- Email: [email protected]
Experimental demonstration of optical MIMO transmission for SCFDM-PON based on polarization interleaving and direct detection
Optics Communications 285 (2012) 5163–5168
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Discussion
Experimental demonstration of optical MIMO transmission for SCFDM-PON based on polarization interleaving and direct detection Bangjiang Lin, Juhao Li n, Hui Yang, Song Jiang, Lixin Zhu, Yongqi He, Zhangyuan Chen State Key Laboratory of Advanced Optical Communication Systems and Networks, School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, China
a r t i c l e i n f o
abstract
Article history: Received 20 April 2012 Received in revised form 19 July 2012 Accepted 20 July 2012 Available online 23 August 2012
The OFDM-PON and SCFDM-PON based on powerful digital signal processing (DSP) are promising candidates for next-generation optical access networks. Recently polarization-division-multiplexing (PDM) transmission with direct detection has been proposed for OFDM-PON to effectively reduce the bandwidth requirement of optical and electrical components. However, the PDM scheme has high algorithm complexity. In this paper, we propose a polarization interleaving (PI) approach, which can significantly reduce the bandwidth requirement for components while achieving a similar 2 2 MIMO algorithm with coherently-detected PDM-OFDM scheme. Downstream PI-SCFDM transmission is experimentally demonstrated. The scheme can be easily extended to OFDM-PON. & 2012 Elsevier B.V. All rights reserved.
Keywords: Polarization interleaving (PI) Single carrier frequency division multiplexing (SCFDM) Passive optical network (PON)
1. Introduction Passive optical networks (PONs) have been largely recognized as a cost effective fiber to the home (FTTH) infrastructure [1–9]. Current passive optical network (PON) architectures including (EPON, GPON) need complex scheduling algorithms and framing technologies to support multiple services. With the rapid increase of bandwidth demand, the transport capacity of next-generation optical access networks will migrate to 40 Gb/s or even higher per channel in the near future. Among various solutions, the wavelength division multiplexing passive optical network (WDM-PON) is impressive by transparently delivering multiple services through dedicated wavelengths and significantly enhancing the number of users. However, WDM-PON lacks the flexibility to dynamically allocate bandwidth resource and relies on high-cost optical components, such as arrayed waveguide gratings and multiple transceivers [2,8]. Recently, the orthogonal frequency division multiplexing passive optical network (OFDMA-PON) has recently received much attention, due to its great resistance to fiber dispersion, high spectral efficiency, and extreme flexibility on multiple services access [8–11]. The OFDM-PON relies on high-speed analog-todigital converters (ADCs), digital-to-analog converters (DACs) and digital signal processing (DSP) circuits. The transfer of complexity and cost from optics to electronics in OFDM-PON enables
n
Corresponding author. Tel.: þ86 10 62763334; fax: þ 86 10 62751763. E-mail address: [email protected] (J. Li).
0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.07.109
transparent support of arbitrary heterogeneous applications and dynamic bandwidth allocation. In addition, the rapid development of component integration and mass production can notably improve cost-efficiency. However, the OFDM has a multiple-carrier modulation format, in which the power of OFDM signal will fluctuate with the superposition and cancellation of multiple carriers at the same phase. So the OFDM always has a large peak-to-average ratio (PAPR). A large PAPR requires high linearity for optical and electrical components and more quantization bits for ADCs and DACs, which will significantly increase the system cost. Recently, we have proposed and experimentally demonstrated the PON architecture based on single-carrier frequency division multiplexing (SCFDM) [4]. The SCFDM is a modified form of OFDM. Due to its inherent single carrier transmission characteristics, the SCFDM signals have much lower PAPR than that of OFDM. Experiments have shown that the SCFDM can achieve better performance than OFDM and suffers less nonlinear transmission impairments both for access network and long-haul transmission [12–15]. Polarization division multiplexing (PDM) is an effective technique to double fiber capacity and has been proposed in long-haul coherent optical OFDM (CO-OFDM) transmissions [16]. The crosstalk between polarizations induced by polarization mode dispersion (PMD) and rotation of SOP (state of polarization) can be effectively eliminated by the multi-input–multi-output (MIMO) algorithm [16–20] based on coherent detection. However, coherent detection with a local oscillator and an optical hybrider is expensive and not suitable for access networks. Recently a PDM
5164
B. Lin et al. / Optics Communications 285 (2012) 5163–5168
and the optical signals before the PBC can be written as
OFDM-PON scheme with direct detection has been proposed [21–23]. But the scheme strictly requires spectrum overlapping of the two orthogonal sidebands and the 4 4 MIMO channel estimation algorithm is complex. In this paper, we propose a polarization interleaving (PI) approach for SCFDM-PON based on direct detection, which can significantly reduce the bandwidth requirement for optical and electrical components while achieving a similar 2 2 MIMO algorithm with PDM-CO-OFDM. The scheme can be easily extended to OFDM-PON.
