Open Access
Add to BibSonomyAbstract
We demonstrate 400 Gbit/s frequency-division-multiplexed and polarization-division-multiplexed 256 QAM-OFDM transmission over 720 km with a spectral efficiency of 14 bit/s/Hz by using high-resolution frequency domain equalization (FDE) and digital back-propagation (DBP) methods. A detailed analytical evaluation of the 256 QAM-OFDM transmission is also provided, which clarifies the influence of quantization error in the digital coherent receiver on the waveform distortion compensation with DBP.
©2013 Optical Society of America
1. Introduction
To meet the ever-increasing capacity demand in optical fiber communications, research on high-spectral-efficiency optical transmission employing multilevel modulation formats has received considerable attention [1]. Coherent optical orthogonal frequency division multiplexing (OFDM) with higher-level QAM subcarrier modulation has been adopted to achieve a spectral efficiency (SE) exceeding 10 bit/s/Hz [2,3]. For example, a 235.1 Gbit/s OFDM transmission over 800 km has been achieved with an SE of 11.15 bit/s/Hz, which yields an SE-distance product of 8,920 km·bit/s/Hz [2].
We previously reported a 400 Gbit/s, 256 QAM-OFDM 400 km transmission with an SE of 14 bit/s/Hz [3], by adopting distortion compensation methods such as frequency domain equalization (FDE) and digital back-propagation (DBP). In this paper, we greatly improve the frequency resolution of FDE and successfully extend the distance up to 720 km, which is the maximum transmission reach at an SE as high as 14 bit/s/Hz. We also employ a numerical analysis to demonstrate the validity of the experimental results in terms of improved transmission performance by using waveform distortion compensation schemes.
2. Experimental setup for 400 Gbit/s 256 QAM-OFDM transmission
Figure 1 shows the experimental setup for a 400 Gbit/s frequency-division-multiplexed and polarization-division-multiplexed 256 QAM-OFDM transmission. The optical source for the transmitter was a CW, C2H2 frequency-stabilized fiber laser [4]. The laser output was divided into two paths, and one was coupled to a multi-carrier generator that consisted of two Mach-Zehnder modulators (MZM) and an optical frequency shifter (OFS1). Each MZM was driven with a 5.18 GHz sinusoidal signal, and five sidebands were generated. OFS1 fed a frequency downshift of 2.59 GHz against the original frequency f0. As a result, ten optical sidebands separated at 2.59 GHz were generated from the multi-carrier generator. Then, each of the fivesidebands from the MZM was modulated using an IQ modulator (IQM) with an OFDM signal generated by an arbitrary waveform generator (AWG) at a sampling rate of 12 Gsample/s. The AWGs generated the OFDM baseband signals using a fast Fourier transform (FFT) and FDE [5–7]. Table 1 shows the parameters of the OFDM signal. The data rate per 2.59 GHz channel was 2 x 8 x 1714 x 1.465 Mbaud = 40 Gbit/s (38.8 Gbit/s including the training symbol and guard interval). After taking a 7% FEC overhead into account, the achieved SE was 14 bit/s/Hz.
Fig. 1 Experimental setup for 400 Gbit/s frequency-division-multiplexed and polarization-division multiplexed 256 QAM-OFDM transmission.
Table 1. Parameters of the OFDM Signal
After data modulation, the two 5-channel OFDM signals were combined with an optical coupler and then polarization-multiplexed with a polarization beam combiner (PBC). The other path from the laser output led to OFS2, which fed a frequency downshift of 1.3 GHz against f0. This signal was used as the pilot tone signal required for the optical phase tracking of the local oscillator (LO) under optical PLL operation [8]. The polarization of the pilot signal was aligned with one of the two polarization axes of the OFDM signal. The transmission link comprised nine 80-km spans of SSMF, whose loss (0.2 dB/km) was compensated with erbium doped fiber amplifiers (EDFAs) and Raman amplifiers. The Raman amplifier provided a signal gain of 13 dB. The power of the pilot signal was set −6 dB lower than the total power of 10-channel OFDM signals. At the receiver, the OFDM signal was homodyne-detected with a 2x8 port 90-degree optical hybrid using an LO signal from a frequency-tunable tracking fiber laser, whose phase was locked to the pilot signal. It is difficult to demodulate all channels at once due to the bandwidth limitation of the A/D converter. So we demodulated each channel individually by changing the frequency of LO to be set at the center of the demodulation channel. Figure 1 shows an example of demodulation of ch.6 data. After detection with four balanced PDs (B-PD), the data were A/D-converted at 20 Gsample/s and post-processed with a digital signal processor (DSP) in an off-line condition. At the DSP, linear and nonlinear fiber impairments were compensated for with the DBP method [9–12]. Then, each OFDM band was extracted electronically by a digital filter. The OFDM signal was demodulated with an FFT and converted to a binary data sequence.
