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Transmission and reception of PDM dual-subcarrier coherent 16QAM-OFDM signals
Optical Fiber Technology xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Regular Articles
Transmission and reception of PDM dual-subcarrier coherent 16QAM-OFDM signals Fan Li a,b,⇑, Junwen Zhang a,c, Jianjun Yu a,c, Xinying Li a,c a
ZTE TX, NJ 07960, USA Georgia Institute of Technology, Atlanta, GA 30332, USA c Key Laboratory for Information Science of Electromagnetic Waves (MoE), Fudan University, Shanghai, China b
a r t i c l e
i n f o
Article history: Received 17 February 2015 Revised 11 September 2015 Available online xxxx Keywords: OFDM Coherent detection Dual-subcarrier 16QAM 49QAM
a b s t r a c t In this paper, 16-Gbaud polarization-division-multiplexed (PDM) dual-subcarrier coherent optical orthogonal frequency division multiplexing (CO-OFDM) transmission and reception are successfully demonstrated without overhead. The in-phase and quadrature (I/Q) components of dual-subcarrier 16-ary quadrature amplitude modulation (QAM) OFDM signal are both seven-level signals in time domain, and thus can be equalized like a 49 QAM signal in time domain with cascaded multi-modulus algorithm (CMMA) equalization method. The experimental results show that there is no power penalty observed between optical back to back (OBTB) and after 80-km single-mode fiber-28 (SMF-28) with time domain CMMA equalization method. A 0.4 dB optical signal to noise ratio (OSNR) penalty in OBTB is observed when the bandwidth of channel is set at 26 GHz at the BER of 2.0 102. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction Coherent optical orthogonal frequency division multiplexing (CO-OFDM) has been intensively studied due to its high spectral efficiency (SE) and robustness to transmission impairments enabled by advanced frequency domain digital signal processing (DSP) [1–9]. The frequency offset estimation (FOE) and carrier phase estimation (CPE) are both implemented in the frequency domain with the aid of training sequences (TSs) and pilot tones for traditional optical OFDM transmission [3–5,9]. Moreover, channel response estimation and equalization is also done in the frequency domain. Frequency domain equalization (FDE) is simple and effective, but it needs overheads (TSs and pilot tones) during equalization which leads to the SE degradation. Few subcarriers OFDM (dual-subcarrier and quad-subcarrier) transmission and reception was proposed in our previous work [10,11] to avoid overhead during equalization. Compared to conventional OFDM with a large number of subcarriers, an unbeatable advantage of OFDM with a small number of subcarriers is that the peak-toaverage power ratio (PAPR) is quite low [10–13]. However, the FDE based on TSs and pilot tones cannot effectively work anymore in the few subcarriers OFDM scheme for two reasons. First, the frequency resolution decreases dramatically during channel ⇑ Corresponding author at: ZTE TX, NJ 07960, USA. E-mail address: [email protected] (F. Li).
estimation in the few subcarriers OFDM scheme. Second, the SE is significantly reduced as the pilot tones should be inserted in every OFDM symbol with few subcarriers. Fortunately, we found that the Quadrature Phase Shift Keyed (QPSK) OFDM signal with dual-subcarrier and quad-subcarrier are 9QAM [10] and 25QAM [11] in the time domain, respectively. With these special properties, QPSK-OFDM signal with dual-subcarrier and quad-subcarrier can be equalized in the time domain with cascaded multimodulus algorithm (CMMA) equalization method. The additional overhead in the FDE is totally avoided, but the SE is still limited as the modulation format is QPSK. In order to further improve the SE, higher order modulation formats must be applied. In this paper, we realized the transmission and reception of 16-Gbaud dual-subcarrier 16-ary quadrature amplitude modulation (16QAM) OFDM. Dual-subcarrier 16QAM OFDM signal demonstrates as a 49QAM signal in the time domain, and therefore it can also be blindly equalized with CMMA equalization method. The overhead in the traditional optical OFDM transmission system can be completely removed in the dual-subcarrier optical 16QAM OFDM transmission system with blind equalization. The main differences between our scheme with dual-subcarrier all optical 16QAM OFDM exist in the generation and detection. For dualsubcarrier all optical 16QAM OFDM signal generation, two frequency-locked subcarriers should be generated before 16QAM signal modulation. The channel spacing between two subcarriers should be exactly equal to the baud rate of data to make the
http://dx.doi.org/10.1016/j.yofte.2015.09.006 1068-5200/Ó 2015 Elsevier Inc. All rights reserved.
