O that scatterers lie within it. This is shown in Figure 18.20. Now the path loss becomes greater than free space. Bertoni et al. report one measurement program where the scatterers have been simu- lated using absorbing screens. It was recognized that path loss will be highly dependent on nearby scattering objects. Figure 18.19 was developed from this program. The path loss in excess of free space calculated at 900 and 1900 MHz where W = 1.5 m is plotted in Figure 18.20 as a function of path length L. The figure shows that the excess path loss (over LOS) is small at each frequency out to distances of about 20 to 40 m, respectively, where it increases dramatically. Propagation between floors of a modern office building can be very complex. If the floors are constructed of reinforced concrete or prefabricated concrete, transmission loss can be 10 dB or more. Floors constructed of concrete poured over steel panels show much greater loss. In this case (Ref. 17), signals may propagate over other paths involving Ceiling Fresnel zone Floor Figure 18.19 Fresnel zone for propagation between transmitter and receiver in clear space between building furnishings and ceiling fixtures. (From Ref. 17, Figure 35.) 18.9 CORDLESS TELEPHONE TECHNOLOGY Clear space: 1.49 m Excess path loss 1900 MHz 910 MHz Theoretical 900 MHz Theoretical 1800 MHz Separation between transmitter and receiver Figure 18.20 Measured and calculated excess path loss at 900 and 1800 MHz for a large office building having head-high cubical partitions, but no floor-to-ceiling partitions. (From Ref. 17, Figure 36.) diffraction rather than transmission through the floors. For instance, signals can exit the building through windows and reenter on higher floors by diffraction mechanisms along the face of the building. 18.9 CORDLESS TELEPHONE TECHNOLOGY 18.9.1 Background Cordless telephones began to become widely used in North America around 1981. Today their popularity is worldwide with hundreds of millions of units in use. As the technology develops, they will beg