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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 20 — Sep. 24, 2012
  • pp: 22878–22885
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None pilot-tones and training sequence assisted OFDM technology based on multiple-differential amplitude phase shift keying

Bo Liu, Lijia Zhang, Xiangjun Xin, and Jianjun Yu  »View Author Affiliations


Optics Express, Vol. 20, Issue 20, pp. 22878-22885 (2012)
http://dx.doi.org/10.1364/OE.20.022878


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Abstract

This paper proposes a novel none pilot-assisted orthogonal frequency division multiplexing (OFDM) technology based on multi-differential amplitude phase shift keying (mDAPSK) for optical OFDM system. It doesn’t require any bandwidth-consuming pilot tones or training sequence for channel estimation due to the differential detection during demodulation. In the experiment, a 41.31 Gb/s 64DAPSK-OFDM signal without pilot tones is successfully transmitted over 160-km single mode fiber (SMF). The performance comparison between multi-quadrature amplitude modulation (mQAM) and mDAPSK is also given in the experiment, and the results indicate a prospect of this technology in optical OFDM system.

© 2012 OSA

1. Introduction

In this paper, we propose a novel OFDM system employing mDAPSK technology, where none pilot tones or training sequence are needed for signal demodulation. The proposed scheme can reduce the redundancy and complexity while maintaining a high SE for optical OFDM system. In our experiment, a 41.31 Gb/s 64DAPSK-OFDM signal is generated and transmitted over 160-km single mode fiber (SMF) successfully. We also compare the performance of mDAPSK-OFDM signal with mQAM-OFDM signal in the experiment.

2. Principle

Figure 1
Fig. 1 The principle of optical mDAPSK-OFDM system (IFFT: inverse fast Fourier transform; CP: cyclic prefix; Coh. Rx: coherent receiver).
illustrates the principle of the mDAPSK-OFDM system. The input PRBS data is fed into the mDAPSK modulation module after serial to parallel conversion. The modulation diagram of mDAPSK is illustrated in the bottom of Fig. 1. The input bits are divided into amplitude mapped bits and phase mapped bits, which are represented by ma,i and mp,i respectively, and then go through differential modulation. The format of ma,i or mp,i can be written as (dj…d2d1d0), where dj denotes the input bits data. In our scheme, the data is differentially encoded between different subcarriers in the same OFDM symbol.

The mth sample of baseband mDAPSK-OFDM signal in the nth symbol interval can be expressed as
s(t)=k=1NXi,kej2πkt/Ts,t=mTs/N
(1)
where k is the index of the OFDM subcarrier, N is the total number of the subcarriers, Ts is the time duration of each sample and Xi, k means the mDAPSK modulated data symbol on the kth subcarrier. Xi, k is represented as
Xi,k=αikejθik
(2)
Here, i is ith data symbol, αik and θik are the amplitude and absolute phase of data symbol respectively, which can be expressed as
{αik=γkαi(k1),γk(A±(2Ma1),A±(2Ma2),...,A0)θik=θi(k1)+Δθk,Δθk=(2l+1)π/2Mp,l=0,1,...,2Mp1
(3)
In Eq. (3), γk and Δθk are the differential parameters of amplitude and phase, Ma and Mp are the bit numbers of ma,i and mp,i, A is a constant value and αik∈(1, A, A2, …, A2Ma1). For example, Ma = 1 and Mp = 3 are for 16DAPSK modulation, and Ma = 2 and Mp = 4 are for 64DAPSK modulation. The mapping rule from mp,i to Δθk obeys the Gray coding for phase differential coding. For example, when mp,i = (000), Δθk = π/8; when mp,i = (011), Δθk = 5π/8. For amplitude differential coding, ma,i can choose different γk according to the values of αi(k-1) and ma,i. We also adopt Gray coding for the input bits of ma,i. The status transition diagrams for ma,i = (d0) and ma,i = (d1d0) are illustrated in Fig. 2
Fig. 2 The status transition diagrams of amplitude differential mapping: (a) two amplitude levels; (b) four amplitude levels.
.

