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We demonstrate the first real-time adaptive optical coherent QAM transmission with variable multiplicities (4-, 16- and 64-QAM) using an FPGA-based transmitter and receiver. Rate-variable transmission (20~60 Gbit/s) was successfully achieved with a polarization multiplexing scheme at 5 Gsymbol/s over 320 km, where the OSNR margins were increased by 9 and 17 dB, respectively, by changing the modulation level from 64 to 16 and 4.
© 2014 Optical Society of America
1. Introduction
Multilevel coherent optical transmission has been receiving a lot of attention with respect to expanding the transmission capacity in a finite optical amplification bandwidth [1]. Higher-order quadrature amplitude modulation (QAM) and orthogonal frequency-division multiplexing (OFDM) have been adopted to achieve a spectral efficiency exceeding 10 bit/s/Hz [2–8]. However, these QAM transmission experiments were based on off-line processing. The development of a real-time coherent optical receiver for multi-level QAM signals is an important issue if we are to realize highly spectral efficient optical transmission systems. Several groups have demonstrated real-time coherent optical receivers for a 16 QAM modulation format [9–11]. We have also demonstrated the first real-time FPGA-based coherent optical receiver for a 64 QAM transmission with an optical phase-locked loop (OPLL) technique [12].
While conventional systems are operated based on a fixed modulation format, an adaptive modulation system is expected to provide large flexibility under transmission-line conditions including resilient ICT (Information and Communication Technologies) applications and to make full use of spectral resources when the data transmission rate and spectral efficiency are optimized [13]. An FPGA-based real-time multiformat transmitter for optical transmission has been demonstrated [14]. An adaptive modulation system controlled by an OpenFlow-based control plane has also been reported [15]. In these systems, however, the receivers are operated in an off-line condition. No complete on-line transmission system has yet been demonstrated with a variable modulation format.
In this paper, we demonstrate the first real-time adaptive 4-64 QAM optical coherent transmission system with an OPLL technique and an FPGA-based transmitter and receiver. Polarization-multiplexed 5 Gsymbol/s, 4-64 QAM (20-60 Gbit/s) signals were transmitted over 320 km with an optical bandwidth of 6 GHz.
2. Configuration of FPGA-based real-time transmitter and receiver for adaptive 4-64 QAM coherent optical transmission
Figure 1(a) shows a block diagram of an FPGA-based real-time transmitter with a variable modulation format that is capable of generating 4, 16 and 64 QAM signals at a symbol rate of 5 Gsymbol/s. The transmitter consisted of four FPGAs (Stratix V, 3180 multipliers) under parallel processing and two digital-to-analog converters (DACs) operated at 10 GS/s with a 10-bit resolution. In the FPGA, a binary data stream from a 212-1 pseudorandom binary sequence (PRBS) generator was encoded by Reed-Solomon FEC codes of RS(255, 239) and then mapped onto an M-QAM constellation, in which phase predistortion was also employed to mitigate the self-phase modulation that occurred during fiber transmission (see Ref [1], Chapter 3.4.3). The FPGAs and DACs were mutually synchronized with a 10 GHz clock. Here, the multiplicity M can be changed by an external control signal within 1 second. This makes the present system applicable to a resilient adaptive ICT network by monitoring, for example, changes in the OSNR. The multiplicity parameter was located in the header of the data frame with a length of 256 symbols and sent to the receiver together with the QAM data. The bandwidth of the QAM signal was reduced to 6 GHz by adopting a root raised cosine (RRC) Nyquist filter with a roll-off factor of 0.2. In addition, frequency domain equalization (FDE) with an FFT size of 512 was used to compensate for the non-ideal frequency responses of individual electrical and optical components. The frequency response was first measured by monitoring the impulse response of the transmission system under a back-to-back condition, and an inverse function of the impulse response was used as the FDE coefficient. Then, sine and cosine wave signals with a sampling rate of 3 samples per period were added to the QAM signal to generate a 3.33 GHz-shifted single sideband from the carrier frequency, which wasused as a tone signal for tracking the optical phase of a local oscillator (LO) under OPLL operation at the receiver. The frequency of the tone signal was set at 3.33 GHz so that it was located close to the band edge of the QAM signal. The power of the tone signal was set at −17 dB lower than the QAM signal. Finally, the QAM signal with a tone was D/A-converted.
A block diagram of an FPGA-based homodyne real-time receiver is shown in Fig. 1(b). The receiver consisted of four FPGAs under parallel processing and two analog-to-digital converters (ADCs) operated at 10 GS/s with an 8-bit resolution. In the FPGA, the waveform distortion caused by chromatic dispersion during fiber transmission was compensated for by using an adaptive finite impulse response (FIR) filter with 51 taps. Then the compensated QAM signal was converted into binary data by referring to the extracted multiplicity parameter from the data frame. Finally, the bit error rate (BER) was measured on-line after FEC decoding. Here, DACs and ADCs were synchronized with each other by using synchronous 10 and 2.5 GHz external clock signals, respectively.
