OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 26 — Dec. 12, 2011
  • pp: B323–B328
« Show journal navigation

Simple feed-forward wide-range frequency offset estimator for optical coherent receivers

J. C. M. Diniz, J. C. R. F. de Oliveira, E. S. Rosa, V. B. Ribeiro, V. E. S. Parahyba, R. da Silva, E. P. da Silva, L. H. H. de Carvalho, A. F. Herbster, and A. C. Bordonalli  »View Author Affiliations


Optics Express, Vol. 19, Issue 26, pp. B323-B328 (2011)
http://dx.doi.org/10.1364/OE.19.00B323


View Full Text Article

Acrobat PDF (1641 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We propose and experimentally demonstrate a hardware-efficient, feed-forward, wide-range frequency offset estimator for DSP-based optical coherent receivers. Using a simple relationship of signal spectrum, this estimator is capable to estimate offsets in a range compliant with OIF requirements. Obtained results show that this estimator has a high tolerance to spectrum asymmetry caused by electrical and optical signal filtering, even when using return-to-zero pulse shaping.

© 2011 OSA

1. Introduction

We propose a new FOE, robust to electrical and optical filtering. This FOE uses a simple relationship between the powers from two sides of received signal spectrum. Simulation and experimental results are also demonstrated.

2. Operation principle

As a spectrum-based method for estimating frequency offsets, the proposed algorithm relies on the principle that a FO implies a proportional shift in received signal base-band spectrum. Spectral asymmetry can be measured from the ratio between the power of two sides of signal's spectrum. It was empirically determined the following relationship between power ratio and frequency offset:

Δf=αlog10(P+P)=α(log10(P+)log10(P))
(1)

Here, P+ and P- are the powers of each side of spectrum (Fig. 1
Fig. 1 Received power spectrum with Δf > 0, showing two sides of spectrum.
) and α is a previously determined constant that depends on type and cutoff frequency of the receiver low-pass filters (LPF). That simple relation leads to a feed-forward algorithm scheme, able to estimate a wide range of frequency offsets with estimation error far below 3.5 GHz, which can be refined by the mth-power algorithm [3

3. A. Leven, N. K. Kaneda, U.-V. Koc, and Y.-K. Chen, “Frequency Estimation in Intradyne Reception,” IEEE Photon. Technol. Lett. 19(6), 366–368 (2007). [CrossRef]

5

5. M. Morelli and U. Mengali, “Feedforward frequency estimation for PSK: A tutorial review,” Eur. Trans. Telecommun. 9(2), 103–116 (1998). [CrossRef]

]. Figure 2
Fig. 2 Block diagram of proposed FOE.
shows a block diagram of the proposed method. Spectrum is obtained through a Fast Fourier Transform (FFT) from received signal.

In order to improve the FOE accuracy, both polarizations are considered while calculating spectrum power. A moving average is applied, over successive acquired spectra, to reduce the impact of the noise. The sum of the components of each spectrum side leads to P+ and P-. Frequency offset can be obtained applying Eq. (1). As the method gives a value of frequency offset for a simple power relation, the convergence time required to find estimated frequency offset depends only on the number of averages done.

The estimated FO can be used as a coarse estimation to eliminate the frequency ambiguity from the mth-power FOE as described by [10

10. T. Nakagawa, K. Ishihara, T. Kobayashi, R. Kudo, M. Matsui, Y. Takatori, and M. Mizoguchi, “Wide-Range and Fast-Tracking Frequency Offset Estimator for Optical Coherent Receivers,” in European Conference and Exhibition on Optical Communication, Technical Digest (CD), paper We.7.A.2, (2010).

