 cell radius of 5 km (3 miles), but down at 2 or 3 GHz, we can save our worry for other issues. Buildings create three kinds of radio degradation: They create radio shadows when the base station and user terminal are on opposite sides. Their vertical reflective surfaces create alternative radio paths and add to the multipath propagation problem. Subscribers inside a building have walls, windows, wiring, and other impairments, and they still want to use their wireless telephones. 1 10rography is the fancy scientific term for hills and mountains. 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. Propagation Models Propagation Models The urban environment is hostile to wireless telephony, again in three major ways: Urban structures create regions of considerably lower path gain (more path loss from more obstructions) than is typical in more rural areas. The urban environment subjects the radio link to more variation and more rapid varia- tion. The multipath component of the radio path is greater because of building surfaces, broad, smooth streets, and metal vehicles on those streets. Regions of low path gain are an obvious concern. In conventional reuse, low path gain means lousy call quality ("Hello? Hello? Are you still there?"). The CDMA power- control algorithm spreads the ill-effects of a weak radio path onto the calls for an en- tire sector. The signal weakens while thermal noise and interference from other sources stay the same, power-up messages are sent, and the transmit power increases in both directions. This creates more interference to other CDMA calls, and their power levels have to increase, too. Let us assure readers that this is not a runaway power increase; the system soon reaches a new equilibrium with higher power levels all around. Of course, if the path impairment is large enough, then there may not be enough power to compen