Digital Fiber Nonlinearity Compensation

IEEE SIGNAL PROCESSING MAGAZINE [46] MARCh 2014 1053-5888/14/$31.00©2014IEEE T he world is connected by a core network of longhaul optical communication systems that link countries and continents, enabling long-distance phone calls, data-center communications, and the Internet. The demands on information rates have been constantly driven up by applications such as online gaming, highdefinition video, and cloud computing. All over the world, end-user connection speeds are being increased by replacing conventional digital subscriber line (DSL) and asymmetric DSL (ADSL) with fiber to the home. Clearly, the capacity of the core network must also increase proportionally. In the 1980s and 1990s, speeds in the core network were pushed forward by technologies such as external modulation and the erbium doped fiber amplifier (EDFA), which supported wavelength division multiplexing (WDM)—transmission of information using different colors. In the last decade, commercial systems have adopted coherent optical receivers and digital signal processing (DSP) to transmit 100 Gb/s per wavelength on over 80 wavelengths, enabling transmission capacity of 8 $ Tb/s per fiber. However, even with this impressive technological development, such system capacities will soon be insufficient to meet the demands of consumers. The data rate in an optical transmission system is currently limited by amplified spontaneous emission (ASE), which determines the minimum power launched into each fiber span, and the interplay between chromatic dispersion (CD) and Kerr fiber nonlinearity, which limits the maximum launch power [1]. To increase the data rate of current-generation coherent systems, fiber nonlinearity compensation is required to enable higher launch powers, thereby providing enough optical signal-to-noise ratio (OSNR) to support larger constellation sizes [2]. Although [Liang B. Du, Danish Rafique, Antonio Napoli, Bernhard Spinnler, Andrew D. Ellis, Maxim Kuschnerov, and Arthur J. Lowery]