], or to control a numeric controlled oscillator (NCO) that will impose a contrary FO to the signal, letting fine FO compensation for mth compensator [9

9. S. Zhang, L. Xu, J. Yu, M. F. Huang, P. Y. Kam, C. Yu, and T. Wang, “Dual-Stage Cascaded Frequency Offset Estimation for Digital Coherent Receivers,” IEEE Photon. Technol. Lett. 22(6), 401–403 (2010). [CrossRef]

]. Figure 3(a)
Fig. 3 NCO-based implementation of fine FOE.
shows the NCO-based implementation and Fig. 3(b) shows the feed-forward way to act with the coarse FO estimation.

As shown in [10

10. T. Nakagawa, K. Ishihara, T. Kobayashi, R. Kudo, M. Matsui, Y. Takatori, and M. Mizoguchi, “Wide-Range and Fast-Tracking Frequency Offset Estimator for Optical Coherent Receivers,” in European Conference and Exhibition on Optical Communication, Technical Digest (CD), paper We.7.A.2, (2010).

], one major drawback of spectrum-based FOE is the penalty imposed by low cutoff frequency of the electrical low-pass filters (LPF) after signal detection. Table 1

Table 1. Electrical filtered spectra comparison

table-icon
View This Table
summarizes the effect of different filter bandwidths over the signal spectrum. Discrete Fourier transform of signals was shifted by N/2, where N is the FFT block size, for better understanding.

As the electrical filtering induces asymmetry in the signal spectrum, it could impact on frequency offset estimation. Optical filtering has a similar impact, but it can be more striking as the number and bandwidth of the optical filters can change with network reconfiguration. The simulation and experimental results show that the proposed method is robust to both electrical and optical filtering.

3. Algorithm evaluation

To evaluate the proposed method simulations were performed in presence of electrical and optical filtering at 112 Gb/s PM-QPSK system. We considered a PM-QPSK transmitter, an optical amplifier (to control OSNR), an optical loop with a 50 GHz optical filter and a polarization diversity coherent receiver.

In all simulations, frequency offset was set from −5GHz to + 5GHz, tuning simultaneously the local oscillator and the transmission laser in a ± 2.5 GHz range, around 193.4 THz. The signal was corrupted by amplifier's noise and laser's phase noise due to 500 kHz linewidth of each laser. The OSNR used was 15dB (at 0.1 nm resolution). For each simulation, we transmitted 32768 symbols at 2 samples per symbol. First, electrical filtering was performed by fourth order Gaussian filters with cut-off frequencies of 16, 20, 25 and 28 GHz and NRZ pulse shape. Then, optical filtering was performed by second order Gaussian filters with 44 GHz bandwidth, modeling typical ROADM optical filters in operation at 50 GHz grid [11

11. S. Tibuleac and M. Filer, “Transmission Impairments in DWDM Networks With Reconfigurable Optical Add-Drop Multiplexers,” J. Lightwave Technol. 28(4), 557–598 (2010), http://www.opticsinfobase.org/jlt/abstract.cfm?URI=jlt-28-4-557. [CrossRef]

], for NRZ and RZ (50% duty cycle) pulse shapes. FFT block size was N = 128 and α parameter used by the algorithm was 14.3 GHz for NRZ and 21 GHz for RZ50%. Figures 4
Fig. 4 FOE performance for 112 Gb/s PM-QPSK with different receiver LPFs.
and 5
Fig. 5 FOE performance for 112 Gb/s PM-QPSK passing through different number of cascaded 50GHz optical filters. (a) Using NRZ pulse shape and (b) using RZ50% pulse shape.
show the simulation results of proposed method for electrical and optical filtering, respectively.

Electrical filtering has a mild effect over the frequency estimation, even when a 16 GHz LPF is employed. The estimation error reaches 1GHz at the worst case scenario. The proposed method also exhibits high tolerance to optical filtering. When the signal passed through 15 optical filters, the maximum estimation error reach approximately 2.5 GHz, even using a pulse shape with wider spectrum as RZ. The estimation error were always less than the mth-power FOE estimation range of ± 3.5 GHz, enabling this method to be used as a coarse FOE robust to filtering.

