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A field trial of 100-Gbit/s Ethernet over an optical transport network (OTN) is conducted using a real-time digital coherent signal processor. Error free operation with the Q-margin of 3.2 dB is confirmed at a 100 Gbit/s Ethernet analyzer by concatenating a low-density parity-check code with a OTN framer forward error correction, after 80-ch WDM transmission through 6 spans x 70 km of dispersion shifted fiber without inline-dispersion compensation. Also, the recovery time of 12 msec is observed in an optical route switching experiment, which is achieved through fast chromatic dispersion estimation functionality.
©2011 Optical Society of America
1. Introduction
The amount of Internet protocol (IP) based data traffic is continuously increasing driven by broadband application services such as IP-based video streaming and rapid growth in the volume of information treated at data centers. Evolution of cellular phone technology such as long term evolution (LTE) requires higher capacity networks among base stations. In order to support the increase in the volume of data traffic, enhancement of the spectral efficiency in the backbone network is inevitable and the bit rate per channel in optical transport network (OTN) systems will need to be upgraded from 10/40 Gbit/s to 100 Gbit/s. Polarization-division multiplexing (PDM) quadrature phase shift keying (QPSK) is a promising modulation format for 100 Gbit/s/ch systems with the channel spacing of 50 GHz [1,2]. Coherent detection enabled through digital signal processing (DSP) can enhance the level of receiver sensitivity. However, additional measures are required to overcome degradation in the receiver sensitivity due to the increase in the bit rate compared to 10/40 Gbit/s/ch, and to satisfy backward compatibility in long-haul transmission system. For this purpose, enhancement of the FEC coding gain is a promising concept. In addition, survivability for optical channel failures and provisioning flexibility become essential features considering an increase in channel capacity such as 100 Gbit/s/ch, and fast recovery of optical channels in less than 50 msec is required [8].
In this paper, we report the successful proof of concept of a real-time 80-ch wavelength division multiplexing (WDM) transmission of a 100-Gbit/s client signal over 420 km of field-installed fiber with the channel spacing of 50 GHz. After the transmission through 6 spans x 70.4 km of dispersion shifted fiber without inline-dispersion compensation, error free operation is confirmed with the Q-margin of 3.2 dB. Furthermore, the recovery time of 12 msec is observed in optical route switching, which is achieved by fast chromatic dispersion estimation functionality.
2. Concept of second generation DSP
Figure 1 shows a block diagram of the second generation DSP. The encoder block receives optical channel transport unit 4 (OTU4) client data from the OTN framer, inserts forward error correction (FEC) parity bits for a low-density parity-check (LDPC) code with 13% redundancy and a 0.7% dispersion estimator overhead. The encoded data are mapped into a parallel distribution format for optical channel transport lane 4.4 (OTL4.4) [3] for transmission over 4 lanes, sets of in-phase and quadrature-phase lanes for X- and Y-polarizations. On the decoder side, the 4 lanes of the input analog signals are digitized by a 4-channel analog-to-digital converter (ADC) at the over-sampling rate of 2. The frequency domain equalizer (FDE) in the fixed equalizer block compensates for chromatic dispersion [4]. The cumulated chromatic dispersion of the received signal is estimated within 5 msec, by detecting arrival time delay calculated from frequency spectrum of the dispersion estimator overhead. The estimation time does not depend on dispersion amount. An adaptive equalizer fulfills the roles of polarization de-multiplexing, polarization mode dispersion (PMD) compensation, frequency offset compensation, carrier phase recovery, and sampling clock recovery [5]. Lane-alignment and de-skew among the 4 lanes, sets of in-phase and quadrature-phase lanes for X- and Y-polarizations, are performed using frame alignment signal (FAS) and multiframe alignment signal (MFAS) patterns that appear in each lane. Then, the soft-decision (SD) LDPC decoder dramatically reduces the number of bit errors even in a high bit error rate (BER) range such as 10−2, and the remaining errors can be completely corrected by the framer FEC. The concatenation of SD-LDPC and framer FEC achieves the net-coding gain (NCG) of 10.8 dB at the BER of 10−15, which reveals the input Q-limit of 6.4 dB [6]. Iteration count of LDPC decode is 16, and its total latency is less than 10 usec. Frame synchronization of OTL4.4 is established within 50 msec, even in the condition that transmission route of optical channel is switched. Specifications of the employed 100-Gbps digital coherent signal processor large scale integration (LSI) are shown in Table 1 .
