play (i.e., a single carrier's offering of voice, data, and TV services). TDD does have some drawbacks, including a requirement for burst demodula- tors and more difficult synchronization of uplink transmission due to the absence of frequency and clock references. However, due to its superior performance in supporting spectrum efficiency and flexibility, TDD is the preferred solution in broadband fixed wireless access (BFWA) networks. (BFWA systems are discussed in Chapter 15.) Compression Techniques After carving space into cells and applying multiple-access techniques within each cell to make better use of the bandwidth available, it is possible to apply compres- sion to make greater use of the bandwidth within each given channel. Compres- sion is very important because it improves the use of a precious resource: the communications channel. Voice compression techniques use voice coders/decoders (vocoders), of which there are two general types: High-bit-rate vocoders-These vocoders are used by PCS, wireless local loops, and wireless office telecommunication systems applications. These vocoders carry voice by using 32Kbps Adaptive Differential Pulse Code Modulation (ADPCM). A bit rate this high emulates the quality achieved on the PSTN, and no additional error detection and correction is necessary. Low-bit-rate vocoders-These vocoders are used in cellular systems that deal with vehicular traffic, where large cells need to facilitate a large num- ber of conversations, or anywhere bandwidth efficiency is critical, such as densely populated areas. These vocoders reduce the voice down to 8Kbps, Spectrum Utilization using extensive channel coding techniques that help facilitate error cor- rection, such as linear predictive coding (LPC), Quantized Code-Excited Linear Prediction (QCELP), or Vector Sum Excited Linear Prediction (VSLEP). GSM uses Regular Pulse Excitation Long Term Prediction (RPE LTP), which carries digitized voice at 13Kbps, achieving good voice quality (albeit not comparable to that achieved on good old