Figure 13.5 FDMA channels to which other techniques are then applied to enable multiple users to use a channel simultaneously. TDMA, which is used in 2G digital cellular systems, WLANs, and Personal Commu- nications Services (PCS) systems, is actually a combination of FDM and TDM. In TDMA systems, you first divide the available or allocated frequency spectrum into a number of channels by using FDM. Then, within each channel, you apply TDM to carry multiple users, interleaved in time. Therefore, one transceiver can support multiple channels. Various cellular network standards are based on TDMA, includ- ing Global System for Mobile Communications (GSM), Universal Wireless Commu- nications (UWC), and Personal Digital Cellular (PDC). UWC-136 TDMA technology, also referred to as ANSI-136, provides a three-to-one gain in capacity compared to analog technology. Each caller is assigned a specific time slot for transmission. GSM supports eight users per channel, resulting in an eightfold increase in capacity. (These standards are discussed in further detail in Chapter 14.) The digital handsets associated with TDMA are more intelligent than those used with FDMA. For example, they have scanning capabilities, and if the channel you are on is encountering anomalies that are lending themselves to transmission errors, the handset can search for an available channel that provides better performance. The key benefits of TDMA are that it offers greater capacity and spectral effi- ciency than FDMA. As Figure 13.6 shows, in TDMA, everybody is talking on the same frequencies but at different moments in time. The users perceive their con- versations as being continuous, even though each is actually getting very rapid samples of his or her conversation. Spread Spectrum Spread spectrum is a multiple-access technique that is receiving a great deal of attention today. In spread spectrum, everybody is using the same frequency at the Chapter 13 Wireless Communications Basics User 1 User 2 User 3 User 4 Figure 13.6 TDMA same time. This is re