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A flexible optical OFDMA-PON upstream scheme based on modulation format conversion technique
Optics & Laser Technology 90 (2017) 237–241
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
A flexible optical OFDMA-PON upstream scheme based on modulation format conversion technique
MARK
⁎
Cheng Ju , Na Liu, Zhiguo Zhang, Xue Chen State Key Lab of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications), P.O. Box 128, #10 XiTuCheng Road, HaiDian District, Beijing 100876, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Orthogonal frequency division multiple Passive Optical Network Upstream transmission Modulation format conversion
This paper introduces a novel architecture for next generation passive optical network (PON) upstream transmission based on the employment of optical orthogonal frequency division multiple access (OFDMA) and modulation format conversion technique. The upstream data could be allocated according to the time slot, subcarrier, and modulation format, which is more flexible to dynamically allocate the bandwidth among the ONUs. The feasibility of the proposed upstream scheme is verified in a 10 Gbps data rate, BPSK to QPSK conversion and a 20 Gbps data rate, QPSK to 16 QAM conversion experiments, respectively. The main advantages and issues of the proposed scheme that are under research are also discussed.
1. Introduction Generally speaking, current passive optical network (PON) according to the data distribution can be classified into three categories: 1) those based on time division multiplexing (TDM) using single carrier modulation, such as gigabit PON (GPON) [1] and Ethernet PON (EPON) [2], which are the most deployed and mature PON architectures; 2) those based on frequency division multiplexing (FDM) [3,4], which is the upgradeable solution of TDM PON system and has an ability to transmit data at different bit-rates on each subcarrier and with a higher spectral efficiency; 3) those based on time and frequency division multiplexing using orthogonal frequency division multiplexing (OFDM) technique [5–7]. OFDM has been considered a promising candidate for future high-speed optical transmission technology. OFDM is a multicarrier transmission technology that transmits a high-speed data stream by splitting it into multiple parallel low-speed orthogonal channels. Therefore, OFDM has the highest spectral efficiency and enable dynamic bandwidth allocation among various applications both in time and frequency domain dimensions [8,9]. In addition, the digital signal processing (DSP)-based OFDM-PON can provide other highly desirable features such as resilience to chromatic dispersion, polarization mode dispersion (PMD) [10] and nonlinear distortion [5,6]. According to the different structure of optical receivers, the optical OFDM system can be categorized as coherent and direct detection (DD) systems. Coherent optical OFDM has better sensitivity. However, relative to DD system, the coherent receiver requires an optical local
⁎
laser, 90° optical hybrids and additional digital signal processing to compensate the distortion caused by the frequency offset and linewidth [11]. These devices or techniques increase the complexity and the cost of the system. Therefore, in the view of costs, the simple structure of DD-OFDM system is attractive for the future low-cost PON systems [5,12]. In this paper, we propose a novel PON upstream scheme based on DD-OFDM and modulation format conversion technique. The conventional directly intensity modulation and direct detection scheme are used for ONU and OLT side, respectively [13,14]. The upstream data could be allocated according to the time slot, subcarrier, and modulation format. The higher order modulated OFDM signal is generated by direct detection after the superposition of two lower order modulated upstream signals with different optical powers and wavelengths. The upstream signal of each ONU could be distinguished according to the different modulation format, which is more flexible for dynamic bandwidth allocation. The principle of the proposed upstream scheme and the impact of key parameters, such as symbol delay and amplitude deviation of two lower order modulated signals, are introduced and analyzed in the following, respectively. Finally, to validate the architecture feasibility, a 10 Gbps data rate, BPSK to QPSK conversion and a 20 Gbps data rate, QPSK to 16 QAM conversion experiments are carried out, respectively. The impact of symbol delay and amplitude deviation of two superposed signals is evaluated in the following experiment. Compared with symbol delay, the results show that the system performance is more robust to amplitude deviation.
Corresponding author. E-mail address: [email protected] (C. Ju).
http://dx.doi.org/10.1016/j.optlastec.2016.11.017 Received 5 March 2016; Received in revised form 22 July 2016; Accepted 22 November 2016 Available online 29 November 2016 0030-3992/ © 2016 Elsevier Ltd. All rights reserved.
Optics & Laser Technology 90 (2017) 237–241
C. Ju et al.
Spectrum before IM
Spectrum before IM
ONU3
ONU3
Power
ONU2
ONU1
Freq. ONU1
OFDM TX
Freq. ONU1
(ii)
Freq.
(i)
PD
SSMF
WDM
Freq.
OLT
IM SSMF
Freq.
ONU2 Power
ONU2
ONU3 Freq.
ADC
OFDM RX
(a)
BPSK
BPSK
BPSK
8QAM
QPSK
(i)
(ii)
(iii)
(vi)
16QAM
(b)
BPSK
QPSK
8QAM
QPSK
QPSK
16QAM
Fig. 1. (a) Principle of the proposed OFDMA-PON Upstream scheme. The electrical spectrum of each ONU before intensity modulation for (i) the conventional upstream scheme and (ii) the proposed scheme. (b) The generation principle of higher order modulated constellation. The generation of (i) QPSK, (ii) 8 QAM and (iii, iv) 16 QAM.