Sx ¼ Bx expðj2pf c,xtÞ þ Ax
N X
S1,iðtÞ expðj2pðf c,x þ f uÞtÞ
i¼1
þ Ax
N X
S1,iðtÞexpðj2pðf c,xf uÞtÞ
ð3Þ
i¼1
Sy ¼ By expðj2pf c,ytÞ þ Ay
N X
S2,iðtÞexpðj2pðf c,y þ f uÞtÞ
i¼1
þ Ay
S1,iðtÞ expðj2pf utÞ
ð1Þ
i¼1
0
dy ¼
N X
ð4Þ
where fc,x and fc,y denote the two polarization interleaving optical carriers; Ax, Bx and Ay, By denote the amplitudes of SCFDM signals and optical carriers, respectively after two IMs. Then the signals are combined by the PBC with orthogonal polarizations. The frequency spacing f0 should be larger than 2f1, as shown in Fig. 1(c). f1 denotes the maximum frequency deviation between the optical carrier and the SCFDM signals for each polarization. At the receiver, the optical PI-SCFDM signals are split by the PBS and each output consists of an arbitrary mix of two transmitted SCFDM signals, as shown in Fig. 1(d) and (e). After direct detection by two photodiodes, various frequency components will interfere each other and the two outputs can be expressed as
Fig. 1 shows the schematic diagram of proposed PI approach using direct-detection. At the transmitter, an intensity modulator (IM) is driven by a radio frequency (RF) f0/2 with carrier suppression to generate two optical carriers with a frequency spacing of f0. Then an interleaver is used to separate the two carriers. For each SCFDM transmitter, the baseband SCFDM signals are up-converted by digital or analog IQ modulation. The up-converted signals can be defined as N X
S2,iðtÞexpðj2pðf c,yf uÞtÞ
i¼1
2. Technique principle
dx ¼
N X
Sx ¼ S2,iðtÞ expðj2pf utÞ
N X
a11 ðiÞS1,iðtÞexpðj2pf utÞ
i¼1
ð2Þ
i¼1
þ
N X
a12 ðiÞS2,iðtÞexpðj2pf utÞ þIxexpðj2pf 0 tÞ
i¼1
The S1,i and S2,i denote the baseband SCFDM signals for the ith subcarrier modulated in SCFDM transmitter 1 and 2, respectively. The fu denotes the RF carrier, and N is the number of subcarriers. Then the electrical SCFDM signals are converted to optical double sideband (DSB) signals by optical intensity modulators (IMs). Fig. 1(a) and (b) shows the schematic outputs of the two IMs
þ
N X
bx,1ðiÞS1,iðtÞexpðj2pðf 0 f u ÞtÞ
i¼1
þ
N X
bx,2ðiÞS1,iðtÞexpðj2pðf 0 þ f u ÞtÞ
i¼1
SCFDM Transmitter1
interleaver
Laser
PC
IM
f0 2
IM
POL-X’
a
POL-X
PD
d
c
Transmission Link
PBC IM
g PD
POL-Y’
POL-Y
b
MIMOSCFDM Receiver
PBS
e PC
LPF
f
LPF
SCFDM Transmitter2
f0 f0
f0
Y1
X2
X1
fc, x
λ
fc, y
fc, x
f1 X2
fc, x
fc, x
Y2
λ
Y1
f1 X1
Y2
fc, y
λ
X2
fc, x
Y1
Y2
λ
f1
fu
f0 f1
fc, y
X2
X1
fc, y
f0
X1
f1
fu fu
fu fu
Y1 Y2 X1 X2
f1 Y1
Y2
fc, y
λ
0
f1
fu fu fu X2
X1
Y1
Y2
f1 f 0
X1 X2 Y1 Y2
f
0
f1
fu fu X2
X1
Y1
Y2
f1 f 0
f
Fig. 1. Architecture of PI-SCFDM-PON (IM: intensity modulator, PC: polarization controller, PBC: polarization beam combiner, PBS: polarization beam splitter, PD: photodiode, LPF: low pass filter).
B. Lin et al. / Optics Communications 285 (2012) 5163–5168
þ
N X
expressed as ! 0 a11 ðiÞ d xðiÞ ¼ 0 a21 ðiÞ d yðiÞ
bx,3ðiÞS2,iðtÞexpðj2pðf 0 f u ÞtÞ
i¼1
þ
N X
bx,4ðiÞS2,iðtÞexpðj2pðf 0 þ f u ÞtÞ
0
N X
a21 ðiÞS1,iðtÞexpðj2pf utÞ
N X
a22 ðiÞS2,iðtÞexpðj2pf utÞ þ Iyexpðj2pf 0 tÞ
i¼1
þ
N X
by,1ðiÞS2,iðtÞexpðj2pðf 0 f u ÞtÞ
N X
by,2ðiÞS2,iðtÞexpðj2pðf 0 þ f u ÞtÞ
i¼1
þ
N X
by,3ðiÞS1,iðtÞexpðj2pðf 0 f u ÞtÞ
i¼1
þ
N X
by,4ðiÞS1,iðtÞexpðj2pðf 0 þ f u ÞtÞ
ð6Þ
i¼1
where Ix and Iy denote the beating interference of two optical carriers for each polarization. The beats between the two optical carriers and four signal bands for each polarization result in six groups of electrical signals, as seen in Fig. 1(f) and (g). a11 denotes the channel response coefficients for the signals beating between POL-X carrier and band X1 and X2 in POL-X’. a12 denotes the coefficients for the signals beating between POL-Y carrier and band Y1 and Y2 in POL-X’. bx,1, bx,2 denote the channel response coefficients for the signals beating between POL-Y carrier and band X2, band X1 in POL-X’, respectively. In a similar manner, bx,3, and bx,4, respectively, denote the channel response coefficients for the signals beating between POL-X carrier and band Y1, band Y2 in POL-X’, and a21, a22, by,1, by,2, by,3, by,4 carry analogous meaning for POL-Y’. After two low-pass filters, high-frequency interferences satisfying f4f1 are eliminated, as shown in Fig. 1 (f) and (g). The final received electrical signals can be described as 0
dx ¼
N X
