er to a cell site. Our goal here is to select an "access" method to provide this service given our bandwidth constraints. We will discuss three generic methods of access: FDMA (frequency division multi- ple access), TDMA (time division multiple access), and CDMA (code division multiple access). It might be useful for the reader to review our discussion of satellite access in Section 9.3 where we describe FDMA and TDMA. However, in this section, the concepts are the same, but some of our constraints and operating parameters are different. It also should be kept in mind that the access technique has an impact on overall cellular band- width constraints. TDMA and CDMA are much more efficient, achieving a considerably greater number of users per unit of RF bandwidth than FDMA. 18.6.2 Frequency Division Multiple Access (FDMA) With FDMA our band of RF frequencies is divided into segments and each segment is available for one user access. Half the contiguous segments are assigned to the cell site for outbound traffic (i.e., to mobile users) and the other half to inbound. A guardband is usually provided between outbound and inbound. In North America the guard band at 800 MHz is 20 MHz wide. This FDMA concept is illustrated in Figure 18.7. Because of our concern to optimize the number of users per unit bandwidth, the key question is the actual width of one user segment. The bandwidth of a user segment is greatly determined by the information bandwidth and the modulation type. With AMPS, the information bandwidth was a single voice channel with a nominal bandwidth of 4 kHz. The modulation is FM and the bandwidth is determined by Carson's rule (Section 9.2). As we pointed out, AMPS is not exactly spectrum conservative (requiring 30 kHz per channel). On the other hand, it has a lot of redeeming features that FM provides, such as noise and interference advantage (FM capture effect) Another approach to FDMA would be to convert the voice channel to its digital equiv- alent using CELP, for example (Section 18.5.2), with a transm