4. Experimental validation

Two experiments were carried out to evaluate the proposed method performance. The modulation format was 112 Gb/s PM-QPSK, generated by a tunable laser together with a polarization diversity quadrature modulator and four 28 GBd lines of pseudorandom bit sequence (PRBS) generator. To evaluate FOE performance with electrical filtering, it was performed a back-to-back experiment and to evaluate the performance with optical filtering we used a recirculation loop with EDFAs, 295 km of single mode fiber and a 50GHz grid ROADM. After transmission, the optical signal was converted to baseband signals by a tunable local oscillator (LO) and a 90° hybrid, and then sampled by two synchronized 80 GS/s real time scopes with LPF bandwidth ranging from 16 to 30 GHz. The digitized data were processed offline in a PC. The frequency offset was set manually by varying the tunable LO. Experimental setup is depicted in Fig. 6
Fig. 6 Experimental Setup.
.

In the first experiment, we set OSNR to 20 dB (at 0.1 nm resolution) by controlling pump power of EDFA, before intradyne coherent receiver. We varied FO in a range of ± 5.25GHz and set cutoff frequency of scope to 16 and 30GHz. The α value was set to 14.5GHz and the FFT block size was N = 128. We acquired 40k samples for each FO value. We also used in our experiment the spectrum symmetry-based FOE proposed by [10

10. T. Nakagawa, K. Ishihara, T. Kobayashi, R. Kudo, M. Matsui, Y. Takatori, and M. Mizoguchi, “Wide-Range and Fast-Tracking Frequency Offset Estimator for Optical Coherent Receivers,” in European Conference and Exhibition on Optical Communication, Technical Digest (CD), paper We.7.A.2, (2010).

] for comparison purposes. Figure 7
Fig. 7 Experimental performance of proposed FOE for 112 Gb/s PM-QPSK using different LPF cutoff frequencies.
shows experimental results for electrical filtering in back-to-back configuration.

In the second experiment, it was transmitted the optical signal through 1475km of single mode fiber (5 turns in the loop). We varied FO in a range of ± 6GHz and set the cutoff frequency of scope to 30GHz. The FFT block size was N = 128 and the α parameter was 14.5GHz. We acquired 40k samples for each FO value.

To measure BER penalties imposed by large frequency offsets we processed the experimental data using the DSP sequence shown in Fig. 3. The proposed algorithm was used as a coarse FOE, using FFT information from received signal, available at static equalization algorithm (used for chromatic dispersion compensation). The other algorithms are explained by [1

1. S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express 16(2), 804–817 (2008), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-2-804. [CrossRef] [PubMed]

,2

2. S. J. Savory, “Digital Coherent Optical Receivers: Algorithms and Subsystems,” IEEE J. Sel. Top. Quantum Electron. 16(5), 1164–1179 (2010). [CrossRef]

]. Figure 8
Fig. 8 Experimental performance of proposed FOE for 112Gb/s PM-QPSK passing through loops with 50GHz grid ROADMs. (a) Coarse FOE. (b) Estimated BER.
shows the experimental results for optical filtering.

As we can observe in Fig. 8(a), the proposed FOE also exhibits high tolerance to optical filtering. The estimation error were always below ± 3.5GHz limit, letting fine estimation to mth-power FOE. After fiber transmission at recirculation loop the received data was corrupted by amplifier noise, with OSNR near 15dB (at 0.1nm optical spectrum analyzer resolution), while in back-to-back experiment, the OSNR was about 35dB. In Fig. 8(b), we see that when increasing the FO absolute value in back-to-back configuration there is considerably BER penalty due to receiver bandwidth limitations. However, after fiber transmission, when OSNR was low enough that the system operates at BER close to FEC limit (BER = 2x10−3), the penalty introduced by amplifier noise overcame the penalty caused by receiver bandwidth limitations. Then, penalties in system performance are mainly affected by amplifier noise.

