Overcoming the limitations of fiber

Ron Kline/Ovum
01 Feb 2013

In preparation for March’s OSA OFC/NFOEC conference, Alcatel-Lucent’s New Jersey–based Bell Labs announced several successful lab experiments to increase wavelength capacity and spectral efficiency using multiple subcarrier super-channels, PDM-16QAM modulation formats, spatial division multiplexing (SDM), and MIMO (multiple-input multiple-output) technology.

An hour down the road in Princeton, New Jersey, scientists from NEC America announced they have completed a successful trial of real-time 1Tbps super-channel transmission over a trans-oceanic distance of 7,200km.

Huawei just completed a 2Tbps transmission trial over 3,325km. Though laboratory and field experiments are a far stretch from viable products, they offer an interesting glimpse into possible solutions for overcoming the physical limitations of transmission over optical fiber.

Increase the line rate and the number of bits transmitted

The most common method to increase capacity on WDM systems is to up the line rate on each wavelength, allowing transmission of more bits in the same channel space. This is accomplished by changing the modulation format by which optical channels are encoded and transmitted. Most 100G WDM systems deployed today use polarization-division multiplexing (PDM) quadrature phase-shift keying (QPSK) modulation coupled with coherent digital signal processing (DSP).

Advanced modulation formats such as 16-QAM (quadrature amplitude modulation) that increase the number of bits per symbol transmitted have been in development for several years. These formats will soon make their way into commercial WDM systems, while even more advanced modulation schemes such as 32-QAM, 64-QAM, and 256-IPM (iterative polar modulation) are being investigated by the scientific community.

While upping the channel rate is the most common method to increase capacity, it comes with a requirement for significantly higher optical signal-to-noise ratio (OSNR), which reduces the tolerance to fiber nonlinearity and noise.

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