a11 ðiÞS1,iðtÞexpðj2pf u tÞ þ
i¼1
0
dy ¼
N X
N X
a12 ðiÞS2,iðtÞexpðj2pf u tÞ
ð7Þ
a22 ðiÞS2,iðtÞexpðj2pf u tÞ
ð8Þ
i¼1
a21 ðiÞS1,iðtÞexpðj2pf u tÞ þ
i¼1
N X i¼1
Due to the elimination of unwanted interferences and the symmetry of the two polarizations, the system can be regarded as a MIMO system with two inputs and two outputs. For each subcarrier, the received signal before MIMO processing can be
! þ
dyðiÞ
nxðiÞ nyðiÞ
! ð9Þ
ð10Þ
The frequency spacing between polarizations can be reduced if high-order QAM mapping is applied. In comparison to the PDM scheme, the PI scheme is a modified approach with much less computation complexity at the cost of slightly-decreased frequency spectrum efficiency. It should be pointed out that the proposed PI scheme is different from the WDM scheme. In the PI scheme, the two optical carriers are in the same channel, which can achieve much less frequency spacing than that of two WDM channels. Moreover, the demultiplexing of the two optical carriers is realized with a PBS, which can be easily integrated into an optical transceiver, while a tunable filter is required to separate two WDM channels, the hardware implementation of which is quite difficult.
3. Experimental setup and results Fig. 2 shows the experimental setup for downstream PISCFDM-PON transmission. At the transmitter, two 3.3-Gbaud baseband quadrature phase shift keying (QPSK) SCFDM signals are three times up-sampled and then up-converted to 2.5 GHz by digital I-Q modulation. The total subcarriers are 256, from which 210 subcarriers are used for data transmission, and the cyclic prefix (CP) size is 16. The generated waveforms are uploaded into the arbitrary waveform generator (Tektronix AWG7122B) operating at 10-GS/s. Two tunable lasers are used to facilitate the generation of the two optical carriers with the frequency spacing of 10 GHz. The wavelengths are set to 1549.913 nm and 1549.993 nm, respectively. So the frequency guardband to realize the PI scheme is 10 2.5 2 3.3¼1.7 GHz. Two intensity Mach– Zehnder modulators (MZMs) are utilized to convert the two SCFDM signals to DSB optical signals. The optical distribution network is emulated with a 22.2 km SSMF, a variable optical attenuator (VOA) and a 50/50 coupler. At the receiver, the DSB PI-SCFDM signals are split by a PBS and detected by two photodiodes. The electrical RF SCFDM signals are amplified by two
AWG 7122B
SCFDM Coder
MZM
P B C MZM
LD2
b
f
22.2 km SSMF
a
LD1
SCFDM Coder
dxðiÞ
2f 1 2f u ¼ Rs
i¼1
þ
a22 ðiÞ
!
nx(i) and ny(i) stand for the frequency-domain noise within ith subcarrier for the two received signals. Then the crosstalk between polarizations for each subcarrier induced by the rotation of SOP can be effectively eliminated by the MIMO algorithm, which is similar with that in coherent optical PDM OFDM systems [16–20]. We can see that the bandwidth capacity to transmit a symbol rate of Rs is 2fu in PDM scheme [22], while it is slightly greater than 2f1 in the PI scheme, as shown in Fig. 1(c). 2f1 and 2fu satisfy the equation
i¼1
þ
a12 ðiÞ
ð5Þ
i¼1
Sy ¼
5165
d coupler
c VOA
P B S
PIN
EA
PIN
EA
e
POL_X Digital Storage Oscillosc ope 72004B
g
MIMOSCFDM Decoder POL_Y
AWG 7122B
Fig. 2. Experimental setup for downstream PI-SCFDM-PON transmission (LD: laser diode, SSMF: standard single mode fiber, EA: electrical amplifier, VOA: variable optical attenuator).
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B. Lin et al. / Optics Communications 285 (2012) 5163–5168
Up-sample and Low-pass filter ing
Pr eamble Inser tion
Cyclic pr efix inser tion
Ser ial/Par allel …… M-point DFT …… Subcar r ier mapping …… N-point IDFT …… Par allel/Ser ial
QPSK modulation
Data sour ce: PRBS Sequence
SCFDM Coder I 90°
2.5 GHz Q
QAM demodulation
Data r eceived
QAM demodulation
Data r eceived
Ser ial/Par allel Ser ial/Par allel …… …… N-point DFT N-point DFT …… …… MIMO Channel estimation and equalization …… …… M-point IDFT M-point IDFT …… …… Phase cor r ection Phase cor r ection …… …… Par allel/Ser ial Par allel/Ser ial
2.5 GHz Q
Cyclic pr efix r emove
90°
Cyclic pr efix r emove
Pol_Y
Down-sample
I
Down-sample
2.5 GHz Q
MIMO Synchronization
90°
Low-pass filter ing
I Pol_X
Low-pass filter ing
MIMO-SCFDM Decoder
Fig. 3. DSP block diagrams for (a) SCFDM coder and (b) MIMO-SCFDM receiver.
