A DMT system for a half-duplex two-way link carries Internet protocol encoded video stream on a coaxial cable that also carries a baseband rendition of the same video stream. A plurality of downlink symbols modulated on a subband of subcarriers in a downlink signal are decoded. The symbols may carry data encoded on a subband using a constellation of QAM symbols assigned to the subband. Other subbands may be associated with different QAM constellations. Lower-order constellations of QAM symbols may be assigned to subbands that include higher-frequency subcarriers and higher-order constellations of QAM symbols may be assigned to subbands that include lower-frequency subcarriers. A block error correction decoder may be synchronized based on an identification of the first constellation of QAM symbols and information identifying boundaries between the plurality of downlink symbols.