Page 50 - Complete Wireless Design

Wireless Essentials

Wireless Essentials 49

receiver is accepting both ground waves and sky waves, or sky waves and sur-
face waves, or waves arriving from one-hop and two-hop paths. The two sig-
nals are, as above, alternately in phase and out of phase with each other,
causing severe fading conditions.
Multipath can create high intersymbol interference (ISI) and BER in digital
communications systems, and is magnified by misalignment of the transmit or
receive antenna, by a receive location that is near an RF reflective site (such
as a building or a mountain behind the receiver), or by the surface movement
of trucks or automobiles sending back a secondary signal to the receive anten-
na. This multipath can be examined as a frequency-selective amplitude notch
and/or a tilting and misshaping of the received waveform on the screen of a
spectrum analyzer (Fig. 1.57) in digital communications systems. Without an
equalizer built into the receiver’s baseband circuits to fill-in for these multi-
path effects, small tilts and notches of 1 dB (or less) would quickly render

Figure 1.57 Digital signals on a spectrum analyzer.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.