Please cite this article in press as: F. Li et al., Transmission and reception of PDM dual-subcarrier coherent 16QAM-OFDM signals, Opt. Fiber Technol. (2015), http://dx.doi.org/10.1016/j.yofte.2015.09.006
2
F. Li et al. / Optical Fiber Technology xxx (2015) xxx–xxx
2-subcarrier orthogonal. The generation of such two subcarriers is usually complicated. At receiver, after optical to electrical (O/E) conversion, a digital filter is used to separate the dual-subcarrier, and then DSP is applied for each subcarrier [12–14]. While in our scheme, dual-subcarrier is processed at the same time with the blind CMMA like a 49QAM. Thus, the receiver is simpler. To our knowledge, transmission and reception of 128-Gbit/s p olarization-division-multiplexed (PDM) dual-subcarrier 16QAM OFDM with blind equalization like a 49QAM signal is first demonstrated in this paper. A comparison between optical back to back (OBTB) and after 80-km single-mode fiber-28 (SMF-28) was performed, and the results show that there was no power penalty observed. We also measured the optical signal to noise ratio (OSNR) penalty versus the bandwidth of the channel and the experimental results show that a 0.4 dB optical signal to noise ratio (OSNR) penalty in OBTB is observed when the bandwidth of channel is set at 26 GHz at the BER of 2.0 102. 2. Principle According to our previous work [10], after 2-point Inverse fast Fourier transform (IFFT), the two samples of one dual-subcarrier OFDM symbol can be expressed as
1 sð0Þ ¼ pffiffiffi ðc0 c1 Þ; 2
1 sð1Þ ¼ pffiffiffi ðc0 þ c1 Þ: 2
ð1Þ
where c0 and c1 represent the input data with corresponding modulation formats modulated onto 2 subcarriers, respectively. s(0) and s(1) denote the symbols after IFFT. In the quad-subcarrier OFDM case, the output of 4-point IFFT can be expressed as [11]
sð0Þ ¼ 12 ðc0 þ c1 þ c2 þ c3 Þ; sð1Þ ¼ 12 ðc0 þ jc1 c2 jc3 Þ; sð2Þ ¼ 12 ðc0 c1 þ c2 c3 Þ;
ð2Þ
sð3Þ ¼ 12 ðc0 jc1 c2 þ jc3 Þ: where c0 , c1 , c2 and c3 represent the input data with corresponding modulation formats modulated onto 4-subcarrier, respectively. And s(0), s(1), s(2) and s(3) denote the corresponding 4 symbols after 4-point IFFT. For input data symbols with different modulation formats, we can get the corresponding output data symbols of dual-subcarrier and Quad-subcarrier OFDM according to Eq. (1) and Eq. (2), respectively. Table 1 shows the input data symbols and output data symbols after 2-point IFFT and 4-point IFFT, respectively. The QPSK in the frequency domain is transformed to 9-QAM and 25-QAM in time domain after 2-point and 4-point IFFT, respectively. 21QAM and 88QAM can be obtained with 2-point and 4-point IFFT of rectangle 8QAM. 16QAM input data is changed to 49QAM and 169QAM in the dual-subcarrier and quad-subcarrier scheme. The formats of output data of dual-subcarrier and quad-subcarrier OFDM with 32QAM in the time domain are 109QAM and 401QAM, respectively. 225QAM and 841QAM can be obtained with 2-point and 4-point IFFT of 64QAM. In this paper, we only discuss the transmission and reception of dual-subcarrier 16QAM OFDM, and the dual-subcarrier 16QAM OFDM can be equalized like 49QAM signal in time domain with CMMA equalization method. 3. Experimental setup Fig. 1 shows the experimental setup of the dual-subcarrier coherent optical 16QAM OFDM transmission system. At the transmitter, an external cavity laser (ECL) at 1557.04 nm with less than 100-kHz linewidth and maximum output power of 14.5 dBm is
modulated by an intensity Mach–Zehnder modulator driven by an electrical baseband dual-subcarrier 16QAM OFDM signal. The dual-subcarrier 16QAM OFDM signal is first generated offline in Matlab software, and then 4-time interpolation is implemented to generate the 16-Gbaud dual-subcarrier 16QAM OFDM signal with a 64 GSa/s sampling rate digital-to-analog convertor (DAC). Two linear electrical amplifiers (EAs) are used to boost the dualsubcarrier 16QAM OFDM signal before electrical to optical (O/E) modulation. For optical OFDM modulation, two parallel Mach– Zehnder modulators in the in-phase/quadrature (I/Q) modulator are both biased at the null point and the phase difference between the upper and lower branches of the I/Q modulator is controlled at p/2. Polarization multiplexing is realized by a polarization multiplexer, comprising a polarization-maintaining optical coupler (OC) to halve the signal into two branches, an optical delay line (DL) to remove the correlation between X-polarization and Y-polarization by providing a time delay, an optical attenuator to balance the power of two branches and a polarization beam combiner (PBC) to recombine the signal. The generated signal is boosted via an erbium doped fiber amplifier (EDFA) before launched into 80-km SMF-28. The 80 km SMF-28 has 18 dB average loss and 17 ps/km/nm chromatic dispersion (CD) at 1550 nm without optical dispersion compensation. The output signal is then injected into an integrated coherent receiver to implement optical to electrical detection. The output signal is then injected into the integrated