In a TDMA system comprising a plurality of earth stations communicable with one another by the use of a plurality of carrier waves through a transponder of a satellite which has an available electric power level, bursts carried by the carrier waves are individually and flexibly controlled in accordance with a burst time plan so that selected ones of the carrier waves have on the satellite a total electric power level which does not exceed the available electric power level at any time. The bursts therefore may have different power levels in dependency upon scales of the earth stations, traffics carried by the bursts, and receiving conditions at the earth conditions. The selected carrier waves can be transmitted or received by frequency hopping. Such control of the power level is carried out in each earth station by the use of a combination of an attenuator (67) and an attenuation control circuit (71) operated in accordance with the burst time plan.