s callers do not want to be bothered by other telephone conversations on the same system. Wireless telephone users take their privacy seriously, and maintain- ing separate and confidential calls is an important component of system design. In the earliest days of radio, we used frequency to discriminate among radio signals, and we called the system frequency division multiple access (FDMA). Each radio user gets a frequency range, although there may be other users on other frequencies. Com- mercial radio stations (both AM and FM) are assigned frequencies in their large geo- graphic areas, and our receiver sets easily discriminate among the stations and allow us to exclude all but the one to which we are listening. Like broadcast radio, the first mobile telephone systems and, later, the first cellular systems were FDMA-based. When the leap was made from analog to digital modulation described in Chap. 7, it was more efficient to use a larger frequency band and to divide it up among several sig- nals using time slots. The system is synchronized so that each receiver knows which transmitted time slots belong to that receiver's signal. This time division multiple ac- cess (TDMA) is more complicated than FDMA, but it uses the radio frequency more ef- ficiently. The Global System for Mobility (GSM), which started in Europe and became a worldwide standard, is a TDMA system with eight time slots aggregated into a sin- gle larger frequency band. FDMA and TDMA have some kind of absolute protection from other broadcasts on the signal channels in the radio medium. A single frequency band or time slot gets minimal interference from other frequency bands or time slots because at the exact moment of re- ception, no other transmitter is broadcasting on the specified frequency. However, in the world of spread spectrum, a single stream of radio is shared simultaneously. Code divi- sion multiple access (CDMA), the subject of this book, is a spread-spectrum system that transmits many signals in the same radio band at the same time. All