Time Multiplexed Training Sequence
Synchronization Sequence
Data
Data
Data
...
Data
Data
Data
Dat a
Tr aining
Data
Tr aining Tr aining
Tr aining
Tr aining
POL-Y
Tr aining
Tr aining
POL-X
Tr aining
electrical amplifiers before sampled by a real-time digital storage oscilloscope (Tektronix DPO72004B) operating at 25 GS/s. Since the bandwidths of the two electrical amplifiers are less than 5 GHz, they also act as low pass filters. The sampled SCFDM signals are decoded offline. The sampled SCFDM signals are decoded offline. The total bit rate for the two orthogonal polarizations is 13.3 Gb/s, of which 4.712% is used for preambles, 4.063% is used for the CP, and 16.225% is used for guard band and pilot subcarriers. So the net bit rate is 10 Gb/s. Fig. 3(a) and (b) shows the DSP block diagrams of the MIMOSCFDM coder and decoder. As a modified form, the baseband DSP method of the SCFDM has much in common with that of the OFDM, as shown in [4]. Before the signals are transmitted, preambles are added for synchronization and MIMO channel estimation. At the receiver, two direct-detected SCFDM signals are digitally down-converted. After synchronization, the CP is removed and the SCFDM signals are transformed into frequency domain by 256-point DFTs. The cross-polarization interferences are removed by MIMO channel equalization. The equalized SCFDM signals are transformed into time domain by the 210point IDFTs for further decision. Due to the presence of the DFT in the SCFDM transmitter and the IDFT in the SCFDM receiver, the SCFDM is sometimes referred to as DFT-spread OFDM [13–15]. The SCFDM has lower PAPR than that of OFDM at the cost of computational complexity. And the low PAPR can reduce the requirement of linearity for components and quantization bits for ADCs and DACs, which improve the economical efficiency for access network. The preamble structure is shown in Fig. 4. For each polarization, the preambles include two synchronization sequences and four time-multiplexed training sequences. The total length of synchronization sequence is 38.8 ns, which includes two Chu sequences. Each time-multiplexed training sequence consists of 256 specific symbols, a 16-symbol cyclic prefix (CP), and a 272-symbol zero sequence. The total length of the training sequence is 659.4 ns. The preamble is inserted after every 200 SCFDM symbols.
...
Fig. 4. Block diagram for the preamble structure.
The optical spectra under back-to-back transmission are shown in Fig. 5. Fig. 5(a) and (b) shows the outputs of two MZMs. The difference of the two optical spectra is induced because the two driver amplifiers we use in our experiment have quite different gain shapes. The optical spectrum after the PBC is shown in Fig. 5(c). Fig. 5(d) and (e) shows the optical spectra for Pol-X’ and Pol-Y’, respectively. We can see that each output of the PBS is a mix of both polarization components. Fig. 5(f) and (g) depicts the electrical spectra for the SCFDM signals. Fig. 6 shows the bit error rate (BER) performances versus the received optical power before PBS. Each BER is calculated based on 2000 SCFDM symbols, which include more than 1:7 106 bits for both polarizations. We can see that the received optical power before PBS is about 10 dBm to achieve a BER of 1 103 . Comparing the back-toback and 22.2 km SSMF transmission curves, we can see that the chromatic dispersion induced penalty is negligible.
4. Conclusion We propose a polarization interleaving (PI) approach for SCFDM-PON based on direct detection, which can significantly reduce the bandwidth requirement for optical and electrical
B. Lin et al. / Optics Communications 285 (2012) 5163–5168
5167
Fig. 5. (a) Optical SCFDM signal at Pol-X; (b) optical SCFDM signal at Pol-Y; (c) PI-SCFDM signal after PBC; (d) Pol-X’ optical signal; (e) Pol-Y’ optical signal; (f) received electrical SCFDM signal at Pol-X’ and (g) received electrical SCFDM signal at Pol-Y’.
Acknowledgment This work was supported by the National Basic Research Program of China (973 Program), Nos. 2010CB328201 and 2010CB328202, and the National Natural Science Foundation of China (NSFC), Nos. 60907030, 60877045, 60736003, and 60931160439.
References
Fig. 6. BER performance for MIMO-PI-SCFDM signal under back-to-back and 22.2 km SSMF transmission.
components while achieving similar 2 2 MIMO algorithm with PDM-CO-OFDM. Downstream PI-SCFDM-PON transmission is experimentally demonstrated. Experimental results show that the chromatic dispersion induced penalty is negligible. The scheme can be easily extended to OFDM-PON.