coherent receiver to implement optical to electrical detection. After integrated coherent receiver, the signal is captured by the real-time oscilloscope with 80 GSa/s sampling rate. It is worth noting that here we use 64 GSa/s sampling rate DAC and 80 GSa/s sampling rate analog-to-digital converter (ADC) to generate and capture 16Gbaud dual-subcarrier 16QAM-OFDM signal in the experimental setup. For practical use, DAC with 16 GSa/s sampling rate is enough to generate 16Gbaud dual-subcarrier 16QAM-OFDM signal. Without frequency offset, 16 GSa/s sampling rate ADCs are enough to capture 16Gbaud dual-subcarrier 16QAMOFDM signal, while in the practical implementation the sampling rate of ADCs should be higher than 16 GSa/s as frequency offset is inevitable. The resolutions of the DAC and the ADC in real-time oscilloscope are both 8 bits. The electrical eye diagram of I component of dual-subcarrier 16QAM-OFDM signal is inserted as inset (a) in Fig. 1. The levels of eye-diagrams of I and Q components are both 7, thus the signal constellation type in time domain is 49QAM. The optical spectra before and after 80-km SMF-28 transmission with 0.1-nm resolution are shown in Fig. 1(b). The DSP for receiver (Rx)-offline processing of the dualsubcarrier 16QAM-OFDM signal is shown in Fig. 2. At the receiver, the dual-subcarrier 16QAM OFDM can be equalized with CMMA method like 49QAM without any additional overhead. The main difference of DSP algorithms in the receiver as compared to dualcarrier QPSK-OFDM include CMMA and phase noise compensation. Dual-subcarrier 16QAM-OFDM signal is equalized like a 49 QAM signal in time domain with CMMA. Dual-subcarrier 16QAMOFDM signal coverages onto ten different rings. The phase noise is estimated and compensated after 2-point IFFT. Without noise the output of IFFT should be both 16QAM signal, thus QPSKpartition algorithm is applied to realize CPE. The detailed DSP algorithms are below. After the integrated receiver, four signal components are first captured by the real-time oscilloscope with 80 GSa/s sample rate. Secondly, a T/2-spaced time-domain finite-impulse-response (FIR) filter is firstly used for chromatic dispersion compensation (CDC), where the filter coefficients are calculated from the known fiber CD transfer function using the frequency-domain truncation method. Thirdly, the 10-radius CMMA is used to retrieve the modulus of the PDM dualsubcarrier 16QAM OFDM like a 49-QAM signal and realize polarization de-multiplexing. The subsequent step is to realize the
Please cite this article in press as: F. Li et al., Transmission and reception of PDM dual-subcarrier coherent 16QAM-OFDM signals, Opt. Fiber Technol. (2015), http://dx.doi.org/10.1016/j.yofte.2015.09.006
F. Li et al. / Optical Fiber Technology xxx (2015) xxx–xxx
3
Table 1 The output data of dual-subcarrier and quad-subcarrier OFDM.
Fig. 1. Experimental setup of dual-subcarrier 16QAM-OFDM transmission system. (a) Electrical eye diagram and (b) optical spectra.
Please cite this article in press as: F. Li et al., Transmission and reception of PDM dual-subcarrier coherent 16QAM-OFDM signals, Opt. Fiber Technol. (2015), http://dx.doi.org/10.1016/j.yofte.2015.09.006
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F. Li et al. / Optical Fiber Technology xxx (2015) xxx–xxx
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FOE, with a 4th power algorithm. After these procedures, 2-point FFT is applied to convert the time domain 49-QAM signal into two 16QAM signal in frequency domain. QPSK partition algorithm is used to realize the CPE [15,16] and then the bit-error ratio (BER) can also be obtained with the BER counting. As blind equalization is applied for dual-subcarrier 16QAM-OFDM signal, there is no overhead and the capacity is 128 Gbit/s. In this experiment, the BER is counted over 20 106 bits (20 data sets, and each set contains 106 bits).
4. Experimental results In the single polarization dual-subcarrier 16QAM-OFDM transmission and reception, we set the optical source in the transmitter and receiver to be the same ECL and the linewidth of this ECL is 400 Hz, which means that there is no frequency offset and negligible phase noise during the reception of the single polarization dual-subcarrier 16QAM-OFDM signal. We can see the 49QAM signal after re-timing and CMMA, which is shown
Please cite this article in press as: F. Li et al., Transmission and reception of PDM dual-subcarrier coherent 16QAM-OFDM signals, Opt. Fiber Technol. (2015), http://dx.doi.org/10.1016/j.yofte.2015.09.006
F. Li et al. / Optical Fiber Technology xxx (2015) xxx–xxx
frequency power attenuation caused by the insufficient bandwidth. As dual-subcarrier 16QAM-OFDM demonstrates as a 49QAM in the time domain, such a high-order QAM signal should be vulnerable to high frequency power attenuation as compared to low-order QAM signal. At 26 GHz, the required OSNR is 16 dB, which indicates the OSNR penalty is 0.4 dB compared to the OSNR value obtained in Fig. 5 without WSS in the link at the BER of 2 102.
