Generation and transmission of 512-Gb/s quadcarrier digital super-Nyquist spectral shaped signal Junwen Zhang,1,2,3 Jianjun Yu,1,2* and Nan Chi1 1
Department of Communication Science and Engineering, and Key Laboratory for Information Science of Electromagnetic Waves (MoE), Fudan University, 220 Handan Road, Shanghai 200433, China 2 ZTE (TX) Inc, Morristown, NJ 07960, USA 3 The School of ECE, Georgia Institute of Technology, Atlanta, GA30332, USA * [email protected]
Abstract: A novel digital super-Nyquist signal generation scheme is proposed to further suppress the Nyquist signal bandwidth and reduce the channel crosstalk without using optical pre-filtering. The spectrum of the generated super-Nyquist 9-QAM signal is much more compact compared with regular Nyquist QPSK signal. Therefore, only optical couplers are needed for super-Nyquist WDM channel multiplexing. By using the 64GSa/s high speed DAC, 32-GBaud super-Nyquist 9-QAM signal is generated within 25-GHz grid for quad-carrier 400G channels. We successfully generate and transmit 4 channels quad-carrier 512-Gb/s superNyquist 9-QAM-like signal within 100-GHz grid over 2975-km at a net SE of 4b/s/Hz (after excluding the 20% soft-decision FEC overhead). ©2013 Optical Society of America OCIS codes: (060.1660) Coherent communications; (060.4080) Modulation.
References and links 1.
X. Zhou, L. E. Nelson, P. Magill, R. Isaac, B. Zhu, D. W. Peckham, P. I. Borel, and K. Carlson, “High spectral efficiency 400 Gb/s transmission using PDM time-domain hybrid 32–64 QAM and training-assisted carrier recovery,” J. Lightwave Technol. 31(7), 999–1005 (2013). 2. H. Zhang, J. Cai, H. G. Batshon, M. Mazurczyk, O. Sinkin, D. Foursa, A. Pilipetskii, G. Mohs, and N. Bergano, “200 Gb/s and dual wavelength 400 Gb/s transmission over transpacific distance at 6 b/s/Hz dpectral efficiency,” in Proc. OFC 2013, paper PDP5A.6. 3. T. J. Xia, G. Wellbrock, A. Tanaka, M. Huang, E. Ip, D. Qian, Y. Huang, S. Zhang, Y. Zhang, P. Ji, Y. Aono, S. Murakami, and T. Tajima, “High capacity field trials of 40.5 Tb/s for LH distance of 1,822 km and 54.2 Tb/s for regional distance of 634 km,” in Proc. OFC 2013, paper PDP5A.4. 4. G. Bosco, V. Curri, A. Carena, P. Poggiolini, and F. Forghieri, “On the performance of Nyquist-WDM terabit superchannels based on PM-BPSK, PM-QPSK, PM-8QAM or PM-16QAM subcarriers,” J. Lightwave Technol. 29(1), 53–61 (2011). 5. J. Wang, C. Xie, and Z. Pan, “Generation of spectrally efficient Nyquist-WDM QPSK signals using DSP techniques at transmitter,” in Proc. OFC 2012, OM3H.5. 6. O. Bertran-Pardo, J. Renaudier, P. Tran, H. Mardoyan, P. Brindel, A. Ghazisaeidi, M. T. Salsi, G. Charlet, and S. Bigo, “Submarine transmissions with spectral efficiency higher than 3 b/s/Hz using Nyquist pulse-shaped channels,” in Proc. OFC 2013, OTu2B.1. 7. Q. Juan, B. Mao, N. Gonzalez, N. Binh, and N. Stojanovic, “Generation of 28GBaud and 32GBaud PDMNyquist-QPSK by a DAC with 11.3GHz analog bandwidth,” in Proc. OFC 2013, OTh1F.1. 8. J. Wang, C. Xie, and Z. Pan, “Generation of spectrally efficient Nyquist-WDM QPSK signals using digital FIR or FDE filters at transmitters,” J. Lightwave Technol. 30(23), 3679–3686 (2012). 9. J.-X. Cai, “100G Transmission Over Transoceanic Distance With High Spectral Efficiency and Large Capacity,” J. Lightwave Technol. 30(24), 3845–3856 (2012). 10. J. Yu, J. Zhang, Z. Dong, Z. Jia, H.-C. Chien, Y. Cai, X. Xiao, and X. Li, “Transmission of 8 × 480-Gb/s superNyquist-filtering 9-QAM-like signal at 100 GHz-grid over 5000-km SMF-28 and twenty-five 100 GHz-grid ROADMs,” Opt. Express 21(13), 15686–15691 (2013). 11. H.-C. Chien, J. Yu, Z. Jia, Z. Dong, and X. Xiao, “512-Gb/s quad-carrier PM-QPSK transmission over 2400-km SMF-28 subject to narrowing 100-GHz optical bandwidth,” in Proc. ECOC 2012, paper Th.2.C.4. 12. J. Zhang, J. Yu, N. Chi, Z. Dong, J. Yu, X. Li, L. Tao, and Y. Shao, “Multi-modulus blind equalizations for coherent quadrature duobinary spectrum shaped PM-QPSK digital signal processing,” J. Lightwave Technol. 31(7), 1073–1078 (2013).