[1] P. Chanclou, J.P. Lanquetin, S. Durel, F. Saliou, B. Landousies, N..Genay, Z. Belfqih,Proceedings of the IEEE/OSA Optical Fiber Communication Conference (OFC/NFOEC), 2008, p. 1(Paper OWH1). [2] Leonid G. Kazovsky, Wei-Tao Shaw, David Gutierrez, Ning Cheng, ShingWa Wong, Journal of Lightwave Technology 25 (2007) 3428. [3] P. Chanclou, S. Gosselin, J.F. Palacios, V.L. Alvarez, E. Zouganeli, IEEE Communications Magazine 44 (8) (2006) 29. [4] Cheng Zhang, Juhao Li, Fan Zhang, Yongqi He, Hequan Wu, Zhangyuan Chen, Optics Express 18 (2010) 24556. [5] P.E. Green Jr., IEEE Communications Magazine 42 (9) (2004) 100. [6] C. Lee, W.V. Sorin, B.Y. Kim, Journal of Lightwave Technology 24 (2006) 4568. [7] Soo-Jin Park, Chang-Hee Lee, Ki-Tae Jeong, Hyung-Jin Park, Jeong-Gyun Ahn, Kil-Ho Song, Journal of Lightwave Technology 22 (11) (2004) 2582. [8] Dayou Qian, N. Cvijetic, Junqiang Hu, Ting Wang, Photonics Technology Letters, IEEE 21 (17) (2009) 1265. [9] Thanh-Nga Duong, Naveena Genay, Meryem Ouzzif, Jerome Le Masson, Benoit Charbonnier, Philippe Chanclou, Jean Claude Simon, Photonics Technology Letters, IEEE 12 (12) (2009) 790.
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Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Discussion
Experimental demonstration of optical MIMO transmission for SCFDM-PON based on polarization interleaving and direct detection Bangjiang Lin, Juhao Li n, Hui Yang, Song Jiang, Lixin Zhu, Yongqi He, Zhangyuan Chen State Key Laboratory of Advanced Optical Communication Systems and Networks, School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, China
a r t i c l e i n f o
abstract
Article history: Received 20 April 2012 Received in revised form 19 July 2012 Accepted 20 July 2012 Available online 23 August 2012
The OFDM-PON and SCFDM-PON based on powerful digital signal processing (DSP) are promising candidates for next-generation optical access networks. Recently polarization-division-multiplexing (PDM) transmission with direct detection has been proposed for OFDM-PON to effectively reduce the bandwidth requirement of optical and electrical components. However, the PDM scheme has high algorithm complexity. In this paper, we propose a polarization interleaving (PI) approach, which can significantly reduce the bandwidth requirement for components while achieving a similar 2 2 MIMO algorithm with coherently-detected PDM-OFDM scheme. Downstream PI-SCFDM transmission is experimentally demonstrated. The scheme can be easily extended to OFDM-PON. & 2012 Elsevier B.V. All rights reserved.
Keywords: Polarization interleaving (PI) Single carrier frequency division multiplexing (SCFDM) Passive optical network (PON)
1. Introduction Passive optical networks (PONs) have been largely recognized as a cost effective fiber to the home (FTTH) infrastructure [1–9]. Current passive optical network (PON) architectures including (EPON, GPON) need complex scheduling algorithms and framing technologies to support multiple services. With the rapid increase of bandwidth demand, the transport capacity of next-generation optical access networks will migrate to 40 Gb/s or even higher per channel in the near future. Among various solutions, the wavelength division multiplexing passive optical network (WDM-PON) is impressive by transparently delivering multiple services through dedicated wavelengths and significantly enhancing the number of users. However, WDM-PON lacks the flexibility to dynamically allocate bandwidth resource and relies on high-cost optical components, such as arrayed waveguide gratings and multiple transceivers [2,8]. Recently, the orthogonal frequency division multiplexing passive optical network (OFDMA-PON) has recently received much attention, due to its great resistance to fiber dispersion, high spectral efficiency, and extreme flexibility on multiple services access [8–11]. The OFDM-PON relies on high-speed analog-todigital converters (ADCs), digital-to-analog converters (DACs) and digital signal processing (DSP) circuits. The transfer of complexity and cost from optics to electronics in OFDM-PON enables
n
Corresponding author. Tel.: þ86 10 62763334; fax: þ 86 10 62751763. E-mail address: [email protected] (J. Li).
0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.07.109
transparent support of arbitrary heterogeneous applications and dynamic bandwidth allocation. In addition, the rapid development of component integration and mass production can notably improve cost-efficiency. However, the OFDM has a multiple-carrier modulation format, in which the power of OFDM signal will fluctuate with the superposition and cancellation of multiple carriers at the same phase. So the OFDM always has a large peak-to-average ratio (PAPR). A large PAPR requires high linearity for optical and electrical components and more quantization bits for ADCs and DACs, which will significantly increase the system cost. Recently, we have proposed and experimentally demonstrated the PON architecture based on single-carrier frequency division multiplexing (SCFDM) [4]. The SCFDM is a modified form of OFDM. Due to its inherent single carrier transmission characteristics, the SCFDM signals have much lower PAPR than that of OFDM. Experiments have shown that the SCFDM can achieve better performance than OFDM and suffers less nonlinear transmission impairments both for access network and long-haul transmission [12–15]. Polarization division multiplexing (PDM) is an effective technique to double fiber capacity and has been proposed in long-haul coherent optical OFDM (CO-OFDM) transmissions [16]. The crosstalk between polarizations induced by polarization mode dispersion (PMD) and rotation of SOP (state of polarization) can be effectively eliminated by the multi-input–multi-output (MIMO) algorithm [16–20] based on coherent detection. However, coherent detection with a local oscillator and an optical hybrider is expensive and not suitable for access networks. Recently a PDM
5164
B. Lin et al. / Optics Communications 285 (2012) 5163–5168
and the optical signals before the PBC can be written as
OFDM-PON scheme with direct detection has been proposed [21–23]. But the scheme strictly requires spectrum overlapping of the two orthogonal sidebands and the 4 4 MIMO channel estimation algorithm is complex. In this paper, we propose a polarization interleaving (PI) approach for SCFDM-PON based on direct detection, which can significantly reduce the bandwidth requirement for optical and electrical components while achieving a similar 2 2 MIMO algorithm with PDM-CO-OFDM. The scheme can be easily extended to OFDM-PON.
