Communication system comprising - a transmitter and one or more receivers, said transmitter being arranged to transmit over a communication link a sequence of physical layer frames, each comprising symbols, at a given physical layer symbol rate to said one or more receivers, said sequence of physical layer frames being filtered with a transmit filter, - adaptive coding and modulation optimization means arranged to determine per receiver of said one or more receivers a first modulation and coding for transmitting data to the receiver in question, whereby said transmitter is arranged to map said data for said one or more receivers in frames using an operational modulation and coding based on said first modulation and coding, thereby obtaining encoded and modulated data frames, and arranged to insert said encoded and modulated data frames into said sequence of physical layer frames, characterized in that said transmitter is further arranged to insert into said sequence, prior to said filtering with said transmit filter, at least one fill frame modulated with a first fill modulation or modulated and encoded with a first fill modulation and coding, said first fill modulation selected from a set of fill modulations comprising at least one fill modulation with a modulation order higher than QPSK or said first fill modulation and coding selected from a set of fill modulations and codings comprising at least one fill modulation and coding with a modulation order higher than QPSK, said communication system further comprising link quality detection means arranged to determine a metric of the quality of said communication link from a version of physical layer frames of said sequence received in one or more of said receivers, said physical layer frames comprising said at least one fill frame, and to convey said communication link quality metric to said adaptive coding and modulation optimization means so that said first modulation and coding can be updated based on said quality metric. - a transmitter and one or more receivers, said transmitter being arranged to transmit over a communication link a sequence of physical layer frames, each comprising symbols, at a given physical layer symbol rate to said one or more receivers, said sequence of physical layer frames being filtered with a transmit filter, - adaptive coding and modulation optimization means arranged to determine per receiver of said one or more receivers a first modulation and coding for transmitting data to the receiver in question, whereby said transmitter is arranged to map said data for said one or more receivers in frames using an operational modulation and coding based on said first modulation and coding, thereby obtaining encoded and modulated data frames, and arranged to insert said encoded and modulated data frames into said sequence of physical layer frames, characterized in that said transmitter is further arranged to insert into said sequence, prior to said filtering with said transmit filter, at least one fill frame modulated with a first fill modulation or modulated and encoded with a first fill modulation and coding, said first fill modulation selected from a set of fill modulations comprising at least one fill modulation with a modulation order higher than QPSK or said first fill modulation and coding selected from a set of fill modulations and codings comprising at least one fill modulation and coding with a modulation order higher than QPSK, said communication system further comprising link quality detection means arranged to determine a metric of the quality of said communication link from a version of physical layer frames of said sequence received in one or more of said receivers, said physical layer frames comprising said at least one fill frame, and to convey said communication link quality metric to said adaptive coding and modulation optimization means so that said first modulation and coding can be updated based on said quality metric. characterized in that said transmitter is further arranged to insert into said sequence, prior to said filtering with said transmit filter, at least one fill frame modulated with a first fill modulation or modulated and encoded with a first fill modulation and coding, said first fill modulation selected from a set of fill modulations comprising at least one fill modulation with a modulation order higher than QPSK or said first fill modulation and coding selected from a set of fill modulations and codings comprising at least one fill modulation and coding with a modulation order higher than QPSK, said communication system further comprising link quality detection means arranged to determine a metric of the quality of said communication link from a version of physical layer frames of said sequence received in one or more of said receivers, said physical layer frames comprising said at least one fill frame, and to convey said communication link quality metric to said adaptive coding and modulation optimization means so that said first modulation and coding can be updated based on said quality metric.