Abstract:We investigate a digital back-propagation simplification method to enable computationally-efficient digital nonlinearity compensation for a coherently-detected 112 Gb/s polarization multiplexed quadrature phase shifted keying transmission over a 1,600 km link (20x80km) with no inline compensation. Through numerical simulation, we report up to 80% reduction in required back-propagation steps to perform nonlinear compensation, in comparison to the standard back-propagation algorithm. This method takes into account the correlation between adjacent symbols at a given instant using a weighted-average approach, and optimization of the position of nonlinear compensator stage to enable practical digital backpropagation.
The frequency domain equalizers (FDEs) employing two types of overlap-add zero-padding (OLA-ZP) methods are applied to compensate the chromatic dispersion in a 112-Gbit=s non-return-to-zero polarization division multiplexed quadrature phase shift keying (NRZ-PDM-QPSK) coherent optical transmission system. Simulation results demonstrate that the OLA-ZP methods can achieve the same acceptable performance as the overlapsave method. The required minimum overlap (or zeropadding) in the FDE is derived, and the optimum fast Fourier transform length to minimize the computational complexity is also analyzed.
Keywords. Frequency domain equalizers (FDEs), overlapsave (OLS), overlap-add (OLA), chromatic dispersion (CD), quadrature phase shift keying (QPSK). PACS ® (2010). 42.25.Kb, 42.79.Sz.
PolMux QPSK has emerged as the solution of choice for the first commercial implementations of 100 Gb/s transmission systems. Thanks to coherent detection and digital signal processing (DSP), linear distortions such as chromatic dispersion (CD) and polarisation mode dispersion (PMD) can in principle be completely compensated for. And indeed, effective algorithms have been devised and extensively investigate that allow CD-and PMD-resilient transmission of 100 Gb/s over long distances, leaving optical noise accumulation and non-linear impairments as the factors ultimately limiting reach. In this paper, we present the evaluation of a simple algorithm to compensate for intra-channel Kerr non-linearity (both intra-and cross-polarisation) arising in the transmission of PolMux QPSK signals at 100 Gb/s.
We experimentally demonstrate performance enhancements enabled by weighted digital back propagation method for 28 Gbaud PM-16QAM transmission systems, over a 250 km ultra-large area fibre, using only one back-propagation step for the entire link, enabling up to 3 dB improvement in power tolerance with respect to linear compensation only. We observe that this is roughly the same improvement that can be obtained with the conventional, computationally heavy, non-weighted digital back propagation compensation with one step per span. As a further benchmark, we analyze performance improvement as a function of number of steps, and show that the performance improvement saturates at approximately 20 steps per span, at which a 5 dB improvement in power tolerance is obtained with respect to linear compensation only. Furthermore, we show that coarse-step self-phase modulation compensation is inefficient in wavelength division multiplexed transmission.
In this paper, we investigate a modified split-step Fourier method to enable computationally-efficient digital nonlinearity compensation for a coherently-detected 112 Gb/s polarization multiplexed quadrature phase shifted keying transmission over a 1,600 km link (20×80 km) with no inline compensation. We report up to 80% reduction in required stages to perform nonlinear compensation, in comparison to the conventional backpropagation algorithm. This method takes into account the correlation between adjacent symbols at a given instant using a weighted-average approach to enable practical digital nonlinearity compensation. Keywords: Coherent communications, Kerr nonlinearity, digital signal processing, advanced modulation formats, nonlinear optics
INTRODUCTIONThe optical coherent receiver with digital signal processing (DSP) is a promising technology for next generation 100 Gb/s transmission systems because it can offer higher optical signal-to-noise ratio (OSNR) tolerance and compensation of linear fibre impairments [1,2]. Despite such benefits, practical 100 Gb/s transmission systems may require further performance improvements in order to achieve similar reaches as current systems operating at 10 Gb/s. One possible approach to achieve this goal is to increase the optical power and thus received OSNR while maintaining the nonlinear impairments as low as possible with a help of DSP [3,4]. With the availability of advanced DSP techniques and high-speed analogue-to-digital converters for 40-and 100 Gb/s systems, electronic mitigation of transmission impairments has matured over past few years. In particular, electronic signal processing using digital back-propagation (DBP) with inverse fibre parameters or time inversion has been applied to the compensation of channel nonlinearities [5][6][7]. However, the complexity of DBP is currently exorbitant due to significantly high number of processing steps required in such calculations. In order to address these issues, simplifications in the DBP algorithm employing single-step per span or less via DBP techniques [8][9][10] have already commenced. In this paper we investigate a simplified DBP algorithm based on the correlation of signal power in neighbouring symbols when applying nonlinear phase compensation which requires less than one processing step per transmission span. We test the algorithm on a 112 Gb/s polarization multiplexed quadrature phase shifted keying (PM-QPSK) system, in a 1,600 km (20×80 km) transmission system, and show that a considerable complexity reduction (in terms of reduced back-propagation stages) can be achieved compared to standard DBP methods [8].
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