Impairment mitigation in superchannels with digital backpropagation and MLSD

Abstract

We assess numerically the performance of single-carrier digital backpropagation (SC-DBP) and maximum-likelihood sequence detection (MLSD) for DP-QPSK and DP-16QAM superchannel transmission over dispersion uncompensated links for three different cases of spectral shaping: optical pre-filtering of RZ and NRZ spectra, and digital Nyquist filtering. We investigate the limits for carrier proximity of each spectral shaping technique and the correspondent performance behavior of each algorithm, for both modulation formats. For superchannels with carrier spacing close to the Nyquist limit, it is shown that the maximum performance improvement of 1.0 dB in Q 2 -factor is provided by those algorithms. However, such gain can be highly reduced when the order of the modulation format increases.

Advanced Digital Signal Processing and Variable-Rate Coding for Unrepeatered Optical Transmission

“…1. The standard DSP (purple line) chain, used here as baseline, considers first the resampling to 2 Sa/Symbol in the pre-processing block 1 . Next, orthonormalization compensates the optical front-end distortions and in-phase and quadrature imbalances using the Gram-Schmidt orthogonalization procedure (GSOP).…”

“…Alternatively, the evaluated advanced DSP considers different algorithm combinations. The MIMO/PEF (orange line) adds a post-emphasis filter (PEF) [15] with a 1st-order Gaussian shape 1 Although DAC sampling rates are usually lower than 2 Sa/Symbol, it is a common practice to resample the signal to 2 Sa/Symbol at the beginning of the DSP chain [14]. and 35-GHz cutoff frequency, compensating bandwidth limitation imposed by system components.…”

“…With less impact, eventually, transceiver impairments such as bandwidth limitations and in-phase and quadrature (IQ) skews may also impair the transmission performance. Recently, techniques for nonlinearity compensation have been investigated, and digital back-propagation (DBP) and maximum likelihood sequence estimation (MLSE) arise as possible solutions [1]. DBP requires the precise estimation of the fiber nonlinear parameter γ for optimum performance.…”

Digital Nonlinearity Compensation Techniques for Unrepeatered Optical Systems

“…On one hand, the benefits of DBP, symbol-rate optimization (SRO), and probabilistic shaping (PS) are experimentally evaluated for ultra-long-haul optical transmission [8]. On the other hand, the MLSE performance is numerically investigated for intra-channel NLC [7] and the performance of MLSE and DBP are numerically analyzed for superchannel transmission [9] with different modulation formats and spectral shaping. However, whereas an unrepeatered system was employed in this work, all the previous ones considered repeatered optical transmission scenarios.…”

Fiber Nonlinearity and Tight Filtering Impairments Mitigation of DP-16QAM Nyquist-WDM Systems Using Multiplier-Free MAP Detection

“…Various approaches have been proposed to compensate/mitigate above impairments using Tx-and/or Rx-side DSP. The current mainstream techniques for fiber nonlinear effect compensation include digital back-propagation (DBP) algorithms [13]- [16], time/frequency-domain Volterra series-based equalizers [17]- [19], machine learning-based techniques [20]- [22], nonlinear Fourier transform (NFT) [23], maximum likelihood sequence estimation (MLSE) [24], [25], and regular perturbation-based methods [26]. However, most of the methods mentioned above require detailed knowledge of the transmitted signals and generally groan under the intolerable burden of massive CC, thereby hindering their practical application.…”