5. Conclusions

Acknowledgments

The authors would like to acknowledge the financial support from FUNTTEL, Fund for Technological Development of Telecommunications, of the Brazilian Ministry of Communications, and from FINEP, Financing Agency for Studies and Projects, of the Brazilian Ministry of Science and Technology.

References and links

1.

S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express 16(2), 804–817 (2008), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-2-804. [CrossRef] [PubMed]

2.

S. J. Savory, “Digital Coherent Optical Receivers: Algorithms and Subsystems,” IEEE J. Sel. Top. Quantum Electron. 16(5), 1164–1179 (2010). [CrossRef]

3.

A. Leven, N. K. Kaneda, U.-V. Koc, and Y.-K. Chen, “Frequency Estimation in Intradyne Reception,” IEEE Photon. Technol. Lett. 19(6), 366–368 (2007). [CrossRef]

4.

S. Hoffmann, S. Bhandare, T. Pfau, O. Adamczyk, C. Wördehoff, R. Peveling, M. Porrmann, and R. Noé, “Frequency and Phase Estimation for Coherent QPSK Transmission with Unlocked DFB Lasers,” IEEE Photon. Technol. Lett. 20(18), 1569–1571 (2008). [CrossRef]

5.

M. Morelli and U. Mengali, “Feedforward frequency estimation for PSK: A tutorial review,” Eur. Trans. Telecommun. 9(2), 103–116 (1998). [CrossRef]

6.

Optical Internetworking Forum, Integrable Tunable Laser Assembly MSA, (OIF-ITLA-MSA-01.2), (2008) http://www.oiforum.com/public/documents/OIF-ITLA-MSA-01.2.pdf

7.

L. Li, Z. Tao, S. Oda, T. Hoshida, and J. C. Rasmussen, “Wide-Range, Accurate and Simple Digital Frequency Offset Compensator for Optical Coherent Receivers, ” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper OWT4. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2008-OWT4

8.

Z. Li, X. Chen, W. Zhou, H. Zhu, X. Zhou, and Z. Zhang, “Wide-Range and Fast-Convergence Frequency Offset Estimator by BER-Aiding for Optical Coherent Receivers, ” in Asia Communications and Photonics Conference and Exhibition, Technical Digest (CD) (Optical Society of America, 2009), paper ThT2. http://www.opticsinfobase.org/abstract.cfm?URI=ACP-2009-ThT2

9.

S. Zhang, L. Xu, J. Yu, M. F. Huang, P. Y. Kam, C. Yu, and T. Wang, “Dual-Stage Cascaded Frequency Offset Estimation for Digital Coherent Receivers,” IEEE Photon. Technol. Lett. 22(6), 401–403 (2010). [CrossRef]

10.

T. Nakagawa, K. Ishihara, T. Kobayashi, R. Kudo, M. Matsui, Y. Takatori, and M. Mizoguchi, “Wide-Range and Fast-Tracking Frequency Offset Estimator for Optical Coherent Receivers,” in European Conference and Exhibition on Optical Communication, Technical Digest (CD), paper We.7.A.2, (2010).

11.

S. Tibuleac and M. Filer, “Transmission Impairments in DWDM Networks With Reconfigurable Optical Add-Drop Multiplexers,” J. Lightwave Technol. 28(4), 557–598 (2010), http://www.opticsinfobase.org/jlt/abstract.cfm?URI=jlt-28-4-557. [CrossRef]

12.

E. Ip and J. M. Kahn, “Power Spectra of Return-to-Zero Optical Signals,” J. Lightwave Technol. 24(3), 1610–1618 (2006), http://www.opticsinfobase.org/jlt/abstract.cfm?URI=jlt-24-3-1610. [CrossRef]

OCIS Codes
(060.1660) Fiber optics and optical communications : Coherent communications
(060.5060) Fiber optics and optical communications : Phase modulation

ToC Category:
Subsystems for Optical Networks

History
Original Manuscript: October 3, 2011
Revised Manuscript: October 29, 2011
Manuscript Accepted: November 2, 2011
Published: November 18, 2011

