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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 13, JULY 1, 2011
Nonlinear Phase Noise Mitigation With Soft-Differential Decoding and Nonredundant Error Correction for Hybrid 10G/100G Transmission Fabian N. Hauske, Member, IEEE, Zhuhong Zhang, Member, IEEE, Chuandong Li, Yanjun Zhu, Yanming Li, Fei Zhu, and Yusheng Bai
Abstract—We report experimental results of 112-Gb/s polarization-division-multiplexed return-to-zero quaternary phase-shift-keying (PDM-RZ-QPSK) transmission over dispersion-managed 1200-km links embedded between 10.7-GBaud on–off keying nonreturn-to-zero (OOK-NRZ) neighbors. Taking into account implementation constraints for equalization and synchronization of the digital signal processing (DSP) by means of processing delay and parallelization, the performances without differential decoding, with hard-differential decoding, and with soft-differential decoding are evaluated. The combination of soft-differential decoding with nonredundant error correction is identified to provide the best mitigation of cross-phase modulation (XPM)-induced nonlinear phase noise. Index Terms—Coherent detection, differential decoding, digital receiver, hybrid transmission, phase noise.
I. INTRODUCTION
P
OLARIZATION-DIVISION-MULTIPLEXED (PDM) quaternary phase-shift keying (QPSK) modulation with digital signal processing (DSP) for synchronization and equalization in a coherent receiver provides excellent linear impairment compensation, such as chromatic dispersion (CD) and polarization-mode dispersion (PMD). Due to its high spectral efficiency it is widely considered in research and product development. To meet the demand for increasing bandwidth requirement, existing dispersion managed 10.7 Gb/s on–off keying (OOK) channels with direct detection will be gradually replaced by 112 Gb/s PDM-QPSK channels with coherent detection. It is well known that coherent QPSK channels experience severe cross-phase modulation (XPM) from adjacent OOK channels limiting the performance of multirate wavelength-division multiplexing (WDM) systems [1]. Differential decoding is widely applied to avoid burst errors due to phase-noise-induced cycle slips in the digital carrier recovery stage. Several methods are reported to mitigate XPM by tracking phase variations and suppressing phase noise with Manuscript received February 03, 2011; revised March 16, 2011; accepted April 02, 2011. Date of publication April 15, 2011; date of current version June 08, 2011. F. N. Hauske is with Huawei Technologies Duesseldorf GmbH, D-80992, Munich, Germany. Z. Zhang and C. Li are with Huawei Technologies Canada, Ottawa, ON, K2K 3J1, Canada (e-mail:
[email protected]). Y. Zhu, Y. Li, F. Zhu, and Y. Bai are with Huawei Technologies USA, Santa Clara, CA 95050 USA. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2011.2141127
Fig. 1. Block diagram of experimental setup comprised of 15 dispersion-managed spans (PBS: polarization beam splitter).
various carrier phase estimation algorithms [2]–[4]. However, implementation constraints due to processing delay and parallelization degrading the performance are not considered. In this letter, we experimentally copropagate one 112 Gb/s PDM-QPSK channel with adjacent 10.7 Gb/s nonreturn to zero (NRZ) OOK neighbors through a dispersion managed 1200 km link at typical 10G channel launch powers. Based on implementation-constrained offline data processing, the performance of the 100G channel is investigated for different channel spacing. It is clear that the impact of XPM increases for narrower channel spacing. Subsequent to digital carrier recovery we evaluate the bit-error rate (BER) without differential decoding, with hard-differential (HD) decoding, with soft-differential (SD) decoding, and SD decoding combined with nonredundant error correction (NEC) [5]. The results show that SD combined with NEC performs best for channel spacing below 150 GHz. II. SYSTEM CONFIGURATION The experimental setup is shown in Fig. 1. The 112 Gb/s PDM return-to-zero (RZ) (D)QPSK signal is transmitted together with one 10.7 Gb/s NRZ-OOK channel at 50 GHz, 100 GHz, or 150 GHz channel spacing to