W2A.63.pdf
OFC 2018 © OSA 2018
Noise Prediction and Cancellation Algorithm for the Bandwidth Limited PAM-4 System in the Presence of IntraChannel Homodyne Crosstalk Tianjian Zuo1, Tianyu Song2, Sen Zhang1, Lei Liu1, Weiqiang, Cheng3, and Xiaofei Xu1 (1) Network Technology Laboratory, Huawei Technologies Co., Ltd., Shenzhen, 518129, China (2) Transmission Technology Research Department, Huawei Technologies Co., Ltd., Shenzhen, 518129, China (3) China Mobile Research Institute, Beijing 100053, China
[email protected]
Abstract: As a low-complexity solution, this paper proposes the use of noise prediction and cancellation (NPC) algorithm in a PAM-4 system, which achieves similar performance as the h o r ( s ) MLSE based PAM-4 receiver and increases tolerance towards intra-channel homodyne crosstalk. ©2018TheAut OCIS codes: (060.2330) Fiber optics communications; (060.4080) Modulation 1. Introduction In order to cope with future demands of bandwidth, mobile backhaul and datacenter links operating at rates of 400 Gb/s per module is required. Current and upcoming standards for electrical links, such as CEI-100G-VSR and 100GBASE-CR, are based on 100 Gb/s PAM-4 transmission [1]. This motives the optical module to operate at a lane rate of 100Gb/s per wavelength, which can relax the 400 Gb/s form-factor pluggable (e.g., OSFP and QSFP-DD) requirements on power consumption and footprint. However, the bandwidth of the transmitter optical subassembly (TOSA), receiver optical subassembly (ROSA) and electrical amplifiers are the bottleneck to achieve the 100Gb/s optical transmission. Therefore, digital signal processing (DSP) capable of relaxing bandwidth requirements is garnering increasing interests from academia as well as industry. Recently, several DSP schemes for the 100Gb/s per lane system have been reported including maximum likelihood sequence estimation (MLSE) based multi-level pulse amplitude modulation (PAM) [2,3], discrete multi-tone modulation (DMT) [4,5] and discrete Fourier transform spread (DFT-S) [6]. However, all mentioned approaches require either MLSE or fast Fourier transform (FFT) algorithm to achieve the claimed bitrates, and thus more complex DSP is required in the system. Besides, the intra-channel homodyne crosstalk caused by multiple reflections of the fiber connectors is another limitation for the high speed optical system. This impairment can be reduced by frequent fiber cleaning or employing better fiber connectors, e.g. APC or UPC. However, it will increase both OPEX and CAPX of the system. This paper reports on a low-complexity and crosstalk-tolerant solution for the possible upcoming 100 Gb/s per lane system targeting 10 km reach applications. An experimental demonstration of a single lane with optical transmission over 10 km standard single mode fiber (SSMF) at a 1310 nm wavelength has been carried out considering the influence of the intra-channel homodyne crosstalk. A maximum bitrate of 140 Gb/s using the PAM4 and a novel noise prediction and cancellation (NPC) algorithm is successfully generated, transmitted, and detected. The bit error rate is below the 7% forward error correction (FEC) limit, corresponding to a net bitrate of 130.8 Gb/s error free transmission. 2. Principle of the noise prediction and cancellation module NPC
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Fig. 1. Principle of operation of the noise prediction and cancellation module. Feed-Forward Equalizer (FFE), Finite Impulse Response (FIR), Impulse response function (IRF), Noise prediction and cancellation (NPC), timing recovery (TR).
W2A.63.pdf
OFC 2018 © OSA 2018
The feed-forward equalizer (FFE) enhances the high frequency noise after the equalization for the intensity modulation-direct detection (IMDD) system [7], because the dominate noise arising from the receiver is approximately white and filtered by the FFE with a frequency response of high-pass. A few taps of the noise whitening filter with low pass response can be used to eliminate the high frequency noise [2]. However, this requires MLSE to further reduce the ISI caused by the noise whitening filter. In order to relax the requirement of MLSE, the noise whitening can be achieved by the noise prediction and cancellation (NPC) module as shown in the Fig. 1. After FFE, the noise is obtained by the subtraction of the FFE output signal and the decision output. Few taps of the inverted impulse response of the non-white noise is calculated using the taps from FFE or other methods [8] and then used to predict the spread noise for next few symbols. After the subtraction of the predicted noise, the spectrum of the noise becomes white and SNR is also improved. 3. Experimental set-up PAM demapper Decision NPC FFE
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Fig. 2. Experimental set-up. Digital to analog converter (DAC), Transmitter Optical Subassembly (TOSA), receiver Optical Subassembly (ROSA), polarization controller (PC), variable optical attenuator (VOA), single mode fiber (SMF),
Fig. 2 shows the experimental setup. A stream of pseudo-random bit sequence (PRBS) with a length of 215 − 1 bits, is mapped into a PAM-4 constellation using gray coding. After the up-sampling, the PAM-4 symbol is converted to analog waveform using a DAC with a sampling rate of 84GSa/s. Then, an EML TOSA is biased by a DC source using a bias-T and driven by the electrical amplified PAM-4 signal generated by DAC. The TOSA operates at Oband with an emitting wavelength of 1305 nm and a launch power of 1.5 dBm. After 10-km SSMF transmission, the optical link is split into two paths by a 3-dB splitter. The lower path is to emulate the intra-channel homodyne crosstalk and decorrelated with an extra 50m SMF. The signal-to-crosstalk ratio is adjusted using the VOA1. The polarization controller (PC) is to ensure polarization alignment of the signal and crosstalk, i.e. the worst-case condition. The upper and lower path is then coupled by a 3-dB coupler, followed by the VOA2 which can adjust the received optical power for the performance evaluation. At the receiver side, the PAM-4 signal is directly detected by a PIN photodiode with 25-GHz -3-dB bandwidth. Then, the received signal is sampled and stored by 80-GSa/s real-time sampling oscilloscope for offline processing. After digitalization and timing recovery, the received signal is then equalized using FFE with least-mean-square (LMS) algorithm and 21 T-spaced taps. A NPC module with 5 T-spaced taps performs the noise whitening and reduction function, followed by a decision module and a de-mapper. Finally, the received sequences are regenerated and the errors are countered. A 7% forward error correction (FEC) encoding with a BER threshold of 4e-3 is assumed, corresponding to a net bitrate of 130.8 Gb/s error free transmission. 3. Experimental Results
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Fig. 3. (a) eye-diagram of FFE output, (b) eye-diagram of NPC output, (c) power spectrum of the noise for the output of the FFE, (d) power spectrum of the noise for the output of the NPC.
