November 23, 2010 – The optical replacement costs associated with the cable industry’s ever-expanding narrowcast requirements may be about to fall dramatically, thanks to an innovative approach to designing optical receivers that will soon be introduced to the market by InnoTrans Communications, a relative newcomer to the field.
As previously reported (September, p. 8), InnoTrans has applied proprietary processing techniques to off-the-shelf directly modulated lasers to expand performance for full-spectrum transmission of broadcast and narrowcast cable channels while eliminating the need to precisely tune each transmitter to the distance to a node. Now the company has developed a new generation of optical receivers that will allow operators to extend the life of installed narrowcast transmitters as more narrowcast capacity is added on the narrowcast transmitters, says InnoTrans CEO Mani Ramachandran.
“This is a solution designed for cable networks where a single high-power transmitter is used to deliver the broadcast channels and a separate transmitter delivers narrowcast channels,” Ramachandran says. “While a full-spectrum transmitter is a preferable solution for addressing the migration to more narrowcast channels, it may not be feasible in longer overlay networks, which is why we’ve come up with this new receiver design.”
The problem InnoTrans is addressing has to do with the second-order distortion that’s introduced on the 1550 nanometer wavelength used by the narrowcast transmitter as more RF spectrum bandwidth is added to the transmitter’s output. When the narrowcast signal is combined at the node receiver with the broadcast signal arriving over the 1550 nm. wavelength from the externally modulated high-power broadcast laser, the distortion impacts the carrier-to-noise (C/N) ratio of the arriving broadcast signal to a point where, in some instances, the narrowcast laser must be replaced with a later generation transmitter that transmits with lower “chirp,” which is an effect that can produce variations in optical power across multiple wavelengths to a point where performance is impeded over distance.
“The problem occurs at longer distances, usually at 50 Km or more,” Ramachandran says. “Where you might be getting a 51 or 52 C/N on the broadcast signal when the narrowcast transmitter is carrying just 50 MHz of bandwidth, the broadcast C/N can drop to 44 when you go to 300 MHz on the narrowcast, and it gets worse from there.”
Fifty-plus Km distances are the norm in many cable systems, where the distance traversed from the headend to the hub often is around 50 Km and the distance to the node is another 20-25 Km, notes George Vasilakis, vice president of sales and business development at InnoTrans. “Not all operators are using the overlay architecture, but for those that do, the costs on the optical side of expanding narrowcast capacity are one of the real pain points in service evolution,” Vasilakis says.
The InnoTrans solution applies proprietary processing at the receiver to cancel out the additional distortion so that the C/N on the broadcast signal is maintained. Putting in the new receivers at the affected nodes results in a savings of 60 to 70 percent compared to the cost of replacing the narrowcast transmitter, Ramachandran says.
In fact, the cost savings is even greater over time, Vasilakis notes. This is because, in the traditional mode of operations, with each narrowcast spectrum increase the operator must send technicians to the nodes to disconnect the link and test the received optical power of the narrowcast and broadcast signals in order to accommodate the necessary rebalancing of power output. “Our receiver circuitry automatically balances for future increases in bandwidth on the narrowcast transmitter, so you don’t need to do those truck rolls,” Vasilakis says.
“At this point we have a prototype of the solution, which we’re showing to customers,” Ramachandran says. “This addresses a market that full-band transmitters can’t, which adds up to a lot of nodes. We still recommend full-band using chirp-free transmitters, because it’s a better architecture. But for those who can’t use that architecture, this is the alternative.”