The Internet Encyclopedia (Volume 3)

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Shankar WL040/Bidgolio-Vol I WL040-Sample.cls June 19, 2003 16:49 Char Count= 0

126 PROPAGATIONCHARACTERISTICS OFWIRELESSCHANNELS

Building

Transmitter Receiver

Building

Transmitter

Receiver

Transmitter

Receiver

Figure 2: Various propagation mechanisms are shown: reflection (left), diffraction (center), and scattering (right).

Modeling of Power Loss

Several models are available to predict the median

value of the received power (Har, Xia, & Bertoni, 1999;

Hata, 1980; Lee, 1986; Oda, Tsunkewa, & Hata, 2000;

Okumura, Ohmori, & Fukuda, 1968; Vogel & Hong, 1988).

These are available for propagation outdoors and in-

doors (Bultitude, Mahmoud, & Sullivan, 1989; Durgin,

Rappaport, & Xu, 1998; Harley, 1989; Lott & Forkel, 2001;

Rappaport & Sandhu, 1994). They are also available for

various frequency bands that are of interest in wireless

communications. Instead of concentrating on these mod-

els, I initially describe one of the simple ways of predicting

loss based on the concept of thepath loss exponent.Toun-

derstand this concept, consider the case of a line-of-sight

propagation in free space as shown in Figure 1. IfPtis the

transmitted power inW(Watts), the received powerPd

(W) at a distancedfrom the transmitter is given by Friis

formula (IEEE, 1988),

Pd=PtGtGr

(

λ

4 πd

) 2

W, (1)

whereGt=gain of the transmitting antenna,Gr=gain of

the receiving antenna, andλ=free space wavelength=fc 0 ,

Distance km

Received Power dBm

a. Distancekm

Received Power dBm

Distance m

Received Power dBm

b. Distance m

Received Power dBm

c. Wavelengths

Received Power dBm

Figure 3: The plot of the received signal: (a) attenuation; (b) long-term fading; (c) short-term fading.