#199761 - $15.00 USD Received 21 Oct 2013; revised 27 Nov 2013; accepted 30 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031212 | OPTICS EXPRESS 31212
13. J. Zhang, B. Huang, and X. Li, “Improved quadrature duobinary system performance using multi-modulus equalization,” Photonic Technology Letters 25(16), 1630–1633 (2013). 14. J. Li, E. Tipsuwannakul, T. Eriksson, M. Karlsson, and P. A. Andrekson, “Approaching Nyquist limit in WDM systems by low-complexity receiver-side duobinary shaping,” J. Lightwave Technol. 30(11), 1664–1676 (2012). 15. Z. Jia, J. Yu, H. Chien, Z. Dong, and D. Huo, “Field transmission of 100 G and beyond: multiple baud rates and mixed line rates using Nyquist-WDM technology,” J. Lightwave Technol. 30(24), 3793–3804 (2012). 16. M. Yan, Z. Tao, L. Dou, L. Li, Y. Zhao, T. Hoshida, and J. C. Rasmussen, “Digital clock recovery algorithm for Nyquist signal,” in Proc. OFC 2013, paper OTu2I.7. 17. Y. Lu, Y. Fang, B. Wu, K. Wang, W. Wan, F. Yu, L. Li, X. Shi, and Q. Xiong, “Experimental comparison of 32Gbaud electrical-OFDM and Nyquist-WDM transmission with 64GSa/s DAC”, in Proc. ECOC 2013, paper We.1.C.3. 18. J. Qi, B. Mao, N. Gonzalez, L. N. Binh, and N. Stojanovic, “Generation of 28GBaud and 32GBaud PDMNyquist-QPSK by a DAC with 11.3GHz analog bandwidth,” in Proc. OFC 2013, paper OTh1F.1.
1. Introduction With the further maturation of high speed digital-to-analog converter (DAC), Nyquist wavelength-division-multiplexing (N-WDM) super-channels with much higher spectrum efficiency (SE) based digital signal processing (DSP) at both transmitter and receiver sides has attracted a great deal of interest for the transmission of 100G beyond [1–9]. Generally, the 400G transmission can be realized by increasing the symbol rate, increasing the number of bonded channels, increasing the modulation levels, or their combinations [1–3, 10, 11]. Bonding a number of established PDM-QPSK channels in the way of N-WDM is considered a practical 400G solution with high spectrum efficiency (SE) and lower bandwidth requirement [10, 11]. Using high speed DAC, Nyquist pulse is digitally generated to achieve the Nyquist limit of SE, which enables the channel spacing approaching equal to the symbol rate with negligible crosstalk and inter-symbol-interference (ISI) [1–8]. However, when considering the forward error correction (FEC) overhead, the transmission of 100G channels on existing optical line systems based on a 25-GHz ITU grid presents a difficult challenge due to the limited bandwidth available for each channel [9]. The excess bandwidth causes severe crosstalk. For 128-Gb/s Nyquist PDM-QPSK, the required channel spacing should be larger than 32GHz, which exceeds the ITU 25-GHz grid limit. Therefore, additional processing is needed for higher SE to counter the inter-channel-interference (ICI) impairment in order to maintain the reasonable long-haul transmission distance with FEC overhead. Using algorithms based on fixed or adaptive digital filtering with multi-symbol detection to equalize the both ISI and ICI impairments, one can transmit a super-Nyquist ((channel occupancy much less than signal baud rate) signal, of which the channel spacing can be smaller than the symbol rate without much penalty [9–15]. In our previous works [10–13], super-Nyquist WDM can be achieved by using the wavelength selective switch (WSS) for optical pre-filtering. However, the discrete and expensive WSS is not easy to integrate into the conventional optical transponder especially for multi-channels system, moreover, the stability of center window of filtering might significantly deteriorate the system performance. Furthermore, the WDM channels can be easily multiplexed by using optical couplers. In this paper, we propose and experimentally demonstrate a novel digital super-Nyquist signal generation scheme to further suppress the Nyquist signal bandwidth and reduce the channel crosstalk without using optical pre-filtering. Only about 1.5-dB OSNR penalty is observed for the 32GBaud super-Nyquist signal in the 25-GHz grid WDM case compared with single carrier case. Finally, 4 channels quad-carrier 512-Gb/s super-Nyquist 9-QAM-like signal within 100-GHz grid are successfully transmitted over 2975-km at a net SE of 4b/s/Hz (after excluding the 20% soft-decision FEC overhead). 2. The principle of digital super-Nyquist 9-QAM (QDB) signal generation Figure 1 shows the principle of the proposed novel DAC-based super-Nyquist 9-QAM signal generation compared with regular Nyquist QPSK signal. For regular Nyquist filtering, a raised cosine (RC) or square root raised cosine (SRRC) filter with roll-off factor of 0 is used for Nyquist pulse generation. However, as shown in Fig. 1, when the channel spacing is less than the baud rate, the excess bandwidth causes severe crosstalk. In order to realize the super-
#199761 - $15.00 USD Received 21 Oct 2013; revised 27 Nov 2013; accepted 30 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031212 | OPTICS EXPRESS 31213
Nyquist transmission, additional low pass filter (LPF) is added for super-Nyquist pulse generation. In this way, the signal spectrum is further suppressed to reduce the channel crosstalk.
Fig. 1. The principle of DAC-based Nyquist and super-Nyquist 9-QAM signal generation and the crosstalk impairments in SN-WDM.
In our case, the low pass filter can be simply realized by the quadrature duobinary (QDB) delay and add filter, of which the transfer function in z-transform is given by H QDB ( z ) = 1 + z −1
(1)
It can be simply implemented by a two-tap finite impulse response (FIR) digital filter with good performance and turn the QPSK to the 9-QAM signal [10–15]. The super-Nyquist digital filter in the time domain by cascading the QDB and SRRC filters is hSN (t ) = hQBD (t ) ⊗ hsrrc (t )
(2)
where the hssrc(t) is the typical time domain impulse response of SRRC filter as described in [6]. The hQDB(t) is the impulse response of QDB filter HQDB described in Eq. (1). When the roll-off factor is zero, the SRRC filter has the same pulse and frequency response with RC. Therefore, the Nyquist filtering can also be implemented with RC filter here.
Fig. 2. The impulse response of (a) the Nyquist filter, (d) the super-Nyquist filter; The eye diagrams of the (b) Nyquist QPSK signal and (e) the super-Nyquist 9-QAM signal; The FFT spectrum of the generated (c) Nyquist QPSK signal and (f) super-Nyquist 9-QAM signal.
Figure 2(a) and 2(d) show the time domain impulse response of the regular Nyquist filter based on SRRC and the super-Nyquist filter based on cascading QDB and SRRC filters, respectively. Here, the roll-off factor of SRRC is set at 0. Less oscillations and faster convergence can be observed for the super-Nyquist digital filter compared with Nyquist filter. Figure 2(b) and 2(e) show eye diagrams of the generated Nyquist QPSK 2-level baseband signal and the super-Nyquist 9-QAM 3-level baseband signal, respectively. The electrical power spectra of Nyquist QPSK and super-Nyquist 9-QAM signals are shown in Fig. 2(c) and 2(f), respectively. We can see that the power spectrum of super-Nyquist signal is significantly suppressed compared with Nyquist signal, and the spectral side lobes are also greatly suppressed. The 3-dB bandwidth is less than 0.5 baud rate. Insets (i) and (ii) show the #199761 - $15.00 USD Received 21 Oct 2013; revised 27 Nov 2013; accepted 30 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031212 | OPTICS EXPRESS 31214
constellations of Nyquist QPSK and super-Nyquist 9-QAM signal, respectively. In this way, a super-Nyquist signal is generated by the Nyquist filtering of the QDB 9-QAM signal. 3. Experiment setup for 4 × 512Gb/s quad-carrier super-Nyquist channels and results analysis Figure 3 shows the experimental setup for 4 × 512Gb/s quad carrier super-Nyquist signal generation, transmission and coherent detection. In each 512-Gb/s, 100-GHz-wide superNyquist channel, four sub-channels with carrier spacing of 25GHz are used, each carrying 128-Gb/s data. In this way, 16 tunable external cavity lasers (ECLs) ECL1 to ECL16 are used as 16 sub-channels in our system with the linewidth less than 100 kHz, the output power of 14.5 dBm and carrier-spacing of 25-GHz from 1548.20 to 1551.20 nm. The odd and even channels are implemented with two sets of polarization-maintaining optical couplers (PMOCs) before the independent in-phase and quadrature (I/Q) modulations. The super-Nyquist signals are generated by a 64GSa/s DAC, in which the I and Q data is generated by operations described in Fig. 1. The data after QPSK mapping is up-sampled by 2 times, then passes through the super-Nyquist filter as described in Eq. (2). The roll-off factor of SRRC filter is 0. The 3-level baseband signal is of 32GBaud, with a word length of 215. The I/Q modulator is biased at the null point. The odd and even channels are independent modulated by the four port outputs of the DAC. The polarization multiplexing of the signal is realized by the polarization multiplexer, which comprises a PM-OC to halve the signal, an optical delay line to provide a delay of 150 symbols, and a polarization beam combiner (PBC) to recombine the signal. Different from the regular optical spectrum shaping schemes in [10–15], in our experiment, the even and odd channels are combined by a 2x1 optical coupler without using a WSS or interleaver. ECL 1 … ECL 15 ECL 2 … ECL 16
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Fig. 3. Experimental setup of 4 × 512Gb/s quad-carrier super-Nyquist channels signal generation, transmission and receiving. (MOD: modulator; OC: optical coupler; P-MUX: polarization multiplexing; WSS: wavelength-selective switch; TOF: tunable optical filter).
Figure 4 shows the optical spectrum of generated single channel (SC) 32-GBaud PDMQPSK, Nyquist PDM-QPSK, PDM-9QAM (with QDB filter only) and proposed superNyquist PDM-9-QAM signal. We can see that, PDM-QPSK signal without any operation occupies the largest bandwidth. The bandwidth of Nyquist PDM-QPSK is equal to the baud rate, which is 32-GHz and exceeds the 25-GHz carrier spacing. The 9-QAM signal with only QDB filer has two side-lobes. Our proposed super-Nyquist PDM-9-QAM signal shows the narrowest bandwidth, compared with other three types of signals. The 3-dB bandwidth is less than 0.5 baud rate. The generated 4 × 512-Gb/s, 100-GHz grid channel signals, as shown as in Fig. 4(b), are then launched into a re-circulating transmission loop, which consists of 5 spans of 85-km conventional SMF-28 with average loss of 18.5 dB and chromatic dispersion (CD) of 17 ps/km/nm, loop switches (SWs), optical coupler (OC), and Erbium-doped fiber
#199761 - $15.00 USD Received 21 Oct 2013; revised 27 Nov 2013; accepted 30 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031212 | OPTICS EXPRESS 31215
amplifier (EDFA)-only amplification without optical dispersion compensation. The noise figure of EDFA is about 5.5-dB. One WSS is placed in the loop, which is programmed to work as an optical band-pass filter to suppress the ASE noise. At the receiver, one tunable optical filter (TOF) with 3-dB bandwidth of 0.33 nm is employed to choose the measured channel. The linewidth of LO at the receiver is around 100 kHz. The analog-to-digital conversion (ADC) is realized in the digital oscilloscope with the sample rate of 80 GSa/s and 30-GHz bandwidth. The data is first resampled to 64-GSa/s with CD compensation, and then processed by the proposed multi-modulus equalization (MMEQ) algorithms with MLSE as described in Fig. 3 [12, 13]. A modified power-law digital clock recovery algorithm designed for Nyquist signal are used here [16].The QDB 9-QAM signal is directly recovery by using the MMEQ scheme. The frequency offset estimation and carrier phase recovery are also based on the 9-QAM like constellation. After phase recovery, the 9-QAM signal is converted back to the QPSK signal by the multi-symbol equalization and detection algorithm MLSE. The FEC encoding is applied to the original QPSK signal. 0
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Fig. 4. (a)The optical spectrum of single channel 32-Gbaud PDM-QPSK, Nyquist PDM-QPSK, PDM-9QAM and super-Nyquist PDM-9-QAM signal; (b) The back to back optical spectrum of 4 channels quad-carrier super-Nyquist PDM-9QAM with 16 sub-channel. 0.1
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Fig. 5. (a) The BTB BER performance of 32-GBaud Nyquist-QPSK, super-Nyquist 9-QAM and 32GBaud 8QAM signals versus OSNR in single channel case and WDM case; (b) The BER of QC-Ch. 2 versus the transmission distance.
Figure 5(a) shows the back-to-back (BTB) bit-error-ratio (BER) performance of 32GBaud Nyquist PDM-QPSK and super-Nyquist PDM-9 QAM signals versus the optical signal to noise ratio (OSNR) in single channel case and 25-GHz grid WDM case. In single channel case, the Nyquist signal shows the best BER performance. Due to the narrow digital QDB filtering, the super-Nyquist 9-QAM signal shows about 1.5-dB OSNR penalty at BER of 1x10−3 compared with SC Nyquist QPSK. However, for 25-GHz grid WDM case, the 32GBaud Nyquist signal cannot be recovered due to large crosstalk from adjacent channels. There is an error floor at 4 × 10−2 for Nyquist QPSK signal even under large OSNR condition. Only about 1.5-dB OSNR penalty is observed for the 32GBaud super-Nyquist signal in the 25-GHz grid WDM case compared with SC case. Insets (i) and (ii) show the constellations of Nyquist QPSK and super-Nyquist 9-QAM signal in WDM case with OSNR at 21-dB, #199761 - $15.00 USD Received 21 Oct 2013; revised 27 Nov 2013; accepted 30 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031212 | OPTICS EXPRESS 31216
respectively. We also measure the BER of SC 32GBaud PDM-8QAM using the same experiment setup. It shows about 2.5-dB OSNR penalty compared with the QDB 9-QAM signal. Thus, 9-QAM signal has better BER performance compared with 8-QAM, we believe there are two reasons. First, the minimum Euclidean distance of the QDB 9-QAM signal is larger than that of the 8-QAM signal; second, a multi-symbol equalization and decision based on MLSE is used for 9-QAM signal. Figure 5(b) shows the measured BER of the 512-Gb/s quad channel 2 versus transmission distance ranging from 425 km to 3400 km. The measured BER of QC-Ch. 2 after 2975-km transmission is 1.8 × 10−2. The optical spectrum of WDM signal after 2975-km transmission is inset in Fig. 5(b). It is worth noting that, the 3-dB analog bandwidth of the DAC used in our experiment is only 11.3GHz. The performance of generated Nyquist and super-Nyquist signals is affected by the bandwidth limitation effect [17, 18]. Further investigations on this bandwidth limitation impairment for super-Nyquist WDM will be carried out in future. The BER performance of sub-Ch. 7 after 2975-km transmission versus the fiber input power per channel is shown in Fig. 6(a). It shows that optimal input power per sub-channel is −1-dBm. The measured BER of all 16 WDM sub-channels of 32GBaud 128-Gb/s and the averaged BER of all quad-carrier 512-Gb/s channels after 2975-km transmission are presented in Fig. 6(b). After 2975-km transmission, the BER for all super-Nyquist-WDM channels are below the 2.4x10−2 BER threshold for 20% soft-decision FEC [1]. The constellations of the received signal in X and Y polarization of Ch. 9 after transmission processed by the proposed MMEQ are shown in Fig. 6(b) as insets. It shows that our proposed super-Nyquist signal has good performances for long-haul 400G transmission. 1
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Fig. 6. (a) The BER of sub-ch. 7 v.s. the input power per sub-channel; (b)The BER of all subchannels and averaged BER of all QC channels after 2975-km transmission.
4. Conclusions We propose and experimentally demonstrate a novel digital super-Nyquist signal generation scheme to further suppress the bandwidth of the Nyquist signal. The channel crosstalk is reduced due to the narrower bandwidth. Only about 1.5-dB OSNR penalty is observed for the 32GBaud super-Nyquist signal in the 25-GHz grid WDM case compared with SC case. Finally, 4 channels quad-carrier 512-Gb/s super-Nyquist 9-QAM-like signal within 100-GHz grid are successfully transmitted over 2975-km at a net SE of 4b/s/Hz. Acknowledgments This work was partially supported by the NNSF of China (No. 61250018, No. 61177071), NHTRDP (863 Program) of China (2011AA010302, 2012AA011302, 2012AA011303 and 2013AA010501), The National Key Technology R&D Program (2012BAH18B00), Key Program of Shanghai Science and Technology Association (12dz1143000)), and the China Scholarship Council (201206100076).
#199761 - $15.00 USD Received 21 Oct 2013; revised 27 Nov 2013; accepted 30 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031212 | OPTICS EXPRESS 31217
Department of Communication Science and Engineering, and Key Laboratory for Information Science of Electromagnetic Waves (MoE), Fudan University, 220 Handan Road, Shanghai 200433, China 2 ZTE (TX) Inc, Morristown, NJ 07960, USA 3 The School of ECE, Georgia Institute of Technology, Atlanta, GA30332, USA * [email protected]
Abstract: A novel digital super-Nyquist signal generation scheme is proposed to further suppress the Nyquist signal bandwidth and reduce the channel crosstalk without using optical pre-filtering. The spectrum of the generated super-Nyquist 9-QAM signal is much more compact compared with regular Nyquist QPSK signal. Therefore, only optical couplers are needed for super-Nyquist WDM channel multiplexing. By using the 64GSa/s high speed DAC, 32-GBaud super-Nyquist 9-QAM signal is generated within 25-GHz grid for quad-carrier 400G channels. We successfully generate and transmit 4 channels quad-carrier 512-Gb/s superNyquist 9-QAM-like signal within 100-GHz grid over 2975-km at a net SE of 4b/s/Hz (after excluding the 20% soft-decision FEC overhead). ©2013 Optical Society of America OCIS codes: (060.1660) Coherent communications; (060.4080) Modulation.
References and links 1.
X. Zhou, L. E. Nelson, P. Magill, R. Isaac, B. Zhu, D. W. Peckham, P. I. Borel, and K. Carlson, “High spectral efficiency 400 Gb/s transmission using PDM time-domain hybrid 32–64 QAM and training-assisted carrier recovery,” J. Lightwave Technol. 31(7), 999–1005 (2013). 2. H. Zhang, J. Cai, H. G. Batshon, M. Mazurczyk, O. Sinkin, D. Foursa, A. Pilipetskii, G. Mohs, and N. Bergano, “200 Gb/s and dual wavelength 400 Gb/s transmission over transpacific distance at 6 b/s/Hz dpectral efficiency,” in Proc. OFC 2013, paper PDP5A.6. 3. T. J. Xia, G. Wellbrock, A. Tanaka, M. Huang, E. Ip, D. Qian, Y. Huang, S. Zhang, Y. Zhang, P. Ji, Y. Aono, S. Murakami, and T. Tajima, “High capacity field trials of 40.5 Tb/s for LH distance of 1,822 km and 54.2 Tb/s for regional distance of 634 km,” in Proc. OFC 2013, paper PDP5A.4. 4. G. Bosco, V. Curri, A. Carena, P. Poggiolini, and F. Forghieri, “On the performance of Nyquist-WDM terabit superchannels based on PM-BPSK, PM-QPSK, PM-8QAM or PM-16QAM subcarriers,” J. Lightwave Technol. 29(1), 53–61 (2011). 5. J. Wang, C. Xie, and Z. Pan, “Generation of spectrally efficient Nyquist-WDM QPSK signals using DSP techniques at transmitter,” in Proc. OFC 2012, OM3H.5. 6. O. Bertran-Pardo, J. Renaudier, P. Tran, H. Mardoyan, P. Brindel, A. Ghazisaeidi, M. T. Salsi, G. Charlet, and S. Bigo, “Submarine transmissions with spectral efficiency higher than 3 b/s/Hz using Nyquist pulse-shaped channels,” in Proc. OFC 2013, OTu2B.1. 7. Q. Juan, B. Mao, N. Gonzalez, N. Binh, and N. Stojanovic, “Generation of 28GBaud and 32GBaud PDMNyquist-QPSK by a DAC with 11.3GHz analog bandwidth,” in Proc. OFC 2013, OTh1F.1. 8. J. Wang, C. Xie, and Z. Pan, “Generation of spectrally efficient Nyquist-WDM QPSK signals using digital FIR or FDE filters at transmitters,” J. Lightwave Technol. 30(23), 3679–3686 (2012). 9. J.-X. Cai, “100G Transmission Over Transoceanic Distance With High Spectral Efficiency and Large Capacity,” J. Lightwave Technol. 30(24), 3845–3856 (2012). 10. J. Yu, J. Zhang, Z. Dong, Z. Jia, H.-C. Chien, Y. Cai, X. Xiao, and X. Li, “Transmission of 8 × 480-Gb/s superNyquist-filtering 9-QAM-like signal at 100 GHz-grid over 5000-km SMF-28 and twenty-five 100 GHz-grid ROADMs,” Opt. Express 21(13), 15686–15691 (2013). 11. H.-C. Chien, J. Yu, Z. Jia, Z. Dong, and X. Xiao, “512-Gb/s quad-carrier PM-QPSK transmission over 2400-km SMF-28 subject to narrowing 100-GHz optical bandwidth,” in Proc. ECOC 2012, paper Th.2.C.4. 12. J. Zhang, J. Yu, N. Chi, Z. Dong, J. Yu, X. Li, L. Tao, and Y. Shao, “Multi-modulus blind equalizations for coherent quadrature duobinary spectrum shaped PM-QPSK digital signal processing,” J. Lightwave Technol. 31(7), 1073–1078 (2013).
