New Megabits, Same Megahertz: Plant Evolution Dividends (2009)

By Robert L. Howald, Ph.D., Amarildo Vieira, Ph.D., Michael Aviles, Motorola Home & Networks Mobility

The use of 256-QAM in the downstream path has nearly completely replaced 64-QAM as the modulation of choice for MSO’s for two simple reasons: (1) 256-QAM is more bandwidth efficient, providing a 33% increase in spectrum efficiency compared to 64- QAM (2) 256-QAM has been proven to work reliably. A natural question to ask, given the relatively smooth transition from 64-QAM to 256-QAM, is whether there is a convenient next step in terms of improved bandwidth efficiency. The industry has yet to make a significant move towards 1024-QAM. Prior papers, including by this author, have pointed out some of the potential hurdles. However, as the HFC network has evolved in support of new service demands, and the downstream multiplex has done the same, variables that affect the ability to reliably implement 1024- QAM are beginning to work in favor of this more bandwidth efficient approach. It is now time to understand and quantify the practical performance and the potential limitations in order that it may be deployed properly. This paper will examine the required specification of impairments for successful transmission of 1024-QAM. The discussion will summarize the effect of HFCspecific impairments on 1024-QAM and compare them to 256-QAM and 64-QAM. Finally, we will present some conclusions, along with supporting measured data, on the proper architecture limitations and system thresholds to ensure high-performance delivery of 1024-QAM. These guidelines can be used to enable MSOs to reliably extract another 25% more bandwidth from their digital multiplex.

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