2. Principle of OFDMA-PON upstream scheme
after direct detection can be expressed as
The proposed OFDMA-PON upstream scheme based on modulation format conversion technique is illustrated in Fig. 1(a). For the conventional upstream scheme, different ONU uses different subcarrier. However, in the proposed scheme, the two ONUs could use the same subcarrier, which are distinguished by modulation format. The transmitter at each ONU contains a DSP module and optics intensity modulation module [13,14]. Each ONU is equipped with a tunable laser source. The tunable laser can be tunable to any of wavelengths. Different wavelength for different ONU in the same OFDM symbol period is necessary for preventing from optical beating interference. The output double-sideband (DSB) optical signals of two ONUs can be written as
R = E1, CD 2 + E2, CD 2
⎛ E1=e jω1t ⎜1 + ⎝
N
∑n =1
⎞ v1, ncos (nωt +θ1, n ) ⎟ ⎠
(1)
⎞ v2, ncos (nωt +θ2, n ) ⎟ ⎠
(2)
N
N
∑n =1
(5) where nθτ represents the phase offset caused by symbol delay of two ONUs, rn=v1, ncos (n2θ1, D ) e jθ1, n +v2, ncos (n2θ2, D ) e j (θ 2, n+ nθτ ) , and θn is the compound angle of rn . The subcarrier powers of two ONUs are proportional to cos 2 (n2θ1, D ) and cos 2 (n2θ2, D ) [15,16]. Then, the higher order constellation constructed by rn is obtained. To demodulate the higher order constellation signal correctly, the signal power and the initial phase of the two lower order upstream signals must be controlled reasonably. The principle of the modulation format conversion is shown in Fig. 1(b). As shown in (i), the QPSK signal could be achieved by combining two BPSK signals, where two BPSK signals have the same amplitudes, and BPSK2 has a phase rotation of 90°. 8 QAM signal could be obtained by combining BPSK1 and QPSK2 signals as shown in (ii), where the amplitude of BPSK1 should be double that of QPSK2, v1, n : v2, n=2 : 1. 16 QAM signal could be achieved by combining BPSK and 8 QAM, as shown in (iii), or two QPSK signals as shown in (iv). The amplitude of BPSK should be double that of 8 QAM, and BPSK has an initial phase rotation of 90°. The amplitude of QPSK1 should be double that of QPSK2, v1, n : v2, n=2 : 1. Each point of the higher order constellation is obtained as the product of a point from the first constellation and a point from the second constellation. That is to say the higher order constellation is generated by the superposition of these two lower order signals. The amplitude of two lower order signals could be controlled by adjusting the optical modulation index (OMI) or the launch power of two ONUs. The initial phase of two ONUs could be controlled at the symbol mapping module in the transmitter. In the actual network, the signals from different transmitters are difficult to achieve absolute symbol synchronization, which is caused by the phase noise of symbol timing error or equipment aging. Therefore, a special rotation of higher order constellation is happened as shown in Fig. 2(a) and (b). The impact of amplitude deviation is shown in Fig. 2(c) and (d). Obviously, the Euclidean distance is reduced. For
where ω1 and ω 2 are the angular frequency of optical carriers, N denotes the number of subcarriers in an OFDM symbol, v1, n , v2, n , and θ1, n , θ2, n are the amplitudes and phases of the nth subcarrier, and the second and higher-order terms after power series expansion are disregarded [15]. After transmission with the distance of L1 and L 2 , the group-velocity dispersion parameter of β2 and no fiber loss, then the optical signals become
⎛ E1, CD=e jω1t ⎜1 + ⎝
N
∑n =1
v1, n e j (n
2θ 1, D ) cos (nωt + θ
⎞
1, n ) ⎟
⎠
(3)
and
⎛ E2, CD=e jω2 t ⎜1 + ⎝
N
∑n =1 ω 2 /2 ,
v2, n e j (n
2θ
2, D ) cos (nωt + θ
⎞
2, n ) ⎟
⎠
N
+ ∑n =1 v2, ncos (n2θ2, D ) cos (nωt +θ2, n+nθτ )= ∑n =1 rn cos (nωt +θn )
and
⎛ E2=e jω2 t ⎜1 + ⎝
N
= ∑n =1 v1, ncos (n2θ1, D ) cos (nωt +θ1, n )
(4)
ω 2 /2
and θ2, D=β2 L 2 [15]. The conventional DDwhere θ1, D=β2 L1 OFDM received structure is used for OLT side. Then the received signal 238
Optics & Laser Technology 90 (2017) 237–241
C. Ju et al.
BPSK1
BPSK2
QPSK Constellation with rotation
QPSK1
QPSK2
16QAM Constellation with rotation
BPSK1
BPSK2
QPSK Constellation
QPSK1
QPSK2
16QAM Constellation
Fig. 2. The rotated constellation of (a) QPSK signal and (b) 16 QAM signal caused by symbol delay. The constellation of (c) QPSK signal and (d) 16 QAM signal with amplitude deviation.
