. For a transponder operating in the FDMA mode, the power level Signals which are to be transmitted over a TDMA system must be of each up-link carrier reaching the satellite must be set with two digital. Bits within a frame are stored at the transmitting earth objectives. The first is to obtain at the output of the amplifier the station, then assembled into a burst with the necessary preamble bits optimum ratio between useful carrier power and noise due to the and transmitted at high speed at the appropriate time. At the receiver distortion products in the vicinity of the carriers. This involves the reverse process puts the signal bits into store, then reads them backing-off the aggregate input level from the point where the out at the appropriate lower speed, frame by frame. The charac- amplifier would be driven to maximum total output, in order to teristics of TDMA systems vary over a wide range because the obtain a larger reduction in distortion products. The output backoff principle can be applied in many different circumstances, ranging necessary for TWTs is typically in the range 6dB to 10dB, although from the transmission of low information rate monitoring or control the available useful power output can be increased above that level signals with an aggregate bit rate of a few kbit/s, probably trans- by optimising the assignment of frequencies to carriers and by the mitted on a frequency assigned within a FDMA system, to the high use of TWT linearising networks. The second objective is to divide capacity international telecommunications network TDMA systems the available output carrier power between the carriers in accord- operating at 120Mbit/s in the INTELSAT and EUTELSAT systems ance with their down-link transmission needs. (INTELSAT, 1972; Eutelsat, 1981; Hills and Evans, 1973). Guard time Traffic burst Traffic burst Traffic burst Traffic burst 2 ms frame Preamble Traffic bits Carrier and DSI (DNI) DSI (DNI) bit timing Unique Order wire and sub-burst sub-burst recovery service channels sequence Symbols