Maxim Kuschnerov

DSP for Coherent Single-Carrier Receivers

In this paper, we outline the design of signal processing (DSP) algorithms with blind estimation for 100-G coherent optical polarization-diversity receivers in single-carrier systems. As main degrading optical propagation effects, we considered chromatic dispersion (CD), polarization-mode dispersion (PMD), polarization-dependent loss (PDL), and cross-phase modulation (XPM). In the context of this work, we developed algorithms to increase the robustness of the single DSP receiver modules against the aforesaid propagation effects. In particular, we first present a new and fast algorithm to perform blind adaptive CD compensation through frequency-domain equalization. This low complexity equalizer component inherits a highly precise estimation of residual dispersion independent from previous or subsequent blocks. Next, we introduce an original dispersion-tolerant timing recovery and illustrate the derivation of blind polarization demultiplexing, capable to operate also in condition of high PDL. At last, we propose an XPM-mitigating carrier phase recovery as an extension of the standard Viterbi-Viterbi algorithm.

73.7 Tb/s (96 x 3 x 256-Gb/s) mode-division-multiplexed DP-16QAM transmission with inline MM-EDFA

Transmission of a 73.7 Tb/s (96 x 3 x 256-Gb/s) DP-16QAM mode-division-multiplexed signal over 119 km of few-mode fiber transmission line incorporating an inline multi mode EDFA and a phase plate based mode (de-)multiplexer is demonstrated. Data-aided 6 x 6 MIMO digital signal processing was used to demodulate the signal. The total demonstrated net capacity, taking into account 20% of FEC-overhead and 7.5% additional overhead (Ethernet and training sequences), is 57.6 Tb/s, corresponding to a spectral efficiency of 12 bits/s/Hz.

Optical Performance Monitoring in Digital Coherent Receivers

In this paper, we present a method for combined fiber parameter estimation from digital filter coefficients of a polarization diverse coherent receiver. All deterministic linear optical channel parameters like residual chromatic dispersion (CD), polarization-mode dispersion (PMD), and polarization-dependent loss (PDL) are continuously monitored by analysis of the filter impulse response of the adaptive equalizer. After deriving the according equations from the theoretical linear fiber channel model, we demonstrate robust estimation for a joint combination of all impairments.

Data-Aided Versus Blind Single-Carrier Coherent Receivers

Fiber-optic research in signal processing for the first generation of coherent communication systems was dominated by receivers with blind adaptation. Next-generation systems will require a scalable and modular design for higher order modulation formats. Due to the nature of the fiber channel and the required parallelization in high-speed receivers, data-aided and blind algorithms call for a general reassessment when used in coherent optic receivers employing higher order modulation formats. In this paper, blind and data-aided receivers are compared for coherent single-carrier optical systems in terms of complexity, tracking ability, and convergence speed. Methods for equalization are discussed for time-domain- and frequency-domain-based receivers covering the most important algorithms. The general superiority of data-aided frequency-domain equalization is demonstrated.

DSP complexity of mode-division multiplexed receivers

The complexities of common equalizer schemes are analytically analyzed in this paper in terms of complex multiplications per bit. Based on this approach we compare the complexity of mode-division multiplexed digital signal processing algorithms with different numbers of multiplexed modes in terms of modal dispersion and distance. It is found that training symbol based equalizers have significantly lower complexity compared to blind approaches for long-haul transmission. Among the training symbol based schemes, OFDM requires the lowest complexity for crosstalk compensation in a mode-division multiplexed receiver. The main challenge for training symbol based schemes is the additional overhead required to compensate modal crosstalk, which increases the data rate. In order to achieve 2000 km transmission, the effective modal dispersion must therefore be below 6 ps/km when the OFDM specific overhead is limited to 10%. It is concluded that for few mode transmission systems the reduction of modal delay is crucial to enable long-haul performance.

Reduced Complexity Digital Back-Propagation Methods for Optical Communication Systems

Next-generation optical communication systems will continue to push the ( bandwidth · distance) product towards its physical limit. To address this enormous demand, the usage of digital signal processing together with advanced modulation formats and coherent detection has been proposed to enable data-rates as high as 400 Gb/s per channel over distances in the order of 1000 km. These technological breakthroughs have been made possible by full compensation of linear fiber impairments using digital equalization algorithms. While linear equalization techniques have already matured over the last decade, the next logical focus is to explore solutions enabling the mitigation of the Kerr effect induced nonlinear channel impairments. One of the most promising methods to compensate for fiber nonlinearities is digital back-propagation (DBP), which has recently been acknowledged as a universal compensator for fiber propagation impairments, albeit with high computational requirements. In this paper, we discuss two proposals to reduce the hardware complexity required by DBP. The first confirms and extends published results for non-dispersion managed link, while the second introduces a novel method applicable to dispersion managed links, showing complexity reductions in the order of 50% and up to 85%, respectively. The proposed techniques are validated by comparing results obtained through post-processing of simulated and experimental data, employing single channel and WDM configurations, with advanced modulation formats, such as quadrature phase shift keying (QPSK) and 16-ary quadrature amplitude modulation (16-QAM). The considered net symbol rate for all cases is 25 GSymbol/s. Our post-processing results show that we can significantly reduce the hardware complexity without affecting the system performance. Finally, a detailed analysis of the obtained reduction is presented for the case of dispersion managed link in terms of number of required complex multiplications per transmitted bit.

Multi-rate (111-Gb/s, 2×43-Gb/s, and 8×10.7-Gb/s) transmission at 50-GHz channel spacing over 1040-km field-deployed fiber

111-Gb/s transmission combined with 2 times 43-Gb/s and 8 times 10.7-Gb/s on a 50-GHz grid over 1,040-km field fiber and two ROADMs is demonstrated, showing the feasibility of 100G overlaying existing 10G/40G commercial systems.

High Capacity Mode-Division Multiplexed Optical Transmission in a Novel 37-cell Hollow-Core Photonic Bandgap Fiber

We present the first demonstration of combined wavelength-division multiplexed (WDM) and mode-division multiplexed (MDM) optical transmission in a hollow-core photonic bandgap fiber (HC-PBGF). For this purpose a novel low loss, broadband 310 m long HC-PBGF with a 37 cell (37c) core geometry is used. The modal properties of the HC-PBGF are characterized in detail, showing an absence of surface modes and low modal crosstalk, which enable WDM and MDM transmission with record high capacity (73.7 Tb/s) for a HC-PBGF. Several modulation formats have been tested, showing very good and stable performance. The transmission properties are assessed by looking into both single-mode transmission and MDM transmission, showing good agreement with the modal characterization of the 37c HC-PGBF.

Adaptive equalizer complexity in coherent optical receivers

We review structures for adaptive equalization in coherent receivers with polarization multiplex and compare them in terms of computation complexity. We cover single and multi carrier approaches.

On the Mitigation of Optical Filtering Penalties Originating From ROADM Cascade

Wavelength selective switches (WSSs) that are integrated in reconfigurable optical add-drop multiplexers (ROADMs) induce penalties on the optical signal due to tight optical filtering, which increases as several ROADMs are cascaded in a meshed network. In this letter, we propose and analyze possible configurations for the mitigation of these penalties in optical domain using optical wave shaper (WS). Including one WS in every ROADM node allows transmission of 28 and 32 GBd signals, which are QPSK, 8-QAM, or 16-QAM modulated, through a cascade of 32 and 14 WSS filters, respectively. With an average bandwidth of 33 GHz per WSS, an optical signal to noise ratio penalty below 1 dB at BER=1×10-3 is observed.