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In this paper, 16-Gbaud PDM dual-subcarrier 16QAM-OFDM signal transmission and reception are successfully demonstrated with blind equalization like a 49QAM signal for the first time. 128-Gbit/s PDM dual-subcarrier 16QAM-OFDM signal is successfully transmitted over 80-km SMF-28 without penalty and 0.4 dB OSNR penalty is observed in OBTB when the bandwidth of the channel is set at 26 GHz at the BER of 2 102. Acknowledgment
OSNR Penalty (dB)
OSNR Requirment (dB)
Fig. 5. Measured BER versus OSNR for 16-Gbaud dual-subcarrier 16QAM-OFDM signal.
18.0
5
0.0 17 18 19 20 21 22 23 24 25 26 27
Bandwidth (GHz) Fig. 6. Measured OSNR requirement and OSNR penalty versus bandwidth of WSS in OBTB at BER of 2.0 102.
in Fig. 3. The constellations are rotated due to the slight phase noise. Fig. 4 shows the constellations in different stages during DSP for PDM dual-subcarrier 16QAM-OFDM with OSNR of 28 dB in different stages of the offline DSP, which is described in detail in Fig. 2. FOE should be implemented before 2-point FFT according to our previous work as FFT may lead to the spread of noise induced by frequency offset [17]. The CPE is achieved after 2-point FFT with QPSK partition algorithm. Fig. 5 shows the measured BER of 16Gbaud dual-subcarrier 16QAM-OFDM signal versus OSNR. There is nearly no OSNR penalty observed after 80-km SMF-28 transmission. The BERs for the 128-Gbit/s PDM dual-subcarrier 16QAM-OFDM signal are less than hard-decision forward-error-correction (HD-FEC) threshold of 3.8 103 and soft-decision FEC (SD-FEC) threshold of 2.0 102 when the OSNR are higher than 18.4 dB and 15.6 dB, respectively. The constellations of 16-Gbaud PDM dual-subcarrier 16QAMOFDM signal after phase recovery with OSNR of 19.5 dB after 80-km SMF-28 transmission are shown in the inset of Fig. 5. In the OBTB case, we insert a wavelength selective switch (WSS) to adjust the bandwidth of channel to find the OSNR penalty for the 128-Gbit/s dual-subcarrier 16QAM-OFDM signal transmission with different optical channel bandwidth at the BER of 2 102. Fig. 6 shows measured OSNR requirement and OSNR penalty versus bandwidth of WSS in OBTB. OSNR penalty increases rapidly when the bandwidth of WSS decreases. We think the dual-subcarrier 16QAM-OFDM has a very low tolerance to high
This work is partly supported by China 863 project under grant number of 2012AA011303 and 2013AA010501. References [1] J. Armstrong, OFDM for optical communications, J. Lightwave Technol. 27 (3) (2009) 189–204. [2] S.L. Jansen, I. Morita, T.C.W. Schenk, N. Takeda, H. Tanaka, Coherent optical 25.8-Gb/s OFDM transmission over 4160-km SSMF, J. Lightwave Technol. 26 (1) (2008) 6–15. [3] W.-R. Peng, H. Takahashi, I. Morita, T. Tsuritani, Per-symbol-based digital backpropagation approach for PDM-CO-OFDM transmission systems, Opt. Express 21 (2) (2013) 1547–1554. [4] Z. Cao, H.P.A. van den Boom, E. Tangdiongga, T. Koonen, Interleaved and partial transmission interleaved optical coherent orthogonal frequency division multiplexing, Opt. Lett. 39 (7) (2014) 2179–2182. [5] M.-F. Huang, S. Zhang, E. Mateo, D. Qian, F. Yaman, T. Inoue, Y. Inada, T. Wang, EDFA-only WDM 4200-km transmission of OFDM-16QAM and 32QAM, IEEE Photonics Tech. Lett. 24 (17) (2012) 1466–1468. [6] J. Zhao, H. Shams, Fast dispersion estimation in coherent optical 16QAM fast OFDM systems, Opt. Express 21 (2) (2013) 2500–2505. [7] C. Zhao, C. Yang, F. Yang, F. Zhang, Z. Chen, A CO-OFDM system with almost blind phase noise suppression, IEEE Photon. Technol. Lett. 25 (17) (2013) 1723–1726. [8] J. Zhao, DFT-based offset-QAM OFDM for optical communications, Opt. Express 22 (1) (2014) 1114–1126. [9] Z. Cao, F. Li, Y. Liu, J. Yu, Q. Wang, C.W. Oh, Y. Jiao, N.C. Tran, H.P.A. van den Boom, E. Tangdiongga, A.M.J. Koonen, 61.3-Gbps hybrid fiber-wireless in-home network enabled by optical heterodyne and polarization multiplexing, J. Lightwave Technol. 32 (19) (2014) 3227–3233. [10] F. Li, J. Zhang, J. Yu, X. Li, Blind equalization for dual-polarization twosubcarrier coherent QPSK-OFDM signals, Opt. Lett. 39 (2) (2014) 201–204. [11] F. Li, J. Zhang, Z. Cao, J. Yu, X. Li, L. Chen, Y. Xia, Y. Chen, Transmission and reception of quad carrier QPSK-OFDM signal with blind equalization and overhead-free operation, Opt. Express 21 (25) (2013) 30999–31005. [12] T. Kobayashi, A. Sano, E. Yamada, E. Yoshida, Y. Miyamoto, Over 100 Gb/s electro-optically multiplexed OFDM for high-capacity optical transport network, J. Lightwave Technol. 