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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 12, JUNE 15, 2009
132.2-Gb/s PDM-8QAM-OFDM Transmission at 4-b/s/Hz Spectral Efficiency Sander L. Jansen, Member, IEEE, Abdullah Al Amin, Hidenori Takahashi, Itsuro Morita, and Hideaki Tanaka
Abstract—In this letter, we investigate 132.2-Gb/s polarizationdivision-multiplexed orthogonal frequency-division-multiplexing (PDM-OFDM) transmission at 25-GHz channel spacing. We show that the nonlinear tolerance is dependent on the OFDM symbol length. By using 14.4-ns-long OFDM symbols, 7 132.2-Gb/s transmission of PDM-OFDM at 4-b/s/Hz spectral efficiency is reported over 1300-km standard single-mode fiber. Index Terms—Chromatic dispersion compensation, fast Fourier transform (FFT), long-haul transmission, orthogonal frequencydivision multiplexing (OFDM).
I. INTRODUCTION
O
NE HUNDRED Gigabit Ethernet (100 GbE) is considered to become the next-generation Ethernet standard for high-capacity backbone networks. For the implementation of this high data rate, the combination of coherent detection and digital signal processing has been proposed as it provides a virtually unlimited chromatic dispersion and polarization-mode dispersion tolerance [1]–[6]. Several modulation formats have been proposed either using a single carrier [1], [2], or multiple orthogonal frequency-division-multiplexed (OFDM) carriers [3]–[6]. Single carrier modulation formats typically employ blind channel estimation whereas multicarrier modulation formats like OFDM typically use many (more than 50) subcarriers that are equalized by using training symbols (TS). Blind channel estimation requires careful design of the channel estimation algorithm so that the system converges under all conditions, whereas the main disadvantage of TS-based channel estimation is that the overhead is increased by about 8% for an optimally designed system configuration [5]. With respect to single-carrier modulation formats, the use of OFDM offers some distinct advantages. First of all, OFDM has a significantly lower symbol size that relaxes the requirements on the clock recovery. Second, OFDM is easily scalable to higher level modulation formats as the TS-based channel estimation is independent on the constellation size [5], [6]. Finally, OFDM has, because Manuscript received December 18, 2008; revised February 24, 2009. First published April 03, 2009; current version published May 22, 2009. This work was supported in part by a project of the National Institute of Information and Communications Technology of Japan. S. L. Jansen was with KDDI R&D Laboratories, Saitama 356-8502, Japan. He is now with Nokia Siemens Networks, 81541 Munich, Germany (e-mail:
[email protected]). A. Al Amin, H. Takahashi, I. Morita, and H. Tanaka are with KDDI R&D Laboratories, 356-8502 Fujimino, Saitama, Japan (e-mail:
[email protected];
[email protected];
[email protected];
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2009.2018637
of its well-defined spectral shape, inherently negligible linear crosstalk. Therefore, the OFDM modulation format is very suitable for realizing transmission systems with a high spectral efficiency [6]. Another way to use this advantage is using multiband OFDM signals in order to realize a very high channel data rate with multiple low-bandwidth digital-to-analog and analog-to-digital converters (DACs/ADCs). In [5], we reported transmission of 120-Gb/s polarization-division-multiplexed OFDM (PDM-OFDM) per wavelength-division-multiplexed (WDM) channel by electronically multiplexing four bands. In this letter, we demonstrate a method to generate and individually modulate multiple OFDM bands in the optical domain. Using three bands, we show transmission of 7 132.2 Gb/s at high spectral efficiency (4-bit/s/Hz). We also compare the nonlinear tolerance for the 25-GHz-spaced channels for different OFDM symbol lengths, and find that for shorter symbol lengths the nonlinear tolerance is improved, allowing error-free transmission over 1300-km standard single-mode fiber (SSMF). II. EXPERIMENTAL SETUP Two Tektronix AWG7102 arbitrary waveform generators (AWGs) are used at a sampling rate of 10 GSamples/s to emulate the generation of the OFDM baseband signals for two neighboring bands. Each OFDM band is 7.34 GHz wide and modulated with eight-level quadrature amplitude modulation (8QAM), providing a nominal data rate of 22.03 Gb/s. Per WDM channel, three OFDM bands are used so that after polarization multiplexing, the nominal data rate per WDM channel is 132.2 Gb/s. The experimental setup or the transmitter is shown in Fig. 1. Instead of using individual lasers per WDM channel, a frequency comb is used in this experiment to generate all seven WDM channels at the same time [7]. This comb is generated by feeding the output of an external cavity laser (ECL) to a push–pull Mach–Zehnder modulator (MZM), driven with a 12-V peak-to-peak sinusoidal signal of 12.5 GHz (inset A of Fig. 1). In a real (commercial) transmission system all WDM channels would have to be separated by a demultiplexer so that the OFDM bands can be modulated individually. Because of a limited amount of modulators and AWGs available in this experiment only two complex MZM (CMZM) chains were used to modulate all OFDM bands. However, care was taken that the neighboring OFDM bands are modulated with the different data. The implementation for our transmitter is as follows. Instead of a demultiplexer, a 25-GHz optical interleaver (INT) is used after the frequency comb generator to split the frequency comb into even and odd WDM channels. After the INT a polarization-maintaining coupler splits the optical signal in two arms. One of the arms is used to generate the optical
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JANSEN et al.: 132.2-Gb/s PDM-8QAM-OFDM TRANSMISSION AT 4-b/s/Hz SPECTRAL EFFICIENCY
Fig. 1. Optical spectra and experimental setup of the 7
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2 132.2-Gb/s PDM-OFDM transmitter and transmission line.
