Cheng Liu,Jie Pan,Thomas Detwiler,Andrew Stark,Yu-Ting Hsueh,Gee-Kung Chang,and Stephen E.Ralph

(School of Electrical and Computer Engineering,Georgia Institute of Technology,Atlanta,GA 30332,USA)

Abstract In this paper,we propose a novel super-receiver architecture for Nyquist-wavelength-division-multiplexing(WDM)superchannel optical coherent systems.As opposed to a conventionalcoherent receiver,where each subchannel is demodulated independently,the proposed super-receiver jointly detects and demodulates multiple subchannels simultaneously.By taking advantage of information from side channels that use joint DSPto cancel interchannel interference(ICI),the proposed super-receiver performs much better than a conventional receiver.This architecture also has the potential to compensate for cross-channel impairments caused by linear and nonlinear effects.We examine the proposed architecture through experiment and simulation.OSNRis improved by more than 5 d B after 1280 km fiber transmission with narrow channel spacing.

Keyw ords superchannel;joint DSP;ICI;coherent receiver

1 Introduction

S uperchanneltransmission of 1 Tb/s and beyond has recently been proposed as an alternative to electrical OFDM for satisfying the bandwidth requirments of future optical networks.Nyquist wavelength-division multiplexing(WDM)[1]-[4]and coherent optical OFDM(CO-OFDM)[5]are the main technologies used to achieve ultrahigh spectral efficiency in superchannel optical coherent systems.In Nyquist WDM optical coherent systems,conventional WDM carriers are packed tightly for near-baud-rate or baud-rate spacing.In such systems,interchannel interference(ICI)significantly degrades system performance.The conventional way to mitigate ICIin a Nyquist WDM system is to apply strong electrical or optical filtering to each channeland use a digital signal processor(DSP)to cancel the induced ISI[6].However,there is still strong ISIwhen the channelspacing is tight near the baud rate.Therefore,we propose a novel coherent receiver architecture called super-receiver for Nyquist WDM systems.The super-receiver detects and demodulates multiple channels simultaneously.Taking advantage of information from side channels that use joint DSPto cancel ICI,the super-receiver performs much better than a conventional receiver,which processes each channel individually.Because all the side-channel information is available,other cross-channelimpairments,such as nonlinear cross-phase modulation(XPM),cross-polarization modulation(XPolM),and four wave mixing(FWM),can be compensated for.We propose using the super-receiver to jointly estimate carrier phase from the side channel information in carrier-locked Nyquist systems.In this paper,we introduce the super-receiver architecture and describe the joint DSPalgorithms that compensate for linear ICI.We assess the algorithms using experimental and simulated data.There is more than 5 d B optical signal-to-noise ratio(OSNR)gain at BER=10-3when the channel spacing is at baud rate.

2 Principle and Design

Fig.1 shows the proposed super-receiver architecture.In Nyquist WDM systems,tightly spaced optical carriers are modulated by independent data and packed together by an optical multiplexer(where ICIis incurred).After optical fiber transmission and opticaldemultiplexing,each channel is separated and sent to its corresponding coherent receiver for O/Econversion and digital sampling.The local oscillators(LOs)for coherent receivers are generated in the same way as at the transmitter side.The synchronized information across the channels is captured for future joint signal processing.For this reason,the optical path and electrical path of each demultiplexed channelneed to be equal in length,and digital sampling must be synchronized across the channels.However,these requirements can be relaxed in a joint DSPblock if time-domain memory size is increased(Fig.2).In a joint DSPblock,information is available from multiple channels,and this enables joint signal processing to compensate for both linear and nonlinear impairments between channels.This also enables joint carrier phase recovery in carrier-locked Nyquist WDM systems.

?Figure 1.Proposed super-receiver architecture.

