It seems a simple enough concept — the higher the QAM, the more bandwidth there is for cable's bigger video and data payloads.
But while stirrings of technical work have been done to range quadrature amplitude modulation schemes beyond 256 QAM, those in the technical know say several key hurdles stand in the way, not the least of which are other, less-expensive alternatives for extending bandwidth.
The idea of higher QAM modulation comes as many cable operators try to squeeze more bandwidth from their cable plants. Many MSOs are already beginning work to upgrade from 64 QAM to 256 QAM, a step that boosts bandwidth for each 6-megahertz channel from 28 megabits per second to 38.8 mbps. It also increases the number of video signals that can fit onto one channel from eight to 12, depending on whether rate-shaping is used.
One of the early pioneers in the technology is Broadcom Corp., which debuted its first series of QAM 1024 silicon for modulators and demodulators in June 2001. QAM 1024 will raise the throughput of one 6-MHz channel to about 50 mbps.
Broadcom has since created a second generation of QAM 1024 silicon for cable modems, and is now in talks with the top U.S. MSOs to start field trials this year, according to cable modem product line manager Jay Kirchoff.
But there are still some significant hurdles. To raise the QAM modulation in a plant, cable operators must adjust three basic elements — the modulator at the headend, the demodulator in the set-top box and the network itself.
"We think it is a very promising technology, but on the headend side there are still some peripheral components that still need to be improved, whether it is an upconverter to take it from baseband to the actual channel or, in some cases, there may be some plant issues in terms of plant quality," Kirchoff said.
The QAM transmission environment is not unlike a multi-lane highway that carries a heavy traffic load. As more data is funneled down the channel, there's a possibility for collisions, as packets crash into each other or stray into other lanes.
"These are all things that the higher you go in the QAM, the harder it is to maintain," said Ron Levin, senior marketing solutions manager with Harmonic Inc.'s convergent systems division. "At 256 QAM, some networks found they had to review their entire plant — not only review, but go node by node in every part of the network and check it physically. This is very costly, and I'm only talking about migration from 64 to 256."
That's one reason Harmonic isn't putting much stock in 1024 QAM right now. Instead, the transmission-gear supplier is looking to other alternatives to boost bandwidth without as much expense or headache.
They includes Harmonic's closed-loop statistical multiplexing technology, which links the content encoder directly to a cable operator's statistical multiplexer. This allows operators to make real-time compression adjustments to fit the available bandwidth.
Not only does that increase the number of streams per channel from eight to 18, but it also doesn't force operators to make changes to the network or to set-top boxes.
"At the end of the day, it's all about bandwidth efficiency, and operators as well as us are looking for different ways to improve that efficiency," Levin said. "If you can get from point A to point B with much less pain and much more cost-effectively, we are going for it and, obviously, MSOs are going to go for it."
Walk, not run
The strain on the existing plant also is a concern for BigBand Networks Inc., a provider of digital video-delivery systems.
BigBand recently introduced an upgrade that allows 256 QAM signals to be put directly into its digital signal groomer. But there are no immediate plans to design a 1024 QAM input, according to BigBand vice president of corporate development Seth Kenvin.
"It comes up more in discussion, and it is a consideration more around this company and our discussions with people today than it was months ago," he said. "But it is still early-stage, speculative and scientific, and all indicators are it is not at the top of the list of ways to get more bandwidth out of the plant. But it merits study and better understanding."
Kirchoff, meanwhile, noted that such doubts are nothing new in the QAM-development world.
"I think it is a situation where most people want to walk before they run," he said. "We are beyond the crawling stage, and are walking. 1024 is yet another level of performance increase, and so I think people like incremental improvements.
"It wasn't that long ago when people were saying you could never do 256 QAM on a cable plant. That where we are today with 1024. Some people are saying it will never be able to be done, but on the other hand, don't get in the way of the guy who is doing it."
Indeed, stepping up to higher levels of QAM is likely a "walk-before-you-run" process, noted Michael Harris, president of cable technology analysis firm Kinetic Strategies Inc.
"Especially as they get more experience with 256, and its stability on their networks, they would have a better idea in terms of what they could really support with 1024," Harris said. "It's definitely a graded or phased opportunity, especially right now — there isn't honestly a huge bottleneck in terms of downstream bandwidth yet.
"If you got into a fully deployed [video-on-demand] model, yeah. But right now downstream isn't yet the problem."
With development in the early stages, Kirchoff said QAM would not become much of a deployment reality until about 2004.
"People don't quite understand it yet," he said. "They don't have a good feel for everything that has to be done to make 1024 QAM work, and with people just now starting to deploy 256 QAM, some may not see a strong demand or a strong need for it."
"But when we talk to MSOs what they tell us is from a VOD standpoint they really need to drive down the cost of a video stream, and 1024 is one way to do that all within the confines of a 6-MHz channel."
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