Signal Modulation is Boosting Fiber Capacity

Optical fiber and emerging modulation schemes are making it possible to transmit ever-larger amounts of data through fiber-optic pipes. Recent industry demonstrations and government research efforts also have demonstrated novel approaches for moving more data through optical fiber networks as well as boosting the efficiency of scare spectrum.

Researchers reported during a recent industry conference that they achieved a new data-transmission efficiency record over a transatlantic cable using a framework called 16-QAM, for quadrature amplitude modulation. Engineers from Facebook (NASDAQ: FB) and networking vendor Infinera (NASDAQ: INFN) said they achieved data rates of up to 26.2 terabits per second over the MAREA cable that runs between Virginia and Spain. (Facebook and Microsoft [NASDAQ: MSFT] co-own MAREA.)

The current bandwidth capacity of 6,600-km cable had been pegged at up to 20 terabits per second on each of eight fiber pairs.

The industry engineers told IEEE Spectrum this week theirs’ was the first demonstration of 16-QAM on a transatlantic cable. The majority of traffic carried by those data pipes originates from datacenters.

The demonstration was significant because it illustrated how modulation techniques like 16-QAM can be used to divvy up signals to squeeze more efficiency from a finite amount of bandwidth.

In the case of 16-QAM, 4-bit code can be modulated to have 16 values—”0000,” “0001” and so on, up to “1111”. This partitioning allows each value to transmit multiple bits per second for each modulation interval. Hence, Infinera’s new transmitting and receiving equipment based on 16-QAM modulation was able to achieve 6.21 bits per second transmission per channel.

The transatlantic cable demonstration expands on earlier photonics research that seeks to leverage to the potential of 16-QAM and other modulation schemes. For example, the Defense Advanced Research Projects Agency launched a photonics initiative last year that seeks to integrate high-capacity optical signaling technology onto multichip modules. Photonics in the Package for Extreme Scalability, or PIPES, is part of DARPA’s $1.5 billion Electronics Resurgence Initiative.

Using optical rather than electrical components for data transfer is increasingly seen as a reliable approach for reducing power consumption while increasing data capacity and reducing latency.

“Efficient photonic signaling offers a path to disruptive system scalability because it eliminates the need to keep data local, and it promises to impact data-intensive applications, including machine learning, large scale emulation and advanced sensors,” Gordon Keeler, a DARPA program manager, noted in announcing PIPES.

In one scenario, DARPA officials said teams demonstrate how 15 software-defined radios could share the same frequency bands and “figure out how they are going to cooperate.” In the DARPA demonstration, 16-QAM was used to improve spectrum efficiency.

“They learned how to communicate using 16-QAM without being programmed, [doing so] in less than a minute,” said William Chappell, hard of DARPA’s Microelectronics Technology Office. “This means that systems that weren’t coordinated have the ability to learn how to interact.”

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