pread-spectrum radios. Generally, for spread-spectrum CDMA systems, cell-site radios provide "master" control of "slave" mobile radios. Although power control and voice activation circuits may be complex, they are typically implemented in digi- tal processing chips for which the cost keeps dropping as technology advances. 3.5.3 Time Synchronization and Orthogonal Codes Another factor affecting the level of interference experienced by base sta- tion receivers is the time synchronization of all mobile radios. If all mobile radio signals arriving at the cell site are synchronized to within a fraction of a chip-time interval, then it is possible to reduce the level of mutual inter- ference dramatically. For such synchronized CDMA (S-CDMA) star net- Commercial Applications works, the use of orthogonal codewords effectively reduces interference to zero, given sufficient orthogonal codes for distribution among mobile radios. Even with some timing errors, interference can be significantly reduced with a careful selection of codewords and time synchronization of all radios in a cell. Codes (collections of codewords) used for such S-CDMA applications have been studied extensively and are discussed in Part 1, Chapter 5, Section 5.7. With S-CDMA, the number of non-interfering spread-spectrum mobile radios can be roughly the same as with TDMA. This is SO because for the same time and bandwidth, the number of orthogonal dimensions is also the same, regardless of whether the dimensions use orthogonal frequencies (FDMA), time slots (TDMA), or spreading codes (S-CDMA). In practice, differences reflect implementation. (As we shall see in Section 3.7, S-CDMA systems can exceed the dimension limit by using more complex optimal mul- tiple-access receivers.) With S-CDMA, it is theoretically possible to have as many non-interfering mobile users as with FDMA or TDMA. If this is truly the case, why does spread-spectrum CDMA offer more capacity? As illus- trated by Qualcomm, the answer is that the 100 percent frequency of reuse allowed wi