After the insertion of the CP and the parallel to serial conversion, the mDAPSK-OFDM signal is produced through digital to analog (D/A) conversion. Then it can be modulated onto the optical carrier with the help of optical I/Q modulator. At the receiver, it executes the corresponding inverse processing to recover the bits stream. The received mDAPSK symbol of the kth subcarrier can be expressed as
Ri,k=Hi,kSi,k+Ni,k
(4)
where Hi,k is the channel transfer function of the subcarrier, Si,k is the frequency domain information of transmitted signal and Ni,k is the noise. There we assume the channel in the optical fiber is a kind of slow time-varying channel, and the differential factor can be expressed as
Di,k=Ri,kRi1,k=α'i,kejθ'i,k
(5)
Here, α’i,k is the received amplitude parameter and θ’i,k is the received phase parameter. We can get the original bits stream with the two parameters. Due to the differential coding, there is no need to know the channel function Hi,k, which can reduce the complexity and redundancy of the receiver.

The QAM-OFDM and DAPSK-OFDM frame structures are shown in Fig. 3
Fig. 3 The structures of QAM-OFDM and DAPSK-OFDM frames.
. In both of the structures, the synchronization sequence is indispensable. But for QAM-OFDM signal, it still needs additional training sequence and pilots to execute the channel estimation and equalization. Compared with QAM-OFDM signal, the DAPSK-OFDM signal could keep a lower complexity while mantaining a higher SE. For a real-time optical QAM-OFDM signal, we take Ref [5

5. N. Kaneda, Q. Yang, X. Liu, W. Shieh, and Y.-K. Chen, “Realizing real-time implementation of coherent optical OFDM receiver with FPGAs,” in Proc. ECOC’2009, paper.5.4.4 (2009).

]. as an example. It inserts 16 training symbols every 496 data symbols. Out of 128 subcarriers, 10 pilot subcarriers are added into 107 data subcarriers. The SE will reduce by 11% compared with DAPSK-OFDM signal, which is enough for forward error correction (FEC) coding. Without channel estimation and equalization, it can also reduce the complexity and cost at both the transmitter and receiver.

3. Experimental setup and results

At the receiver, an optical hybrid is adopted for the coherent detection of OFDM signal. A real-time sampling oscilloscope (TDS) with two embedded 20-GS/s ADCs is used for the sampling of received 64DAPSK-OFDM signal. The signal processing and demodulation are executed offline on the computer, and Monte-Carlo analysis based on Matlab has been carried out in our experiment. Figure 5
Fig. 5 Measured BER curves and constellation of 64DAPSK-OFDM signal before and after transmission (resolution: 0.1nm).
shows the bit rate error (BER) as a function of the optical signal-to-noise ratio (OSNR). The single channel without pilot tones is measured before and after 160 km transmission. The required OSNR to achieve a BER of 10−3 are 19.87 dB and 22.5 dB respectively. An additional penalty of 2.63 dB is incurred in Fig. 5, which is mainly due to the fiber dispersion as well as nonlinearity during transmission. Although the CP of 1/32 can resolve the channel dispersion-induced inter-carrier interference (ICI) and inter-symbol interference (ISI), the phase noise on each subcarrier cannot be eliminated; furthermore, the signal suffers both nonlinearity and fiber dispersion, and no equalization is adopted for signal demodulation.

Then we compare the nonlinearity during the transmission of 64QAM-OFDM and 64DAPSK-OFDM in single channel case. Figure 8
Fig. 8 The measured BER after transmission vs. launced optical power for 64QAM-OFDM and 64DAPSK signals (at OSNR of 25dB, resolution: 0.1nm).
illustrates the BER versus launched optical power with 160 km transmission. We can see that the optimum optical powers for both the signals are almost the same. However, as the input power increasing, the BER of 64QAM-OFDM signal degrades faster than 64DAPSK-OFDM signal. The DAPSK-OFDM signal shows higher tolerance towards fiber nonlinearity.

4. Conclusion

We have proposed and experimentally demonstrated a novel no pilot tones and training sequence assisted OFDM technology based on mDAPSK modulation. The differential modulation can mitigate the phase noise caused by the fiber dispersion during transmission. There is no need to execute the channel estimation and can reduce the complexity of the optical OFDM system. A 41.31 Gb/s 64DAPSK-OFDM signal has been successfully transmitted over 160 km fiber in the experiment. The performance comparision between mQAM-OFDM and mDAPSK-OFDM is also executed in the experiment. Considering the penelty during channel equalization, the performance of 64DAPSK-OFDM is almost the same as pilots and training sequence inserted 64QAM-OFDM signal. The results show the prospect of DAPSK-OFDM in optical OFDM system.

