 of carriers which have been multiple access without unacceptable interference between the modulated in any way, analogue or digital. Some of the carriers various signals involved. assigned frequencies in an FDMA system may themselves be In frequency division multiple access (FDMA) the carriers that multiple access systems, using time division multiple access will be relayed by a transponder are assigned carrier frequencies (TDMA). Furthermore, if the C/N ratio in the output of the trans- within the transmission band of the transponder, the frequency ponder is not too high, it may be feasible to overlay the FDMA separation between assigned frequencies being sufficient to avoid signals with spread spectrum signals, forming, in effect, a code overlap of emission spectra. The travelling wave tube (TWT) and division multiple access (CDMA) system. solid state power amplifiers which are used in transponders have A time division multiple access (TDMA) system, operating alone relatively constant gain characteristics within a certain range of in a transponder, allows the full power to the transponder to be used, drive levels, but they become non-linear, then saturate, as an upper that is, no backoff is required. This is because only one carrier is limit is approached. Therefore the output of the transponder will present in the transponder at any instant in time. Each earth station contain the input carriers, amplified, plus distortion products, such in the system transmits its signals in turn, in bursts, in assigned time as harmonics of the carriers and the products of intermodulation slots, typically using PSK modulation, a brief guard time being between them, the level of which will be high if the input carrier assigned between each pair of burst slots to ensure that the bursts do aggregate is powerful enough to drive the amplifier close to satura- not overlap even if small timing errors arise. Figure 51.11 illustrates tion (Westcott, 1972; Chitre and Fuenzalida, 1972). the frame structure of a high capacity TDMA system