]. Each reference frequencies [Bult02b]. The validity of the of these models has it owns advantages and drawbacks. assumption of US in indoor environments has been Hence, path loss in outdoor macro- and microcells treated by Kattenbach [Katt97]. Also, the presence of is addressed, including vegetated residential environ- dominant coherent components, as in Ricean fading ments. Indoor and building penetration models are also processes, can distort results obtained by Fourier presented. Propagation modelling and channel characterisation CHAPTER 4.7 An approach to extend the validity domain of simple where 9 is the dominance probability, i.e. the probability empirical models, e.g. Okumura-Hata [OOKF68], that, at a given distance, d, the dominant component in the [Hata80], in urban areas is proposed in [GCBV02]. It is received field is the one due to vertical propagation, and assumed that propagation in