Return Path Expansion – No Easy Answers, One Clear Winner in Mid-Split “Plus”

Author: Dr. Robert Howald, Fellow of the Technical Staff

Traffic growth has been the story of cable broadband since DOCSIS^® debuted.  While downstream growth has aggressively continued, lagging growth upstream has put its evolution on the back burner.  However, as connected home services evolve, user generated content explodes, and machine-to-machine interactions swell among everything IP-ready, growth will rebound.  The time to prepare is now before it becomes an emergency.  Proper preparation will avoid a “Napster” moment.

Unlike the downstream where many knobs and levers are available to manage spectrum, most weapons on upstream have been fired or are being activated today—64-QAM, laser upgrades, S-CDMA.  Serious new upstream capacity is inevitably about finding new spectrum.  And, cable must plan for the best use of this most precious asset—downstream or upstream.  Using it most efficiently guarantees the longest HFC lifespan.

Of course, while long term objectives are important, an efficient, low-risk  transition strategy is an operational imperative.  One strategy stands out as offering both—a mid-split “plus” approach. 

Mid-split, in this case, refers to extending to an 85 MHz upstream band.  The “plus” means enabling new bandwidth on demand, for example, up to 200 MHz, or greater.  Mid-split plus is the best of both—available today to provide many years of lifespan for even aggressive CAGR, and long term additional bandwidth as required.  This is true just considering technology available today. Also, this approach maintains the fundamental diplex architecture already in place, yielding the most efficient overall use of spectrum.

The industry has been considering both extended diplex and “top split” architectures, whereby new upstream is placed above the forward band.  A pro-con comparison heavily favors extending the low-band return (and extending the forward too, if necessary).  Consider these advantages: clear path to 15 years of capacity lifespan (see Figure), 100 Mbps peak rates, DOCSIS 3.0 compatibility, supports legacy STB out-of-band channel, and easily handled by existing return path optics.

What about a “Plus” band?  Advantageously, this can also use HFC optics, ensure Gbps services, and be implemented as a smooth transition.  Moreover, both single-carrier techniques (simple DOCSIS extensions) or multi-carrier approaches can be used.  This approach carries almost no technology or implementation risk.  The amount of additional spectrum MSOs may need depends on many factors, but 1 Gbps of upstream is a common objective.  With modern forward error-correction, such as that used in MoCA and DVB-C2, upstream spectrum could be limited to 200 MHz or less.  These same tools, applied to mid-split, could also further extend the 15-year lifespan identified above, at least in terms of aggregate capacity.

Mid-split “plus” delivers the best of both – many comforting years of immediate lifespan, additional years enabled by modern error correction tools, and a strategy for effectively dealing with the continuous traffic growth beyond.