Sx ¼ Bx expðj2pf c,xtÞ þ Ax
N X
S1,iðtÞ expðj2pðf c,x þ f uÞtÞ
i¼1
þ Ax
N X
S1,iðtÞexpðj2pðf c,xf uÞtÞ
ð3Þ
i¼1
Sy ¼ By expðj2pf c,ytÞ þ Ay
N X
S2,iðtÞexpðj2pðf c,y þ f uÞtÞ
i¼1
þ Ay
S1,iðtÞ expðj2pf utÞ
ð1Þ
i¼1
0
dy ¼
N X
ð4Þ
where fc,x and fc,y denote the two polarization interleaving optical carriers; Ax, Bx and Ay, By denote the amplitudes of SCFDM signals and optical carriers, respectively after two IMs. Then the signals are combined by the PBC with orthogonal polarizations. The frequency spacing f0 should be larger than 2f1, as shown in Fig. 1(c). f1 denotes the maximum frequency deviation between the optical carrier and the SCFDM signals for each polarization. At the receiver, the optical PI-SCFDM signals are split by the PBS and each output consists of an arbitrary mix of two transmitted SCFDM signals, as shown in Fig. 1(d) and (e). After direct detection by two photodiodes, various frequency components will interfere each other and the two outputs can be expressed as
Fig. 1 shows the schematic diagram of proposed PI approach using direct-detection. At the transmitter, an intensity modulator (IM) is driven by a radio frequency (RF) f0/2 with carrier suppression to generate two optical carriers with a frequency spacing of f0. Then an interleaver is used to separate the two carriers. For each SCFDM transmitter, the baseband SCFDM signals are up-converted by digital or analog IQ modulation. The up-converted signals can be defined as N X
S2,iðtÞexpðj2pðf c,yf uÞtÞ
i¼1
2. Technique principle
dx ¼
N X
Sx ¼ S2,iðtÞ expðj2pf utÞ
N X
a11 ðiÞS1,iðtÞexpðj2pf utÞ
i¼1
ð2Þ
i¼1
þ
N X
a12 ðiÞS2,iðtÞexpðj2pf utÞ þIxexpðj2pf 0 tÞ
i¼1
The S1,i and S2,i denote the baseband SCFDM signals for the ith subcarrier modulated in SCFDM transmitter 1 and 2, respectively. The fu denotes the RF carrier, and N is the number of subcarriers. Then the electrical SCFDM signals are converted to optical double sideband (DSB) signals by optical intensity modulators (IMs). Fig. 1(a) and (b) shows the schematic outputs of the two IMs
þ
N X
bx,1ðiÞS1,iðtÞexpðj2pðf 0 f u ÞtÞ
i¼1
þ
N X
bx,2ðiÞS1,iðtÞexpðj2pðf 0 þ f u ÞtÞ
i¼1
SCFDM Transmitter1
interleaver
Laser
PC
IM
f0 2
IM
POL-X’
a
POL-X
PD
d
c
Transmission Link
PBC IM
g PD
POL-Y’
POL-Y
b
MIMOSCFDM Receiver
PBS
e PC
LPF
f
LPF
SCFDM Transmitter2
f0 f0
f0
Y1
X2
X1
fc, x
λ
fc, y
fc, x
f1 X2
fc, x
fc, x
Y2
λ
Y1
f1 X1
Y2
fc, y
λ
X2
fc, x
Y1
Y2
λ
f1
fu
f0 f1
fc, y
X2
X1
fc, y
f0
X1
f1
fu fu
fu fu
Y1 Y2 X1 X2
f1 Y1
Y2
fc, y
λ
0
f1
fu fu fu X2
X1
Y1
Y2
f1 f 0
X1 X2 Y1 Y2
f
0
f1
fu fu X2
X1
Y1
Y2
f1 f 0
f
Fig. 1. Architecture of PI-SCFDM-PON (IM: intensity modulator, PC: polarization controller, PBC: polarization beam combiner, PBS: polarization beam splitter, PD: photodiode, LPF: low pass filter).