each side. The link is composed of 10 km pre- and 15 80 km dispersion managed standard single-mode fiber (SSMF) with close to 100% per span compensation by dispersion compensating modules (DCM) ps/nm net residual CD at receiver. The 100G resulting in channel is detected by a polarization-diverse intradyne coherent receiver front-end using single-ended detection with four u2t XPRV2022A receivers. The resulting four data paths are fed into the analogue/digital conversion (ADC) stage and sampled at 50 GS/s with Tektronixs’ DPO72004 digital sampling oscilloscope (DSO). Digital equalization and synchronization for data recovery is processed offline in a PC. The local oscillator
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HAUSKE et al.: NONLINEAR PHASE NOISE MITIGATION WITH SOFT-DIFFERENTIAL DECODING
(LO) laser and the signal laser are tunable lasers with 100 kHz linewidth. Our DSP algorithms include the following key stages: Resampling the captured data to 56 GS/s, CD compensation in the frequency-domain including correction of transmitter and receiver device imperfections, multi-input multi-output (MIMO) equalizer accomplishing polarization tracking and PMD compensation followed by carrier frequency and carrier phase recovery. The carrier recovery is realized by a digital Costas phase-locked loop (PLL). The bandwidth of this loop is clearly governed by the processing delay for each phase estimation. Soft-values representing the signal with one sample per symbol are obtained after carrier recovery. The BER is calculated based on error counting. In the following, we refer to the QPSK BER performance based on the hard decisions of without differential decoding. Applying HD decoding to leads to the HD-DQPSK performance respectively. Furthermore, the soft values are employed for SD decoding by computing . Hard-decisions on provide the SD-DQPSK BER performance. Finally, the decision and the decision from a second soft-differential stage with a two-symbol delay are employed for NEC, which allows correcting error patterns based on simple 1-bit operations. The improved decisions for the SD-NEC-DQPSK BER performance come at the cost of an additional soft-differential operation to yield [3], [5]. Since the data throughput rate is much higher than the processing rate of commercially available integrated circuits in the rage of 500 MHz, our DSP is processed offline at a reduced rate emulating a realistic chip implementation taking into account processing latency and parallelization delays. As reported in [6] and [7], this “reduced rate DSP” leads to a limited bandwidth of polarization and carrier phase tracking with significant performance degradation. It is clear that the implementation of the carrier recovery largely influences the ability to mitigate phase noise, which in turn affects the performance after the differential decoding stage.
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Fig. 2. QPSK BER versus OSNR with different spacing.
Fig. 3. HD-DQPSK BER versus OSNR with different spacing.
III. EXPERIMENTAL RESULTS The performance of this transmission platform has been studied extensively by simulations and offline experiments to optimize the parameters of the DSP algorithms like the phase averaging in the carrier recovery. Implementation constrained carrier recovery turned out to be very critical limiting the performance in presence of 10G OOK neighbors. In particular, parallel block-by-block processing and processing latency leading to delayed phase correction limit the ability to track fast XPM-induced phase fluctuations. Figs. 2–5 show the waterfall curves of QPSK, HD-DQPSK, SD-DQPSK and SD-NEC-DQPSK respectively, each for 150 GHz (diamond), 100 GHz (cross) and 50 GHz (square) channel spacing. Additionally, the single channel performance is given (line without marker). Each data point refers to the average BER over at least 5 DSO captures. At 1 dBm launch power, our system achieved 18 dB optical signal-to-noise ratio (OSNR) at the receiver. At pre-forward error correction (FEC) BER, the required OSNR for 100G single channel transmission is 13.8 dB (Fig. 2). At
Fig. 4. SD-DQPSK BER versus OSNR with different spacing.