Figure 2 shows our experimental setup for polarization-multiplexed 5 Gsymbol/s, 4-64 QAM coherent optical transmission. We used a CW, C2H2 frequency-stabilized fiber laser operating at 1538.8 nm (ν1 = 194.96 THz) with a 4 kHz linewidth as a transmitter. The signal was coupled to an IQ modulator and a polarization multiplexer, where a polarization-multiplexed QAM signal was generated. Part of the laser output was divided in front of the IQ modulator, and its frequency was downshifted by 4 GHz against the carrier frequency. This signal was used as a second tone signal for polarization demultiplexing at the receiver. The polarization of the second tone signal was the same as that of the polarization axes (forexample, the Y axis in the present case) of the two QAM signals. Furthermore, we used a CW LD operating at 1540 nm (ν2 = 194.81 THz) to send a 3.33 GHz clock to realize both OPLL operation at the receiver and synchronous operation between the real-time transmitter and receiver. The powers of the second tone and the 3.33 GHz clock signals were set at −14 and −10 dB lower than the QAM signal, respectively. The transmission link was composed of four 80 km spans of standard single-mode fiber (SSMF). The power launched into each span was set at −3 dBm, which was optimally chosen to maximize the OSNR and minimize the nonlinear impairments.
Fig. 2 Experimental setup for polarization-multiplexed 5 Gsymbol/s, 4-64 QAM coherent optical transmission over 320 km with real-time transmitter and receiver.
At the receiver, the transmitted signals were split into two arms. One part was combined with an LO and received by a polarization-diverse 90-degree optical hybrid and four balanced photodiodes (BPDs). Since the second tone signal was set with a Y-polarization, the QAM signals were able to be polarization-demultiplexed with an automatic polarization controller (PC) by minimizing the second tone signal level at 4 GHz, which was detected at the X-polarization port. The detected signals were coupled to two real-time receivers and demodulated on-line. The other part was coupled to a 3.33 GHz clock extraction circuit, and the extracted clock was used as both a reference signal in the OPLL circuit and a synchronization signal for two real-time receivers, where the clock frequency is converted from 3.33 to 2.5 GHz by combining a frequency tripler and a 1/4 divider. In Fig. 2, the inset in the bottom right shows the electrical spectrum of the demodulated signal at the DSP. The demodulation bandwidth was set at 6 GHz due to the adoption of a Nyquist filter.
3. Experimental results
We first measured the OSNR and phase noise of the transmitted data. Figures 3(a) and 3(b) show the optical spectra of the QAM signal before and after a 320 km transmission, respectively. The OSNR, which we measured with a 0.1 nm resolution before transmission, was 42 dB, and it had degraded to 30.5 dB after a 320 km transmission.
Figure 4(a) shows the setup we used to measure the long-term phase error in the OPLL circuit. The phase difference between the IF signal and the 3.33 GHz clock signal was monitored long term with a double balance mixer (DBM) and an oscilloscope. The phase errors before and after a 320 km transmission are shown in Figs. 4(b-1) and 4(b-2), respectively. The root mean square values of the phase error before and after the 320 km transmission were 0.6 and 1.4 degrees, respectively. The phase error was smaller than the angle of 9.5 degrees between adjacent symbols of the 64 QAM format. The OPLL-based coherent optical transmission system is particularly advantageous for adaptive modulation because it does not require digital phase error compensation, which generally depends strongly on the data multiplicity. Therefore, the OPLL-based system enables us to realize a QAM multiplicity-free real-time adaptive coherent optical receiver. A low phase noise of less than 1 degree can also be achieved with an OPLL circuit based on external cavity laser diodes (ECLDs) with a 4 kHz linewidth [16].
Fig. 4 Measurement of long-term phase error in OPLL circuit. (a) Measurement setup, (b-1), (b-2) measurement results before and after 320 km transmission, respectively.
Figure 5 shows the measured BER for a 64 QAM signal under a back-to-back condition at an OSNR of 33 dB as a function of the power ratio of the tone signal for OPLL to the QAM signal. There is a tradeoff between the OSNR of the QAM signal and the OPLL performance depending on the OSNR of the tone signal. In our system, the optimum power ratio was −17 dB. Figure 6 shows the measured BER for a polarization-multiplexed 64 QAM signal after a320 km transmission as a function of the fiber launched power. The optimum launched power was −3 dBm, which can be increased by compensating for the cross phase modulation (XPM) between the two polarizations in the receiver.
Fig. 5 BER performance for 64 QAM signal under a back-to-back condition at an OSNR of 33 dB as a function of the power ratio of the tone signal to the QAM signal.
Figure 7(a) shows the BER performance of a polarization-multiplexed 5 Gsymbol/s, 64 QAM transmission. Error-free operations (defined as BER < 10−11) with FEC were obtained at OSNRs of more than 22 and 26 dB under a back-to-back condition and after a 320 km transmission, respectively. Here, the FEC threshold of the RS(255, 239) codes corresponds to a BER of approximately 2 x 10−4 [17]. The OSNR penalties between back-to-back and after a 320 km transmission were 4 dB, and this was caused by XPM between the two polarizations and an increase in the phase error in the OPLL circuit. 60 Gbit/s data were transmitted over 320 km with an optical bandwidth of 6 GHz on-line, resulting in a potential spectral efficiency of 9.3 bit/s/Hz when the 7% FEC overhead is taken into account. Figures 7(b-1), 7(b-2), and 7(b-3) show constellation maps of X-polarization data under a back-to-back condition, and after 160 and 320 km transmissions, respectively. In a back-to-back condition, a clear constellation was obtained and the error vector magnitude (EVM) was as low as 2.1%, where error free operation was achieved without FEC as shown by the closed rectangles. Here, EVM is defined as a percentage of the error vector magnitude to the peak signal magnitude. After the 160 and 320 km transmissions, the EVMs had degraded to 2.6 and 3.6%, respectively, because of the OSNR degradation and the increase in the phase noise. Error-free transmission was achieved with FEC even over 320 km.
Fig. 7 (a) BER performance for polarization-multiplexed 5 Gsymbol/s, 64 QAM coherent optical transmission, and (b-1), (b-2), and (b-3) constellations of X-polarization data for back-to-back, after 160 and 320 km transmissions, respectively.
Figure 8(a) shows the BER performance of polarization-multiplexed 5 Gsymbol/s, 4-64 QAM signals after a 320 km transmission. Error-free operations with FEC were obtained at OSNRs of more than 9, 17 and 26 dB for 4, 16 and 64 QAM signals, respectively. By changing the modulation level from 64 to 16 and 4, the OSNR margins were increased by 9 and 17 dB, respectively, which means that more robust transmissions can be achieved over longer distances. Figures 8(b-1), 8(b-2), and 8(b-3) show constellation maps of X-polarization data after a 320 km transmission for 4, 16, and 64 QAM signals with EVMs of 5.4, 4.1 and 3.6%, respectively. By changing the modulation level to less than 16, error free operations were obtained even without FEC.
Fig. 8 (a) BER performance for polarization-multiplexed 5 Gsymbol/s, 4-64 QAM coherent optical transmission over 320 km, and (b-1), (b-2), and (b-3) constellations of X-polarization data for 4, 16 and 64 QAM signals, respectively.
In this system, we used a bandwidth of 7.66 GHz to set four tones for OPLL, polarization tracking and clock recovery. As a result, the spectral efficiency was 4.1 bit/s/Hz when we take account of both the extra bandwidth for the tones and the 7% FEC overhead. In WDM transmission, these tones can be set as follows. One 3.33 GHz clock signal delivered with two tones can be commonly used for the demodulation of each WDM channel. So only two tones are transmitted with a WDM signal. To insert the other tones for OPLL and polarization recovery, the required guard band is less than 10 MHz since these tones can be easily extracted in the receiver with narrow bandpass electrical filters. Therefore, the decrease in the spectral efficiency caused by using multi-tones is negligible, resulting in a potential spectral efficiency of 9.3 bit/s/Hz.
4. Conclusion
We demonstrated an FPGA-based real-time transmitter and homodyne receiver for 5 Gsymbol/s adaptive 4-64 QAM transmission. Error-free performance with a 7% overhead FEC was achieved for 20~60 Gbit/s adaptive data transmission over 320 km with an optical bandwidth of 6 GHz. As a result, the potential spectral efficiency reached as high as 9.3 bit/s/Hz when the 7% FEC overhead is taken into account in the on-line system. Furthermore, by changing the modulation level from 64 to 16 and 4, the OSRN margins were increased by 9 and 17 dB, respectively. This may be useful for resilient optical networks.
In this system, a real-time adaptive QAM coherent optical transmission can be realized with a QAM multiplicity as high as 64. However, compared with a standard digital coherent optical transmission system, many tones are used for OPLL, polarization and clock recovery, and this increases system complexity. In future work, a simpler system can be expected by employing FPGA based polarization and clock recovery circuits, which will enable us to reduce the number of tones in the system.
Acknowledgment
This work is supported by a grant from the Ministry of Internal Affairs and Communications.
References and links
1. M. Nakazawa, K. Kikuchi, and T. Miyazaki, eds., High Spectral Density Optical Transmission Technologies (Springer, 2010).
2. D. Qian, M. Huang, E. Ip, Y. Huang, Y. Shao, J. Hu, and T. Wang, “101.7-Tb/s (370×294-Gb/s) PDM-128QAM-OFDM transmission over 3×55-km SSMF using pilot-based phase noise mitigation,” in OFC (2011), PDPB5.
3. M. -F. Huang, D. Qian, and E. Ip, “50.53-Gb/s PDM-1024QAM-OFDM transmission using pilot-based phase noise mitigation,” in OECC (2011), PDP1.
4. T. Omiya, M. Yoshida, and M. Nakazawa, “400 Gbit/s 256 QAM-OFDM transmission over 720 km with a 14 bit/s/Hz spectral efficiency by using high-resolution FDE,” Opt. Express 21(3), 2632–2641 (2013). [CrossRef] [PubMed]
5. R. Schmogrow, D. Hillerkuss, S. Wolf, B. Bäuerle, M. Winter, P. Kleinow, B. Nebendahl, T. Dippon, P. C. Schindler, C. Koos, W. Freude, and J. Leuthold, “512QAM Nyquist sinc-pulse transmission at 54 Gbit/s in an optical bandwidth of 3 GHz,” Opt. Express 20(6), 6439–6447 (2012). [CrossRef] [PubMed]
6. Y. Koizumi, K. Toyoda, M. Yoshida, and M. Nakazawa, “1024 QAM (60 Gbit/s) single-carrier coherent optical transmission over 150 km,” Opt. Express 20(11), 12508–12514 (2012). [CrossRef] [PubMed]
7. D. Qian, E. Ip, M. -F. Huang, M. -J. Li, and T. Wan, “698.5-Gb/s PDM-2048QAM transmission over 3 km multicore fiber,” in ECOC (2013), Th.1.C.5.
8. S. Beppu, M. Yoshida, K. Kasai, and M. Nakazawa, “2048 QAM (66 Gbit/s) single-carrier coherent optical transmission over 150 km with a potential SE of 15.3 bit/s/Hz,” in OFC (2014), W1A.6.
9. S. Chen, Q. Yang, Y. Ma, and W. Shieh, “Real-time multi-gigabit receiver for coherent optical MIMO-OFDM signals,” J. Lightwave Technol. 27(16), 3699–3704 (2009). [CrossRef]
10. A. Al-Bermani, C. Wördehoff, S. Hoffmann, D. Sandel, U. Rückert, and R. Noé, “Real-time phase-noise-tolerant 2.5 Gb/s synchronous 16-QAM transmission,” IEEE Photon. Technol. Lett. 22(24), 1823–1825 (2010). [CrossRef]
11. T. Pfau, N. Kaneda, S. Corteselli, A. Leven, and Y. K. Chen, “Real-time FPGA-based intradyne coherent receiver for 40 Gbit/s polarization-multiplexed 16-QAM,” in OFC (2011), OTuN4.
12. M. Yoshida, S. Okamoto, T. Omiya, K. Kasai, and M. Nakazawa, “Real-time FPGA-based coherent optical receiver for 1 Gsymbol/s, 64 QAM transmission,” in OFC (2011), OTuN3.
13. W. T. Webb and R. Steele, “Variable rate QAM for mobile radio,” IEEE Trans. Commun. 43(7), 2223–2230 (1995). [CrossRef]
14. R. Schmogrow, D. Hillerkuss, M. Dreschmann, M. Huebner, M. Winter, J. Meyer, B. Nebendahl, C. Koos, J. Becker, W. Freude, and J. Leuthold, “Real-time software-defined multiformat transmitter generating 64QAM at 28 GBd,” IEEE Photon. Technol. Lett. 22(21), 1601–1603 (2010). [CrossRef]
15. H. Y. Choi, L. Liu, T. Tsuritani, and I. Morita, “Demonstration of BER-adaptive WSON employing flexible transmitter/receiver with an extended OpenFlow-based control plane,” IEEE Photon. Technol. Lett. 25(2), 119–121 (2013). [CrossRef]
16. Y. Wang, K. Kasai, T. Omiya, and M. Nakazawa, “120 Gbit/s, polarization-multiplexed 10 Gsymbol/s, 64 QAM coherent transmission over 150 km using an optical voltage controlled oscillator,” Opt. Express 21(23), 28290–28296 (2013). [CrossRef] [PubMed]
17. “Forward Error Correction for Submarine Systems,” Telecommunication Standardization Section, ITU, G.975 (1996).