5.5 GHz is inserted after AWG to prevent high frequency residual images. The electrical signal after microwave amplifier is used to drive a single drive MachZehnder modulator (MZM), which is biased at its quadrature point to generate an optical DSB signal. The wavelength of the laser1 and laser2 are set to be 1550.32 nm and 1549.12 nm, respectively. To simplify the complexity of experiment, the amplitude of constellation for each ONU is adjusted by controlling the launch power and the OMI of two transmitters are set to be 0.25. Specially designed training symbols with subcarrier interleaving [17] and lower peak to average power ratio (PAPR) are used to protect the training symbols from being affected by subcarrier-to-subcarrier intermixing interference (SSII), which would improve the accuracy of channel estimation. A 3 dB optical coupler is used after 50 km SSMF transmission. An erbium-doped fiber amplifier (EDFA) is inserted after optical coupler. After 100 km SSMF transmission, a fiber Bragg grating (FBG) filter is used to mitigate amplified spontaneous emission (ASE) noise of EDFA, and an optical attenuator is used to adjust the received power. The received electrical signal is captured by a LeCroy® digital oscilloscope WaveMaster813ZI-A with a sampling rate of 40 GSa/s. The digital signal is down-sampling to 12 GSa/s, and off-line process using MATLAB® digital signal processing program. The demodulation process includes removing CP, serial-to-parallel conversion, FFT, linear equalization, symbol demapping, parallel-to-serial conversion, and bit error rate (BER) calculation. To verify the feasibility, two examples are carried out. The modulation formats of two ONUs used in the first example are both of BPSK. The launch power for two ONUs are both of 3 dB m. The phase rotation of ONU2 after symbol mapping is set to be 90°. After
sake of brevity, the description is shown only for QPSK and 16 QAM. The other modulation formats follow the same trend. For each subcarrier, the ideal constellation with amplitude and phase deviation could be obtained by using training symbols. The demodulation of data subcarrier is realized by the calculation and comparison of the minimum Euclidean distance between the ideal constellation and the signal after linear equalization. It is emphasized that the demodulation process would be failed when two rotated constellation points are overlapped. Therefore, the initial phase and the amplitude of two ONUs should be accurately controlled. The impact of amplitude and phase deviation on system performance is experimentally evaluated in the following.
3. Experimental setup and results The experimental setup for the proposed OFDMA-PON upstream transmission system over 100 km SSMF is shown in Fig. 3(a). In this work, two transmitters are employed to simulate the scene of upstream transmission. The baseband electrical OFDM signals are generated by an arbitrary waveform generator (AWG M8190A) using the MATLAB® program. The signal processing of OFDM transmitter consists of serialto-parallel conversion, symbol mapping, phase rotation, hardware imperfection compensation [5], IFFT, parallel-to-serial conversion, and adding cyclic prefix (CP). The sampling rate and digital to analog converter (DAC) resolution of AWG are 12 GSa/s and 8 bits, respectively. The OFDM signal contains 107 subcarriers to occupy 5 GHz bandwidth with an inverse fast Fourier transform (IFFT) size of 256. CP of 1/8 is used. A low pass filter (LPF) with 3 dB bandwidth of
ONU1
ONU2
Fig. 3. (a) The experimental setup of the proposed OFDMA-PON upstream scheme. (b) The QPSK constellation after equalization at subcarrier number of (i) n=1, (ii) n=50, (iii) n=100, and the 16 QAM constellation at subcarrier number of (iv) n=1, (v) n=50, (vi) n=100. (Red points: constellation of training symbols. Blue points:constellation of data symbols). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. The BER versus the received power after 100 km SSMF at different symbol delay for (a) QPSK and (b) 16 QAM. The BER versus the received power at different power deviation for (c) QPSK and (d) 16 QAM. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
case, there are only 0.7 dB power penalty for two QPSK upstream case at BER of 3. 8 × 10−3 with symbol delay of 1 T, 2 T, and 4 T. The SNR of each subcarrier has not changed for different symbol delay cases at the same received power. The reason of power penalty is that special rotation of 16 QAM constellation decreases the Euclidean distance. The BER of QPSK and 16 QAM signals versus the received power at different power deviation after 100 km SSMF transmission are shown in Fig. 4(c) and (d). The symbol delay of two coupled optical signal for two examples is set to be zero by adjusting the time delay of AWG output signal. As shown in Fig. 4(c), the three curves with power ratio of P1:P2=0.75, 1, and 1.25 are overlapped in the first example. For the second example, the three curves with power ratio of P1:P2=1.75, 2, and 2.25 are almost overlapped. Therefore, the proposed OFDMA-PON upstream scheme with BPSK–QPSK conversion and QPSK–16 QAM conversion is robust to amplitude deviation of two coupled signals, but sensitive to symbol delay. Finally, the BER performance of QPSK and 16 QAM for ONU1 and ONU2 are shown in Fig. 5(a) and (b). The amplitude ratios of two BPSK signals and two QPSK signal are fixed to v1, n : v2, n=1 : 1 and v1, n : v2, n=2 : 1, respectively. The symbol delay is set to be zero. For the case of two BPSK upstream transmission, the BER curves of ONU1 and ONU2 at B2B and after 100 km SSMF transmission are overlapped, respectively. The power penalty after transmission over 100 km SSMF at FEC limit is 0.6 dB. For the case of two QPSK upstream transmission, the FEC limit for ONU1 and ONU2 at B2B can be obtained at the received power of −14.2 dB m and −13.2 dB m. After 100 km SSMF transmission, the FEC limit can be obtained at the received power of −12.1 dB m and −10.8 dB m, respectively. Obviously, ONU1 has a better performance. The reason is that the optical signal power of ONU1 is twice larger than that of ONU2.