Table 1. Specifications of 100-Gbps digital coherent signal processor LSI
3. Field fiber transmission demonstration with optical route switching
We tested the transmission performance of the 100 Gbit/s digital coherent signal processor LSI in a field-installed fiber environment. The evaluated configuration is shown in Fig. 2 . The transmitted 100 Gbit/s client data from the Ethernet analyzer are fed into the OTN framer. The 100 Gbit/s digital coherent signal processor LSI receives the OTU4 frame data, and its encoder outputs 127.156-Gbps OTU4v data in serializer/deserializer framer interface (SFI) format of SFI-S. The data are multiplexed into 31.789 Gbps x 4 lanes by a 20:4 multiplexer, and finally, are converted into a PDM-QPSK optical signal through optical modulators. In order to evaluate performance of the LSI in WDM situation, 79 dummy channels of 127.156-Gbps PDM-QPSK signals are also generated. In-phase and quadrature phase lanes of even and odd channels of the dummy signal are individually modulated using the pseudo random binary sequence (PRBS) pattern with a 215-1 sequence length with different delays. Even and odd channels are multiplexed, and then polarization multiplexed with a delay of a few tens of symbols to obtain 79-channel PDM-QPSK dummy signal. The test channel is multiplexed with the dummy 79-channel signal through a wavelength selector switch (WSS) to obtain 80 channel WDM signal. The wavelength of the real-time channel under test is 1586.623 nm, which is the center of the WDM signal. Then, the 80 channel WDM signal is launched into the dispersion shifted fiber of a terrestrial network. The average fiber launch power is set at −2 dBm/ch. The field transmission line consists of 6 spans with a 70.4-km repeater section length without distributed Raman amplifiers. The average loss of the 6 spans is 23.2 dB and the accumulated chromatic dispersion is 1,097 ps/nm at 1586.623 nm. On the receiver side, the test channel is WDM-demultiplexed through a WSS, and it is mixed with a local oscillator and converted into a 4 lane electrical signal by an optical front-end based on the planer lightwave circuit technology [7]. The 4 lanes of analog signals are input into the digital coherent signal processor LSI. The output is fed into the OTN framer and bit errors that remain after SD-LDPC are corrected by generic FEC (GFEC) function of the OTN framer. Finally the BER is monitored by the 100 G Ethernet analyzer.
Fig. 2 Experimental configuration for 80-ch WDM 100-Gbps field fiber transmission. Picture shows used optical front-end and digital coherent signal processor LSI.
Figure 3 shows the measured BER results after applying the framer FEC + LDPC code and the BER after applying LDPC code only as a function of the input BER before LDPC decoding, each with a simulation reference. The inset to Fig. 3 shows the measured BER before LDPC versus the received optical signal-to-noise ratio (OSNR). In both cases of LDPC + GFEC and LDPC only, the measured results agree well with the simulation references. As a consequence, an NCG of 10.8 and 10.4 dB are obtained by concatenation with Enhanced-FEC (EFEC) and GFEC, respectively. For the OSNR tolerance, degradation of 0.3 dB is observed compared to that for the offline measurement results, which are measured using a Tektronix DPO 72004B oscilloscope, at OSNR of 14dB. The degradation increases with OSNR increase, and difference in bandwidth limitation in frequency response of optical front-end and ADC are estimated to be dominant reasons. At the OSNR of 13.7 dB, the BER before LDPC reaches the Q-limit of 6.4 dB, which yields error free operation after the concatenation of LDPC and EFEC.
