Electronic dispersion compensation for 107 Gb/s coherent detection with multi-level modulation formats

Abstract

We investigate the performance of electronic dispersion compensation for coherent detection with a zero-forcing equalizer approach, using the minimum mean-square error criterion, for RZ-QPSK, RZ-8PSK and Star-RZ-16QAM for the linear and nonlinear channel at 107Gb/s. Introduction The next bitrate in the Ethernet hierarchy is expected to be 100Gb/s. For this, multi-level modulation formats can be used to reduce the bandwidth requirement. In conjunction with a coherent receiver the multi-level formats can be demodulated in the electrical domain with digital signal processing (DSP), due to the availability of high-speed digital signal processors. Similarly carrier and phase recovery as well as the equalization can be done by the DSP [1]. In this contribution we investigate numerically the performance of electronic dispersion compensation (EDC) after coherent reception for RZ-QPSK, RZ8PSK and Star-RZ-16QAM at 107Gb/s for the linear and nonlinear channel. EDC is achieved by a zeroforcing (ZF) approach, using the minimum meansquare error (MMSE) criterion to derive the coefficients. We investigate the performance in terms of dispersion tolerance by Monte-Carlo simulations. Simulation setup At the transmitter side either 53.5Gbaud RZ-QPSK [2], 35.7Gbaud RZ-8PSK [3] or 26.75Gbaud Star-RZ16QAM are generated to achieve 107Gb/s for all modulation formats according to fig. 1 (top). Star-RZ16QAM is generated from RZ-8PSK with an additional Mach-Zehnder modulator (MZM). The advantage of this setup is the necessity of only binary driving signals, contrary to multi-level driving signals in the case of pure I/Q-modulation. All data signals are differentially encoded due to the phase ambiguity. The transmission channel is modelled as a single span with variable length to adjust the chromatic dispersion (CD). A linear channel (only CD, D=17ps/nm/km) as well as a nonlinear channel (selfphase-modulation (SPM) and CD, γ=1.6215 1/W/km, α=0.21dB/km, Pfiber in, average=5dBm) is assumed. At the receiver side an EDFA with a Gaussian optical bandpass filter (BP, f3dB= 4.1x baudrate) is used for noise loading. The received signal is combined with the signal of a local oscillator (LO) in a 2x4 90o hybrid (for this contribution ideal homodyne detection is assumed) and detected with two balanced photodetectors. Afterwards the resulting electrical inphase and quadrature signals are lowpass (LP) filtered (Butterworth 3 order, f3dB= 1x baudrate), sampled twice per symbol and then processed in a digital signal processing unit (fig. 2).