100 km SSMF transmission and equalization, the QPSK constellation at subcarrier number of n=1, 50 and 100 are shown in Fig. 3(b) (i–iii). The constellation of training symbols and data symbols are shown by red and blue points, respectively. The received power is set to be −3 dB m. The two rotated QPSK constellation points for the case of n=100 are overlapped. In this situation, the two constellation points could not be distinguished. The modulation formats of two ONUs used in the second example are both of QPSK. The launch power for ONU1 and ONU2 are 4 dB m and 1 dB m, respectively. The 16 QAM constellation of data symbols and training symbols at subcarrier number of n=1, 50 and 100 are shown by blue and red points in Fig. 4(b) (iv–vi). The phase rotation of 16 QAM constellation, as shown in (v) and (vi), decreases the Euclidean distance. Nonetheless, the constellation points could be distinguished between each other. To give an analysis of the influence of symbol delay and amplitude deviation on system performance, hardware imperfection compensator [5] is used before IFFT to get a flat signal-to-noise ratio (SNR) curves for all data subcarriers. BER of QPSK and 16 QAM signals versus the received power at different symbol delay after 100 km SSMF transmission are shown in Fig. 4(a) and (b). The amplitude ratios of two BPSK signals and two QPSK signal are fixed to v1, n : v2, n=1 : 1 and v1, n : v2, n=2 : 1, respectively. In the experiment, the signal of ONU1 is synchronized, which is used as a time reference. The symbol delay is adjusted by setting the sampling time delay of ONU2 signal at AWG. For the case of two BPSK upstream transmission with symbol delay of 1 T, 2 T, and 4 T, the BER fails to meet the requirements of FEC limit 1 (3. 8 × 10−3), where T = 12 ×10−9 (s) is the sampling interval of DAC. The reason is that the two constellation points of rotated QPSK signal for some subcarriers are overlapped as shown in Fig. 3(b) (iii), which would lead to symbol decision error. Compared with zero symbol delay
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Fig. 5. The BER versus the received power at B2B and after 100 km SSMF transmission for (a) QPSK and (b) 16 QAM.
4. Conclusions [6]
In this paper, we propose a flexible PON upstream scheme based on direct detection OFDMA and modulation format conversion techniques. The upstream data could be allocated according to the time slot, subcarrier, and modulation format. Data distribution in three-dimension could improve the flexibility of dynamic bandwidth allocation. The feasibility of the proposed scheme is confirmed by two experimental examples. The impacts of symbol delay and amplitude deviation of the two upstream signals on system performance are analyzed and evaluated.
[7]
[8]
[9] [10]
Acknowledgement [11]
This study is supported by National Natural Science Foundation of China (No. 61571061).
[12]
[13]
References
[14]
[1] F. Selmanovic, E. Skaljo, GPON in telecommunication network, in: Proceedings of the ICUMT, Oct. 18–20, 2010, pp. 1012–1016 [2] R. Davey, J. Kani, F. Bourgart, K. McCammon, Options for future optical access networks, IEEE Commun. Mag. 44 (10) (2006). [3] B. Charbonnier et al., Low Complexity FDM/FDMA Approach for Future PON, OFC/NFOEC, OThA3.7, 2013 [4] S.D. Le1, A. Lebreton1, F. Saliou, Up to 60 km bidirectional transmission of a 16 channels×10 Gb/s FDM-WDM PON based on self-seeded reflective semiconductor optical amplifiers, in: Proceesings of the OFC 2014, Paper Th3G.8, San Francisco, CA, March 2014 [5] C.Ju, X.Chen, N.Liu, Q.Zhang, H.Wang, OFDM PON Downstream Scheme with
[15] [16]
[17]
241
Symbol Predistortion and Scalable Receiver Frontend, in: Proceeding of the Optical Fiber Communication Conference Paper Th1H.4, 2015. Cheng Ju, Na Liu, Xue Chen, Zhiguo Zhang, SSBI mitigation in A-RF-tone-based VSSB-OFDM system with a frequency-domain volterra series equalizer, J. Light. Technol. 33 (23) (2015) 4997–5006. Hui Yang, Juhao Li, Bangjiang Lin, Yangsha Wan, Yong Guo, Lixin Zhu, Li Li, Yongqi He, Zhangyuan Chen, DSP-based evolution from conventional TDM-PON– TDM-OFDM-PON, J. Light. Technol. 31 (16) (2015) 3035–3041. W.Wei, J.Hu, L.Zeng, Optical Orthogonal Frequency Division Multiple Access (OFDMA)-based optical access/metro ring networks, in Proceedings OFC Paper JWA123, 2008. Guoying Zhang, Marc De Leenheer, Annalisa Morea, "A survey on OFDM-based elastic core optical networking," IEEE, Commun. Surv. Tutor. 15 (1) (2013). N.Cvijetic, L.Xu, T.Wang, Adaptive PMD compensation using OFDM in long-haul 10 Gb/s DWDM systems, in: Proceedings of the 2007 IEEE/OSA Optical Fiber Communication conference (OFC/NFOEC), Anaheim, CA, Mar paper OTuA5, 2007. W. Shieh, C. Athaudage, Coherent optical orthogonal frequency division multiplexing, Electron. Lett. 42 (2006) 587–589. B.J.C. Schmidt, A.J. Lowery, J. Armstrong, Experimental demonstrations of electronic dispersion compensation for long-haul transmission using direct-detection optical OFDM,, J. Light. Technol. 26 (1) (2008) 196–203. D.Qian, J.Hu, J.Yu, and et al., Experimental demonstration of a novel OFDM-A Based 10 Gb/s PON Architecture, in Proceedings of the European Conference and Exhibition on Optical Communication (ECOC), Paper. 5.4. 1, 2007. Cheng Ju, Na Liu, Xue Chen, Iteration SSII cancellation in DD-OFDM PON upstream scheme, Opt. Commun. 364 (2016) 9–12. Chia-Chien Wei, Small-signal analysis of OOFDM signal transmission with directly modulated laser and direct detection, Opt. Lett. 36 (2) (2011) 151–153. Bangjiang Lin, Juhao Li, Hui Yang, Yangsha Wan, Yongqi He, Zhangyuan Chen, Comparison of DSB and SSB Transmission for OFDM-PON, J. Opt. Commun. Netw. 4 (11) (2012). W.R. Peng, B. Zhang, K.M. Feng, X. Wu, A.E. Willner, S. Chi, Spectrally efficient direct-detected OFDM transmission incorporating a tunable frequency gap and an iterative detection techniques, J. Light. Technol. 27 (24) (2009) 5723–5735.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
A flexible optical OFDMA-PON upstream scheme based on modulation format conversion technique
MARK
⁎
Cheng Ju , Na Liu, Zhiguo Zhang, Xue Chen State Key Lab of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications), P.O. Box 128, #10 XiTuCheng Road, HaiDian District, Beijing 100876, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Orthogonal frequency division multiple Passive Optical Network Upstream transmission Modulation format conversion
This paper introduces a novel architecture for next generation passive optical network (PON) upstream transmission based on the employment of optical orthogonal frequency division multiple access (OFDMA) and modulation format conversion technique. The upstream data could be allocated according to the time slot, subcarrier, and modulation format, which is more flexible to dynamically allocate the bandwidth among the ONUs. The feasibility of the proposed upstream scheme is verified in a 10 Gbps data rate, BPSK to QPSK conversion and a 20 Gbps data rate, QPSK to 16 QAM conversion experiments, respectively. The main advantages and issues of the proposed scheme that are under research are also discussed.