Fig. 3 Measured results of output BER as a function of input BER for LDPC-only case and LDPC + EFEC/GFEC concatenation case, with simulation references. Inset shows measured results of Q factor before LDPC as a function of received OSNR.
Next, we measured the Q factor after transmitting an 80 ch WDM signal over 6 spans x 70.4 km fiber. The Q factor before LDPC for the test channel of 1586.623 nm is measured using the real-time digital coherent signal processor LSI, other channels are measured using the offline configuration. The results are plotted in Fig. 4 . The received optical power spectrum is also shown. Compensation for the chromatic dispersion is achieved using the dispersion estimator included in the LSI. The received Q-factor before LDPC is 9.6 dB, which is 3.2 dB higher than the Q-limit of 6.4 dB. Received OSNR was approximately 19 dB, and we estimate transmission penalty is approximately 1.5 dB, referring the OSNR tolerance curve. This indicates that DSP functionalities of chromatic dispersion compensation, PDM demultiplexing, frequency offset compensation, carrier phase recovery, and sampling clock recovery work properly. The measured Q-factor after the LDPC decode block is 16.1 dB, which yields error free operation after GFEC.
Fig. 4 Measured Q factor before/after FEC after 80-ch WDM transmission through 6 spans x 70.4 km of dispersion shifted fiber, and received optical power spectrum. Q factor for test channel of 1586.623 nm was measured using the real-time digital coherent signal processor, and others were measured using offline configuration.
Finally, we evaluate the recovery time, when the optical route is switched. The received signal is switched from the second span to the 6-th span using an optical switch, as shown in Fig. 2. The cumulated amount of chromatic dispersion changes from 350 ps/nm to 1,097 ps/nm after concavity in the optical input level, as shown in Fig. 5 . The sampling clock recovery block detects the channel disconnection, and informs the sequence controller of the LSI by an alarm pulse. The difference in the cumulated dispersion is estimated, and the sequence controller renews the compensated dispersion value in the FDE block according to the estimated value. The OTL4.4 lane alignment alarm disappears after the recovery of the optical input signal with the time delay of 12 msec, as shown in lower part of the Fig. 5. This includes re-convergence of adaptive filters, frame synchronization, and frame alignment among the OTL4.4 multilanes in addition to the chromatic dispersion estimation and FDE block renewal. In this experiment, total outage time was approximately 15 msec, including the period of no input signal light of 3 msec.
Fig. 5 Time waveforms of optical input level received by optical front-end, alarm pulse from sampling clock recovery block, and OTL lane alignment alarm generated from the digital coherent signal processor LSI. Cumulated chromatic dispersion of received signal changes from 350 ps/nm to 1,097 ps/nm after concavity in the optical input level.
4. Conclusion
Real-time field fiber transport of 100-Gbit/s Ethernet client data was shown using a digital coherent signal processor LSI, which enables coherent detection of 127.156 Gbps PDM-QPSK and soft-decision LDPC encoding/decoding. The LSI achieves a tolerance level similar to that for OSNR tolerance with offline measurement, NCG of 10.8 dB, and the Q-limit of 6.4 dB by concatenating SD-LDPC with the framer FEC. After 80-ch WDM transmission through 6 spans x 70 km of dispersion shifted fiber without inline-dispersion compensation, error free operation was confirmed at the 100 Gbit/s Ethernet analyzer with the Q-margin of 3.2 dB. Furthermore, the recovery time of 12 msec was achieved in optical route switching, using the fast chromatic dispersion estimation functionality. As a result, 100 Gbit/s x 80 ch WDM transmission technology was confirmed to be feasible.
Acknowledgement
This work is partly supported by the R&D project on “High-speed Optical Edge Node Technologies” by the Ministry of Internal Affairs and Communications (MIC) of Japan.
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