27 (16) (2009) 3714–3720. [13] A. Sano, E. Yamada, H. Masuda, E. Yamazaki, T. Kobayashi, E. Yoshida, Y. Miyamoto, R. Kudo, K. Ishihara, Y. Takatori, No-guard-interval coherent optical OFDM for 100-Gb/s Long-Haul WDM transmission, J. Lightwave Technol. 27 (16) (2009) 3705–3713. [14] J. Yu, Z. Dong, N. Chi, 1.96 Tb/s (21 100 Gb/s) OFDM optical signal generation and transmission over 3200-km fiber, IEEE Photon. Technol. Lett. 23 (15) (2011) 1061–1063. [15] J. Zhang, J. Yu, N. Chi, Z. Dong, J. Yu, X. Li, L. Tao, Y. Shao, Multi-modulus blind equalizations for coherent quadrature duobinary spectrum shaped PMQPSK digital signal processing, J. Lightwave Technol. 31 (7) (2013) 1073–1078. [16] I. Fatadin, D. Ives, S.J. Savory, Laser linewidth tolerance for 16-QAM coherent optical systems using QPSK partitioning, IEEE Photon. Technol. Lett. 22 (9) (2010) 631–633. [17] L. Tao, J. Yu, Y. Fang, J. Zhang, Y. Shao, N. Chi, Analysis of noise spread in optical DFT-S OFDM systems, J. Lightwave Technol. 30 (20) (2012) 3219–3225.
Please cite this article in press as: F. Li et al., Transmission and reception of PDM dual-subcarrier coherent 16QAM-OFDM signals, Opt. Fiber Technol. (2015), http://dx.doi.org/10.1016/j.yofte.2015.09.006
Contents lists available at ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Regular Articles
Transmission and reception of PDM dual-subcarrier coherent 16QAM-OFDM signals Fan Li a,b,⇑, Junwen Zhang a,c, Jianjun Yu a,c, Xinying Li a,c a
ZTE TX, NJ 07960, USA Georgia Institute of Technology, Atlanta, GA 30332, USA c Key Laboratory for Information Science of Electromagnetic Waves (MoE), Fudan University, Shanghai, China b
a r t i c l e
i n f o
Article history: Received 17 February 2015 Revised 11 September 2015 Available online xxxx Keywords: OFDM Coherent detection Dual-subcarrier 16QAM 49QAM
a b s t r a c t In this paper, 16-Gbaud polarization-division-multiplexed (PDM) dual-subcarrier coherent optical orthogonal frequency division multiplexing (CO-OFDM) transmission and reception are successfully demonstrated without overhead. The in-phase and quadrature (I/Q) components of dual-subcarrier 16-ary quadrature amplitude modulation (QAM) OFDM signal are both seven-level signals in time domain, and thus can be equalized like a 49 QAM signal in time domain with cascaded multi-modulus algorithm (CMMA) equalization method. The experimental results show that there is no power penalty observed between optical back to back (OBTB) and after 80-km single-mode fiber-28 (SMF-28) with time domain CMMA equalization method. A 0.4 dB optical signal to noise ratio (OSNR) penalty in OBTB is observed when the bandwidth of channel is set at 26 GHz at the BER of 2.0 102. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction Coherent optical orthogonal frequency division multiplexing (CO-OFDM) has been intensively studied due to its high spectral efficiency (SE) and robustness to transmission impairments enabled by advanced frequency domain digital signal processing (DSP) [1–9]. The frequency offset estimation (FOE) and carrier phase estimation (CPE) are both implemented in the frequency domain with the aid of training sequences (TSs) and pilot tones for traditional optical OFDM transmission [3–5,9]. Moreover, channel response estimation and equalization is also done in the frequency domain. Frequency domain equalization (FDE) is simple and effective, but it needs overheads (TSs and pilot tones) during equalization which leads to the SE degradation. Few subcarriers OFDM (dual-subcarrier and quad-subcarrier) transmission and reception was proposed in our previous work [10,11] to avoid overhead during equalization. Compared to conventional OFDM with a large number of subcarriers, an unbeatable advantage of OFDM with a small number of subcarriers is that the peak-toaverage power ratio (PAPR) is quite low [10–13]. However, the FDE based on TSs and pilot tones cannot effectively work anymore in the few subcarriers OFDM scheme for two reasons. First, the frequency resolution decreases dramatically during channel ⇑ Corresponding author at: ZTE TX, NJ 07960, USA. E-mail address: [email protected] (F. Li).
