S pecial Topic Field Transmission of 100G and Beyond: Multiple Baud Rates and Mixed Line Rates Using Nyquist-WDM Technology Zhensheng Jia, Jianjun Yu, Hung-Chang Chien, Ze Dong, and Di Huo
chromatic dispersion. The filter coefficients are calculated from a Transmitter known fiber CD transfer function PBC PBS EDFA using the frequency-domain pc DWDM Signals truncation method. to the Field Link I/Q Mod IM Pol. Mux Step 3: Two complex-valued, OC DL2 13-tap, T/2-spaced adaptive FIR λ filter banks, found on a classic CMA, λ is used to demultiplex the QPSK WSS λ Darmstadt signal and compensate for the to λ Nuernberg residual CD and polarization-mode with 10 ps/div 10 ps/div dispersion (PMD). LO frequency Added Fiber in offset compensation and Stuttgart carrier-phase estimation (CPE) and Polarization-Diversity follow using Viterbi-Viterbi algorithm. Loop back Real-time 90-Degree Hybrid As a result of linear equalization of Off-Line Signal Processing PD Oscilloscope x pol. the spectral shaping signals, the ADC 90̊ noise and linear crosstalk at the Hybrid ADC high-frequency edge of signals is PBS TOF enhanced, which leads to the OSNR ADC 90̊ reduction. Hybrid LO ADC Step 4: A digital postfilter, as y pol. discussed in section 2, is added and Coherent Receiver followed by the MLSE for the partial response estimation. CD: chromatic dispersion IM: intensity modulator PBS: polarization beam splitter Because of the known pattern of CMA: constant-modulus algorithm LO: local oscillator Pol. Mux: polarization multiplexing DL: optical time delay line OC: optical coupler TOF: tunable optical filter duobinary signaling, only two states I/Q Mod: QPSK modulator PBC: polarization beam combiner WSS: wavelength selective switch with 1 bit memory length) are needed for the signal decoding of the ▲Figure 8. Experimental setup for generating and transmitting 8 × 216.8 Gbit/s PDM-CSRZ-QPSK over 1750 km fiber. Eye diagrams of PDM-CSRZ-QPSK before (a) and after (b) CSRZ pulse curving. independent electrical lane. In this field experiment, errors are counted combiner (PBC) to recombine the signal. The odd and even over 12 ×106 bits (12 data sets, each data set contains 106 channels are spectrally filtered and combined using a bits), and differential decoding is used to solve π/2 phase programmable WSS with 50 GHz fixed grid. The optical ambiguity. spectrum of the single-channel 54.2 Gbaud Fig. 10 shows the back-to-back BER as a function of PDM-CSRZ-QPSK signal before and after 50 GHz grid WSS OSNR with noise measured at 0.1 nm bandwidth. We is shown in Fig. 9. The spectrum shape at the high-frequency measure the performance of the middle channel at edge is significantly affected by the strong optical filtering. 1551.51 nm while temporarily turning off the adjacent The generated 8 × 216.8 Gbit/s PDM-CSRZ-QPSK signal Before WSS is launched into a 22-span G.652 fiber transmission link 50-GHz WSS (Fig. 7). At the receiver, a tunable optical filter (TOF) with 3 dB After WSS bandwidth of 1 nm is used to select the desired channel to be further evaluated. An ECL with a linewidth of less than 100 kHz is used as the local oscillator (LO). 10 dB A polarization-diverse 90° hybrid is used for polarization-diverse and phase-diverse front end of the balanced coherent detection. Sampling and digitization (A/D) is performed in the digital scope with 80 GSa/s sampling rate and 30 GHz electrical analog bandwidth. DSP is performed offline. Electrical impairment equalization and polarization demultiplexing is performed using the following blind equalization steps: Step 1: The clock is extracted using the square-and-filter method, and the digital signal is resampled at twice the baud 1552 1553 1554 Wavelength (nm) rate of the recovery clock. WSS: wavelength selective switch Step 2: A T/2-spaced time-domain finite ▲Figure 9. Spectrum before and after WSS (0.1 nm resolution). impulse-response (FIR) filter is used to compensate for 54.2 Gb/s
ECLs λ1 λ3 λ5 λ7
27.1 GHz
DL1
2 4 6
Front-end Compensation
CD Compensation
CMA Equalization
Carrier Recovery
Digital Filter
MLSE Decoding
BER Counting
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September 2012 Vol.10 No.3
ZTE COMMUNICATIONS
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