B. Lin et al. / Optics Communications 285 (2012) 5163–5168
þ
N X
expressed as ! 0 a11 ðiÞ d xðiÞ ¼ 0 a21 ðiÞ d yðiÞ
bx,3ðiÞS2,iðtÞexpðj2pðf 0 f u ÞtÞ
i¼1
þ
N X
bx,4ðiÞS2,iðtÞexpðj2pðf 0 þ f u ÞtÞ
0
N X
a21 ðiÞS1,iðtÞexpðj2pf utÞ
N X
a22 ðiÞS2,iðtÞexpðj2pf utÞ þ Iyexpðj2pf 0 tÞ
i¼1
þ
N X
by,1ðiÞS2,iðtÞexpðj2pðf 0 f u ÞtÞ
N X
by,2ðiÞS2,iðtÞexpðj2pðf 0 þ f u ÞtÞ
i¼1
þ
N X
by,3ðiÞS1,iðtÞexpðj2pðf 0 f u ÞtÞ
i¼1
þ
N X
by,4ðiÞS1,iðtÞexpðj2pðf 0 þ f u ÞtÞ
ð6Þ
i¼1
where Ix and Iy denote the beating interference of two optical carriers for each polarization. The beats between the two optical carriers and four signal bands for each polarization result in six groups of electrical signals, as seen in Fig. 1(f) and (g). a11 denotes the channel response coefficients for the signals beating between POL-X carrier and band X1 and X2 in POL-X’. a12 denotes the coefficients for the signals beating between POL-Y carrier and band Y1 and Y2 in POL-X’. bx,1, bx,2 denote the channel response coefficients for the signals beating between POL-Y carrier and band X2, band X1 in POL-X’, respectively. In a similar manner, bx,3, and bx,4, respectively, denote the channel response coefficients for the signals beating between POL-X carrier and band Y1, band Y2 in POL-X’, and a21, a22, by,1, by,2, by,3, by,4 carry analogous meaning for POL-Y’. After two low-pass filters, high-frequency interferences satisfying f4f1 are eliminated, as shown in Fig. 1 (f) and (g). The final received electrical signals can be described as 0
dx ¼
N X
a11 ðiÞS1,iðtÞexpðj2pf u tÞ þ
i¼1
0
dy ¼
N X
N X
a12 ðiÞS2,iðtÞexpðj2pf u tÞ
ð7Þ
a22 ðiÞS2,iðtÞexpðj2pf u tÞ
ð8Þ
i¼1
a21 ðiÞS1,iðtÞexpðj2pf u tÞ þ
i¼1
N X i¼1
Due to the elimination of unwanted interferences and the symmetry of the two polarizations, the system can be regarded as a MIMO system with two inputs and two outputs. For each subcarrier, the received signal before MIMO processing can be
! þ
dyðiÞ
nxðiÞ nyðiÞ
! ð9Þ
ð10Þ
The frequency spacing between polarizations can be reduced if high-order QAM mapping is applied. In comparison to the PDM scheme, the PI scheme is a modified approach with much less computation complexity at the cost of slightly-decreased frequency spectrum efficiency. It should be pointed out that the proposed PI scheme is different from the WDM scheme. In the PI scheme, the two optical carriers are in the same channel, which can achieve much less frequency spacing than that of two WDM channels. Moreover, the demultiplexing of the two optical carriers is realized with a PBS, which can be easily integrated into an optical transceiver, while a tunable filter is required to separate two WDM channels, the hardware implementation of which is quite difficult.
3. Experimental setup and results Fig. 2 shows the experimental setup for downstream PISCFDM-PON transmission. At the transmitter, two 3.3-Gbaud baseband quadrature phase shift keying (QPSK) SCFDM signals are three times up-sampled and then up-converted to 2.5 GHz by digital I-Q modulation. The total subcarriers are 256, from which 210 subcarriers are used for data transmission, and the cyclic prefix (CP) size is 16. The generated waveforms are uploaded into the arbitrary waveform generator (Tektronix AWG7122B) operating at 10-GS/s. Two tunable lasers are used to facilitate the generation of the two optical carriers with the frequency spacing of 10 GHz. The wavelengths are set to 1549.913 nm and 1549.993 nm, respectively. So the frequency guardband to realize the PI scheme is 10 2.5 2 3.3¼1.7 GHz. Two intensity Mach– Zehnder modulators (MZMs) are utilized to convert the two SCFDM signals to DSB optical signals. The optical distribution network is emulated with a 22.2 km SSMF, a variable optical attenuator (VOA) and a 50/50 coupler. At the receiver, the DSB PI-SCFDM signals are split by a PBS and detected by two photodiodes. The electrical RF SCFDM signals are amplified by two
AWG 7122B
SCFDM Coder
MZM
P B C MZM
LD2
b
f
22.2 km SSMF
a
LD1
SCFDM Coder
dxðiÞ
2f 1 2f u ¼ Rs
i¼1
þ
a22 ðiÞ
!
nx(i) and ny(i) stand for the frequency-domain noise within ith subcarrier for the two received signals. Then the crosstalk between polarizations for each subcarrier induced by the rotation of SOP can be effectively eliminated by the MIMO algorithm, which is similar with that in coherent optical PDM OFDM systems [16–20]. We can see that the bandwidth capacity to transmit a symbol rate of Rs is 2fu in PDM scheme [22], while it is slightly greater than 2f1 in the PI scheme, as shown in Fig. 1(c). 2f1 and 2fu satisfy the equation
i¼1
þ
a12 ðiÞ
ð5Þ
i¼1
Sy ¼
5165
d coupler
c VOA
P B S
PIN
EA
PIN
EA
e
POL_X Digital Storage Oscillosc ope 72004B
g
MIMOSCFDM Decoder POL_Y
AWG 7122B
Fig. 2. Experimental setup for downstream PI-SCFDM-PON transmission (LD: laser diode, SSMF: standard single mode fiber, EA: electrical amplifier, VOA: variable optical attenuator).
