Fred Buchali

Intra-symbol frequency-domain averaging based channel estimation for coherent optical OFDM

We present an efficient channel estimation method for coherent optical OFDM (CO-OFDM) based on intra-symbol frequency-domain averaging (ISFA), and systematically study its robustness against transmission impairments such as optical noise, chromatic dispersion (CD), polarization-mode dispersion (PMD), polarization-dependent loss (PDL), and fiber nonlinearity. Numerical simulations are performed for a 112-Gb/s polarization-division multiplexed (PDM) CO-OFDM signal, and the ISFAbased channel estimation and the subsequent channel compensation are found to be highly robust against these transmission impairments in typical optical transport systems.

Electronic Dispersion Compensation

The performance of different electronic equalization and processing schemes for 40- and 10-Gb/s optical transmission over single-mode fiber (SMF) are discussed, from the point of their ability to compensate chromatic dispersion (CD) and polarization mode dispersion (PMD). In addition, the impact of fiber nonlinearity and modulation format on equalization is also investigated. The main objective of this paper is to present an overview and a comparison of the performances rather than a detailed explanation of the principles of the different equalization schemes. The equalizers which will be covered are analog equalizer (feedforward and decision feedback type), maximum likelihood sequence estimator (MLSE), electronic precompensation, coherent/intradyne detection with digital signal processing (DSP) equalization, DSP-based optical orthogonal frequency division multiplexing (OFDM), and turbo equalization.

Adaptive PMD compensation by electrical and optical techniques

Adaptive polarization-mode dispersion (PMD) compensation in 10 and 40 Gb/s transmission systems might become necessary if the link PMD causes strong signal distortions, which results in an unacceptable high system outage. The application of PMD compensation has the potential to lead to a drastic improvement of system robustness. Optical and electrical compensators will be reported, their operation principle explained, and their benefits will be quantified. The discussion is focused on the most important structures for practical system application including adaptive feedback loops. Experimental investigations proof compensator concepts which performance conforms with simulations. The electronical feed forward equalizer and the decision feedback equalizer are assessed in comparison to the optical simple and the two-stage compensator and the differences in performance are discussed for statistical PMD limited systems.

Improving the Nonlinear Tolerance of Polarization-Division-Multiplexed CO-OFDM in Long-Haul Fiber Transmission

We present digital signal processing techniques and algorithms for improving the tolerance of polarization-division multiplexed (PDM) coherent optical orthogonal frequency-division multiplexing (CO-OFDM) to fiber nonlinear effects, in both single-channel and wavelength-division multiplexed (WDM) transmission. For single-channel transmission, we discuss a self-phase modulation (SPM) compensation method that jointly processes two polarization components of a PDM CO-OFDM signal. For WDM transmission, we describe a novel OFDM channel estimation scheme that reduces the cross-phase-modulation (XPM) penalty among the WDM channels. The nonlinear tolerance improvements enabled by these signal processing techniques are quantified through numerical simulations for both dispersion-unmanaged and dispersion-managed long-haul optical fiber transmission with 112-Gb/s PDM CO-OFDM wavelength channels.

Fast eye monitor for 10 Gbit/s and its application for optical PMD compensation

The excellent correlation between high speed eye monitor signal and BER shows the potential for fast tracking of eye fluctuations and performance monitoring. PMD compensation experiments at 10 Gbit/s demonstrate the performance of a fast eye monitor used as feedback signal.

Transmission of 1.2 Tb/s continuous waveband PDM-OFDM-FDM signal with spectral efficiency of 3.3 bit/s/Hz over 400 km of SSMF

We demonstrate generation, transmission and reception of a 1.21 Tb/s continuous waveband PDM-OFDM-FDM signal with spectral efficiency of 3.33 bit/s/Hz. After DCF-free transmission over 400-km SSMF a considerable Q-factor margin of 2dB vs. EFEC limit was achieved.

Quantifying spectrum, cost, and energy efficiency in fixed-grid and flex-grid networks [Invited]

Single and multi-carrier networks offering channel rates up to 400 Gb/s are evaluated under realistic reach parameters. It is found that efficient spectrum utilization and fine bit-rate granularity are essential to achieve cost and energy efficiency. Additionally, the break-even cost of flexible orthogonal frequency division multiplexing transponders is examined under different settings. The break-even cost of a flexible transponder corresponds to the cost value for which the total cost of the network is equal to that of the related single-line-rate network. The impact of the traffic load, the additional cost required for flex-grid optical cross connects, the cost of spectrum, as well as the cost of fixed-grid transponders is examined.

Rate Adaptation and Reach Increase by Probabilistically Shaped 64-QAM: An Experimental Demonstration

A transmission system with adjustable data rate for single-carrier coherent optical transmission is proposed, which enables high-speed transmission close to the Shannon limit. The proposed system is based on probabilistically shaped 64-QAM modulation formats. Adjustable shaping is combined with a fixed-QAM modulation and a fixed forward-error correction code to realize a system with adjustable net data rate that can operate over a large reach range. At the transmitter, an adjustable distribution matcher performs the shaping. At the receiver, an inverse distribution matcher is used. Probabilistic shaping is implemented into a coherent optical transmission system for 64-QAM at 32 Gbaud to realize adjustable operation modes for net data rates ranging from 200 to 300 Gb/s. It is experimentally demonstrated that the optical transmission of probabilistically shaped 64-QAM signals outperforms the transmission reach of regular 16-QAM and regular 64-QAM signals by more than 40% in the transmission reach.

Limits of Spectral Efficiency and Transmission Reach of Optical-OFDM Superchannels for Adaptive Networks

Limits of information spectral efficiency and transmission reach are analyzed by an optical orthogonal frequency- division- multiplexing (O-OFDM) dense wavelength- division- multiplexed (DWDM) system with bit rates of 10-600 Gb/s and quadrature phase-shift keying (QPSK) or m -quadrature amplitude modulation ( m-QAM). In addition, using the International Telecommunication Union (ITU) 50-GHz grid transmission of OFDM superchannels is studied.

Performance of turbo equalizers for optical PMD channels

This paper investigates the performance of iterative (turbo) equalization to mitigate the effects of a polarization-mode dispersion (PMD) in nonreturn-to-zero (NRZ) intensity-modulated optical-fiber transmission systems. A PMD can lead to severe distortions in the received electrical signal and is a key limiter for the development of high-bit-rate transmission over currently used fibers. In order to reduce the distortions due to a PMD, the performance of symbol-by-symbol maximum a posteriori (sbs-MAP) soft-in/soft-out (SISO) decoders is studied. The SISO algorithms are adapted to the noise statistics of the optical channel where the photo detector leads to a non-Gaussian signal-dependent noise at the receiver side. The modified SISO algorithms are successfully employed for turbo equalization and results show that iterative (turbo) equalization and decoding for the compensation of a PMD can lead to a tremendous reduction in the bit error ratio (BER). Moreover, it is shown that, due to the robustness of mutual information, the extrinsic information transfer (EXIT) chart can be applied for the design of iterative receivers in optical transmission systems even with a non-Gaussian noise