 electrical voltage in a conducting antenna, by putting a metal object in the air and sending pulses of electricity that become radio waves. Similarly, a radio signal is received by measuring electrical voltage changes in an antenna, by putting another metal object in the air and detecting the very tiny pulses of electricity generated by the varying electrical field of the radio waves. The technology of radio transmission is developing the ability to transmit a radio signal containing some desired information and developing a receiver to pick up just that particular signal and to extract that desired information. One of the latest tech- nologies to do this is code division multiple access (CDMA), a long way from the dit-dah Morse code transmission of the earliest wireless equipment. Both Morse code and CDMA, however, are digital radio technologies. We use radio to get some kind of information, a signal, from one place to another us- ing a radio wave. We put that signal onto the radio medium, the carrier we call it, with some kind of scheme that we call modulation. The Morse code sender uses the simple modulation scheme of a short transmission burst as a dit and a longer transmission burst as a dah. The demodulation scheme does the reverse: The telegraph receiver makes audible noise during a radio burst, and the listener hears short and long bursts of noise as dits and dahs. Morse code is simple and elegant, and it used the technology of its day efficiently. All the components of the process of radio communications were already present in the telegraph. There is a meaningful message to be sent that was coded into a specific for- mat, the letters of the alphabet. The formatted message, the signal, is then modulated by the telegraph operator into a radio message that is then demodulated into something that looks like the original signal. The receiver restores the message's meaning, reading Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill C