- Email: [email protected]
Demonstration of C-band 112G PAM-4 transmission over 80-km stand-single mode fiber based on direct-detection
Optics Communications 424 (2018) 32–36
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Demonstration of C-band 112G PAM-4 transmission over 80-km stand-single mode fiber based on direct-detection Yiran Wei a , Jianjun Yu b , Jianyang Shi b , Junwen Zhang c , Jing He a , Lin Chen a, * a
College of Computer Science and Electronic Engineering, Hunan University, Changsha, Hunan, China Shanghai Institute for Advanced Communication and Data Science, Key Laboratory for Information Science of Electromagnetic Waves (MoE), Fudan University, Shanghai 200433, China c ZTE (TX) Inc., Morristown, NJ 07960, USA b
a r t i c l e
i n f o
Keywords: Optical fiber communication Intensity modulation and direct-detection Training based pre-equalization Nonlinear look-up-table based pre-distortion Pulse amplitude modulation-4
a b s t r a c t In this paper, we experimentally demonstrate the intensity modulation and direct-detection (IM/DD) 112-Gb/s single carrier pulse amplitude modulation (PAM)-4 transmission system over 80-km standard signal-mode fiber (SSMF) without any optical chromatic dispersion (CD) compensation at the 3.8×10−3 BER threshold for 7% the hard-decision forward error correction (HD-FEC). The training based pre-equalization (pre-EQ) and the nonlinear look-up-table (LUT) based pre-distortion (pre-DT) in the transmitter (Tx)-side are employed to reduce the computation complexity and deal with the modulation bandwidth and the nonlinearity impairments of electro-optical modulators and detectors. On the other hand, the system without optical CD compensation modules is obviously more simple and flexible. Therefore, the nonlinear LUT pre-DT simultaneously used for CD compensation will further simplify the system. The BER is about 20% without any digital signal processing (DSP) such as equalization, pre-distortion, pre-equalization or compensation. After signal recovery, the BERs of BTB and 80-km SSMF system can achieve 3.07×10−5 and 1.50×10−3 respectively. The principle of the LUT based pre-DT and the pre-EQ are introduced in the paper.
1. Introduction In order to satisfy the rapidly increasing bandwidth demand for widespread multimedia services, cloud services and gigabit Ethernet, 400G and even more data rate transmission based on the cost-efficient transceivers has become a focused issue in short-reach links, access network, data-center-interconnections (DCI) and so on [1–4]. Directdetection (DD) is an excellent solution for these system owing to the low system cost and power consumption. In the inter-DCI applications, 80 km transmission distance is the basic requirement [1,3,4]. Recently, several 100 Gbit/s per lane transmissions have been proposed and experimentally demonstrated, concluding pulse amplitude modulation (PAM), multi-band carrier-less amplitude phase (multi-CAP) modulation and discrete multi-tone (DMT) [2,5,6]. One of the most promising approach to support 400G data connection is 4-lane×100 Gbit/s/λ net rate based on PAM [7], considering the transceiver complexity, power consumption and cost. However, most systems are facing the same two problem, the modulation bandwidth and the nonlinearity impairments of electro-optical modulators and detectors [8]. Many *
schemes have been proposed to solve these problem, such as the decision feedback equalizations [3], the nonlinear Volterra equalizations [2] and so on. But most of them are implemented in the receiver (Rx)-side and require high computation complexity. In this case, the training-based pre-equalization (pre-EQ) and the nonlinear look-up-table (LUT) predistortion (pre-DT) [9] in the transmitter (Tx)-side are proposed to deal with the issues effectively. On the other hand, the system without optical chromatic dispersion (CD) compensation modules is obviously more simple and flexible. Thus, the nonlinear LUT pre-DT simultaneously used for CD compensation would simplify the system. In this paper, the intensity modulation and direct-detection (IM/DD) 112 Gb/s double-side band (DSB) single carrier PAM-4 transmission system over 80-km standard signal-mode fiber (SSMF) has been demonstrated without any optical DC compensation. The combination of the linear adaptive time-domain pre-EQ and the nonlinear LUT-based preDT are employed. The pre-EQ filter tap number is further tested in the experiment. Thanks to the Tx-side DSP, performance is improved significantly.
Corresponding author. E-mail address: [email protected] (L. Chen).
https://doi.org/10.1016/j.optcom.2018.02.032 Received 2 September 2017; Received in revised form 11 February 2018; Accepted 12 February 2018 0030-4018/© 2018 Published by Elsevier B.V.
Y. Wei et al.
Optics Communications 424 (2018) 32–36
Fig. 1. Block diagram of (a) the LUT-based pre-DT and (b) the pre-EQ.
2. Principle
emission (ASE) noise suppression is applied as a pre-amplifier before the 50-GHz photodetector (PD). The signal is sampled by a digital real-time oscilloscope (OSC) with 80-GSa/s sampling rate and 36-GHz electrical bandwidth. Firstly, the training sequence is sent to estimate the LUT and the pre-EQ filter as the pre-processing training. Before the transmission test, the 5-symbols LUT-based pre-DT is carried out to mitigate the channel nonlinearity impairment. In order to obtain the reasonable signal quality for effective LUT creation, we have initially estimated the channel response by the training sequence to perform process. The pre-EQ filter is also generated based on the transfer function of the Rx-side adaptive CMMA equalizer. For the offline digital signal process (DSP) transmission test at the Txside, the gray-coded data is first mapped to PAM-4 symbols, followed by the nonlinear LUT-based pre-distortion and pre-equalization algorithm. Then the signal is resampled from 56-GSa/s to 81.92-GSa/s and output by the DAC. At the Rx-side, the signal sampled by the OSC is first resampled from 80-GSa/s to 56-GSa/s, and the clock recovery is carried out. Then the signal is processed by an 80-taps CMMA adaptive equalizer. After that, PAM-4 decision and bit-error-rate (BER) calculation are employed successively for the transmission test. Fig. 2(a) is the spectrum of the signal before transmission. Fig. 2(b)∼(d) are the spectra of received signal after 80-km SSMF without LUT-based pre-DT and pre-EQ, with pre-DT, and with pre-DT and pre-EQ respectively
2.1. LUT-based pre-DT The nonlinear distortion in the system are the limitation of the amplifiers and optical modulator, and the fiber nonlinearity. This can exhibit pattern-dependent distortion or non-linear inter symbol interference with memory. The LUT-based pre-DT has been considered as an effective method to mitigate these issues [10,11]. Fig. 1(a) shows the process of the LUT-based pre-DT in the communication system. X is the PAM-4 signal at the TX-side and Y is the received signal at RX-side. At first, the LUT amplitude correction are all zero. For an M-length PAM-4 pattern ∶ 𝑘 + 𝑀−1 ), there are 4𝑀 different permutations, namely 𝑋(𝑘 − 𝑀−1 2 2 𝑀 4 LUTs. A training sequence containing all these patterns is generated and transmitted in the system. At the RX-side, the transmitted training sequence is compared with the corresponding received sequence Y(k) and the distortion is defined as 𝑒 (𝑘) = 𝑌 (𝑘) − 𝑋(𝑘). Then the average distortion LUT_e(k) is written to the LUT. The LUT is completely finished when all the patterns are trained in the channel. In practice, before transmission, an M -length sliding window is implemented for the signal to identify the 𝑀-length pattern to locate the address of LUT and precorrect the signal as 𝑋 ′ (𝑘) = 𝑋 (𝑘) − 𝐿𝑈 𝑇 _𝑒(𝑘). As the sliding window moves forward, the LUT-based pre-DT is employed. 2.2. Pre-equalization
4. Experimental results and discussions
The pre-EQ finite impulse response (FIR) is generated based on the transfer function of the RX-side adaptive equalizer, as shown in Fig. 1(b). After the transmission of the training sequence, the pre-filter H(n) for the pre-EQ is calculated from the modified cascaded multi-modulus algorithm (CMMA) [12] normalized filter when the CMMA equalizer achieves the steady state. The training sequence in our experiment is simultaneously used for the LUT based pre-DT. After then, at the TXside, the signal X(n) is pre-equalized as 𝑋 ′ (𝑛) = 𝑋 (𝑛) ∗𝐻(𝑛) before transmission.
We have first generate the 5-symbols LUT and the pre-EQ FIR. The finite impulse response (FIR) filter for the pre-EQ for 80 km SSMF is generated from the CMMA equalizer as shown in Fig. 3(a). The PAM-4 symbols with 5-symbols LUT based pre-DT are shown in Fig. 3(b). Larger distortion correction can be obtained by using the pre-DT. We also test the BER performances versus the adaptive filter tap number as shown in Fig. 3(c) The performance is improved with the increase of the filter tap number. With pre-EQ, both BER of BTB and 80 km SSMF get drop around 80 taps. It is because that pre-EQ compensates part of the distortion of the system. However, no more obvious improvement can be achieved when the tap number exceeds 80. Pre-EQ is mainly used to mitigate the frequency domain linear distortion. Since the main distortion caused by chromatic dispersion is nonlinear, pre-EQ does not work well. The frequency response of the system after 80-km SSMF transmission with and without pre-EQ is shown in Fig. 4(a) and (c) respectively. Fig. 4(b) is the inverse channel response from receiver-side CMMA equalizer. The channel response becomes relatively flat after pre-EQ. The measured back to back (BTB) BER performances versus received optical power are shown in Fig. 5(a). The LUT based pre-DT mitigate about 4-dBm received optical power penalty with pre-EQ from −2.5dBm to 1.5-dBm. Inter symbol interference (ISI) in the system is mainly resulted from CD. Blind equalization techniques are devised to overcome
3. Experimental setup Fig. 2 shows the experimental setup for the IM/DD 112 Gb/s single carrier PAM-4 transmission system over 80 km SSMF without any optical CD-compensation. An external cavity laser (ECL) in C-band is used in our system for optical signal modulation. The 56-Gbaud PAM-4 signal is generated from an 81.92-GSa/s digital-to-analog converter (DAC) with a 3 dB analog bandwidth of 40-GHz to drive the single ended Mach– Zehnder modulator (MZM). The bias of MZM is set at 4.8 V to satisfy the linear range of MZM. A combination of electrical amplifier (EA) and 3 dB attenuator (ATT) is used to adjust the signal in the linear region of the modulator. The signal are launched into the fiber link after an erbium-doped fiber amplifier (EDFA) following the MZM. Another EDFA followed by a tunable-optical-filter (TOF) for amplified spontaneous 33
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Optics Communications 424 (2018) 32–36
Fig. 2. IM/DD 112-Gbit/s PAM-4 transmission over 80-km SSMF without any optical CD compensations. (a) is the spectrum of signal before transmission, and (b)–(d) are the spectra of received signal after 80-km SSMF without LUT-based pre-DT or pre-EQ, only with pre-DT, and with both pre-DT and pre-EQ respectively. We will clarify it in the manuscript.
Fig. 3. (a) The FIR for pre-EQ; (b) the PAM-4 symbols after 5-symbols LUT pre-DT; (c) The BER performances versus the adaptive filter tap number.
Fig. 4. (a) The frequency response of the system after 80-km SSMF transmission without pre-EQ; (b) The frequency response of the CMMA equalizer; (c) the frequency response of the system after 80-km SSMF transmission with pre-EQ.
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Optics Communications 424 (2018) 32–36
Fig. 5. (a) The BER performance versus received optical power for BTB. (b) and (c) are the optical spectra without pre-EQ and with pre-EQ respectively. (d) recovered PAM-4 without CMMA, without LUT pre-DT and without pre-EQ. (e) recovered PAM-4 with CMMA, without LUT pre-DT and without pre-EQ. (f) recovered PAM-4 with CMMA, with LUT pre-DT and without pre-EQ. (g) recovered PAM-4 with CMMA, with LUT pre-DT and with pre-EQ.
Fig. 6. (a) The BER performance versus received optical power for 80-km SSMF. (b) and (c) are the optical spectra without pre-EQ and with pre-EQ respectively. (d) recovered PAM-4 without CMMA, without LUT pre-DT and without pre-EQ. (e) recovered PAM-4 with CMMA, without LUT pre-DT and without pre-EQ. (f) recovered PAM-4 with CMMA, with LUT pre-DT and without pre-EQ. (g) recovered PAM-4 with CMMA, with LUT pre-DT and with pre-EQ.
the ISI problems. In the experiment, both LUT pre-DT and CMMA are used to overcome the ISI. By using the combination of LUT based preDT and pre-EQ, the performance can be achieved much lower than the 3.8×10-3 BER threshold valid for 7% the hard-decision forward error correction (HD-FEC). The BERs at 1.5-dBm received power are 1.11×10−2 , 1.30×10−2 , 5.74×10−5 and 3.07×10−5 from the highest to the lowest. Fig. 5(b) and (c) are the optical spectra without pre-EQ and with pre-EQ respectively. Fig. 5(d) is the recovered PAM-4 without CMMA, without LUT pre-DT and without pre-EQ. The BER of (d) is about 20%. Fig. 5(e) is the recovered PAM-4 with CMMA, without LUT pre-DT and without pre-EQ. Fig. 5(f) is the recovered PAM-4 with CMMA, with LUT pre-DT and without pre-EQ. Fig. 5(g) is the recovered PAM-4 with CMMA, LUT pre-DT and pre-EQ. The PAM-4 symbols gradually become easy to identify from (d) to (g).
Fig. 6(a) indicates the measured 80-km SSMF BER performances versus received optical power. The BER with CMMA, LUT based pre-DT and pre-EQ can achieve 3.8×10−3 . After the pre-DT and the pre-EQ, the BER performances are all lower than 3.8×10−3 BER threshold from −2.5dBm to 1.5-dBm received optical power. The BERs at 1.5 dBm received power are 1.36×10−2 , 1.29×10−2 , 1.90×10−3 and 1.50×10−3 from the highest to the lowest. Fig. 6(b) and (c) are the optical spectra without pre-EQ and with pre-EQ respectively. Fig. 6(d) is the recovered PAM-4 without CMMA, without LUT pre-DT and without pre-EQ. The BER of (d) is also around 20%. Fig. 6(e) is the recovered PAM-4 with CMMA, without LUT pre-DT and without pre-EQ. Fig. 6(f) is the recovered PAM4 with CMMA, with LUT pre-DT and without pre-EQ. Fig. 6(g) is the recovered PAM-4 with CMMA, LUT pre-DT and pre-EQ. The same as the BTB performance, the symbols is gradually clear to recognize from (d) to (g). 35
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Thus, we have experimentally demonstrated the 112 Gbits/s PAM-4 over 80-km SSMF without optical CD compensation at the FEC threshold of 3.8×10−3 . Hence, the net data rate after the 80-km SSMF transmission is more than 100 Gb/s assuming the use of 7% overhead FEC.
[2] Y. Gao, J.C. Cartledge, S.S. Yam, A. Rezania, Y. Matsui, 112 Gb/s PAM-4 using a directly modulated laser with linear pre-compensation and nonlinear postcompensation, in: European Conference on Optical Communication, 2016, pp. 121– 123, no. 1. [3] A. Dochhan, N. Eiselt, H. Griesser, M. Eiselt, J. José, V. Olmos, Solutions for 400 Gbit/s inter data center WDM transmission, in: European Conference on Optical Communication, 2016, pp. 680–682, no. 1. [4] N. Eiselt, J. Wei, H. Griesser, A. Dochhan, M. Eiselt, J. Elbers, First real-time 400G PAM-4 demonstration for inter-data center transmission over 100 km of SSMF at 1550 nm, in: Optical Fiber Communication Conference, 2016, p. W1K.5. [5] M.I. Olmedo, et al., Multiband carrierless amplitude phase modulation for high capacity optical data links, J. Lightwave Technol. 32 (4) (2014) 798–804. [6] Y. Wang, N. Chi, S.M.F. With, I.M. Dd, Demonstration of 4x128-Gb/s DFT-S OFDM signal transmission over 320-km SMF with IM/DD, IEEE Photon. J. 8 (2) (2016) 1–9. [7] P. Gou, L. Zhao, K. Wang, W. Zhou, J. Yu, Nonlinear look-up table predistortion and chromatic dispersion precompensation for IM/DD PAM-4 transmission precompensation for IM / DD, IEEE Photon. J. 9 (5) (2017). [8] J. Zhang, J. Yu, H.-C. Chien, EML-based IM/DD 400G (4x112.5-Gbit/s) PAM-4 over 80 km SSMFbased on linear pre-equalization and nonlinear LUT pre-distortion for inter-DCI applications, in: Optical Fiber Communication Conference, 2017, p. W4I.4. [9] Z. Jia, et al., Experimental demonstration of PDM-32QAM single-carrier 400G over 1200-km transmission enabled by training-assisted pre-equalization and look-up table, in: Optical Fiber Communication Conference, 2016, p. Tu3A.4. [10] J.H. Ke, Y. Gao, J.C. Cartledge, 400 Gbit/s single-carrier and 1 Tbit/s three- carrier superchannel signals using dual polarization 16-QAM with look-up table correction and optical pulse shaping, Opt. Express 22 (1) (2014) 71. [11] P.J. Winzer, High-spectral-efficiency optical modulation formats, J. Lightwave Technol. 30 (24) (2012) 3824–3835. [12] J. Ma, D. Wang, C. Guo, A modified cascaded multi-modulus algorithm for blind polarization de-multiplexing of high order QAM optical signals, in: Asia Communications and Photonics Conference, vol. 1, 2013, p. AW3F.5.
5. Conclusion In this paper, we experimentally demonstrated the IM/DD 112 Gb/s single carrier PAM-4 transmission system over 80-km SSMF without any optical DC compensation at the 3.8×10−3 BER threshold for 7% the HD-FEC. The combination of linear adaptive time-domain pre-EQ and nonlinear LUT-based pre-DT are employed. The BERs of BTB and 80-km SSMF systems can achieve 3.07×10−5 and 1.50×10−3 respectively. Acknowledgments This work was partially supported by the NNSF of China (61325002, 61250018, 61527801, 61720106015), NSFC project of China (61377079, 61775054), and Science and Technology Project of Hunan Province (2016GK2011). References [1] D. Sadot, G. Dorman, A. Gorshtein, E. Sonkin, O. Vidal, Single channel 112 Gbit/sec PAM4 at 56 Gbaud with digital signal processing for data centers applications, in: Optical Fiber Communications Conference and Exhibition, vol. 1, 2015, p. Th2A.67.
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Demonstration of C-band 112G PAM-4 transmission over 80-km stand-single mode fiber based on direct-detection Yiran Wei a , Jianjun Yu b , Jianyang Shi b , Junwen Zhang c , Jing He a , Lin Chen a, * a
College of Computer Science and Electronic Engineering, Hunan University, Changsha, Hunan, China Shanghai Institute for Advanced Communication and Data Science, Key Laboratory for Information Science of Electromagnetic Waves (MoE), Fudan University, Shanghai 200433, China c ZTE (TX) Inc., Morristown, NJ 07960, USA b
a r t i c l e
i n f o
Keywords: Optical fiber communication Intensity modulation and direct-detection Training based pre-equalization Nonlinear look-up-table based pre-distortion Pulse amplitude modulation-4
a b s t r a c t In this paper, we experimentally demonstrate the intensity modulation and direct-detection (IM/DD) 112-Gb/s single carrier pulse amplitude modulation (PAM)-4 transmission system over 80-km standard signal-mode fiber (SSMF) without any optical chromatic dispersion (CD) compensation at the 3.8×10−3 BER threshold for 7% the hard-decision forward error correction (HD-FEC). The training based pre-equalization (pre-EQ) and the nonlinear look-up-table (LUT) based pre-distortion (pre-DT) in the transmitter (Tx)-side are employed to reduce the computation complexity and deal with the modulation bandwidth and the nonlinearity impairments of electro-optical modulators and detectors. On the other hand, the system without optical CD compensation modules is obviously more simple and flexible. Therefore, the nonlinear LUT pre-DT simultaneously used for CD compensation will further simplify the system. The BER is about 20% without any digital signal processing (DSP) such as equalization, pre-distortion, pre-equalization or compensation. After signal recovery, the BERs of BTB and 80-km SSMF system can achieve 3.07×10−5 and 1.50×10−3 respectively. The principle of the LUT based pre-DT and the pre-EQ are introduced in the paper.
1. Introduction In order to satisfy the rapidly increasing bandwidth demand for widespread multimedia services, cloud services and gigabit Ethernet, 400G and even more data rate transmission based on the cost-efficient transceivers has become a focused issue in short-reach links, access network, data-center-interconnections (DCI) and so on [1–4]. Directdetection (DD) is an excellent solution for these system owing to the low system cost and power consumption. In the inter-DCI applications, 80 km transmission distance is the basic requirement [1,3,4]. Recently, several 100 Gbit/s per lane transmissions have been proposed and experimentally demonstrated, concluding pulse amplitude modulation (PAM), multi-band carrier-less amplitude phase (multi-CAP) modulation and discrete multi-tone (DMT) [2,5,6]. One of the most promising approach to support 400G data connection is 4-lane×100 Gbit/s/λ net rate based on PAM [7], considering the transceiver complexity, power consumption and cost. However, most systems are facing the same two problem, the modulation bandwidth and the nonlinearity impairments of electro-optical modulators and detectors [8]. Many *
schemes have been proposed to solve these problem, such as the decision feedback equalizations [3], the nonlinear Volterra equalizations [2] and so on. But most of them are implemented in the receiver (Rx)-side and require high computation complexity. In this case, the training-based pre-equalization (pre-EQ) and the nonlinear look-up-table (LUT) predistortion (pre-DT) [9] in the transmitter (Tx)-side are proposed to deal with the issues effectively. On the other hand, the system without optical chromatic dispersion (CD) compensation modules is obviously more simple and flexible. Thus, the nonlinear LUT pre-DT simultaneously used for CD compensation would simplify the system. In this paper, the intensity modulation and direct-detection (IM/DD) 112 Gb/s double-side band (DSB) single carrier PAM-4 transmission system over 80-km standard signal-mode fiber (SSMF) has been demonstrated without any optical DC compensation. The combination of the linear adaptive time-domain pre-EQ and the nonlinear LUT-based preDT are employed. The pre-EQ filter tap number is further tested in the experiment. Thanks to the Tx-side DSP, performance is improved significantly.
Corresponding author. E-mail address: [email protected] (L. Chen).
https://doi.org/10.1016/j.optcom.2018.02.032 Received 2 September 2017; Received in revised form 11 February 2018; Accepted 12 February 2018 0030-4018/© 2018 Published by Elsevier B.V.
Y. Wei et al.
Optics Communications 424 (2018) 32–36
Fig. 1. Block diagram of (a) the LUT-based pre-DT and (b) the pre-EQ.
2. Principle
emission (ASE) noise suppression is applied as a pre-amplifier before the 50-GHz photodetector (PD). The signal is sampled by a digital real-time oscilloscope (OSC) with 80-GSa/s sampling rate and 36-GHz electrical bandwidth. Firstly, the training sequence is sent to estimate the LUT and the pre-EQ filter as the pre-processing training. Before the transmission test, the 5-symbols LUT-based pre-DT is carried out to mitigate the channel nonlinearity impairment. In order to obtain the reasonable signal quality for effective LUT creation, we have initially estimated the channel response by the training sequence to perform process. The pre-EQ filter is also generated based on the transfer function of the Rx-side adaptive CMMA equalizer. For the offline digital signal process (DSP) transmission test at the Txside, the gray-coded data is first mapped to PAM-4 symbols, followed by the nonlinear LUT-based pre-distortion and pre-equalization algorithm. Then the signal is resampled from 56-GSa/s to 81.92-GSa/s and output by the DAC. At the Rx-side, the signal sampled by the OSC is first resampled from 80-GSa/s to 56-GSa/s, and the clock recovery is carried out. Then the signal is processed by an 80-taps CMMA adaptive equalizer. After that, PAM-4 decision and bit-error-rate (BER) calculation are employed successively for the transmission test. Fig. 2(a) is the spectrum of the signal before transmission. Fig. 2(b)∼(d) are the spectra of received signal after 80-km SSMF without LUT-based pre-DT and pre-EQ, with pre-DT, and with pre-DT and pre-EQ respectively
2.1. LUT-based pre-DT The nonlinear distortion in the system are the limitation of the amplifiers and optical modulator, and the fiber nonlinearity. This can exhibit pattern-dependent distortion or non-linear inter symbol interference with memory. The LUT-based pre-DT has been considered as an effective method to mitigate these issues [10,11]. Fig. 1(a) shows the process of the LUT-based pre-DT in the communication system. X is the PAM-4 signal at the TX-side and Y is the received signal at RX-side. At first, the LUT amplitude correction are all zero. For an M-length PAM-4 pattern ∶ 𝑘 + 𝑀−1 ), there are 4𝑀 different permutations, namely 𝑋(𝑘 − 𝑀−1 2 2 𝑀 4 LUTs. A training sequence containing all these patterns is generated and transmitted in the system. At the RX-side, the transmitted training sequence is compared with the corresponding received sequence Y(k) and the distortion is defined as 𝑒 (𝑘) = 𝑌 (𝑘) − 𝑋(𝑘). Then the average distortion LUT_e(k) is written to the LUT. The LUT is completely finished when all the patterns are trained in the channel. In practice, before transmission, an M -length sliding window is implemented for the signal to identify the 𝑀-length pattern to locate the address of LUT and precorrect the signal as 𝑋 ′ (𝑘) = 𝑋 (𝑘) − 𝐿𝑈 𝑇 _𝑒(𝑘). As the sliding window moves forward, the LUT-based pre-DT is employed. 2.2. Pre-equalization
4. Experimental results and discussions
The pre-EQ finite impulse response (FIR) is generated based on the transfer function of the RX-side adaptive equalizer, as shown in Fig. 1(b). After the transmission of the training sequence, the pre-filter H(n) for the pre-EQ is calculated from the modified cascaded multi-modulus algorithm (CMMA) [12] normalized filter when the CMMA equalizer achieves the steady state. The training sequence in our experiment is simultaneously used for the LUT based pre-DT. After then, at the TXside, the signal X(n) is pre-equalized as 𝑋 ′ (𝑛) = 𝑋 (𝑛) ∗𝐻(𝑛) before transmission.
We have first generate the 5-symbols LUT and the pre-EQ FIR. The finite impulse response (FIR) filter for the pre-EQ for 80 km SSMF is generated from the CMMA equalizer as shown in Fig. 3(a). The PAM-4 symbols with 5-symbols LUT based pre-DT are shown in Fig. 3(b). Larger distortion correction can be obtained by using the pre-DT. We also test the BER performances versus the adaptive filter tap number as shown in Fig. 3(c) The performance is improved with the increase of the filter tap number. With pre-EQ, both BER of BTB and 80 km SSMF get drop around 80 taps. It is because that pre-EQ compensates part of the distortion of the system. However, no more obvious improvement can be achieved when the tap number exceeds 80. Pre-EQ is mainly used to mitigate the frequency domain linear distortion. Since the main distortion caused by chromatic dispersion is nonlinear, pre-EQ does not work well. The frequency response of the system after 80-km SSMF transmission with and without pre-EQ is shown in Fig. 4(a) and (c) respectively. Fig. 4(b) is the inverse channel response from receiver-side CMMA equalizer. The channel response becomes relatively flat after pre-EQ. The measured back to back (BTB) BER performances versus received optical power are shown in Fig. 5(a). The LUT based pre-DT mitigate about 4-dBm received optical power penalty with pre-EQ from −2.5dBm to 1.5-dBm. Inter symbol interference (ISI) in the system is mainly resulted from CD. Blind equalization techniques are devised to overcome
3. Experimental setup Fig. 2 shows the experimental setup for the IM/DD 112 Gb/s single carrier PAM-4 transmission system over 80 km SSMF without any optical CD-compensation. An external cavity laser (ECL) in C-band is used in our system for optical signal modulation. The 56-Gbaud PAM-4 signal is generated from an 81.92-GSa/s digital-to-analog converter (DAC) with a 3 dB analog bandwidth of 40-GHz to drive the single ended Mach– Zehnder modulator (MZM). The bias of MZM is set at 4.8 V to satisfy the linear range of MZM. A combination of electrical amplifier (EA) and 3 dB attenuator (ATT) is used to adjust the signal in the linear region of the modulator. The signal are launched into the fiber link after an erbium-doped fiber amplifier (EDFA) following the MZM. Another EDFA followed by a tunable-optical-filter (TOF) for amplified spontaneous 33
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Fig. 2. IM/DD 112-Gbit/s PAM-4 transmission over 80-km SSMF without any optical CD compensations. (a) is the spectrum of signal before transmission, and (b)–(d) are the spectra of received signal after 80-km SSMF without LUT-based pre-DT or pre-EQ, only with pre-DT, and with both pre-DT and pre-EQ respectively. We will clarify it in the manuscript.
Fig. 3. (a) The FIR for pre-EQ; (b) the PAM-4 symbols after 5-symbols LUT pre-DT; (c) The BER performances versus the adaptive filter tap number.
Fig. 4. (a) The frequency response of the system after 80-km SSMF transmission without pre-EQ; (b) The frequency response of the CMMA equalizer; (c) the frequency response of the system after 80-km SSMF transmission with pre-EQ.
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Fig. 5. (a) The BER performance versus received optical power for BTB. (b) and (c) are the optical spectra without pre-EQ and with pre-EQ respectively. (d) recovered PAM-4 without CMMA, without LUT pre-DT and without pre-EQ. (e) recovered PAM-4 with CMMA, without LUT pre-DT and without pre-EQ. (f) recovered PAM-4 with CMMA, with LUT pre-DT and without pre-EQ. (g) recovered PAM-4 with CMMA, with LUT pre-DT and with pre-EQ.
Fig. 6. (a) The BER performance versus received optical power for 80-km SSMF. (b) and (c) are the optical spectra without pre-EQ and with pre-EQ respectively. (d) recovered PAM-4 without CMMA, without LUT pre-DT and without pre-EQ. (e) recovered PAM-4 with CMMA, without LUT pre-DT and without pre-EQ. (f) recovered PAM-4 with CMMA, with LUT pre-DT and without pre-EQ. (g) recovered PAM-4 with CMMA, with LUT pre-DT and with pre-EQ.
the ISI problems. In the experiment, both LUT pre-DT and CMMA are used to overcome the ISI. By using the combination of LUT based preDT and pre-EQ, the performance can be achieved much lower than the 3.8×10-3 BER threshold valid for 7% the hard-decision forward error correction (HD-FEC). The BERs at 1.5-dBm received power are 1.11×10−2 , 1.30×10−2 , 5.74×10−5 and 3.07×10−5 from the highest to the lowest. Fig. 5(b) and (c) are the optical spectra without pre-EQ and with pre-EQ respectively. Fig. 5(d) is the recovered PAM-4 without CMMA, without LUT pre-DT and without pre-EQ. The BER of (d) is about 20%. Fig. 5(e) is the recovered PAM-4 with CMMA, without LUT pre-DT and without pre-EQ. Fig. 5(f) is the recovered PAM-4 with CMMA, with LUT pre-DT and without pre-EQ. Fig. 5(g) is the recovered PAM-4 with CMMA, LUT pre-DT and pre-EQ. The PAM-4 symbols gradually become easy to identify from (d) to (g).
Fig. 6(a) indicates the measured 80-km SSMF BER performances versus received optical power. The BER with CMMA, LUT based pre-DT and pre-EQ can achieve 3.8×10−3 . After the pre-DT and the pre-EQ, the BER performances are all lower than 3.8×10−3 BER threshold from −2.5dBm to 1.5-dBm received optical power. The BERs at 1.5 dBm received power are 1.36×10−2 , 1.29×10−2 , 1.90×10−3 and 1.50×10−3 from the highest to the lowest. Fig. 6(b) and (c) are the optical spectra without pre-EQ and with pre-EQ respectively. Fig. 6(d) is the recovered PAM-4 without CMMA, without LUT pre-DT and without pre-EQ. The BER of (d) is also around 20%. Fig. 6(e) is the recovered PAM-4 with CMMA, without LUT pre-DT and without pre-EQ. Fig. 6(f) is the recovered PAM4 with CMMA, with LUT pre-DT and without pre-EQ. Fig. 6(g) is the recovered PAM-4 with CMMA, LUT pre-DT and pre-EQ. The same as the BTB performance, the symbols is gradually clear to recognize from (d) to (g). 35
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Thus, we have experimentally demonstrated the 112 Gbits/s PAM-4 over 80-km SSMF without optical CD compensation at the FEC threshold of 3.8×10−3 . Hence, the net data rate after the 80-km SSMF transmission is more than 100 Gb/s assuming the use of 7% overhead FEC.
[2] Y. Gao, J.C. Cartledge, S.S. Yam, A. Rezania, Y. Matsui, 112 Gb/s PAM-4 using a directly modulated laser with linear pre-compensation and nonlinear postcompensation, in: European Conference on Optical Communication, 2016, pp. 121– 123, no. 1. [3] A. Dochhan, N. Eiselt, H. Griesser, M. Eiselt, J. José, V. Olmos, Solutions for 400 Gbit/s inter data center WDM transmission, in: European Conference on Optical Communication, 2016, pp. 680–682, no. 1. [4] N. Eiselt, J. Wei, H. Griesser, A. Dochhan, M. Eiselt, J. Elbers, First real-time 400G PAM-4 demonstration for inter-data center transmission over 100 km of SSMF at 1550 nm, in: Optical Fiber Communication Conference, 2016, p. W1K.5. [5] M.I. Olmedo, et al., Multiband carrierless amplitude phase modulation for high capacity optical data links, J. Lightwave Technol. 32 (4) (2014) 798–804. [6] Y. Wang, N. Chi, S.M.F. With, I.M. Dd, Demonstration of 4x128-Gb/s DFT-S OFDM signal transmission over 320-km SMF with IM/DD, IEEE Photon. J. 8 (2) (2016) 1–9. [7] P. Gou, L. Zhao, K. Wang, W. Zhou, J. Yu, Nonlinear look-up table predistortion and chromatic dispersion precompensation for IM/DD PAM-4 transmission precompensation for IM / DD, IEEE Photon. J. 9 (5) (2017). [8] J. Zhang, J. Yu, H.-C. Chien, EML-based IM/DD 400G (4x112.5-Gbit/s) PAM-4 over 80 km SSMFbased on linear pre-equalization and nonlinear LUT pre-distortion for inter-DCI applications, in: Optical Fiber Communication Conference, 2017, p. W4I.4. [9] Z. Jia, et al., Experimental demonstration of PDM-32QAM single-carrier 400G over 1200-km transmission enabled by training-assisted pre-equalization and look-up table, in: Optical Fiber Communication Conference, 2016, p. Tu3A.4. [10] J.H. Ke, Y. Gao, J.C. Cartledge, 400 Gbit/s single-carrier and 1 Tbit/s three- carrier superchannel signals using dual polarization 16-QAM with look-up table correction and optical pulse shaping, Opt. Express 22 (1) (2014) 71. [11] P.J. Winzer, High-spectral-efficiency optical modulation formats, J. Lightwave Technol. 30 (24) (2012) 3824–3835. [12] J. Ma, D. Wang, C. Guo, A modified cascaded multi-modulus algorithm for blind polarization de-multiplexing of high order QAM optical signals, in: Asia Communications and Photonics Conference, vol. 1, 2013, p. AW3F.5.
5. Conclusion In this paper, we experimentally demonstrated the IM/DD 112 Gb/s single carrier PAM-4 transmission system over 80-km SSMF without any optical DC compensation at the 3.8×10−3 BER threshold for 7% the HD-FEC. The combination of linear adaptive time-domain pre-EQ and nonlinear LUT-based pre-DT are employed. The BERs of BTB and 80-km SSMF systems can achieve 3.07×10−5 and 1.50×10−3 respectively. Acknowledgments This work was partially supported by the NNSF of China (61325002, 61250018, 61527801, 61720106015), NSFC project of China (61377079, 61775054), and Science and Technology Project of Hunan Province (2016GK2011). References [1] D. Sadot, G. Dorman, A. Gorshtein, E. Sonkin, O. Vidal, Single channel 112 Gbit/sec PAM4 at 56 Gbaud with digital signal processing for data centers applications, in: Optical Fiber Communications Conference and Exhibition, vol. 1, 2015, p. Th2A.67.
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