1. Introduction Generally speaking, current passive optical network (PON) according to the data distribution can be classified into three categories: 1) those based on time division multiplexing (TDM) using single carrier modulation, such as gigabit PON (GPON) [1] and Ethernet PON (EPON) [2], which are the most deployed and mature PON architectures; 2) those based on frequency division multiplexing (FDM) [3,4], which is the upgradeable solution of TDM PON system and has an ability to transmit data at different bit-rates on each subcarrier and with a higher spectral efficiency; 3) those based on time and frequency division multiplexing using orthogonal frequency division multiplexing (OFDM) technique [5–7]. OFDM has been considered a promising candidate for future high-speed optical transmission technology. OFDM is a multicarrier transmission technology that transmits a high-speed data stream by splitting it into multiple parallel low-speed orthogonal channels. Therefore, OFDM has the highest spectral efficiency and enable dynamic bandwidth allocation among various applications both in time and frequency domain dimensions [8,9]. In addition, the digital signal processing (DSP)-based OFDM-PON can provide other highly desirable features such as resilience to chromatic dispersion, polarization mode dispersion (PMD) [10] and nonlinear distortion [5,6]. According to the different structure of optical receivers, the optical OFDM system can be categorized as coherent and direct detection (DD) systems. Coherent optical OFDM has better sensitivity. However, relative to DD system, the coherent receiver requires an optical local
⁎
laser, 90° optical hybrids and additional digital signal processing to compensate the distortion caused by the frequency offset and linewidth [11]. These devices or techniques increase the complexity and the cost of the system. Therefore, in the view of costs, the simple structure of DD-OFDM system is attractive for the future low-cost PON systems [5,12]. In this paper, we propose a novel PON upstream scheme based on DD-OFDM and modulation format conversion technique. The conventional directly intensity modulation and direct detection scheme are used for ONU and OLT side, respectively [13,14]. The upstream data could be allocated according to the time slot, subcarrier, and modulation format. The higher order modulated OFDM signal is generated by direct detection after the superposition of two lower order modulated upstream signals with different optical powers and wavelengths. The upstream signal of each ONU could be distinguished according to the different modulation format, which is more flexible for dynamic bandwidth allocation. The principle of the proposed upstream scheme and the impact of key parameters, such as symbol delay and amplitude deviation of two lower order modulated signals, are introduced and analyzed in the following, respectively. Finally, to validate the architecture feasibility, a 10 Gbps data rate, BPSK to QPSK conversion and a 20 Gbps data rate, QPSK to 16 QAM conversion experiments are carried out, respectively. The impact of symbol delay and amplitude deviation of two superposed signals is evaluated in the following experiment. Compared with symbol delay, the results show that the system performance is more robust to amplitude deviation.
Corresponding author. E-mail address: [email protected] (C. Ju).
http://dx.doi.org/10.1016/j.optlastec.2016.11.017 Received 5 March 2016; Received in revised form 22 July 2016; Accepted 22 November 2016 Available online 29 November 2016 0030-3992/ © 2016 Elsevier Ltd. All rights reserved.
Optics & Laser Technology 90 (2017) 237–241
C. Ju et al.
Spectrum before IM
Spectrum before IM
ONU3
ONU3
Power
ONU2
ONU1
Freq. ONU1
OFDM TX
Freq. ONU1
(ii)
Freq.
(i)
PD
SSMF
WDM
Freq.
OLT
IM SSMF
Freq.
ONU2 Power
ONU2
ONU3 Freq.
ADC
OFDM RX
(a)
BPSK
BPSK
BPSK
8QAM
QPSK
(i)
(ii)
(iii)
(vi)
16QAM
(b)
BPSK
QPSK
8QAM
QPSK
QPSK
16QAM
Fig. 1. (a) Principle of the proposed OFDMA-PON Upstream scheme. The electrical spectrum of each ONU before intensity modulation for (i) the conventional upstream scheme and (ii) the proposed scheme. (b) The generation principle of higher order modulated constellation. The generation of (i) QPSK, (ii) 8 QAM and (iii, iv) 16 QAM.
