Evolution ofSpaceTWT Requirements andSpecifications forModern Communication Satellites

Complex, non-constant envelope, highorder digital modulation schemes usedinmodern communication satellites bring newrequirements forthelinearity and powerefficiency oftheTraveling-wave Tubes (TWT). Asa result, areassessment ofthetypical TWTspecifications is inorder. A digital system-level figure-of-merit, theTotal Degradation (TD),isproposed Itaccounts forthe characteristics ofanycodeddigital systemandthe performance oftheoverall communication link, including co-channel andadjacent channel interference. This figure- of-merit allows re-optimizing theoperating pointof currently usedTWTs,comparing theend-to-end performance ofdifferent tubes andrevising thestandard TWTlinearity-versus -efficiency requirements.