Acknowledgment

The financial supports from National Basic Research Program of China with No. 2010CB328300, National High Technology 863 Program of China with No.2012AA011300, National International Technology Cooperation with No.2012DFG12110, National NSFC with No. 60932004, 61077050, 61077014, 61205066 and BUPT Excellent Ph. D. Students Foundation are gratefully acknowledged. The project is also supported by the Fundamental Research Funds for the Central Universities with No. 2012RC0311.

References and links

1.

W. Shieh, “OFDM for flexible high-speed optical networks,” J. Lightwave Technol. 29(10), 1560–1577 (2011). [CrossRef]

2.

N. Cvijetic, M. Cvijetic, M.-F. Huang, E. Ip, Y.-K. Huang, and T. Wang, “Terabit optical access networks based on WDM-OFDMA-PON,” J. Lightwave Technol. 30(4), 493–503 (2012). [CrossRef]

3.

J. L. Wei, C. Sánchez, R. P. Giddings, E. Hugues-Salas, and J. M. Tang, “Significant improvements in optical power budgets of real-time optical OFDM PON systems,” Opt. Express 18(20), 20732–20745 (2010). [CrossRef] [PubMed]

4.

D. Qian, M.-F. Huang, E. Ip, Y.-K. Huang, Y. Shao, J. Hu, and T. Wang, “High capacity/spectral efficiency 101.7-Tb/s WDM transmission using PDM-128QAM-OFDM over 165-km SSMF within C- and L-bands,” J. Lightwave Technol. 30(10), 1540–1548 (2012). [CrossRef]

5.

N. Kaneda, Q. Yang, X. Liu, W. Shieh, and Y.-K. Chen, “Realizing real-time implementation of coherent optical OFDM receiver with FPGAs,” in Proc. ECOC’2009, paper.5.4.4 (2009).

6.

A. J. Lowery and L. B. Du, “Optical orthogonal division multiplexing for long haul optical communications: A review of the first five years,” Opt. Fiber Technol. 17(5), 421–438 (2011). [CrossRef]

7.

J. Zhao and A. Ellis, “Transmission of 4-ASK optical fast OFDM with chromatic dispersion compensation,” IEEE Photon. Technol. Lett. 24(1), 34–36 (2012). [CrossRef]

8.

X. Liu, F. Buchali, and R. W. Tkach, “Improving the nonlinear tolerance of polarization-division-multiplexed CO-OFDM in long-haul fiber transmission,” J. Lightwave Technol. 27(16), 3632–3640 (2009). [CrossRef]

9.

Q. Zhuge, M. Morsy-Osman, and D. V. Plant, “Analysis of dispersion-enhanced phase noise in CO-OFDM systems with RF-pilot phase compensation,” Opt. Express 19(24), 24030–24036 (2011). [CrossRef] [PubMed]

10.

S. L. Jansen, I. Morita, N. Takeda, and H. Tanaka, “Pre-emphasis and RF-pilot tone phase noise compensation for coherent OFDM transmission systems,” in Proc. CLEO 2007, paper. MA1.2 (2007).

11.

A. Sano, E. Yamada, H. Masuda, E. Yamazaki, T. Kobayashi, E. Yoshida, Y. Miyamoto, R. Kudo, K. Ishihara, and Y. Takatori, “No-Guard-Interval coherent optical OFDM for 100-Gb/s long-haul WDM transmission,” J. Lightwave Technol. 27(16), 3705–3713 (2009). [CrossRef]

12.

N. Toender and H. Rohling, “DAPSK schemes for low-complexity OFDM systems,” IEEE 16th International Symposium on Personal, Indoor and Mobile Radio Communications, 735–739 (2006).

13.

C.-C.Fang, Y.-J. Lin, S.-W. Wei, and J.-F. Chang, “Performance analyses of DAPSK in a very high mobility environment,” in Proc.WIRLES 2005, 570–575 (2005).

14.