2. Principle of OFDMA-PON upstream scheme
after direct detection can be expressed as
The proposed OFDMA-PON upstream scheme based on modulation format conversion technique is illustrated in Fig. 1(a). For the conventional upstream scheme, different ONU uses different subcarrier. However, in the proposed scheme, the two ONUs could use the same subcarrier, which are distinguished by modulation format. The transmitter at each ONU contains a DSP module and optics intensity modulation module [13,14]. Each ONU is equipped with a tunable laser source. The tunable laser can be tunable to any of wavelengths. Different wavelength for different ONU in the same OFDM symbol period is necessary for preventing from optical beating interference. The output double-sideband (DSB) optical signals of two ONUs can be written as
R = E1, CD 2 + E2, CD 2
⎛ E1=e jω1t ⎜1 + ⎝
N
∑n =1
⎞ v1, ncos (nωt +θ1, n ) ⎟ ⎠
(1)
⎞ v2, ncos (nωt +θ2, n ) ⎟ ⎠
(2)
N
N
∑n =1
(5) where nθτ represents the phase offset caused by symbol delay of two ONUs, rn=v1, ncos (n2θ1, D ) e jθ1, n +v2, ncos (n2θ2, D ) e j (θ 2, n+ nθτ ) , and θn is the compound angle of rn . The subcarrier powers of two ONUs are proportional to cos 2 (n2θ1, D ) and cos 2 (n2θ2, D ) [15,16]. Then, the higher order constellation constructed by rn is obtained. To demodulate the higher order constellation signal correctly, the signal power and the initial phase of the two lower order upstream signals must be controlled reasonably. The principle of the modulation format conversion is shown in Fig. 1(b). As shown in (i), the QPSK signal could be achieved by combining two BPSK signals, where two BPSK signals have the same amplitudes, and BPSK2 has a phase rotation of 90°. 8 QAM signal could be obtained by combining BPSK1 and QPSK2 signals as shown in (ii), where the amplitude of BPSK1 should be double that of QPSK2, v1, n : v2, n=2 : 1. 16 QAM signal could be achieved by combining BPSK and 8 QAM, as shown in (iii), or two QPSK signals as shown in (iv). The amplitude of BPSK should be double that of 8 QAM, and BPSK has an initial phase rotation of 90°. The amplitude of QPSK1 should be double that of QPSK2, v1, n : v2, n=2 : 1. Each point of the higher order constellation is obtained as the product of a point from the first constellation and a point from the second constellation. That is to say the higher order constellation is generated by the superposition of these two lower order signals. The amplitude of two lower order signals could be controlled by adjusting the optical modulation index (OMI) or the launch power of two ONUs. The initial phase of two ONUs could be controlled at the symbol mapping module in the transmitter. In the actual network, the signals from different transmitters are difficult to achieve absolute symbol synchronization, which is caused by the phase noise of symbol timing error or equipment aging. Therefore, a special rotation of higher order constellation is happened as shown in Fig. 2(a) and (b). The impact of amplitude deviation is shown in Fig. 2(c) and (d). Obviously, the Euclidean distance is reduced. For
where ω1 and ω 2 are the angular frequency of optical carriers, N denotes the number of subcarriers in an OFDM symbol, v1, n , v2, n , and θ1, n , θ2, n are the amplitudes and phases of the nth subcarrier, and the second and higher-order terms after power series expansion are disregarded [15]. After transmission with the distance of L1 and L 2 , the group-velocity dispersion parameter of β2 and no fiber loss, then the optical signals become
⎛ E1, CD=e jω1t ⎜1 + ⎝
N
∑n =1
v1, n e j (n
2θ 1, D ) cos (nωt + θ
⎞
1, n ) ⎟
⎠
(3)
and
⎛ E2, CD=e jω2 t ⎜1 + ⎝
N
∑n =1 ω 2 /2 ,
v2, n e j (n
2θ
2, D ) cos (nωt + θ
⎞
2, n ) ⎟
⎠
N
+ ∑n =1 v2, ncos (n2θ2, D ) cos (nωt +θ2, n+nθτ )= ∑n =1 rn cos (nωt +θn )
and
⎛ E2=e jω2 t ⎜1 + ⎝
N
= ∑n =1 v1, ncos (n2θ1, D ) cos (nωt +θ1, n )
(4)
ω 2 /2
and θ2, D=β2 L 2 [15]. The conventional DDwhere θ1, D=β2 L1 OFDM received structure is used for OLT side. Then the received signal 238
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BPSK1
BPSK2
QPSK Constellation with rotation
QPSK1
QPSK2
16QAM Constellation with rotation
BPSK1
BPSK2
QPSK Constellation
QPSK1
QPSK2
16QAM Constellation
Fig. 2. The rotated constellation of (a) QPSK signal and (b) 16 QAM signal caused by symbol delay. The constellation of (c) QPSK signal and (d) 16 QAM signal with amplitude deviation.
5.5 GHz is inserted after AWG to prevent high frequency residual images. The electrical signal after microwave amplifier is used to drive a single drive MachZehnder modulator (MZM), which is biased at its quadrature point to generate an optical DSB signal. The wavelength of the laser1 and laser2 are set to be 1550.32 nm and 1549.12 nm, respectively. To simplify the complexity of experiment, the amplitude of constellation for each ONU is adjusted by controlling the launch power and the OMI of two transmitters are set to be 0.25. Specially designed training symbols with subcarrier interleaving [17] and lower peak to average power ratio (PAPR) are used to protect the training symbols from being affected by subcarrier-to-subcarrier intermixing interference (SSII), which would improve the accuracy of channel estimation. A 3 dB optical coupler is used after 50 km SSMF transmission. An erbium-doped fiber amplifier (EDFA) is inserted after optical coupler. After 100 km SSMF transmission, a fiber Bragg grating (FBG) filter is used to mitigate amplified spontaneous emission (ASE) noise of EDFA, and an optical attenuator is used to adjust the received power. The received electrical signal is captured by a LeCroy® digital oscilloscope WaveMaster813ZI-A with a sampling rate of 40 GSa/s. The digital signal is down-sampling to 12 GSa/s, and off-line process using MATLAB® digital signal processing program. The demodulation process includes removing CP, serial-to-parallel conversion, FFT, linear equalization, symbol demapping, parallel-to-serial conversion, and bit error rate (BER) calculation. To verify the feasibility, two examples are carried out. The modulation formats of two ONUs used in the first example are both of BPSK. The launch power for two ONUs are both of 3 dB m. The phase rotation of ONU2 after symbol mapping is set to be 90°. After
sake of brevity, the description is shown only for QPSK and 16 QAM. The other modulation formats follow the same trend. For each subcarrier, the ideal constellation with amplitude and phase deviation could be obtained by using training symbols. The demodulation of data subcarrier is realized by the calculation and comparison of the minimum Euclidean distance between the ideal constellation and the signal after linear equalization. It is emphasized that the demodulation process would be failed when two rotated constellation points are overlapped. Therefore, the initial phase and the amplitude of two ONUs should be accurately controlled. The impact of amplitude and phase deviation on system performance is experimentally evaluated in the following.