W2A.63.pdf
OFC 2018 © OSA 2018
Fig.3 depicts the measured eye-diagrams of the signal and the power spectrum of the noise for the output of the FFE (Fig. 2(a, c)) and the output of the NPC module (Fig. 2(b d)). The clear eye-opening improvement is observed for the NPC output in contrast to the FFE output, i.e. the SNR is improved after the NPC module. Furthermore, the obvious flat noise spectrum is also achieved by the NPC module (Fig. 2(c, d)).
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Fig. 4. (a) BER curves without intra-channel homodyne crosstalk, (b) BER curves for 35-dB signal-to-crosstalk ratio, (c) Power penalties measured at the 1e-3 BER threshold of FEC.
Fig. 4 provides a comparative analysis of how NDC structure performs with respect to conventional FFE structure and FFE combined with noise whitening filter and MLSE (16-states) structure regarding received optical power and signal-to-crosstalk ratio. For the case without intra-channel homodyne crosstalk (Fig.4 (a)), NDC shows similar performance compared with MLSE for 56-Gb/s and 112-Gb/s transmission but 2.8 dB power penalty is observed for 140-Gb/s transmission at the 4e-3 BER threshold. For the case for 35-dB signal-to-crosstalk ratio (Fig.4 (b)), NDC outperforms MLSE for 56-Gb/s transmission and has a similar performance for 112-Gb/s transmission. Fig. 4(c) shows that the NDC can significantly reduce the crosstalk penalty in contrast to MLSE. However, for the bandwidth with critical limitations, e.g. 140-Gb/s transmission using 25G-TOSA, the MLSE is still effective and can be utilized to deal with the residual ISI after FFE. Besides, the conventional FFE is only suitable for the low bite rate scenario, because the FFE induced noise amplification significantly decreases the system performance. 4. Conclusions A low-complexity approach named NPC has been proposed as performance-enhanced solution for the 100 Gb/s per lane optical module with obvious intra-channel homodyne crosstalk tolerance. The successful experimental demonstration realizes a 10-km optical transmission with a maximum bitrate of 140 Gb/s using only a single 25GTOSA. A similar performance and better crosstalk tolerance has been shown for the NPC in contrast to the MLSE based receiver side DSP for the 56Gb/s and 112Gb/s transmission. 5. References [1] T. Palkert, et al., “SERDES for 100Gbps,” IEEE 802.3 NEA Ad Hoc, May (2017). [2] K. Zhong, et al.,” Experimental demonstration of 500Gbit/s short reach transmission employing PAM4 signal and direct detection with 25Gbps device,” Proc. OFC, Th3a.3, Los Angeles (2015). [3] Q. Zhang, et al.,” Single-Lane 180 Gb/s SSB-Duobinary-PAM-4 Signal Transmission over 13 km SSMF,” Proc. OFC, Tu2D.2, Los Angeles (2017). [4] L. Zhang, et al.,"Beyond 100-Gb/s transmission over 80-km SMF using direct-detection SSB-DMT at C-band." J. Lightw. Technol., vol. 34, no. 2, pp. 723-729, 2016. [5] J. Zhou, et al, “Transmission of 100-Gb/s DSB-DMT over 80-km SMF Using 10-G class TTA and Direct-Detection,” Proc. ECOC, Tu3.F, Düsseldorf (2016). [6] W. A. Ling et al., “Single-Channel 50G and 100G Discrete Multitone Transmission With 25G VCSEL Technology,” J. Lightwave Technol., Vol. 33, no. 4, p.761 (2015). [7] T. Zuo, et al., “Single Lane 150-Gb/s, 100-Gb/s and 70-Gb/s 4-PAM Transmission over 100-m, 300-m and 500-m MMF Using 850nm VCSEL” Proc. ECOC, Th1.C, Düsseldorf (2016). [8] K. Steven, Modern Spectral Estimation: Theory and Application (Prentice Hall, 1988).