sideband continuous-wave (CW) tones by using an MZM that is driven with a 7.8-GHz sinusoid. This modulator is biased in the transmission null such that the optical carrier is suppressed and two sideband CW tones are generated at 7.8 and 7.8 GHz relative to the channel’s center frequency. After the creation of the sideband CW tones, a second passive coupler is used to add the center CW tone of the even WDM channels with the sideband CW tones of the odd WDM channels and vice versa. Then these tones are modulated using two CMZMs. The CMZM in the upper branch modulates the two sideband CW tones of the odd channels and the center CW tone of the even channels (inset B). The CMZM in the lower branch modulates the opposite, i.e., the sideband CW tones of the even channels and the center CW tone of the odd channels. Therefore, no direct neighbors are modulated by the same modulator. Finally all OFDM bands are combined with a 3-dB coupler (inset C of Fig. 1). Polarization multiplexing is emulated by splitting the dense WDM OFDM signal with a 3-dB coupler and combining it afterwards with a polarization beam combiner. In order for the training symbol estimation to work, the symbols in both polarizations must be exactly aligned; therefore, one arm is delayed by exactly one OFDM symbol for decorrelation. The recirculating loop consists of four spans of 82-km SSMF without optical dispersion compensation. After every span, amplification is provided by a Raman/erbium-doped fiber amplifier structure with an average ON–OFF Raman gain of 8 dB. A dynamic gain equalizer is used for power equalization and a loop-synchronous polarization scrambler (LSPS) is employed to reduce loop-induced polarization effects. After the recirculating loop, the transmission line is extended with 25-km SSMF. At the receiver, the signal is split in two random polarizations and detected with a polarization-diverse 90 optical hybrid. An ECL with 100-kHz linewidth is used as free running local oscillator (LO) and four single-ended 20-GHz
Pin/TIA modules are used for detection. Similar to [5], the LO is placed in between OFDM sub-bands and used to detect the complete 132-Gb/s channel at the same time. A real-time digital storage oscilloscope (Tektronix DPO72004) is used to sample the four outputs of the optical hybrid. The bandwidth of the oscilloscope is 16 GHz and the sampling frequency is 50 GHz. After detection, the data is postprocessed off-line. TS are periodically inserted into the OFDM signal, so that polarization derotation at the receiver can be realized through multiple-input–multiple-output (MIMO) processing. The full description for the receiver structure and the algorithms used for postprocessing are discussed in more detail in [3]. In order to compensate for the phase noise of the LO, radio-frequency (RF)-aided phase noise compensation is implemented. For all reported bit-error-rate (BER) five sets with each 2.4 million bits have been evaluated, each set with different polarization state settings in the LSPS. III. NONLINEAR TOLERANCE The nonlinear tolerance of the system was assessed by varying the SSMF input power from 10 to 3 dBm per channel and evaluating the BER after 1000-km transmission. In this experiment, we set the OFDM symbol length to 14.4, 27.2 and 104.0 ns by setting the fast Fourier transform (FFT) size to 128, 256, and 1024, respectively (while keeping a constant OFDM bandwidth). The results for the center channel are shown in Fig. 2. At low input power, a negligible difference in BER performance is observed for the different OFDM symbol lengths, indicating a similar performance for all measured OFDM symbol lengths in the amplified spontaneous emission limited regime. However, in the nonlinear regime (at high input powers) a significantly larger nonlinear tolerance is observed for short OFDM symbol lengths. This finding stands in contrast to for instance [8] in which it has been found in both theory and simulations that the variation of the OFDM symbol length has
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 12, JUNE 15, 2009
Fig. 2. BER as a function of the SSMF input power after 1000-km transmission for different OFDM symbol lengths (center channel).
Fig. 3. BER after 1000- and 1300-km transmission.
