er T of height hix and at geometric distance W2 and radial distance d2 from the receiver R x of height hrx as shown in Figures 6.5, 6.6, 6.7 and 6.8. When transmitter T propagates radio waves to the receiver R via the rooftop of the knife edge obstacles, waves get diffracted from the rooftop of the knife edge obstacles. These diffracted radio waves take longer paths than the direct ray path (namely, the line-of-sight path). In this situation we have to determine the excess path length (l) between the diffracted ray and the direct ray path, such that h eff<<dy, and hoeff>>2 where a is the wavelength. (6.19) Now its corresponding phase difference is given as P=27((A) (6.20) From Equations (6.18) and (6.19), we get (6.21) (6.22) UTD-PO and FKE Models for Path Loss Prediction (6.23) Equations (6.22) and (6.23), show that the direct ray and the diffracted ray path are T radians out of phase. This means that there is 1 cycle a difference in the path of both rays at any point in time. (6.24) (6.25) Equation (6.23) is simplified in Equation (6.24) by using dimensionless Fresnel- Kirchhoff diffraction parameter V given in Equation (6.25). Equation (6.24) shows that the phase difference between both rays is a function of effective height and effective distance of the obstacle with respect to the position of the T and R When the receiver RX is in the shadowed region, namely, behind the obstacle where no line-of-sight exists, the received field at R is the vector sum of all the fields from the rooftop of the knife edge obstacle. For this, the v-parameter is used to determine the received field at the receiver R Categorization of an irregular terrain scenario based on effective knife edge obs- tacle height hoeff relative height hix of the transmitter, and height h of the receiver. Case 1: The height hix of the transmitter T x and height h__ of the receiver Rx x are below the effective height hoeff of an obstacle and no line-of-sight exists between the Tx and the R as shown in Figure 6.5. hoeff 0-face (0-face = n-face) in-face