estimation in the few subcarriers OFDM scheme. Second, the SE is significantly reduced as the pilot tones should be inserted in every OFDM symbol with few subcarriers. Fortunately, we found that the Quadrature Phase Shift Keyed (QPSK) OFDM signal with dual-subcarrier and quad-subcarrier are 9QAM [10] and 25QAM [11] in the time domain, respectively. With these special properties, QPSK-OFDM signal with dual-subcarrier and quad-subcarrier can be equalized in the time domain with cascaded multimodulus algorithm (CMMA) equalization method. The additional overhead in the FDE is totally avoided, but the SE is still limited as the modulation format is QPSK. In order to further improve the SE, higher order modulation formats must be applied. In this paper, we realized the transmission and reception of 16-Gbaud dual-subcarrier 16-ary quadrature amplitude modulation (16QAM) OFDM. Dual-subcarrier 16QAM OFDM signal demonstrates as a 49QAM signal in the time domain, and therefore it can also be blindly equalized with CMMA equalization method. The overhead in the traditional optical OFDM transmission system can be completely removed in the dual-subcarrier optical 16QAM OFDM transmission system with blind equalization. The main differences between our scheme with dual-subcarrier all optical 16QAM OFDM exist in the generation and detection. For dualsubcarrier all optical 16QAM OFDM signal generation, two frequency-locked subcarriers should be generated before 16QAM signal modulation. The channel spacing between two subcarriers should be exactly equal to the baud rate of data to make the
http://dx.doi.org/10.1016/j.yofte.2015.09.006 1068-5200/Ó 2015 Elsevier Inc. All rights reserved.
Please cite this article in press as: F. Li et al., Transmission and reception of PDM dual-subcarrier coherent 16QAM-OFDM signals, Opt. Fiber Technol. (2015), http://dx.doi.org/10.1016/j.yofte.2015.09.006
2
F. Li et al. / Optical Fiber Technology xxx (2015) xxx–xxx
2-subcarrier orthogonal. The generation of such two subcarriers is usually complicated. At receiver, after optical to electrical (O/E) conversion, a digital filter is used to separate the dual-subcarrier, and then DSP is applied for each subcarrier [12–14]. While in our scheme, dual-subcarrier is processed at the same time with the blind CMMA like a 49QAM. Thus, the receiver is simpler. To our knowledge, transmission and reception of 128-Gbit/s p olarization-division-multiplexed (PDM) dual-subcarrier 16QAM OFDM with blind equalization like a 49QAM signal is first demonstrated in this paper. A comparison between optical back to back (OBTB) and after 80-km single-mode fiber-28 (SMF-28) was performed, and the results show that there was no power penalty observed. We also measured the optical signal to noise ratio (OSNR) penalty versus the bandwidth of the channel and the experimental results show that a 0.4 dB optical signal to noise ratio (OSNR) penalty in OBTB is observed when the bandwidth of channel is set at 26 GHz at the BER of 2.0 102. 2. Principle According to our previous work [10], after 2-point Inverse fast Fourier transform (IFFT), the two samples of one dual-subcarrier OFDM symbol can be expressed as
1 sð0Þ ¼ pffiffiffi ðc0 c1 Þ; 2
1 sð1Þ ¼ pffiffiffi ðc0 þ c1 Þ: 2
ð1Þ
where c0 and c1 represent the input data with corresponding modulation formats modulated onto 2 subcarriers, respectively. s(0) and s(1) denote the symbols after IFFT. In the quad-subcarrier OFDM case, the output of 4-point IFFT can be expressed as [11]
sð0Þ ¼ 12 ðc0 þ c1 þ c2 þ c3 Þ; sð1Þ ¼ 12 ðc0 þ jc1 c2 jc3 Þ; sð2Þ ¼ 12 ðc0 c1 þ c2 c3 Þ;
ð2Þ
sð3Þ ¼ 12 ðc0 jc1 c2 þ jc3 Þ: where c0 , c1 , c2 and c3 represent the input data with corresponding modulation formats modulated onto 4-subcarrier, respectively. And s(0), s(1), s(2) and s(3) denote the corresponding 4 symbols after 4-point IFFT. For input data symbols with different modulation formats, we can get the corresponding output data symbols of dual-subcarrier and Quad-subcarrier OFDM according to Eq. (1) and Eq. (2), respectively. Table 1 shows the input data symbols and output data symbols after 2-point IFFT and 4-point IFFT, respectively. The QPSK in the frequency domain is transformed to 9-QAM and 25-QAM in time domain after 2-point and 4-point IFFT, respectively. 21QAM and 88QAM can be obtained with 2-point and 4-point IFFT of rectangle 8QAM. 16QAM input data is changed to 49QAM and 169QAM in the dual-subcarrier and quad-subcarrier scheme. The formats of output data of dual-subcarrier and quad-subcarrier OFDM with 32QAM in the time domain are 109QAM and 401QAM, respectively. 225QAM and 841QAM can be obtained with 2-point and 4-point IFFT of 64QAM. In this paper, we only discuss the transmission and reception of dual-subcarrier 16QAM OFDM, and the dual-subcarrier 16QAM OFDM can be equalized like 49QAM signal in time domain with CMMA equalization method. 3. Experimental setup Fig. 1 shows the experimental setup of the dual-subcarrier coherent optical 16QAM OFDM transmission system. At the transmitter, an external cavity laser (ECL) at 1557.04 nm with less than 100-kHz linewidth and maximum output power of 14.5 dBm is
