which is in keeping with the subject of this book. By doing so, we do not wish to imply that digital CDMA has been universally adopted as a standard for cellular telephones either in the United States or elsewhere. In fact, the European Community (EC) has already elected to follow dig- ital time division multiple access (TDMA), a non-spread-spectrum approach. Commercial Applications 3.1 KEY EVENTS IN THE COMMERCIAL MARKET In commercial applications, the bandwidth efficiency-with a high number of bits per second per hertz-is often an important criterion. Consequently, it had generally been assumed that spread-spectrum radios had no practi- cal commercial applications. Why is it that today a considerable commercial interest exists in the use of spread-spectrum radios? The interest is largely the result of two events: In 1985, the FCC, in its Part 15 rules, allowed the unlicensed use of spread- spectrum radios in ISM bands for power levels up to 1 watt (W). Qualcomm developed a well-publicized spread-spectrum CDMA system for digital mobile cellular phone applications [15]. This digital cellular sys- tem provides as much as 20 times the capacity of existing analog cellular radios, which use frequency division multiple access (FDMA). It also has a higher potential capacity than the first U.S. digital cellular standard, IS- 54, based on TDMA, using a conventional narrowband M-ary phase-shift- keying (MPSK) radio. Qualcomm's effort has led to the adoption of a second U.S. digital cellular standard, IS-95. In the next sections, we discuss the impact of these two events, which are primarily responsible for the widespread interest in commercial applications of spread-spectrum radios today, and the advantages of spread-spectrum CDMA in multicell voice networks. 3.2 THE FCC PART 15 RULES Prior to 1985, conventional radios with very low power (less than a milli- watt) were allowed by the FCC to operate in certain bands without licenses. To stimulate innovation, Dr. Michael Marcus of the FCC cham- pioned the use of higher powe