T. May, H. Rohling, and V. Engels, “Performance analysis of Viterbi decoding for 64-DAPSK and 64-QAM modulated OFDM signals,” IEEE Trans. Commun. 46(2), 182–190 (1998). [CrossRef]

OCIS Codes
(060.2330) Fiber optics and optical communications : Fiber optics communications
(060.4080) Fiber optics and optical communications : Modulation
(060.4250) Fiber optics and optical communications : Networks

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: July 25, 2012
Revised Manuscript: September 14, 2012
Manuscript Accepted: September 18, 2012
Published: September 20, 2012

Citation
Bo Liu, Lijia Zhang, Xiangjun Xin, and Jianjun Yu, "None pilot-tones and training sequence assisted OFDM technology based on multiple-differential amplitude phase shift keying," Opt. Express 20, 22878-22885 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-20-22878


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References

  1. W. Shieh, “OFDM for flexible high-speed optical networks,” J. Lightwave Technol.29(10), 1560–1577 (2011). [CrossRef]
  2. N. Cvijetic, M. Cvijetic, M.-F. Huang, E. Ip, Y.-K. Huang, and T. Wang, “Terabit optical access networks based on WDM-OFDMA-PON,” J. Lightwave Technol.30(4), 493–503 (2012). [CrossRef]
  3. J. L. Wei, C. Sánchez, R. P. Giddings, E. Hugues-Salas, and J. M. Tang, “Significant improvements in optical power budgets of real-time optical OFDM PON systems,” Opt. Express18(20), 20732–20745 (2010). [CrossRef] [PubMed]
  4. D. Qian, M.-F. Huang, E. Ip, Y.-K. Huang, Y. Shao, J. Hu, and T. Wang, “High capacity/spectral efficiency 101.7-Tb/s WDM transmission using PDM-128QAM-OFDM over 165-km SSMF within C- and L-bands,” J. Lightwave Technol.30(10), 1540–1548 (2012). [CrossRef]
  5. N. Kaneda, Q. Yang, X. Liu, W. Shieh, and Y.-K. Chen, “Realizing real-time implementation of coherent optical OFDM receiver with FPGAs,” in Proc. ECOC’2009, paper.5.4.4 (2009).
  6. A. J. Lowery and L. B. Du, “Optical orthogonal division multiplexing for long haul optical communications: A review of the first five years,” Opt. Fiber Technol.17(5), 421–438 (2011). [CrossRef]
  7. J. Zhao and A. Ellis, “Transmission of 4-ASK optical fast OFDM with chromatic dispersion compensation,” IEEE Photon. Technol. Lett.24(1), 34–36 (2012). [CrossRef]
  8. X. Liu, F. Buchali, and R. W. Tkach, “Improving the nonlinear tolerance of polarization-division-multiplexed CO-OFDM in long-haul fiber transmission,” J. Lightwave Technol.27(16), 3632–3640 (2009). [CrossRef]
  9. Q. Zhuge, M. Morsy-Osman, and D. V. Plant, “Analysis of dispersion-enhanced phase noise in CO-OFDM systems with RF-pilot phase compensation,” Opt. Express19(24), 24030–24036 (2011). [CrossRef] [PubMed]
  10. S. L. Jansen, I. Morita, N. Takeda, and H. Tanaka, “Pre-emphasis and RF-pilot tone phase noise compensation for coherent OFDM transmission systems,” in Proc. CLEO 2007, paper. MA1.2 (2007).
  11. A. Sano, E. Yamada, H. Masuda, E. Yamazaki, T. Kobayashi, E. Yoshida, Y. Miyamoto, R. Kudo, K. Ishihara, and Y. Takatori, “No-Guard-Interval coherent optical OFDM for 100-Gb/s long-haul WDM transmission,” J. Lightwave Technol.27(16), 3705–3713 (2009). [CrossRef]
  12. N. Toender and H. Rohling, “DAPSK schemes for low-complexity OFDM systems,” IEEE 16th International Symposium on Personal, Indoor and Mobile Radio Communications, 735–739 (2006).
  13. C.-C.Fang, Y.-J. Lin, S.-W. Wei, and J.-F. Chang, “Performance analyses of DAPSK in a very high mobility environment,” in Proc.WIRLES 2005, 570–575 (2005).
  14. T. May, H. Rohling, and V. Engels, “Performance analysis of Viterbi decoding for 64-DAPSK and 64-QAM modulated OFDM signals,” IEEE Trans. Commun.46(2), 182–190 (1998). [CrossRef]

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