Multichannel 64-QAM transmission in AM/digital and all-digital hybrid-fiber-coax based video dial-tone systems

A hybrid-fiber-coax (HFC) system is operated with a combination of 63 AM and 21 64-QAM subcarriers as well as with the 21 64-QAM subcarriers alone. The 64-QAM subcarriers are 6-MHz wide, decorrelated, and of the type intended for compressed digital video. The AM signal levels (over both the optical and coaxial links) are set to produce standard carrier-to-noise ratios (CNRs), composite second order distortion (CSO), and composite triple beat (CTB). The optical modulation depth (OMD) is independently varied and transmission tests are carried out over the optical link alone, as well as over the entire system (including two coaxial amplifiers, or actives). The relative degradation of the system due to the optical link versus the actives is investigated. For an OMD of 24% or less and with the raw bit-error ratio (BER) set to 10/sup -5/ (as anticipated for use with forward error correction (FEC) systems), the performance is limited by the actives. There is an approximate 3.5 dB penalty in the necessary CNR for 64-QAM reception, due to the presence of the AM signals. On the other hand, when the system is operated with all-digital subcarriers, the optical modulation depth may be increased from 24% to 37% with no penalty in the required digital CNR.