100 GHz and 50 GHz channel spacing, we observed many phase noise induced cycle slips. Shown in Fig. 2, the realistic hardware-constrained DSP (solid lines) fails to reach the FEC limit
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 13, JULY 1, 2011
Fig. 6. Power spectral density (PSD) of carrier phase noise at different spacing and different processing rates. Fig. 5. SD-NEC-DQPSK BER versus OSNR with different spacing. TABLE I REQUIRED OSNR AT PRE-FEC BER
than at 100 GHz spacing. Improving the digital bandwidth by doubling the processing rate to , we observed fewer cycle slips and the QPSK performance improved, with the 100 GHz scenario being slightly better than the 50 GHz scenario (dashed lines). We attribute the phase noise within the 500 MHz bandwidth to be the major contributor to cycle slips. IV. CONCLUSION
with 50 GHz and 100 GHz channel spacing, in contrast to unconstrained DSP without parallelization and latency due to processing delay (dashed lines). Even, HD-DQPSK is not able to bring the BER down below the FEC threshold (Fig. 3). Only SD decoding tolerates phase noise such that even the 50 GHz scenario nearly reaches the single-channel performance (Fig. 4). Comparing the single-channel scenario of QPSK (Fig. 2) to SD-QPSK (Fig. 4), the 2.5 dB OSNR penalty at the FEC limit due to the noise enhancement of SD decoding can be observed. By use of NEC, this penalty can be efficiently reduced with about 1 dB OSNR improvement compared to SD-DQPSK for all channel spacing (Fig. 5). Table I summarizes the required OSNR to achieve pre-FEC BER for each decoding scheme. For a channel spacing of 150 GHz with negligible phase noise, QPSK performs best, but for 100 GHz and 50 GHz channel spacing with significant XPM, SD-NEC-DQPSK provides the best solution. Analyzing the error patterns, the scenario with 100 GHz spacing turned out to be more prone to cycle-slip-induced burst errors than the 50 GHz scenario, which is the root cause of QPSK performance reversing between 100 GHz and 50 GHz (Fig. 2). Fig. 6 shows the power spectrum of the carrier phase noise after carrier recovery, where could be considered as 56 GS/s and being the reduction factor of the DSP processing rate. At a processing rate of with , the phase noise power spectrum clearly indicates low frequency components resulting from nonlinear phase noise. For 50 GHz spacing, the total phase noise (black) is larger than that at 100 GHz spacing (gray), but below a frequency of 500 MHz, the phase noise contributions for 50 GHz spacing are lower
We compare the performance of copropagating 100G PDM-QPSK with 10G OOK channels over dispersion managed links for different decoding schemes. Nonlinear phase noise was characterized to be colored with dominant low frequency components. For channel spacing below 100 GHz, hard-differential decoding was found to be insufficient for suppressing the impact of nonlinear phase noise with a realistic hardware constrained implementation. Instead, soft-differential decoding is required. We demonstrate that NEC efficiently mitigates the noise enhancement of soft-differential decoding providing the best nonlinear phase noise tolerance, which enables 50 GHz spaced copropagation of 100G phase modulated channels with 10G OOK channels at typical 10G launch power. REFERENCES [1] M. S. Alfiad et al., “111-Gb/s Polmux-RZ- DQPSK transmission over 1140 km of SSMF with 10.7-Gb/s NRZ-OOK neighbours,” in Proc. ECOC, Brussels, Belgium, 2008, Paper Mo.4.E.2. [2] A. J. Viterbi and A. M. Viterbi, “Nonlinear estimation of PSK-modulated carrier phase with application to burst digital transmission,” IEEE Trans. Inf. Theory, vol. IT-29, no. 4, pp. 543–551, Jul. 1983. [3] K. Piyawanno, M. Kuschnerov, F. N. Hauske, B. Spinnler, E.-D. Schmidt, and B. Lankl, “Correlation-based carrier estimation for WDM DP-QPSK transmission,” IEEE Photon. Technol. Let., vol. 20, no. 4, pp. 2090–2092, Dec. 2008. [4] M. Kuschnerov et al., “Joint-polarization carrier phase estimation for XPM-limited coherent polarization-multiplexed QPSK transmission with OOK-neighbors,” in Proc. ECOC, Brussels, Belgium, 2008, Paper Mo.4.D.2. [5] H. C. Schroeder and J. R. Sheehan, “Nonredundant Error Detection and Correction System,” U.S. Patent 3 529 290, 1970. [6] T. Pfau, S. Hoffmann, and R. Noe, “Hardware-efficient coherent digital receiver concept with feedforward carrier recovery for M-QAM constellations,” J. Lightw. Technol., vol. 27, no. 8, pp. 989–999, Apr. 15, 2009. [7] C. R. S. Fludger, J. C. Geyer, T. Duthel, and C. Schulien, “Digital signal processing—From simulation to silicon,” in Proc. ECOC, Torino, Italy, 2010, Paper Tu.5.A.1.