modulated by an intensity Mach–Zehnder modulator driven by an electrical baseband dual-subcarrier 16QAM OFDM signal. The dual-subcarrier 16QAM OFDM signal is first generated offline in Matlab software, and then 4-time interpolation is implemented to generate the 16-Gbaud dual-subcarrier 16QAM OFDM signal with a 64 GSa/s sampling rate digital-to-analog convertor (DAC). Two linear electrical amplifiers (EAs) are used to boost the dualsubcarrier 16QAM OFDM signal before electrical to optical (O/E) modulation. For optical OFDM modulation, two parallel Mach– Zehnder modulators in the in-phase/quadrature (I/Q) modulator are both biased at the null point and the phase difference between the upper and lower branches of the I/Q modulator is controlled at p/2. Polarization multiplexing is realized by a polarization multiplexer, comprising a polarization-maintaining optical coupler (OC) to halve the signal into two branches, an optical delay line (DL) to remove the correlation between X-polarization and Y-polarization by providing a time delay, an optical attenuator to balance the power of two branches and a polarization beam combiner (PBC) to recombine the signal. The generated signal is boosted via an erbium doped fiber amplifier (EDFA) before launched into 80-km SMF-28. The 80 km SMF-28 has 18 dB average loss and 17 ps/km/nm chromatic dispersion (CD) at 1550 nm without optical dispersion compensation. The output signal is then injected into an integrated coherent receiver to implement optical to electrical detection. The output signal is then injected into the integrated coherent receiver to implement optical to electrical detection. After integrated coherent receiver, the signal is captured by the real-time oscilloscope with 80 GSa/s sampling rate. It is worth noting that here we use 64 GSa/s sampling rate DAC and 80 GSa/s sampling rate analog-to-digital converter (ADC) to generate and capture 16Gbaud dual-subcarrier 16QAM-OFDM signal in the experimental setup. For practical use, DAC with 16 GSa/s sampling rate is enough to generate 16Gbaud dual-subcarrier 16QAM-OFDM signal. Without frequency offset, 16 GSa/s sampling rate ADCs are enough to capture 16Gbaud dual-subcarrier 16QAMOFDM signal, while in the practical implementation the sampling rate of ADCs should be higher than 16 GSa/s as frequency offset is inevitable. The resolutions of the DAC and the ADC in real-time oscilloscope are both 8 bits. The electrical eye diagram of I component of dual-subcarrier 16QAM-OFDM signal is inserted as inset (a) in Fig. 1. The levels of eye-diagrams of I and Q components are both 7, thus the signal constellation type in time domain is 49QAM. The optical spectra before and after 80-km SMF-28 transmission with 0.1-nm resolution are shown in Fig. 1(b). The DSP for receiver (Rx)-offline processing of the dualsubcarrier 16QAM-OFDM signal is shown in Fig. 2. At the receiver, the dual-subcarrier 16QAM OFDM can be equalized with CMMA method like 49QAM without any additional overhead. The main difference of DSP algorithms in the receiver as compared to dualcarrier QPSK-OFDM include CMMA and phase noise compensation. Dual-subcarrier 16QAM-OFDM signal is equalized like a 49 QAM signal in time domain with CMMA. Dual-subcarrier 16QAMOFDM signal coverages onto ten different rings. The phase noise is estimated and compensated after 2-point IFFT. Without noise the output of IFFT should be both 16QAM signal, thus QPSKpartition algorithm is applied to realize CPE. The detailed DSP algorithms are below. After the integrated receiver, four signal components are first captured by the real-time oscilloscope with 80 GSa/s sample rate. Secondly, a T/2-spaced time-domain finite-impulse-response (FIR) filter is firstly used for chromatic dispersion compensation (CDC), where the filter coefficients are calculated from the known fiber CD transfer function using the frequency-domain truncation method. Thirdly, the 10-radius CMMA is used to retrieve the modulus of the PDM dualsubcarrier 16QAM OFDM like a 49-QAM signal and realize polarization de-multiplexing. The subsequent step is to realize the
Please cite this article in press as: F. Li et al., Transmission and reception of PDM dual-subcarrier coherent 16QAM-OFDM signals, Opt. Fiber Technol. (2015), http://dx.doi.org/10.1016/j.yofte.2015.09.006
F. Li et al. / Optical Fiber Technology xxx (2015) xxx–xxx
3
Table 1 The output data of dual-subcarrier and quad-subcarrier OFDM.
