I'm not an RF engineer, but I do know that 256 QAM runs easily on modern HFC plants with 35-39db MER at the modem. The DOCSIS spec says a modem must be able to operate at +/- 15db rx power. Obviously, modems closer to 0db have the best signal and the most amount of headroom.
The data rate for 64 QAM is 30.34 Mbps, and the data rate for 256 QAM is 42.88 Mbps. There's overhead associated with the DOCSIS protocol itself so 256 QAM downstreams are realistically limited to 38Mbps, 64 QAM to 27Mbps.
I have the following in my notes:
o MER at CMTS output should be a minimum of 27 (some say 25) dB for 64-QAM and 31 dB for 256-QAM, but generally they are in the 34 to 36 dB range at the headend (Cisco Systems, “Implementation of 16-QAM to Increase the Value of Cable Modems”, http://www.cisco.com/en/US/tech/tk86/tk319/technologies_white_paper09186...)
o The digitally modulated carrier’s average power level should be 6 to 10 dB below that the level of an analog TV visual carrier amplitude on the same frequency would be. Many cable operators set 64-QAM signals at 10 dBc, and the 256-QAM at 5 to 6 dBc. Higher orders of modulation (i.e. 256-QAM) have a higher peak-t-average power ration and could cause occasional, intermittent downstream laser clipping. The analog TV channel levels might need to be decreased slightly into the laser transmitters, if several 256-QAM signals are present (i.e. digital TV). (Cisco Systems, “Implementation of 16-QAM to Increase the Value of Cable Modems”, http://www.cisco.com/en/US/tech/tk86/tk319/technologies_white_paper09186...)
I'm not an RF engineer, but I do know that 256 QAM runs easily on modern HFC plants with 35-39db MER at the modem. The DOCSIS spec says a modem must be able to operate at +/- 15db rx power. Obviously, modems closer to 0db have the best signal and the most amount of headroom.
The data rate for 64 QAM is 30.34 Mbps, and the data rate for 256 QAM is 42.88 Mbps. There's overhead associated with the DOCSIS protocol itself so 256 QAM downstreams are realistically limited to 38Mbps, 64 QAM to 27Mbps.
I have the following in my notes:
o MER at CMTS output should be a minimum of 27 (some say 25) dB for 64-QAM and 31 dB for 256-QAM, but generally they are in the 34 to 36 dB range at the headend (Cisco Systems, “Implementation of 16-QAM to Increase the Value of Cable Modems”, http://www.cisco.com/en/US/tech/tk86/tk319/technologies_white_paper09186...)
o The digitally modulated carrier’s average power level should be 6 to 10 dB below that the level of an analog TV visual carrier amplitude on the same frequency would be. Many cable operators set 64-QAM signals at 10 dBc, and the 256-QAM at 5 to 6 dBc. Higher orders of modulation (i.e. 256-QAM) have a higher peak-t-average power ration and could cause occasional, intermittent downstream laser clipping. The analog TV channel levels might need to be decreased slightly into the laser transmitters, if several 256-QAM signals are present (i.e. digital TV). (Cisco Systems, “Implementation of 16-QAM to Increase the Value of Cable Modems”, http://www.cisco.com/en/US/tech/tk86/tk319/technologies_white_paper09186...)
Frank