 (formerly called IS-95 and developed by Qual- comm) as it is deployed across North America and elsewhere. 8.1 Spread Spectrum In contrast to conventional reuse, the spread-spectrum channel is no more efficient for one serving antenna but is enormously more resistant to interference. The effect of this is a more efficient system. The origin of spread spectrum is military, where resistance to deliberate interference, or jamming, was the major consideration, with radio spectrum efficiency a nice side effect. Picture a communication link in a hostile environment. If we send a signal with a traditional analog or digital modulation scheme, then a hostile party can broadcast a signal on the same carrier frequency, and our own receiver loses the message. Now sup- pose that we hop from one frequency to another in a prearranged pattern. The actress Heddy Lamar suggested this, and the patent for frequency hopping bears her name. So long as the hostile party does not know the frequency pattern, it would take an enor- mous amount of power over a broad band of frequencies to jam the signal. More sophisticated spread-spectrum systems use a large frequency band continu- ously and typically at low power levels. Not only does this make it hard to interfere 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. The CDMA Principle Key Radio Concepts with the signal, but it also makes it hard even to detect the signal. The safest way to protect something is to have the hostile party not even know it exists. Ten watts of ra- dio power in 10 kHz is easy prey, easily detected and easily jammed. The same 10 W spread over 10 MHz is 1000 times less efficient in bandwidth usage but is a lot safer. Assuming that the enemy detects such a spread-out signal, the spread-spectrum sys- tem can be nearly 1000 times more resistant to interference. The same technology that protects mi