QAM and QPRS digital broadband cable systems

The performance of high‐level modulation schemes in non‐linear broadband digital communication systems is investigated. The tolerance of 16‐, 64‐, 256‐QAM and 49‐, 225‐QPRS schemes to these non‐linearities is given in terms of their phase margins. Our research results show that high level QAM schemes are less sensitive to non‐linearities when operated in offset mode. Comparison of non‐linearly amplified broadband QAM and QPRS schemes reveals that QPRS schemes need less carrier‐to‐noise ratio (CNR) than corresponding QAM schemes for identical operating conditions. Also, when the non‐linearities are not severe, the degradation in performance is controlled by AM/PM characteristic of the broadband amplifier.