discussed earlier, this time slotted S-CDMA technique can be con- sidered as a bit-level TDMA scheme in which the CDMA signals are con- verted to TDMA signals. The inverse filter for the common codeword treated as an impulse response, however, has infinite duration. Ruprecht considered various levels of time truncation for the inverse filter and found that in most cases truncation to 3 times the codeword duration resulted in "slight" interference among signals at the filter output samples. He also con- ducted an exhaustive search for short codewords and their truncated inverse filters and found the best ones when white Gaussian noise is in the channel. Although the use of a truncated inverse filter results in a single filter rather than a bank of matched filters for separating all mobile units' signals at the base station, implementation of such filters is complicated because, unlike codewords, they are not binary levels. Many ways exist for developing eas- Commercial Applications ier-to-implement approximations to the inverse filters at the cost of some mutual interference. 3.6.3 A High-Density Voice PCS System We consider here an S-CDMA high-density network of microcells for a voice application of PCS similar in principle to the IS-95 digital cellular standard but less complex and optimized for microcellular networks. There are two fundamental differences between the high-density network of microcells and the IS-95 digital cellular standard: Full-duplex operation is achieved by using time division duplexing (TDD) rather than the FDD dictated by the given cellular frequency band allo- cation. A 32-kbps voice compression is assumed rather than the lower-data-rate compression techniques for digital cellular, which require more digital sig- nal processing with more time delays. Both these differences reduce cost and complexity. Using TDD is consis- tent with PCS bands which are generally not allocated as separate transmit and receive bands, as in the cellular bands. Using 32-kbps voice compression reduces processin