3 Joint DSP Based on Adaptive LMS Algorithm for Cancelling Linear ICI

ICI.Three channels are considered,and ICIequalization for the center channel(channel two)is shown.After the three-channel signal is sampled with synchronized ADCs,each channelundergoes conventional DSP,that is,chromatic dispersion compensation,polarization demultiplexing,timing recovery,and carrier phase estimation.After timing recovery and carrier phase recovery,an adaptive ICIequalizer based on adaptive least mean square(LMS)algorithm is used across all three channels for both Xand Ypolarizations.For each polarization,side channels(channels one and three)are shifted in the frequency domain by the amount of channel spacing.By shifting the side channels into the original spectral location,the overlapped spectra are aligned for subsequent ICIequalizer.After then the signals from the three channels are fed into the ICIfilter,where the filter coefficients W12,W22,and W32are jointly and adaptively updated according to the slicing error.Each filter coefficient Wijrepresents the

▲Figure 2.Joint DSPbased on adaptive LMSalgorithm for cancelling linear ICI.

Fig.2 shows the proposed joint DSPfor cancelling linear weighted crosstalk from channel i to channel j.For each Wij,there are a few taps in time domain so that timing offset between channels,caused by imperfect synchronized sampling or path mismatch,can be compensated for.After the ICIequalizer,ISIequalizers are used for each polarization to compensate for the residual ISIinduced by optical or electrical filtering in the link.Conventional DSPin Nyquist WDM systems uses an ISIequalizer to compensate for the penalty introduced by strong filtering.However,a joint ICI equalizer relaxes the narrow filter bandwidth and adaptively cancels a significant part of the linear crosstalk.Channel one and three are processed in the same way using information from neighboring channels.

▲Figure 3.Comparison of proposed joint LMSICIequalizer with conventionalmethod.

4 Experiment and Simulation Results

We conducted a proof-of-concept experiment on the super-receiver architecture and joint ICIalgorithm and compared the results with those using simulated data.Two independent lasers each carrying 28 Gbaud dual-polarization quadrature phase-shift keying(DP-QPSK)data were MUXed together without optical filters.At the receiver side,the signals were split into two paths and were received by two coherent receivers with synchronized 40 GSa/s Agilent sampling oscilloscopes.The electrical filters at the sampling head were 16 GHz.Fig.3(a)and(b)shows the OSNRrequired to reach a BERof 10-3at different channel spacings.The results are shown for experimental and simulated data.When channel spacing decreases,absolute performance degrades in all cases.However,with a joint ICI equalizer,performance degradation is reduced.For this simple two-channel case,there is a 2 d Brequired OSNR penalty at 30 GHz channel spacing when conventional ISI equalization is used.Joint ICIequalization reduces this penalty by 1 d B.

To give a more complete picture of the proposed system with three-channel conditions,we simulated a 3×32 Gbaud(128 Gb/s)DP-QPSKNyquist WDM opticalcoherent system by using RSOFTOptSim.The bandwidths of optical filters at optical multiplexing were set to 43.75 GHz in order to shape the signalspectrum.After optical multiplexing,the three channel signals were transmitted through 16 spans of 80 km SSMFwith launch power of 0 d Bm per channel.The digital sampling rate was 80 GSa/s per channel,and the bandwidth of the electrical filters at the sampling head were 22 GHz.Fig.3(c)shows the performance of joint ICIequalization versus that of conventional ICIequalization.We compared the required OSNRfor BER=10-3and BER=10-2at different channel spacings after 1280 km standard single-mode fiber(SSMF)transmission.At BER=10-3and channel spacing of 32 GHz(baud rate),the required OSNRusing the conventional algorithm was more than 5 d B higher than that using the LMSalgorithm in joint ICIequalization.After fiber transmission,the relative OSNRgain was maintained,and absolute performance degraded with longer transmission distance because of increased nonlinearity.Fig.3(d)shows the timing offset between channels when different filter lengths are used.The ICIequalizer operates with two samples per symbol,so a filter length of five taps spans 2.5 symbols.As long as the number of offset symbols is within the filter memory range,the ICIequalizer is effective.By increasing the filter tap length,the equalizer is more tolerant of timing offset.

5 Conclusion

Anovel coherent receiver architecture has been proposed for Nyquist-WDM super-channelsystems.Ajoint DSP algorithm for linear ICIcancellation has been developed and verfied using the proposed super-receiver architecture.When the channel spacing is at baud rate,the required OSNR using conventional ICIequalization is more than 5 d B higher than that using joint ICIequalization.As a result,the proposed super-receiver architecture can greatly enhance coherent receiver performance in Nyquist-WDM super-channel systems with high spectral efficiency.

Acknowledgement

This work was supported by the Georgia Tech 100G Consortium.