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B. Lin et al. / Optics Communications 285 (2012) 5163–5168
Up-sample and Low-pass filter ing
Pr eamble Inser tion
Cyclic pr efix inser tion
Ser ial/Par allel …… M-point DFT …… Subcar r ier mapping …… N-point IDFT …… Par allel/Ser ial
QPSK modulation
Data sour ce: PRBS Sequence
SCFDM Coder I 90°
2.5 GHz Q
QAM demodulation
Data r eceived
QAM demodulation
Data r eceived
Ser ial/Par allel Ser ial/Par allel …… …… N-point DFT N-point DFT …… …… MIMO Channel estimation and equalization …… …… M-point IDFT M-point IDFT …… …… Phase cor r ection Phase cor r ection …… …… Par allel/Ser ial Par allel/Ser ial
2.5 GHz Q
Cyclic pr efix r emove
90°
Cyclic pr efix r emove
Pol_Y
Down-sample
I
Down-sample
2.5 GHz Q
MIMO Synchronization
90°
Low-pass filter ing
I Pol_X
Low-pass filter ing
MIMO-SCFDM Decoder
Fig. 3. DSP block diagrams for (a) SCFDM coder and (b) MIMO-SCFDM receiver.
Time Multiplexed Training Sequence
Synchronization Sequence
Data
Data
Data
...
Data
Data
Data
Dat a
Tr aining
Data
Tr aining Tr aining
Tr aining
Tr aining
POL-Y
Tr aining
Tr aining
POL-X
Tr aining
electrical amplifiers before sampled by a real-time digital storage oscilloscope (Tektronix DPO72004B) operating at 25 GS/s. Since the bandwidths of the two electrical amplifiers are less than 5 GHz, they also act as low pass filters. The sampled SCFDM signals are decoded offline. The sampled SCFDM signals are decoded offline. The total bit rate for the two orthogonal polarizations is 13.3 Gb/s, of which 4.712% is used for preambles, 4.063% is used for the CP, and 16.225% is used for guard band and pilot subcarriers. So the net bit rate is 10 Gb/s. Fig. 3(a) and (b) shows the DSP block diagrams of the MIMOSCFDM coder and decoder. As a modified form, the baseband DSP method of the SCFDM has much in common with that of the OFDM, as shown in [4]. Before the signals are transmitted, preambles are added for synchronization and MIMO channel estimation. At the receiver, two direct-detected SCFDM signals are digitally down-converted. After synchronization, the CP is removed and the SCFDM signals are transformed into frequency domain by 256-point DFTs. The cross-polarization interferences are removed by MIMO channel equalization. The equalized SCFDM signals are transformed into time domain by the 210point IDFTs for further decision. Due to the presence of the DFT in the SCFDM transmitter and the IDFT in the SCFDM receiver, the SCFDM is sometimes referred to as DFT-spread OFDM [13–15]. The SCFDM has lower PAPR than that of OFDM at the cost of computational complexity. And the low PAPR can reduce the requirement of linearity for components and quantization bits for ADCs and DACs, which improve the economical efficiency for access network. The preamble structure is shown in Fig. 4. For each polarization, the preambles include two synchronization sequences and four time-multiplexed training sequences. The total length of synchronization sequence is 38.8 ns, which includes two Chu sequences. Each time-multiplexed training sequence consists of 256 specific symbols, a 16-symbol cyclic prefix (CP), and a 272-symbol zero sequence. The total length of the training sequence is 659.4 ns. The preamble is inserted after every 200 SCFDM symbols.
...
Fig. 4. Block diagram for the preamble structure.
The optical spectra under back-to-back transmission are shown in Fig. 5. Fig. 5(a) and (b) shows the outputs of two MZMs. The difference of the two optical spectra is induced because the two driver amplifiers we use in our experiment have quite different gain shapes. The optical spectrum after the PBC is shown in Fig. 5(c). Fig. 5(d) and (e) shows the optical spectra for Pol-X’ and Pol-Y’, respectively. We can see that each output of the PBS is a mix of both polarization components. Fig. 5(f) and (g) depicts the electrical spectra for the SCFDM signals. Fig. 6 shows the bit error rate (BER) performances versus the received optical power before PBS. Each BER is calculated based on 2000 SCFDM symbols, which include more than 1:7 106 bits for both polarizations. We can see that the received optical power before PBS is about 10 dBm to achieve a BER of 1 103 . Comparing the back-toback and 22.2 km SSMF transmission curves, we can see that the chromatic dispersion induced penalty is negligible.
4. Conclusion We propose a polarization interleaving (PI) approach for SCFDM-PON based on direct detection, which can significantly reduce the bandwidth requirement for optical and electrical
B. Lin et al. / Optics Communications 285 (2012) 5163–5168
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Fig. 5. (a) Optical SCFDM signal at Pol-X; (b) optical SCFDM signal at Pol-Y; (c) PI-SCFDM signal after PBC; (d) Pol-X’ optical signal; (e) Pol-Y’ optical signal; (f) received electrical SCFDM signal at Pol-X’ and (g) received electrical SCFDM signal at Pol-Y’.
Acknowledgment This work was supported by the National Basic Research Program of China (973 Program), Nos. 2010CB328201 and 2010CB328202, and the National Natural Science Foundation of China (NSFC), Nos. 60907030, 60877045, 60736003, and 60931160439.
References
Fig. 6. BER performance for MIMO-PI-SCFDM signal under back-to-back and 22.2 km SSMF transmission.
components while achieving similar 2 2 MIMO algorithm with PDM-CO-OFDM. Downstream PI-SCFDM-PON transmission is experimentally demonstrated. Experimental results show that the chromatic dispersion induced penalty is negligible. The scheme can be easily extended to OFDM-PON.
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