Occupied bandwidth (MHz) Source: Ref. 4. With TDMA operation, earth stations use digital modulation and transmit with bursts of information. The duration of a burst lasts for the time period of the slot assigned. Timing synchronization is a major problem. A frame, in digital format, may be defined as a repeating cycle of events. It occurs in a time period containing a single digital burst from each accessing earth station and the guard periods or guard times between each burst. A sample frame is shown in Figure 9.19 for earth stations 1, 2, and 3 to earth station N. Typical frame periods are 750 usec for INTELSAT and 250 usec for the Canadian Telesat. The reader should appreciate that timing is crucial to effective TDMA operation. The greater N becomes (i.e., the more stations operating in the frame period), the more clock timing affects the system. The secret lies in the carrier and clock timing recovery pattern, as shown in Figure 9.19. One way to ensure that all stations synchronize to a master clock is to place a synchronization burst as the first element in the frame format. INTELSAT's TDMA does just this. The burst carries 44 bits, starting with 30 bits carrier and bit timing recovery, 10 bits for the unique word, and 4 bits for the station identification code in its header. Frame Period Station 1 Station 2 Station 3 N - 1 Preamble Information Guard Carrier & clock Burst start & Engineering recovery pattern identification service channel Figure 9.19 Example of TDMA burst frame format. CONCEPTS IN TRANSMISSION TRANSPORT Why use TDMA in the first place? It lies in a major detraction of FDMA. Satellites use traveling-wave tubes (TWTs) in their transmitter final amplifiers. A TWT has the undesirable property of nonlinearity in its input-output characteristics. When there is more than one carrier accessing the transponder simultaneously, high levels of intermodulation (IM) products are produced, thus increasing noise and crosstalk. When a transponder is operated at full power output, such noise can be excessive 