Downstream transmitter and cable modem receiver for 1024 QAM

摘要

A headend transmitter that transmits 1024 QAM including a 256 QAM modulator which has been modified to have more aggressive forward error correction processing. The 256 QAM modulator outputs 256 QAM points to a summer. Another data modulator receives additional data to be transmitted in a separate, substantially less complex constellation. This modulator processes the additional data to do forward error correction thereon and then maps the encoded data into a less complex constellation such as QPSK, 16 QAM etc. The additional data constellation points are then amplified in a variable gain amplifier and fed to a summer where each additional data point is added by vector summation to one 256 QAM point. The output 1024 QAM point is filtered and shifted to the desired transmission frequency. Legacy cable modem receivers can still receive the 256 QAM point since the addition of the new data just appears to be noise which they can overcome using the parity bits encoded in the transmitted symbols. 1024 QAM cable modem receivers receive both the 256 QAM points and the new data points and output both.