Coherent QPSK Systems Research Articles

Post-FEC performance evaluation of coherent QPSK system with an enhanced pilot-aided CPE scheme

In this paper, a simple and robust pilot-aided carrier phase estimation scheme with a modified phase unwrapping is proposed. The principle of the proposed phase unwrapping is shown with a step-by-step procedure. Further, we test the post-FEC BER performance for the pilot-aided carrier phase estimation scheme with the traditional and proposed phase unwrapping. For the proposed phase unwrapping, a cycle slip rate reduction of about 10 times is achieved than the case using the traditional phase unwrapping through simulations in the presence of Wiener phase noise. Finally, we present a 2.2 Eb/N0 gain and post-FEC error-free performance with only 0.78 % pilot overhead in a 32-Gbaud QPSK system using soft-decision forward error correction.

Phase noise analysis of injected gain switched comb source for coherent communications

We present experimentally and analytically the phase noise characterization of an externally injected gain switched comb source. The results reveal the residual high frequency FM noise in the comb lines, which stays unnoticed in the optical linewidth value but leads to an increased phase-error variance. The potential impact of the residual phase noise is investigated in a 10.7 GBaud optical DQPSK system where a 2 dB power penalty is recorded at BER of 10(-9). In a 10.7 GBaud digital coherent QPSK system no penalty is observed but with 5 GBaud 16-QAM format a 3 dBpenalty exists at the FEC limit of 4.4e-3.

DGD monitoring issues in high-speed polarisation multiplexed coherent QPSK systems

Optical performance monitoring based on digital signal processing (DSP) of a polarisation diverse coherent receiver can allow the estimation of the differential group delay (DGD) of the link fibre. In this reported work, it was found that the DGD estimation can be affected by substantial errors, since the DSP can introduce or leave uncompensated any symbol-time delay between orthogonal polarisations and cannot discriminate the in-line DGD from any other link delay. These issues are investigated and a method is proposed to partially overcome the problems.

Experimental demonstration of intra-channel nonlinearity mitigation in coherent QPSK systems with nonlinear electrical equaliser

The mitigation of intra-channel nonlinearities in 10 Gbaud coherent NRZ-QPSK systems via a nonlinear electrical equaliser based on Volterra theory is experimentally demonstrated. Without the requirement of prior knowledge of the fibre link, the equaliser significantly improves the system performance compared to digital dispersion compensation only.

Experimental and Theoretical Assessment of Multipath Effects on QPSK

Qualitative and quantitative agreement between experimental and theoretical results for binary error probability for a coherent QPSK system degraded by intersymbol interference is given. The source of intersymbol interference is a delayed and attenuated replica of the transmitted signal which may represent several forms of observed multipath. Although the experimental results pertain to a particular 1 Gbit/s modem, the theory is applicable for any data rate.