Virtual Issues
European Conference on Optical Communication 2011 (2011) Optics Express

Citation
J. C. M. Diniz, J. C. R. F. de Oliveira, E. S. Rosa, V. B. Ribeiro, V. E. S. Parahyba, R. da Silva, E. P. da Silva, L. H. H. de Carvalho, A. F. Herbster, and A. C. Bordonalli, "Simple feed-forward wide-range frequency offset estimator for optical coherent receivers," Opt. Express 19, B323-B328 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-26-B323


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express16(2), 804–817 (2008), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-2-804 . [CrossRef] [PubMed]
  2. S. J. Savory, “Digital Coherent Optical Receivers: Algorithms and Subsystems,” IEEE J. Sel. Top. Quantum Electron.16(5), 1164–1179 (2010). [CrossRef]
  3. A. Leven, N. K. Kaneda, U.-V. Koc, and Y.-K. Chen, “Frequency Estimation in Intradyne Reception,” IEEE Photon. Technol. Lett.19(6), 366–368 (2007). [CrossRef]
  4. S. Hoffmann, S. Bhandare, T. Pfau, O. Adamczyk, C. Wördehoff, R. Peveling, M. Porrmann, and R. Noé, “Frequency and Phase Estimation for Coherent QPSK Transmission with Unlocked DFB Lasers,” IEEE Photon. Technol. Lett.20(18), 1569–1571 (2008). [CrossRef]
  5. M. Morelli and U. Mengali, “Feedforward frequency estimation for PSK: A tutorial review,” Eur. Trans. Telecommun.9(2), 103–116 (1998). [CrossRef]
  6. Optical Internetworking Forum, Integrable Tunable Laser Assembly MSA, (OIF-ITLA-MSA-01.2), (2008) http://www.oiforum.com/public/documents/OIF-ITLA-MSA-01.2.pdf
  7. L. Li, Z. Tao, S. Oda, T. Hoshida, and J. C. Rasmussen, “Wide-Range, Accurate and Simple Digital Frequency Offset Compensator for Optical Coherent Receivers, ” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper OWT4. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2008-OWT4
  8. Z. Li, X. Chen, W. Zhou, H. Zhu, X. Zhou, and Z. Zhang, “Wide-Range and Fast-Convergence Frequency Offset Estimator by BER-Aiding for Optical Coherent Receivers, ” in Asia Communications and Photonics Conference and Exhibition, Technical Digest (CD) (Optical Society of America, 2009), paper ThT2. http://www.opticsinfobase.org/abstract.cfm?URI=ACP-2009-ThT2
  9. S. Zhang, L. Xu, J. Yu, M. F. Huang, P. Y. Kam, C. Yu, and T. Wang, “Dual-Stage Cascaded Frequency Offset Estimation for Digital Coherent Receivers,” IEEE Photon. Technol. Lett.22(6), 401–403 (2010). [CrossRef]
  10. T. Nakagawa, K. Ishihara, T. Kobayashi, R. Kudo, M. Matsui, Y. Takatori, and M. Mizoguchi, “Wide-Range and Fast-Tracking Frequency Offset Estimator for Optical Coherent Receivers,” in European Conference and Exhibition on Optical Communication, Technical Digest (CD), paper We.7.A.2, (2010).
  11. S. Tibuleac and M. Filer, “Transmission Impairments in DWDM Networks With Reconfigurable Optical Add-Drop Multiplexers,” J. Lightwave Technol.28(4), 557–598 (2010), http://www.opticsinfobase.org/jlt/abstract.cfm?URI=jlt-28-4-557 . [CrossRef]
  12. E. Ip and J. M. Kahn, “Power Spectra of Return-to-Zero Optical Signals,” J. Lightwave Technol.24(3), 1610–1618 (2006), http://www.opticsinfobase.org/jlt/abstract.cfm?URI=jlt-24-3-1610 . [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited