Scheme bridges cable and DSL service gaps
By Patrick Mannion, EE Times
Mar 29, 2001 (3:56 PM)
URL: eetimes.com
MANHASSET, N.Y. — Looking to fill the gaps left by today's cable and digital subscriber line (DSL) deployments, BridgeWave Communications (Sunnyvale, Calif.) has devised a new modulation scheme that capitalizes on the wealth of available millimeter-wave bandwidth to wirelessly deploy any broadband-access service on a multimegabit/second basis throughout the metropolitan area. The goal of the two-year-old company is not to compete with cable or DSL, but to complement these services by allowing them to surpass the 60 to 80 percent penetration they currently have in first-tier markets.
To accomplish this, the company has combined the high-bandwidth availability in the millimeter-wave (MMW) region with its own signal-code modulation (SCM) scheme. The result is a system that allows, for example, a cable head-end or DSL central office to wirelessly transmit its signal on a point-to-multipoint basis, over typical MMW distances, without any loss of signal integrity — with low-cost radios.
The use of low-cost radios is significant, considering how the data rate required for a data over cable system interface spec (Docsis) cable transmission is 40 Mbits/s downstream and 10 Mbits/s upstream. The company's SCM scheme makes the wireless conversion totally transparent to both the head-end and the customer.
"We're looking at a model that completely changes the economics of MMW deployment so it can be used pervasively throughout metro areas — rather than just in the densest downtown areas, as is the case today," said Gregg Levin, senior vice president of marketing at BridgeWave. "In fact, many of our employees come from companies that have a history with MMW. They joined us as they've lost faith in the original model being all that it was hyped to be."
Levin pointed to work being done to deploy local, multipoint distribution service (LMDS) to get high data rates wirelessly over short distances, and lumps it in with free-space optics as a fiber extension.
"The problem is that the electronics and infrastructure costs for these are so high, you need a building with 50 to 100 potential subscribers just to get 10 to 20 per building — then you'd need 10 to 20 of those buildings around a basestation. Only then can you make it work economically," he said. This isn't a scenario that's feasible outside of the densest parts of downtown.
For the lower-cost multichannel, multipoint distribution service (MMDS), Levin points to problems with getting the cost of the customer premises equipment below $500. "They're talking about under $500 now, but they really need to go below $300," he said. "Also," he added, "they're spectrum-limited, with only about 200 MHz to play with, and they're spreading that over a 35-m radius." The result is a marketing of sub-rate service or else "best-effort" rates of 5 Mbits/s — with no promises.
MMDS operators can go to micro or picocells, Levin said, but then they face a backhaul problem. "The same applies to operators in the unlicensed ISM [industrial, scientific and medical] band," he said. "They've really gotten the cost down. But they have at most 300 MHz of shared spectrum," he said. While they too can go to picocells to reduce the interference issues, "they have an even greater backhaul problem [than MMDS]," he said.
It was this backhaul issue that spurred BreezeCOM (Tel Aviv, Israel) to back BridgeWave's development in that company's third round of financing, as BreezeCOM's model is to deploy picocells for unlicensed radios.
"There's an obvious need for microcells, but fiber won't cut it for the backhaul, as it's not pervasive enough," Levin said.
BridgeWave is targeting price points that can deliver service to single, small businesses that can generate $200 to $300 in revenue. "We're looking at MMW as the only place where there's sufficient frequency allocation," Levin said.
The other problem with introducing an alternate wireless, high-speed transport methodology — aside from the cost of the radios — is getting the system integrated into what's already out there. These systems include cable head-end equipment, cable modems and DSL central offices, and also with the networks directly, such as through hybrid fiber/coaxial connections. "We need the radio to be just another media option that puts no requirements on the network itself, and works with equipment that doesn't come from us," said Levin.
To answer all the above requirements of high bandwidth, low cost, easy integration and support for all end-user services and applications, BridgeWave came up with a new generic modulation scheme, SCM, that takes advantage of the high bandwidth availability of MMW bands.
The scheme takes the complex, modulated 64- or 256-QAM cable signal, for example, and remodulates it using a combination of transparent analog modulation and bandwidth-efficient, noise-tolerant, digital modulation with its forward error correction (FEC) capabilities. The combination signal is then demodulated on the receiving end to recover the original complex waveform with all its associated parameters intact.
The result is a scheme that efficiently trades abundant MMW bandwidth for noise tolerance by allowing a complex cable (64 or 256 QAM) or VDSL signal to be transmitted using 16 QAM modulation in a MMW band. "We can take the 256 QAM signal, increase its bandwidth in the air, relative to the wired network, and improve its noise performance to the point where it has the same propagation and noise performance as a 16-QAM signal — without even demodulating it," said Levin. |