3. Experimental setup and results The experimental setup for the proposed OFDMA-PON upstream transmission system over 100 km SSMF is shown in Fig. 3(a). In this work, two transmitters are employed to simulate the scene of upstream transmission. The baseband electrical OFDM signals are generated by an arbitrary waveform generator (AWG M8190A) using the MATLAB® program. The signal processing of OFDM transmitter consists of serialto-parallel conversion, symbol mapping, phase rotation, hardware imperfection compensation [5], IFFT, parallel-to-serial conversion, and adding cyclic prefix (CP). The sampling rate and digital to analog converter (DAC) resolution of AWG are 12 GSa/s and 8 bits, respectively. The OFDM signal contains 107 subcarriers to occupy 5 GHz bandwidth with an inverse fast Fourier transform (IFFT) size of 256. CP of 1/8 is used. A low pass filter (LPF) with 3 dB bandwidth of
ONU1
ONU2
Fig. 3. (a) The experimental setup of the proposed OFDMA-PON upstream scheme. (b) The QPSK constellation after equalization at subcarrier number of (i) n=1, (ii) n=50, (iii) n=100, and the 16 QAM constellation at subcarrier number of (iv) n=1, (v) n=50, (vi) n=100. (Red points: constellation of training symbols. Blue points:constellation of data symbols). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. The BER versus the received power after 100 km SSMF at different symbol delay for (a) QPSK and (b) 16 QAM. The BER versus the received power at different power deviation for (c) QPSK and (d) 16 QAM. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
case, there are only 0.7 dB power penalty for two QPSK upstream case at BER of 3. 8 × 10−3 with symbol delay of 1 T, 2 T, and 4 T. The SNR of each subcarrier has not changed for different symbol delay cases at the same received power. The reason of power penalty is that special rotation of 16 QAM constellation decreases the Euclidean distance. The BER of QPSK and 16 QAM signals versus the received power at different power deviation after 100 km SSMF transmission are shown in Fig. 4(c) and (d). The symbol delay of two coupled optical signal for two examples is set to be zero by adjusting the time delay of AWG output signal. As shown in Fig. 4(c), the three curves with power ratio of P1:P2=0.75, 1, and 1.25 are overlapped in the first example. For the second example, the three curves with power ratio of P1:P2=1.75, 2, and 2.25 are almost overlapped. Therefore, the proposed OFDMA-PON upstream scheme with BPSK–QPSK conversion and QPSK–16 QAM conversion is robust to amplitude deviation of two coupled signals, but sensitive to symbol delay. Finally, the BER performance of QPSK and 16 QAM for ONU1 and ONU2 are shown in Fig. 5(a) and (b). The amplitude ratios of two BPSK signals and two QPSK signal are fixed to v1, n : v2, n=1 : 1 and v1, n : v2, n=2 : 1, respectively. The symbol delay is set to be zero. For the case of two BPSK upstream transmission, the BER curves of ONU1 and ONU2 at B2B and after 100 km SSMF transmission are overlapped, respectively. The power penalty after transmission over 100 km SSMF at FEC limit is 0.6 dB. For the case of two QPSK upstream transmission, the FEC limit for ONU1 and ONU2 at B2B can be obtained at the received power of −14.2 dB m and −13.2 dB m. After 100 km SSMF transmission, the FEC limit can be obtained at the received power of −12.1 dB m and −10.8 dB m, respectively. Obviously, ONU1 has a better performance. The reason is that the optical signal power of ONU1 is twice larger than that of ONU2.
