ip. A sig- nal v(t) consisting of a single continuous unchanging wave v(t) = sin(2nft) has only one frequency f, as shown in Fig. 1.2. An important asymmetry in the time-frequency duality is the notion of phase shift. Consider the waveforms shown in Fig. 1.3. In each case we have two frequencies, one twice the other. However, the phase relationship between the two waves is different in the two cases, and their pictures are quite different. Frequency is often a more natural representation of our radio world than voltage amplitude. To put this another way, it is often easy and natural to work with frequency in radio system design. For example, we can build frequency filters that restrict a re- ceiver to a certain range of frequency so that the received signal is not affected by ac- tivity at other frequencies. This all seems very natural today, hardly worth going over, but the core technology CDMA, the subject of this book, pushes radio technology very hard and tests these basic ideas. Thus, understanding the fundamental concept of ra- dio frequency is a prerequisite to having a thorough understanding of CDMA. Technology has changed, and VHF has become an anachronistic acronym. The ultra- high frequency (UHF) band is the upper hundreds of megahertz, and it was allocated in the United States to television stations. As cable television has reduced the need for 70 UHF TV stations, the UHF band has been reallocated to other services, including the first North American cellular telephone service. Since then, the competition for the UHF band has become severe, and wireless telephony has moved into the microwave band, above 1 GHz. It has been a constant V (t) = sin(2nft) Figure 1.2 A sine wave is just one frequency. Downloaded from Digital Engineering Library @ McGraw-Hil (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. Radio Engineering Concepts Radio Engineering Concepts volts volts Figure 1.3 Phase relatio