In the present case, we greatly improved the equalization capability of FDE by increasing the FFT size for FDE operation while keeping the FFT size for the OFDM signal fixed, and thus improving the FDE resolution of each subcarrier. Figure 2 shows the schematic configuration of the FDE for N = 1 and 2, where N is defined as the number of points for each subcarrier in the frequency domain. The case of N = 1 corresponds to a conventional FDE. When the FFT is operated over a length of twice the symbol period, the frequency resolution can be doubled. Figure 3 shows the FDE performance results obtained with a back-to-back 256 QAM-OFDM signal for N = 1 ~32 when the FFT size of the OFDM signal was fixed at 8,192. The FFT sizes of the FDE were 8,192 ~262,144 for N = 1 ~32. The error vector magnitude (EVM) value was 1.14% for conventional FDE (N = 1). As N increases, EVM was gradually reduced and improved to 0.82% at N = 16. Figures 4(a) and 4(b) show the back-to-back constellations when N = 1 and 16. The optimal N value was set at 16 because the EVM improvement was saturated for N > 16. When N = 16, the frequency resolution was 91.5 kHz. Figure 5 shows the BER performance as a function of the OSNR under back-to-back condition. The power penalty at a BER of 2 x 10−3 was improved by 1 dB with the FDE improvement.
3. Experimental result
Figures 6(a) and 6(b) show the optical spectra of an OFDM signal before and after a 720 km transmission with a fiber launched power of −2 dBm. In Fig. 6(a), the residual harmonic components on both sides were less than −32 dB. The optical signal-to-noise ratios (OSNR) before and after the 720 km transmission were 39 and 29.3 dB, respectively, which were measured with a 0.1 nm resolution. Figure 7 shows the measured OSNR values as a function of the transmission distance.
Figure 8 shows the single side-band noise power spectrum of a heterodyne beat note between the LO and the pilot tone after a 720 km transmission. By integrating this spectrum, the phase noise was estimated to be 0.4 degrees, whereas it was 0.3 degrees before transmission. Figure 9 shows the phase noise of an IF signal as a function of the transmission distance. This slight increase in phase noise was mainly caused by the OSNR degradation. On the other hand, the phase tolerance for 256 QAM, determined by the phase difference between the two nearest symbols, was ± 2.04 degrees. This implies that the OPLL successfully achieved a phase noise sufficiently below the phase tolerance for 256 QAM even after a 720 km transmission.
Fig. 8 Single side-band (SSB) noise power spectrum of a heterodyne beat note between an LO and a pilot tone after a 720 km transmission.
Figure 10 (a) shows the measured BER of ch.6 after a 720 km transmission for various fiber launched powers. The optimum launched power was −2 dBm, which resulted in a BER below the FEC limit. The constellation diagrams are plotted in Fig. 10(b). The EVM had increased from 0.82 to 1.79% after the 720 km transmission, which is mainly due to the OSNR degradation.
Fig. 10 (a) BER after 720 km transmission as a function of fiber launched power and (b) constellations for 256 QAM-OFDM signals after a 720 km transmission for a −2 dBm transmission power.
Figure 11 shows the measured BER of each channel after a 720 km transmission with DBP and with average individual dispersion SPM compensation. By using the DBP including XPM compensation, BER is reduced by one-fourth.
Figures 12(a) and 12(b), respectively, show the BER performance as a function of the received power and the transmission distance at the maximum received power (−8 dBm). The power penalty at a BER of 2 x 10−3 after 320, 640, and 720 km transmissions were 2, 5, and 15 dB, respectively. In Fig. 12(b), BER obtained without and with the FDE improvement (N = 1 and 16, respectively) are plotted. The transmission distance with a BER below the FEC threshold (2 x 10−3) was extended from 560 to 720 km as a result of the improvement in the FDE as shown in Fig. 5, where BER at the OSNR of 29.3 dB was reduced to 1.0 x 10−3 with the FDE improvement. The present result corresponds to an SE-product of 10,080 km·bit/s/Hz.
4. Numerical simulation and discussion
In the transmission experiment described in sections 2 and 3, the BER performance of a 256 QAM-OFDM transmission was successfully improved by adopting distortion compensation with FDE and DBP. In this section, we demonstrate the validity of the experimental results using a numerical analysis. Figure 13 shows the numerical model for a 256 QAM-OFDM transmission. This model takes account of waveform distortions due to the frequency dependent hardware imperfections, the phase noise of the homodyne detected signal, and the quantization error caused by A/D conversion. At the transmitter, distortion caused by the hardware imperfections is applied to an ideal 256 QAM-OFDM signal. The amount of distortion, which can be reduced by adopting FDE, is determined from the EVM value of the demodulated signal under a back-to-back condition as shown in Fig. 4. Figure 14 shows the distribution of the amplitude fluctuation in the constellation shown in Fig. 4(b). This figure shows that the amplitude fluctuation can be approximated by a Gaussian distribution. Therefore, we assumed that the distortion is Gaussian noise. After transmission, phase noise at the homodyne detection with an LO is applied to the transmitted signal. The amount of phase noise is determined by that of the IF signal in the OPLL circuit as shown in Fig. 9. Then, the signal is affected by a quantization error caused by A/D conversion with a 5.8 bit resolution, which corresponds to the effective number of bits (ENOB) of the A/D converter used in our transmission system. After applying distortion compensation with DBP, the received OFDM signal is demodulated with an FFT and the BER is counted.
Figure 15 shows BER performance as a function of launched power after a 720 km transmission. A numerical result obtained with a 5.8 bit resolution (ENOB) at A/D conversion provides a good fit with the experimental results plotted by the squares. In this case, the optimum launched power is −2 dBm. We also carried out a simulation by assuming an ENOB of 7 and 8 bits, whose results are shown by red and green curves, respectively. It can be seen that, by increasing the ENOB of the A/D converter to 8 bits, the optimum launched power can be increased to 2 dBm and the BER is improved to 5.2 x 10−4 as a result of the increased OSNR. This indicates that it is important to increase the ENOB of the A/D converter to improve the distortion compensation with DBP in our high spectral efficiency transmission [13].
5. Conclusion
We successfully demonstrated a 400 Gbit/s 256 QAM-OFDM transmission over 720 km with a 14 bit/s/Hz SE by employing a new high-resolution FDE technique. We also used a numerical analysis to compare the experimental results in terms of transmission performance improvement by distortion compensation schemes. The numerical results show that the ENOB of the A/D converter has a large influence on the DBP compensation capability.
References and links
1. P. J. Winzer, “Modulation and multiplexing in optical communication systems,” IEEE LEOS Newsletter 23(1), 4–10 (2009).
2. X. Liu, S. Chandrasekhar, T. Lotz, P. Winzer, H. Haunstein, S. Randel, S. Corteselli, B. Zhu, and D. W. Peckham “Generation and FEC-decoding of a 231.5-Gb/s PDM-OFDM signal with 256-iterative-polar-modulation achieving 11.15-b/s/Hz intrachannel spectral efficiency and 800-km reach,” OFC2012, PDP5B.3.
3. T. Omiya, K. Toyoda, M. Yoshida, and M. Nakazawa, “400 Gbit/s frequency-division-multiplexed and polarization-multiplexed 256 QAM-OFDM transmission over 400 km with a spectral efficiency of 14 bit/s/Hz,” OFC2012, OM2A.7.
4. K. Kasai, A. Suzuki, M. Yoshida, and M. Nakazawa, “Performance improvement of an acetylene (C2H2) frequency-stabilized fiber laser,” IEICE Electron. Express 3(22), 487–492 (2006). [CrossRef]
5. A. Al Amin, S. L. Jansen, H. Takahashi, I. Morita, and H. Tanaka, “A hybrid IQ imbalance compensation method for optical OFDM transmission,” Opt. Express 18(5), 4859–4866 (2010). [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. Y. Koizumi, K. Toyoda, T. Omiya, M. Yoshida, T. Hirooka, and M. Nakazawa, “512 QAM transmission over 240 km using frequency-domain equalization in a digital coherent receiver,” Opt. Express 20(21), 23383–23389 (2012). [CrossRef] [PubMed]
8. K. Kasai, J. Hongo, M. Yoshida, and M. Nakazawa, “Optical phase-locked loop for coherent transmission over 500 km using heterodyne detection with fiber lasers,” IEICE Electron. Express 4(3), 77–81 (2007). [CrossRef]
9. T. Yamazaki, T. Tanabe, F. Kannari, Y. Shida, and S. Fushimi, “Fiber delivery of ultrashort optical pulses preshaped on the basis of a backward propagation solver,” Jpn. J. Appl. Phys. 1, Regul. Pap. Short Notes 42(12), 7313–7317 (2003).
10. C. Paré, A. Villeneuve, P.-A. Bélanger, and N. J. Doran, “Compensating for dispersion and the nonlinear Kerr effect without phase conjugation,” Opt. Lett. 21(7), 459–461 (1996). [CrossRef] [PubMed]
11. M. Tsang, D. Psaltis, and F. G. Omenetto, “Reverse propagation of femtosecond pulses in optical fibers,” Opt. Lett. 28(20), 1873–1875 (2003). [CrossRef] [PubMed]
12. K. Toyoda, Y. Koizumi, T. Omiya, M. Yoshida, T. Hirooka, and M. Nakazawa, “Marked performance improvement of 256 QAM transmission using a digital back-propagation method,” Opt. Express 20(18), 19815–19821 (2012). [CrossRef] [PubMed]
13. B. Schmauss, C. Lin, and R. Asif, “Progress in digital backward propagation,” ECOC2012, Th.1.D.5.
