ble interest today in spread-spectrum radios for wireless applications. That spread spectrum and CDMA are the same is a frequent but incor- rect assumption. Multiple-access techniques are typically applied in a star network with a hub base station communicating with many remote radio units, as shown in Figure 3.2. FDMA and TDMA techniques can be used with Figure 3.2. Single cell. Commercial Applications any type of modulation, including spread-spectrum signals. However, code division multiple access, as its name implies, applies only to modulation tech- niques associated with a code, specifically spread-spectrum modulation techniques. Since CDMA is used only with spread-spectrum signals, it is often assumed that spread spectrum, a modulation technique, and CDMA, a multiple-access technique, are the same. However, in general, while spread- spectrum signals can be used with or without CDMA, the employment of CDMA requires spread-spectrum signals. Part 5, Chapter 2, discusses the topic of multiple access in greater detail. (See [19] for further discussion of multiple-access communications.) Qualcomm's spread-spectrum CDMA system was optimized under exist- ing U.S. mobile cellular system constraints. Its analysis shows that the CDMA system can achieve about 10 to 20 times the capacity of the existing analog FDMA system and about 3 to 7 times that of the new digital TDMA stan- dard system. The key to this increase in capacity is the ability of the CDMA system to reuse the same frequency in all cells, with capacity defined as the total number of active mobile users in a large area with many cells. Several cellular operators in the United States have committed to installing this CDMA system, beginning in 1993, the year it became a sec- ond digital cellular standard, IS-95. The previous U.S. digital cellular stan- dard, IS-45, is based on TDMA, which is similar to the European digital cellular standard referred to as GSM. 3.3.1 Overview of the CDMA Digital Cellular System (IS-95) The IS-95 digital cellular system operate