V. CONCLUSION no effect on the intrachannel nonlinear tolerance. However, the influence of phase noise and interchannel nonlinear effects was excluded in that work. In this experiment, RF-aided phase noise compensation is used for phase noise compensation. Whereas conventional phase noise compensation techniques are very sensitive towards the length of the OFDM symbol, we have shown in that with RF-aided phase noise compensation is significantly more robust [3]. Fig. 2 shows that in the linear domain the same performance is obtained for all OFDM symbol lengths indicating that the OFDM symbol length does not influence the BER performance. However, at higher input powers, self-phase modulation and cross-phase modulation impair not only the OFDM signal, but the RF-pilot as well (see, for instance, [3, Fig. 8(b)]). Because of these nonlinear distortions, the phase noise compensation algorithm becomes less accurate. Similar to conventional phase noise compensation schemes, the influence of nonperfect phase noise compensation scales with the length of the OFDM symbol. As a result, shorter OFDM symbols provide a better BER performance in the nonlinear regime. The use of shorter OFDM symbols, however, significantly increases the CP-overhead [9]. Another method to reduce the OFDM symbol length is to increase the OFDM baseband bandwidth. The advantage of increasing the OFDM baseband bandwidth instead of reducing the FFT size is that it becomes easier to reduce the cyclic prefix overhead. However, in this experiment the OFDM baseband bandwidth could not be increased as it was limited by the maximum speed of the DACs. IV. MAXIMUM REACH Fig. 3 shows the BER performance after 1000- and 1300-km transmission for all seven WDM channels. The input power is set to the optimum of 6 dBm per channel and the FFT size is 128. The overhead for cyclic prefix and TS is 12.5% and 2%, respectively so that the net data rate is 115 Gb/s. The obtained BER values for all WDM channels show a similar performance and are well below the threshold of a concatenated forward-error . correction code with 7% overhead
In this letter, we investigated transmission of 132 Gb/s by optically multiplexing three-band PDM-OFDM. The nonlinear tolerance of OFDM is found to be severely affected by the length of OFDM symbols, and using a 14.4-ns symbol length, we showed transmission of 7 132.2-Gb/s PDM-8QAM-OFDM at 4-b/s/Hz spectral efficiency over 1300-km of SSMF. ACKNOWLEDGMENT The authors would like to thank M. Alfiad and Dr. D. van den Borne for the many fruitful discussions. Furthermore, the authors thank Dr. S. Akiba and Dr. M. Suzuki for their support. REFERENCES [1] C. R. S. Fludger, T. Duthel, D. van den Borne, C. Schulien, E.-D. Schmidt, T. Wuth, J. Geyer, E. de Man, G. D. Khoe, and H. de Waardt, “10 111 Gbit/s, 50 GHz spaced, POLMUX-RZ-DQPSK transmission over 2375 km employing coherent equalisation,” J. Lightw. Technol., vol. 26, no. 1, pp. 64–72, Jan. 1, 2008. [2] X. Zhou, J. Yu, D. Qian, T. Wang, G. Zhang, and P. D. Magill, “8 114 Gb/s, 25-GHz-spaced, PolMux-RZ-8PSK transmission over 640 km of SSMF employing digital coherent detection and EDFA-only amplification,” in Proc. Optical Fiber Commun. Conf., 2008, Paper PDP 1. [3] S. L. Jansen, I. Morita, T. C. W. Schenk, and H. Tanaka, “Long-haul transmission of 16 52.5-Gb/s polarization division multiplexed OFDM enabled by MIMO processing,” J. Opt. Netw., vol. 7, pp. 173–182, 2008. [4] Q. Yang, Y. Ma, and W. Shieh, “107 Gb/s coherent optical OFDM reception using orthogonal band multiplexing,” in Proc. Optical Fiber Commun. Conf., 2008, Paper PDP 7. [5] S. L. Jansen, I. Morita, T. C. W. Schenk, and H. Tanaka, “121.9-Gb/s PDM-OFDM transmission with 2-b/s/Hz spectral efficiency over 1000 km of SSMF,” J. Lightw. Technol., vol. 27, no. 3, pp. 177–188, Feb. 1, 2009. [6] H. Takahashi, A. Al Amin, S. L. Jansen, I. Morita, and H. Tanaka, “8 66.8-Gbit/s coherent PDM-OFDM transmission over 640 km of SSMF at 5.6-bit/s/Hz spectral efficiency,” in Proc. Eur. Conf. Optical Commun., Brussels, Belgium, 2008, Paper Th.3.E.4. [7] A. D. Ellis, F. C. G. Gunning, and T. Healy, “Coherent WDM: The achievement of high information spectral density through phase control within the transmitter,” in Proc. Optical Fiber Commun. Conf., 2006, p. OThR4. [8] A. J. Lowery, S. Wang, and M. Premaratne, “Calculation of power limit due to fiber nonlinearity in optical OFDM systems,” Opt. Express, vol. 15, no. 20, pp. 13282–13287, 2007. [9] S. L. Jansen, I. Morita, K. Forozesh, S. Randel, D. van den Borne, and H. Tanaka, “Optical OFDM, a hype or is it for real?,” in Proc. Eur. Conf. Optical Commun., Brussels, Belgium, 2008, Paper Mo.3.E.3.
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