ission rate of 4.8 kbps. Let the modulation be BPSK using a raised cosine filter where the bandwidth would be 1.25% of the bit rate or just 6 kHz per voice channel. This alone would increase the voice channel capacity five times over AMPS with its 30 kHz per channel. It should be noted that a radio carrier is normally required for each frequency slot. 18.6.3 Time Division Multiple Access (TDMA) With TDMA we work in the time domain rather than the frequency domain of FDMA. Each user is assigned a time slot rather than a frequency segment and, during the user's turn, the full frequency bandwidth is available for the duration of the user's assigned time slot. Let's say that there are n users and SO there are n time slots. In the case of FDMA, we had n frequency segments and n radio carriers, one for each segment. For the TDMA Transmit Guard band Receive 60 kHz Figure 18.7 A conceptual drawing of FDMA. CELLULAR AND PCS RADIO SYSTEMS Time slot Payload Training Payload Guard sequence period Figure 18.8 A typical TDMA frame. case, only one carrier is required. Each user gains access to the carrier for 1/n of the time and there is generally an ordered sequence of time slot turns. A TDMA frame can be defined as cycling through n users' turns just once. A typical TDMA frame is illustrated in Figure 18.8. One must realize that TDMA is only practical with a digital system such as PCM or any of those discussed Section 18.5.2. As we said in Section 9.3.5.2, TDMA is a store and burst system. Incoming user traffic is stored in memory, and when that user's turn comes up, that accumulated traffic is transmitted in a digital burst. Suppose there are 10 users. Let each user's bit rate be R, then a user's burst must be at least 10R. Of course, the burst will be greater than 10R to accommodate a certain amount of overhead bits as shown in Figure 18.8. We define downlink as outbound, base station to mobile station(s), and define uplink as mobile station to base station. Typical frame periods are: North American IS-54 40 msec for six ti