#199761 - $15.00 USD Received 21 Oct 2013; revised 27 Nov 2013; accepted 30 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031212 | OPTICS EXPRESS 31212
13. J. Zhang, B. Huang, and X. Li, “Improved quadrature duobinary system performance using multi-modulus equalization,” Photonic Technology Letters 25(16), 1630–1633 (2013). 14. J. Li, E. Tipsuwannakul, T. Eriksson, M. Karlsson, and P. A. Andrekson, “Approaching Nyquist limit in WDM systems by low-complexity receiver-side duobinary shaping,” J. Lightwave Technol. 30(11), 1664–1676 (2012). 15. Z. Jia, J. Yu, H. Chien, Z. Dong, and D. Huo, “Field transmission of 100 G and beyond: multiple baud rates and mixed line rates using Nyquist-WDM technology,” J. Lightwave Technol. 30(24), 3793–3804 (2012). 16. M. Yan, Z. Tao, L. Dou, L. Li, Y. Zhao, T. Hoshida, and J. C. Rasmussen, “Digital clock recovery algorithm for Nyquist signal,” in Proc. OFC 2013, paper OTu2I.7. 17. Y. Lu, Y. Fang, B. Wu, K. Wang, W. Wan, F. Yu, L. Li, X. Shi, and Q. Xiong, “Experimental comparison of 32Gbaud electrical-OFDM and Nyquist-WDM transmission with 64GSa/s DAC”, in Proc. ECOC 2013, paper We.1.C.3. 18. J. Qi, B. Mao, N. Gonzalez, L. N. Binh, and N. Stojanovic, “Generation of 28GBaud and 32GBaud PDMNyquist-QPSK by a DAC with 11.3GHz analog bandwidth,” in Proc. OFC 2013, paper OTh1F.1.
1. Introduction With the further maturation of high speed digital-to-analog converter (DAC), Nyquist wavelength-division-multiplexing (N-WDM) super-channels with much higher spectrum efficiency (SE) based digital signal processing (DSP) at both transmitter and receiver sides has attracted a great deal of interest for the transmission of 100G beyond [1–9]. Generally, the 400G transmission can be realized by increasing the symbol rate, increasing the number of bonded channels, increasing the modulation levels, or their combinations [1–3, 10, 11]. Bonding a number of established PDM-QPSK channels in the way of N-WDM is considered a practical 400G solution with high spectrum efficiency (SE) and lower bandwidth requirement [10, 11]. Using high speed DAC, Nyquist pulse is digitally generated to achieve the Nyquist limit of SE, which enables the channel spacing approaching equal to the symbol rate with negligible crosstalk and inter-symbol-interference (ISI) [1–8]. However, when considering the forward error correction (FEC) overhead, the transmission of 100G channels on existing optical line systems based on a 25-GHz ITU grid presents a difficult challenge due to the limited bandwidth available for each channel [9]. The excess bandwidth causes severe crosstalk. For 128-Gb/s Nyquist PDM-QPSK, the required channel spacing should be larger than 32GHz, which exceeds the ITU 25-GHz grid limit. Therefore, additional processing is needed for higher SE to counter the inter-channel-interference (ICI) impairment in order to maintain the reasonable long-haul transmission distance with FEC overhead. Using algorithms based on fixed or adaptive digital filtering with multi-symbol detection to equalize the both ISI and ICI impairments, one can transmit a super-Nyquist ((channel occupancy much less than signal baud rate) signal, of which the channel spacing can be smaller than the symbol rate without much penalty [9–15]. In our previous works [10–13], super-Nyquist WDM can be achieved by using the wavelength selective switch (WSS) for optical pre-filtering. However, the discrete and expensive WSS is not easy to integrate into the conventional optical transponder especially for multi-channels system, moreover, the stability of center window of filtering might significantly deteriorate the system performance. Furthermore, the WDM channels can be easily multiplexed by using optical couplers. In this paper, we propose and experimentally demonstrate a novel digital super-Nyquist signal generation scheme to further suppress the Nyquist signal bandwidth and reduce the channel crosstalk without using optical pre-filtering. Only about 1.5-dB OSNR penalty is observed for the 32GBaud super-Nyquist signal in the 25-GHz grid WDM case compared with single carrier case. Finally, 4 channels quad-carrier 512-Gb/s super-Nyquist 9-QAM-like signal within 100-GHz grid are successfully transmitted over 2975-km at a net SE of 4b/s/Hz (after excluding the 20% soft-decision FEC overhead). 2. The principle of digital super-Nyquist 9-QAM (QDB) signal generation Figure 1 shows the principle of the proposed novel DAC-based super-Nyquist 9-QAM signal generation compared with regular Nyquist QPSK signal. For regular Nyquist filtering, a raised cosine (RC) or square root raised cosine (SRRC) filter with roll-off factor of 0 is used for Nyquist pulse generation. However, as shown in Fig. 1, when the channel spacing is less than the baud rate, the excess bandwidth causes severe crosstalk. In order to realize the super-
#199761 - $15.00 USD Received 21 Oct 2013; revised 27 Nov 2013; accepted 30 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031212 | OPTICS EXPRESS 31213
Nyquist transmission, additional low pass filter (LPF) is added for super-Nyquist pulse generation. In this way, the signal spectrum is further suppressed to reduce the channel crosstalk.
Fig. 1. The principle of DAC-based Nyquist and super-Nyquist 9-QAM signal generation and the crosstalk impairments in SN-WDM.
In our case, the low pass filter can be simply realized by the quadrature duobinary (QDB) delay and add filter, of which the transfer function in z-transform is given by H QDB ( z ) = 1 + z −1
(1)
It can be simply implemented by a two-tap finite impulse response (FIR) digital filter with good performance and turn the QPSK to the 9-QAM signal [10–15]. The super-Nyquist digital filter in the time domain by cascading the QDB and SRRC filters is hSN (t ) = hQBD (t ) ⊗ hsrrc (t )
(2)
where the hssrc(t) is the typical time domain impulse response of SRRC filter as described in [6]. The hQDB(t) is the impulse response of QDB filter HQDB described in Eq. (1). When the roll-off factor is zero, the SRRC filter has the same pulse and frequency response with RC. Therefore, the Nyquist filtering can also be implemented with RC filter here.
