New Lasers Open Way to Combine Analog, Digital in Cable Migration

The emergence of full-spectrum amplitude-modulated lasers for use in cable industry networks is spawning new network migration strategies, including a forward-looking architectural concept from Aurora Networks that is meant to facilitate transition to the converged media platform known as "CMAP."

Aurora, with introduction of a suite of directly modulated lasers that can carry analog and narrowcast digital signals across the entire 1 GHz cable RF spectrum, is proposing a relatively low-cost approach to accommodating current narrowcast needs in a way that also meets the future requirements of CMAP (Converged Media Access Platform) transport. Although, as previously reported (June, p. 8), CMAP is designed to cut modulation and video processing costs in the transition to higher narrowcast channel counts, the platform puts new burdens on the optical network which could eat into those cost savings.

"The key for us is to provide operators with a transport solution that meets their immediate needs cost effectively while providing them the lowest cost migration path when they're ready to implement CMAP," says John Dahlquist, vice president of marketing at Aurora. "Eventually we see operators retiring their broadcast transmitters so that everything goes on the full-spectrum transmitters."

There are many situations currently calling for purchase of new transmitters where it makes sense to exploit the efficiencies of the new 1550 nm. devices, Dahlquist notes. "The need for more narrowcast channels just keeps growing," he says. "With DOCSIS 3.0 we're seeing more and more channels being dedicated to that service. Operators are adding more time-shifted and VOD programming options. And you need capacity to support switched digital video."

Today operators typically rely on high power externally modulated lasers to distribute analog and, in some cases, digital broadcast channels on a dedicated fiber and use directly modulated lasers operating at different wavelengths on a separate fiber to deliver narrowcast digital services. "Many of these broadcast/narrowcast overlay types of systems were deployed in the late '90s or early 2000s when you could only transmit at eight or 16 narrowcast QAM (quadrature amplitude modulated) channels over a single fiber," Dahlquist notes. "Now as those narrowcast transmitters age and operators need more narrowcast channels, they're looking to upgrade to more advanced technology."

One way to go is to load up on purely narrowcast transmitters, exploiting the higher wavelength densities made possible by more recent advances. For example, narrowcast transmitters supplied by Aurora, operating on wavelengths separated at 100 GHz intervals in the 1531-1571 nanometer ITU C band, can transmit up 80 QAM channels at 40 wavelengths per fiber at distances of 110 Km. or more. At the same time the firm's externally modulated high-power lasers transmit the analog channels and possibly some QAM broadcast channels over comparable distances on a separate fiber in the 1565 nm. window.

Aurora's new full-spectrum analog/digital lasers offer another architectural migration path where the costs of delivering broadcast and dedicated signals across the full downstream spectrum are radically reduced compared to the overlay approach, Dahlquist says. "With these new transmitters operators can deliver the full spectrum of broadcast analog and broadcast and narrowcast QAM channels at wavelength densities of up to 16 per fiber at a per-transmitter cost that's just 20 to 25 percent of the cost of an externally modulated analog laser," he notes.

There are two categories of transmitters in the new AT3545G product line. One is designed for "light" analog channel loading (from 0 to 30 analog channels up to 258 MHz) plus loading of as many as 124 QAM channels up to 1002 MHz. Initially, Dahlquist says, operators may want to use these transmitters exclusively for QAM channels, thereby ensuring that all the transport power that will be required to handle ongoing narrowcast channel increases is in place.

Then, as operators reduce the analog channel count to threshold levels, they can employ the 50-258 MHz spectrum segment on these transmitters to carry the analog lineup, allowing them to retire their analog transmitters and reclaim the broadcast fiber or to use that fiber to double the capacity for transporting full-spectrum wavelengths. "When you get to the point of eliminating analog altogether, you'll be able to deliver 154 QAM channels in whatever combination of broadcast and narrowcast you want," he adds.

The other category of full-spectrum transmitter offered by Aurora is meant for operators who want to transmit a larger quantity of analog channels with narrowcast over a single wavelength. These transmitters are designed for full analog channel loading of up to 79 channels, leaving the spectrum between 552 and 1002 MHz for QAM channels, Dahlquist says.

"The migration from 79 analog channels to all-QAM is underway around the world," adds Aurora CTO Oleh Sniezko. "You can see it in Europe, Asia, Latin America and North America. They want the flexibility to allocate more QAM channels to narrowcast as they reduce analog, which means they want to put as many narrowcast channels on each wavelength as possible while maintaining the ability to deliver multiple wavelengths over a single fiber."

Advanced dispersion compensation circuitry in all AT3545G transmitters provides support for reaching required distances with full-spectrum channel loading, Sniezko says. "By improving the lasers and the circuitry we can address these needs with fully loaded transmitters that can deliver 79 analog with 75 QAM channels or 30 analog and 124 QAM channels or 154 QAM channels and anything in between," he explains.

For example, he says, operators deploying the transmitter designed for low analog channel counts to deliver 30 analog and 124 QAM channels can achieve 50 dB carrier-to-noise (C/N) performance at up to 40 Km. at wavelength densities of eight per fiber and the same C/N at up to 50 Km. at four wavelengths per fiber. At 16 wavelengths per fiber the same noise performance is achievable at up to 25 Km.

Higher analog channel loading with the other version of the AT3545G achieves comparable C/N performance levels at shorter distances for any given wavelength density, Sniezko says. With all-QAM loading on either type of full-spectrum transmitter 50 dB C/N can be attained at distances of up to 80 Km., depending on wavelength density, he adds.

"Ultimately, what the industry wants is that all the multi-wavelength systems will be able to carry CMAP," Sniezko says. CMAP, he notes, offers a much more efficient solution for expanding narrowcast channel counts and variations in the services carried over those channels compared to previously devised migration paths such as modular CMTS (cable modem termination system), DOCSIS by-pass and stacking of universal edge QAMs (quadrature amplitude modulators) for each service area.

In essence, CMAP combines CMTS and QAM functionalities at the headend so that all MPEG-2, voice and broadband streams can be switched in IP mode for aggregation into narrowcast and broadcast QAM channels in whatever combinations are needed to meet market requirements at a given time. By configuring service groups by services rather than geographic areas CMAP eliminates unnecessary replications of QAM modules, and, in so doing, creates an opportunity to minimize transport costs through use of wavelength-specific full-spectrum transmitters dedicated to each serving area.

In other words, in order to take advantage of the great efficiency gains made possible by CMAP operators must be able to shift a much larger share of the total downstream spectrum to narrowcast lasers than would have been possible in the past. Owing to their limited modulation depths and relatively high second and third order distortion characteristics, older lasers won't be able to propagate across the wider RF spectrum without introducing unacceptable increases in noise levels.

While CMAP won't be widely deployed until sometime in 2012, it's none too soon to be considering the transport implications as operators act on current needs, Dahlquist says. "We've qualified the new full-spectrum transmitter platform with one major MSO already," he notes. "And we've got a couple of other trials going on as well."