Juan Qi

Delayed Single-Tap Frequency-Domain Chromatic-Dispersion Compensation

A long-haul transmission of 100 Gb/s without optical chromatic-dispersion (CD) compensation provides a range of benefits regarding cost effectiveness, power budget, and nonlinearity tolerance. The channel memory is largely dominated by CD in this case with an intersymbol-interference spread of more than 100 symbol durations. An efficient implementation of digital CD compensation is feasible by frequency-domain (FD) filtering. Still the large size of the Fourier transform requires a high gate-count and a large chip size. We propose a new FD filtering on the basis of a nonmaximally decimated discrete Fourier transform filter bank with a trivial prototype filter and a delayed single-tap equalizer per sub-band. This method, which can be regarded as an extension to the popular overlap-save method, allows us to increase the CD tolerance drastically. At the same time, the implementation complexity is not altered apart from adding simple memory elements realizing sub-band delays. With this technique, the uncompensated trans-Pacific transmission becomes feasible with the digital CD compensation.

Beyond 100G optical channel noise modeling for optimized soft-decision FEC performance

Deviations away from ideal AWGN statistics degrading FEC performance are demonstrated caused by the nonlinear optical channel and by receiver-side DSP. An according noise model is demonstrated for FEC test and optimization.

Multiplier-free carrier phase estimation for low complexity carrier frequency and phase recovery

Carrier phase estimation for QPSK modulation is demonstrated using only real-valued additions without any multiplication or angle computation. Based on simulation and offline data, the same performance as the widely considered 4th-power algorithm is reached.

Timing recovery in nyquist coherent optical systems

Timing recovery in coherent optical systems is a critical issue since residual chromatic dispersion (CD) and polarization mode dispersion (PMD) aggravate the clock tone extraction. The problem becomes more serious in case of a very dense channel packaging using Nyquist pulses. In this paper we discuss the timing problem in Nyquist optical systems and propose an algorithm that can work even with a roll-off-factor (ROF) equal to zero. The algorithm provides almost the same clock tone performance for all modulation formats that are considered to be used in coherent optical transmission systems.

Soft-differential decoding for demodulation without carrier phase recovery

We demonstrate demodulation by low complexity soft-differential decoding. Combined with subsequent non-redundant error correction, the penalty compared to classical carrier recovery with hard-differential decoding is largely reduced providing an attractive low gate-count solution.

Delayed Single-tap Subband Processing for Chromatic Dispersion Compensation

Our method for subband processing frequency domain (FD) chromatic dispersion (CD) compensation is based on a single tap with delay equalizer in each subband. With this technique uncompensated trans-Pacific transmission becomes feasible.

Accelerating chromatic dispersion estimation in coherent optical receivers

We present a fast and robust method for the estimation of residual chromatic dispersion (CD) in coherent optical receivers. The estimation method relies on the clock tone quality in the presence of polarization-mode dispersion (PMD) and CD. The robust estimation is accomplished using estimation cost functions covering a wide range of PMD, while the estimation time is reduced thanks to a rough scanning range and interpolating estimation results. Comparing to the published estimation algorithms, a coarse estimation using CD scanning step of 1 ns/nm combined with a trivial fine tuning (e.g. step 10ps/nm) shortens the estimation time several times with an estimation error less than 5 ps/nm.

On the Mitigation of the Differential Decoding Penalty in 100G PDM-QPSK Digital Coherent Receivers for Multirate WDM Transmission

We evaluate the performance of nonredundant differential decoding schemes under linear and nonlinear channel conditions with various neighbor channels based on measured data. Each method is analyzed with respect to its systematic back-to-back performance, its nonlinear tolerance, its implementation complexity, and its compatibility to soft-decision (SD) forward error correction (FEC). All schemes successfully avoid large error-bursts due to residual phase ambiguity after digital carrier recovery. Only the complex implementation of the recursive multisymbol phase estimation provides SD samples to subsequent SD-FEC and reaches substantial recovery of the differential penalty at the same time. None of the schemes can overcome the hard-decision differential penalty.