100 km SSMF transmission and equalization, the QPSK constellation at subcarrier number of n=1, 50 and 100 are shown in Fig. 3(b) (i–iii). The constellation of training symbols and data symbols are shown by red and blue points, respectively. The received power is set to be −3 dB m. The two rotated QPSK constellation points for the case of n=100 are overlapped. In this situation, the two constellation points could not be distinguished. The modulation formats of two ONUs used in the second example are both of QPSK. The launch power for ONU1 and ONU2 are 4 dB m and 1 dB m, respectively. The 16 QAM constellation of data symbols and training symbols at subcarrier number of n=1, 50 and 100 are shown by blue and red points in Fig. 4(b) (iv–vi). The phase rotation of 16 QAM constellation, as shown in (v) and (vi), decreases the Euclidean distance. Nonetheless, the constellation points could be distinguished between each other. To give an analysis of the influence of symbol delay and amplitude deviation on system performance, hardware imperfection compensator [5] is used before IFFT to get a flat signal-to-noise ratio (SNR) curves for all data subcarriers. BER of QPSK and 16 QAM signals versus the received power at different symbol delay after 100 km SSMF transmission are shown in Fig. 4(a) and (b). The amplitude ratios of two BPSK signals and two QPSK signal are fixed to v1, n : v2, n=1 : 1 and v1, n : v2, n=2 : 1, respectively. In the experiment, the signal of ONU1 is synchronized, which is used as a time reference. The symbol delay is adjusted by setting the sampling time delay of ONU2 signal at AWG. For the case of two BPSK upstream transmission with symbol delay of 1 T, 2 T, and 4 T, the BER fails to meet the requirements of FEC limit 1 (3. 8 × 10−3), where T = 12 ×10−9 (s) is the sampling interval of DAC. The reason is that the two constellation points of rotated QPSK signal for some subcarriers are overlapped as shown in Fig. 3(b) (iii), which would lead to symbol decision error. Compared with zero symbol delay
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Fig. 5. The BER versus the received power at B2B and after 100 km SSMF transmission for (a) QPSK and (b) 16 QAM.
4. Conclusions [6]
In this paper, we propose a flexible PON upstream scheme based on direct detection OFDMA and modulation format conversion techniques. The upstream data could be allocated according to the time slot, subcarrier, and modulation format. Data distribution in three-dimension could improve the flexibility of dynamic bandwidth allocation. The feasibility of the proposed scheme is confirmed by two experimental examples. The impacts of symbol delay and amplitude deviation of the two upstream signals on system performance are analyzed and evaluated.
[7]
[8]
[9] [10]
Acknowledgement [11]
This study is supported by National Natural Science Foundation of China (No. 61571061).
[12]
[13]
References
[14]
[1] F. Selmanovic, E. Skaljo, GPON in telecommunication network, in: Proceedings of the ICUMT, Oct. 18–20, 2010, pp. 1012–1016 [2] R. Davey, J. Kani, F. Bourgart, K. McCammon, Options for future optical access networks, IEEE Commun. Mag. 44 (10) (2006). [3] B. Charbonnier et al., Low Complexity FDM/FDMA Approach for Future PON, OFC/NFOEC, OThA3.7, 2013 [4] S.D. Le1, A. Lebreton1, F. Saliou, Up to 60 km bidirectional transmission of a 16 channels×10 Gb/s FDM-WDM PON based on self-seeded reflective semiconductor optical amplifiers, in: Proceesings of the OFC 2014, Paper Th3G.8, San Francisco, CA, March 2014 [5] C.Ju, X.Chen, N.Liu, Q.Zhang, H.Wang, OFDM PON Downstream Scheme with
[15] [16]
[17]
241
Symbol Predistortion and Scalable Receiver Frontend, in: Proceeding of the Optical Fiber Communication Conference Paper Th1H.4, 2015. Cheng Ju, Na Liu, Xue Chen, Zhiguo Zhang, SSBI mitigation in A-RF-tone-based VSSB-OFDM system with a frequency-domain volterra series equalizer, J. Light. Technol. 33 (23) (2015) 4997–5006. Hui Yang, Juhao Li, Bangjiang Lin, Yangsha Wan, Yong Guo, Lixin Zhu, Li Li, Yongqi He, Zhangyuan Chen, DSP-based evolution from conventional TDM-PON– TDM-OFDM-PON, J. Light. Technol. 31 (16) (2015) 3035–3041. W.Wei, J.Hu, L.Zeng, Optical Orthogonal Frequency Division Multiple Access (OFDMA)-based optical access/metro ring networks, in Proceedings OFC Paper JWA123, 2008. Guoying Zhang, Marc De Leenheer, Annalisa Morea, "A survey on OFDM-based elastic core optical networking," IEEE, Commun. Surv. Tutor. 15 (1) (2013). N.Cvijetic, L.Xu, T.Wang, Adaptive PMD compensation using OFDM in long-haul 10 Gb/s DWDM systems, in: Proceedings of the 2007 IEEE/OSA Optical Fiber Communication conference (OFC/NFOEC), Anaheim, CA, Mar paper OTuA5, 2007. W. Shieh, C. Athaudage, Coherent optical orthogonal frequency division multiplexing, Electron. Lett. 42 (2006) 587–589. B.J.C. Schmidt, A.J. Lowery, J. Armstrong, Experimental demonstrations of electronic dispersion compensation for long-haul transmission using direct-detection optical OFDM,, J. Light. Technol. 26 (1) (2008) 196–203. D.Qian, J.Hu, J.Yu, and et al., Experimental demonstration of a novel OFDM-A Based 10 Gb/s PON Architecture, in Proceedings of the European Conference and Exhibition on Optical Communication (ECOC), Paper. 5.4. 1, 2007. Cheng Ju, Na Liu, Xue Chen, Iteration SSII cancellation in DD-OFDM PON upstream scheme, Opt. Commun. 364 (2016) 9–12. Chia-Chien Wei, Small-signal analysis of OOFDM signal transmission with directly modulated laser and direct detection, Opt. Lett. 36 (2) (2011) 151–153. Bangjiang Lin, Juhao Li, Hui Yang, Yangsha Wan, Yongqi He, Zhangyuan Chen, Comparison of DSB and SSB Transmission for OFDM-PON, J. Opt. Commun. Netw. 4 (11) (2012). W.R. Peng, B. Zhang, K.M. Feng, X. Wu, A.E. Willner, S. Chi, Spectrally efficient direct-detected OFDM transmission incorporating a tunable frequency gap and an iterative detection techniques, J. Light. Technol. 27 (24) (2009) 5723–5735.