ensity of microcells. Although this design follows the paradigm intro- duced by Qualcomm, this design is simpler. We compare it with the digital European cordless telephone (DECT) standard for PCS in Europe. DECT is the European standard for PCS analogous to GSM, the European digital cellular standard. In this PCS application, spread-spectrum radios can be used to achieve synchronous code division multiple access (S-CDMA) in both directions within each microcell. Synchronization is not necessarily maintainable between radios from different microcells, SO the spread-spectrum process- ing gain and error-control coding are used primarily for overcoming inter- ference between radios from different microcells. By carefully selecting the amount of processing gain for these spread-spectrum radios, we can practi- cally achieve a high-capacity system in which each microcell uses S-CDMA and achieves 100 percent frequency of reuse. Since orthogonal codewords are used in this system, it is less sensitive to power control. Consider a star network in which all remote radios are time-synchronized to within a fraction of a chip-time arrival at the base station. In these illus- trations, M remote radios each transmit a spread-spectrum radio signal with a binary spreading code of L chips per data bit. 3.6.1 Binary Orthogonal Codes From Welch's bound (see Part 1, Chapter 5, Section 5.7.2), we know that the number of binary orthogonal codewords of length L is less than or equal to L. On the other hand, it's easy to find L orthogonal codewords of length L. One of the most common is given when L = 2n for any integer n, which yields M = L orthogonal codewords generated by the Hadamard matrices. This shows that, in the binary case, the S-CDMA scheme can achieve as many non-interfering channels as in a TDMA scheme. 3.6.2 S-CDMA Equivalent to Bit-Level TDMA The idea of using a matched filter in direct-sequence spread-spectrum receivers is based on the fact that such filters are known to be optimum for signal detection in white Gaussian no