Fig. 2. The impulse response of (a) the Nyquist filter, (d) the super-Nyquist filter; The eye diagrams of the (b) Nyquist QPSK signal and (e) the super-Nyquist 9-QAM signal; The FFT spectrum of the generated (c) Nyquist QPSK signal and (f) super-Nyquist 9-QAM signal.
Figure 2(a) and 2(d) show the time domain impulse response of the regular Nyquist filter based on SRRC and the super-Nyquist filter based on cascading QDB and SRRC filters, respectively. Here, the roll-off factor of SRRC is set at 0. Less oscillations and faster convergence can be observed for the super-Nyquist digital filter compared with Nyquist filter. Figure 2(b) and 2(e) show eye diagrams of the generated Nyquist QPSK 2-level baseband signal and the super-Nyquist 9-QAM 3-level baseband signal, respectively. The electrical power spectra of Nyquist QPSK and super-Nyquist 9-QAM signals are shown in Fig. 2(c) and 2(f), respectively. We can see that the power spectrum of super-Nyquist signal is significantly suppressed compared with Nyquist signal, and the spectral side lobes are also greatly suppressed. The 3-dB bandwidth is less than 0.5 baud rate. Insets (i) and (ii) show the #199761 - $15.00 USD Received 21 Oct 2013; revised 27 Nov 2013; accepted 30 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031212 | OPTICS EXPRESS 31214
constellations of Nyquist QPSK and super-Nyquist 9-QAM signal, respectively. In this way, a super-Nyquist signal is generated by the Nyquist filtering of the QDB 9-QAM signal. 3. Experiment setup for 4 × 512Gb/s quad-carrier super-Nyquist channels and results analysis Figure 3 shows the experimental setup for 4 × 512Gb/s quad carrier super-Nyquist signal generation, transmission and coherent detection. In each 512-Gb/s, 100-GHz-wide superNyquist channel, four sub-channels with carrier spacing of 25GHz are used, each carrying 128-Gb/s data. In this way, 16 tunable external cavity lasers (ECLs) ECL1 to ECL16 are used as 16 sub-channels in our system with the linewidth less than 100 kHz, the output power of 14.5 dBm and carrier-spacing of 25-GHz from 1548.20 to 1551.20 nm. The odd and even channels are implemented with two sets of polarization-maintaining optical couplers (PMOCs) before the independent in-phase and quadrature (I/Q) modulations. The super-Nyquist signals are generated by a 64GSa/s DAC, in which the I and Q data is generated by operations described in Fig. 1. The data after QPSK mapping is up-sampled by 2 times, then passes through the super-Nyquist filter as described in Eq. (2). The roll-off factor of SRRC filter is 0. The 3-level baseband signal is of 32GBaud, with a word length of 215. The I/Q modulator is biased at the null point. The odd and even channels are independent modulated by the four port outputs of the DAC. The polarization multiplexing of the signal is realized by the polarization multiplexer, which comprises a PM-OC to halve the signal, an optical delay line to provide a delay of 150 symbols, and a polarization beam combiner (PBC) to recombine the signal. Different from the regular optical spectrum shaping schemes in [10–15], in our experiment, the even and odd channels are combined by a 2x1 optical coupler without using a WSS or interleaver. ECL 1 … ECL 15 ECL 2 … ECL 16
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Fig. 3. Experimental setup of 4 × 512Gb/s quad-carrier super-Nyquist channels signal generation, transmission and receiving. (MOD: modulator; OC: optical coupler; P-MUX: polarization multiplexing; WSS: wavelength-selective switch; TOF: tunable optical filter).
Figure 4 shows the optical spectrum of generated single channel (SC) 32-GBaud PDMQPSK, Nyquist PDM-QPSK, PDM-9QAM (with QDB filter only) and proposed superNyquist PDM-9-QAM signal. We can see that, PDM-QPSK signal without any operation occupies the largest bandwidth. The bandwidth of Nyquist PDM-QPSK is equal to the baud rate, which is 32-GHz and exceeds the 25-GHz carrier spacing. The 9-QAM signal with only QDB filer has two side-lobes. Our proposed super-Nyquist PDM-9-QAM signal shows the narrowest bandwidth, compared with other three types of signals. The 3-dB bandwidth is less than 0.5 baud rate. The generated 4 × 512-Gb/s, 100-GHz grid channel signals, as shown as in Fig. 4(b), are then launched into a re-circulating transmission loop, which consists of 5 spans of 85-km conventional SMF-28 with average loss of 18.5 dB and chromatic dispersion (CD) of 17 ps/km/nm, loop switches (SWs), optical coupler (OC), and Erbium-doped fiber
#199761 - $15.00 USD Received 21 Oct 2013; revised 27 Nov 2013; accepted 30 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031212 | OPTICS EXPRESS 31215
amplifier (EDFA)-only amplification without optical dispersion compensation. The noise figure of EDFA is about 5.5-dB. One WSS is placed in the loop, which is programmed to work as an optical band-pass filter to suppress the ASE noise. At the receiver, one tunable optical filter (TOF) with 3-dB bandwidth of 0.33 nm is employed to choose the measured channel. The linewidth of LO at the receiver is around 100 kHz. The analog-to-digital conversion (ADC) is realized in the digital oscilloscope with the sample rate of 80 GSa/s and 30-GHz bandwidth. The data is first resampled to 64-GSa/s with CD compensation, and then processed by the proposed multi-modulus equalization (MMEQ) algorithms with MLSE as described in Fig. 3 [12, 13]. A modified power-law digital clock recovery algorithm designed for Nyquist signal are used here [16].The QDB 9-QAM signal is directly recovery by using the MMEQ scheme. The frequency offset estimation and carrier phase recovery are also based on the 9-QAM like constellation. After phase recovery, the 9-QAM signal is converted back to the QPSK signal by the multi-symbol equalization and detection algorithm MLSE. The FEC encoding is applied to the original QPSK signal. 0
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Fig. 4. (a)The optical spectrum of single channel 32-Gbaud PDM-QPSK, Nyquist PDM-QPSK, PDM-9QAM and super-Nyquist PDM-9-QAM signal; (b) The back to back optical spectrum of 4 channels quad-carrier super-Nyquist PDM-9QAM with 16 sub-channel. 0.1
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Fig. 5. (a) The BTB BER performance of 32-GBaud Nyquist-QPSK, super-Nyquist 9-QAM and 32GBaud 8QAM signals versus OSNR in single channel case and WDM case; (b) The BER of QC-Ch. 2 versus the transmission distance.
Figure 5(a) shows the back-to-back (BTB) bit-error-ratio (BER) performance of 32GBaud Nyquist PDM-QPSK and super-Nyquist PDM-9 QAM signals versus the optical signal to noise ratio (OSNR) in single channel case and 25-GHz grid WDM case. In single channel case, the Nyquist signal shows the best BER performance. Due to the narrow digital QDB filtering, the super-Nyquist 9-QAM signal shows about 1.5-dB OSNR penalty at BER of 1x10−3 compared with SC Nyquist QPSK. However, for 25-GHz grid WDM case, the 32GBaud Nyquist signal cannot be recovered due to large crosstalk from adjacent channels. There is an error floor at 4 × 10−2 for Nyquist QPSK signal even under large OSNR condition. Only about 1.5-dB OSNR penalty is observed for the 32GBaud super-Nyquist signal in the 25-GHz grid WDM case compared with SC case. Insets (i) and (ii) show the constellations of Nyquist QPSK and super-Nyquist 9-QAM signal in WDM case with OSNR at 21-dB, #199761 - $15.00 USD Received 21 Oct 2013; revised 27 Nov 2013; accepted 30 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031212 | OPTICS EXPRESS 31216
respectively. We also measure the BER of SC 32GBaud PDM-8QAM using the same experiment setup. It shows about 2.5-dB OSNR penalty compared with the QDB 9-QAM signal. Thus, 9-QAM signal has better BER performance compared with 8-QAM, we believe there are two reasons. First, the minimum Euclidean distance of the QDB 9-QAM signal is larger than that of the 8-QAM signal; second, a multi-symbol equalization and decision based on MLSE is used for 9-QAM signal. Figure 5(b) shows the measured BER of the 512-Gb/s quad channel 2 versus transmission distance ranging from 425 km to 3400 km. The measured BER of QC-Ch. 2 after 2975-km transmission is 1.8 × 10−2. The optical spectrum of WDM signal after 2975-km transmission is inset in Fig. 5(b). It is worth noting that, the 3-dB analog bandwidth of the DAC used in our experiment is only 11.3GHz. The performance of generated Nyquist and super-Nyquist signals is affected by the bandwidth limitation effect [17, 18]. Further investigations on this bandwidth limitation impairment for super-Nyquist WDM will be carried out in future. The BER performance of sub-Ch. 7 after 2975-km transmission versus the fiber input power per channel is shown in Fig. 6(a). It shows that optimal input power per sub-channel is −1-dBm. The measured BER of all 16 WDM sub-channels of 32GBaud 128-Gb/s and the averaged BER of all quad-carrier 512-Gb/s channels after 2975-km transmission are presented in Fig. 6(b). After 2975-km transmission, the BER for all super-Nyquist-WDM channels are below the 2.4x10−2 BER threshold for 20% soft-decision FEC [1]. The constellations of the received signal in X and Y polarization of Ch. 9 after transmission processed by the proposed MMEQ are shown in Fig. 6(b) as insets. It shows that our proposed super-Nyquist signal has good performances for long-haul 400G transmission. 1
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Fig. 6. (a) The BER of sub-ch. 7 v.s. the input power per sub-channel; (b)The BER of all subchannels and averaged BER of all QC channels after 2975-km transmission.
4. Conclusions We propose and experimentally demonstrate a novel digital super-Nyquist signal generation scheme to further suppress the bandwidth of the Nyquist signal. The channel crosstalk is reduced due to the narrower bandwidth. Only about 1.5-dB OSNR penalty is observed for the 32GBaud super-Nyquist signal in the 25-GHz grid WDM case compared with SC case. Finally, 4 channels quad-carrier 512-Gb/s super-Nyquist 9-QAM-like signal within 100-GHz grid are successfully transmitted over 2975-km at a net SE of 4b/s/Hz. Acknowledgments This work was partially supported by the NNSF of China (No. 61250018, No. 61177071), NHTRDP (863 Program) of China (2011AA010302, 2012AA011302, 2012AA011303 and 2013AA010501), The National Key Technology R&D Program (2012BAH18B00), Key Program of Shanghai Science and Technology Association (12dz1143000)), and the China Scholarship Council (201206100076).
#199761 - $15.00 USD Received 21 Oct 2013; revised 27 Nov 2013; accepted 30 Nov 2013; published 11 Dec 2013 (C) 2013 OSA 16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.031212 | OPTICS EXPRESS 31217