Fig. 1. Experimental setup of dual-subcarrier 16QAM-OFDM transmission system. (a) Electrical eye diagram and (b) optical spectra.
Please cite this article in press as: F. Li et al., Transmission and reception of PDM dual-subcarrier coherent 16QAM-OFDM signals, Opt. Fiber Technol. (2015), http://dx.doi.org/10.1016/j.yofte.2015.09.006
4
F. Li et al. / Optical Fiber Technology xxx (2015) xxx–xxx
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FOE, with a 4th power algorithm. After these procedures, 2-point FFT is applied to convert the time domain 49-QAM signal into two 16QAM signal in frequency domain. QPSK partition algorithm is used to realize the CPE [15,16] and then the bit-error ratio (BER) can also be obtained with the BER counting. As blind equalization is applied for dual-subcarrier 16QAM-OFDM signal, there is no overhead and the capacity is 128 Gbit/s. In this experiment, the BER is counted over 20 106 bits (20 data sets, and each set contains 106 bits).
4. Experimental results In the single polarization dual-subcarrier 16QAM-OFDM transmission and reception, we set the optical source in the transmitter and receiver to be the same ECL and the linewidth of this ECL is 400 Hz, which means that there is no frequency offset and negligible phase noise during the reception of the single polarization dual-subcarrier 16QAM-OFDM signal. We can see the 49QAM signal after re-timing and CMMA, which is shown
Please cite this article in press as: F. Li et al., Transmission and reception of PDM dual-subcarrier coherent 16QAM-OFDM signals, Opt. Fiber Technol. (2015), http://dx.doi.org/10.1016/j.yofte.2015.09.006
F. Li et al. / Optical Fiber Technology xxx (2015) xxx–xxx
frequency power attenuation caused by the insufficient bandwidth. As dual-subcarrier 16QAM-OFDM demonstrates as a 49QAM in the time domain, such a high-order QAM signal should be vulnerable to high frequency power attenuation as compared to low-order QAM signal. At 26 GHz, the required OSNR is 16 dB, which indicates the OSNR penalty is 0.4 dB compared to the OSNR value obtained in Fig. 5 without WSS in the link at the BER of 2 102.
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In this paper, 16-Gbaud PDM dual-subcarrier 16QAM-OFDM signal transmission and reception are successfully demonstrated with blind equalization like a 49QAM signal for the first time. 128-Gbit/s PDM dual-subcarrier 16QAM-OFDM signal is successfully transmitted over 80-km SMF-28 without penalty and 0.4 dB OSNR penalty is observed in OBTB when the bandwidth of the channel is set at 26 GHz at the BER of 2 102. Acknowledgment
OSNR Penalty (dB)
OSNR Requirment (dB)
Fig. 5. Measured BER versus OSNR for 16-Gbaud dual-subcarrier 16QAM-OFDM signal.
18.0
5
0.0 17 18 19 20 21 22 23 24 25 26 27
Bandwidth (GHz) Fig. 6. Measured OSNR requirement and OSNR penalty versus bandwidth of WSS in OBTB at BER of 2.0 102.
in Fig. 3. The constellations are rotated due to the slight phase noise. Fig. 4 shows the constellations in different stages during DSP for PDM dual-subcarrier 16QAM-OFDM with OSNR of 28 dB in different stages of the offline DSP, which is described in detail in Fig. 2. FOE should be implemented before 2-point FFT according to our previous work as FFT may lead to the spread of noise induced by frequency offset [17]. The CPE is achieved after 2-point FFT with QPSK partition algorithm. Fig. 5 shows the measured BER of 16Gbaud dual-subcarrier 16QAM-OFDM signal versus OSNR. There is nearly no OSNR penalty observed after 80-km SMF-28 transmission. The BERs for the 128-Gbit/s PDM dual-subcarrier 16QAM-OFDM signal are less than hard-decision forward-error-correction (HD-FEC) threshold of 3.8 103 and soft-decision FEC (SD-FEC) threshold of 2.0 102 when the OSNR are higher than 18.4 dB and 15.6 dB, respectively. The constellations of 16-Gbaud PDM dual-subcarrier 16QAMOFDM signal after phase recovery with OSNR of 19.5 dB after 80-km SMF-28 transmission are shown in the inset of Fig. 5. In the OBTB case, we insert a wavelength selective switch (WSS) to adjust the bandwidth of channel to find the OSNR penalty for the 128-Gbit/s dual-subcarrier 16QAM-OFDM signal transmission with different optical channel bandwidth at the BER of 2 102. Fig. 6 shows measured OSNR requirement and OSNR penalty versus bandwidth of WSS in OBTB. OSNR penalty increases rapidly when the bandwidth of WSS decreases. We think the dual-subcarrier 16QAM-OFDM has a very low tolerance to high
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Please cite this article in press as: F. Li et al., Transmission and reception of PDM dual-subcarrier coherent 16QAM-OFDM signals, Opt. Fiber Technol. (2015), http://dx.doi.org/10.1016/j.yofte.2015.09.006