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Optics Express

Optics Express

  • Editor: Martijn de Sterke
  • Vol. 16, Iss. 20 — Sep. 29, 2008
  • pp: 15477–15482
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Experimental investigation of SPM in long-haul direct-detection OFDM systems

Yannis Benlachtar, Giancarlo Gavioli, Vitaly Mikhailov, and Robert I. Killey  »View Author Affiliations


Optics Express, Vol. 16, Issue 20, pp. 15477-15482 (2008)
http://dx.doi.org/10.1364/OE.16.015477


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Abstract

We experimentally investigate the effect of self-phase modulation on direct-detection orthogonal frequency division multiplexed (OFDM) transmission at 11.1 Gb/s over 960km and 1600km uncompensated standard single-mode fiber links. We show that for long-haul systems, the penalties due to nonlinear distortion in OFDM systems are comparable to those in links employing electronic predistortion.

© 2008 Optical Society of America

1. Introduction

Orthogonal frequency division multiplexing (OFDM) is widely used in wireless systems such as GSM and WiMax and has been gaining a great deal of attention in the optical networks area because of its advantages [1

1. A. J. Lowery, B. D. Liang, and J. Armstrong, “Performance of Optical OFDM in Ultralong-Haul WDM Lightwave Systems,” J. Lightwave Technol. 25, 131–138 (2007). [CrossRef]

-4

4. S. L. Jansen, I. Morita, T. C. W. Schenk, D. van den Borne, and H. Tanaka, “Optical OFDM - A Candidate for Future Long-Haul Optical Transmission Systems,” Proc. Optical Fiber Commun., paper OMU3 (2008).

], which include the reduction in the baud rate, since high-speed serial information is transmitted through multiple lower-speed sub-channels. As a result, the inter-symbol interference is reduced and the equalization at the receiver is easily achieved. Additionally, OFDM allows for the utilization of more advanced modulation formats and thus can achieve higher spectral efficiency. Synchronization can be achieved relatively easily and the implementation of the system is simplified by the use of FFT/IFFT algorithms. However, a major drawback to this technique is the high peak-to-average power ratio (PAPR), although there are many methods such as clipping and pre-coding to mitigate this problem [5

5. T. Jiang and Y. Wu, “An Overview: Peak-to-Average Power Ratio Reduction Techniques for OFDM Signals,” IEEE Transactions on Broadcasting , 54, 257–268 (2008). [CrossRef]

]. In optics there are two schemes to achieve OFDM: direct-detection (DD-OFDM) and coherent-detection (CO-OFDM). The former is simpler and cheaper to implement as it only requires a single photodiode and digital signal processing (DSP) at the receiver, albeit with lower spectral efficiency [1

1. A. J. Lowery, B. D. Liang, and J. Armstrong, “Performance of Optical OFDM in Ultralong-Haul WDM Lightwave Systems,” J. Lightwave Technol. 25, 131–138 (2007). [CrossRef]

]. Coherent detection, on the other hand, has a higher sensitivity and spectral efficiency, but with additional cost and complexity due to the requirement for phase and polarization tracking [3

3. W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett. 42, 587–589 (2006). [CrossRef]

].

2. OFDM transmitter

The digital signal processing (DSP) required to generate and decode the OFDM signals was carried out off-line using Matlab. The OFDM transmitter DSP is shown in Fig. 1. First an 11.1Gbit/s serial data is mapped onto a QAM-4 format then passed to an N=2048 IFFT block. The data is fed to the first 512 input of the IFFT, while zeros occupy the remainder (2 ×oversampled). The block multiplexes the 512 OFDM subcarriers and generates a 0-5.55GHz OFDM band. A cyclic extension is added to each OFDM symbol, then the data is serialized. Following this, an up-conversion stage is necessary where the OFDM band is shifted to the 5.55-11.1GHz band. This is achieved by multiplying the signal by exp(2iπft) where f=5.55GHz. An alternative method to avoid the up-conversion stage is to feed the data to the [N/4+1: N/2] input of the IFFT block [4

4. S. L. Jansen, I. Morita, T. C. W. Schenk, D. van den Borne, and H. Tanaka, “Optical OFDM - A Candidate for Future Long-Haul Optical Transmission Systems,” Proc. Optical Fiber Commun., paper OMU3 (2008).

]. In this case the OFDM subcarriers are directly multiplexed onto the 5.55-11.1GHz band.

Fig. 1. OFDM transmitter configuration. Inset: Optical spectrum showing the carrier and the SSB OFDM band (0.01nm resolution)

The data is then stored in random access memory on the Nortel eDCO transmitter which is described in detail in [6

6. K. Roberts, C. Li, L. Strawczynski, M. O’Sullivan, and I. Hardcastle, “Electronic precompensation of optical nonlinearity,” Photon. Technol. Lett. 18, 403–405 (2006). [CrossRef]

]. The card is utilized as a 22.2Gs/s arbitrary waveform generator (AWG) consisting of a memory, two 6-bit digital-to-analog converters (DACs), a tunable laser and a polar dual-drive Mach-Zehnder modulator (MZM). The OFDM waveform stored in the memory was adjusted to avoid any clipping due to the DACs. In this work the polar MZM is used to generate a double optical sideband and a sideband suppression stage using an optical filter is employed. The power in the carrier was set to equal the power in the sideband. The measured optical spectrum at the output of the OFDM transmitter is depicted in Fig. 1 with 0.01nm resolution (inset). The modulator was biased so that it operated in the linear region and to accommodate the relatively high peak-to-average power (PAPR) ratio. In the worst case, the PAPR can be as high as 10×log(Nsc) where Nsc is the number of subcarrier (512 in this case) [7

7. O. Bulakci, M. Schuster, C.-A. Bunge, and B. Spinnler, “Precoding based Peak-to-Average Power Ratio Reduction for Optical OFDM demonstrated on Compatible Single-Sideband Modulation with Direct Detection,” Proc. Opt. Fiber Commun Paper JThA56 (OFC) (2008)

].

3. OFDM receiver

Figure 2 shows the direct-detection OFDM receiver model. After the square law detection, the electrical spectrum consists of the transmitted OFDM band (5.55-11.1GHz) and inter-modulation components in the 0-5.55GHz band as shown in the inset in Fig. 2 [1

1. A. J. Lowery, B. D. Liang, and J. Armstrong, “Performance of Optical OFDM in Ultralong-Haul WDM Lightwave Systems,” J. Lightwave Technol. 25, 131–138 (2007). [CrossRef]

]. The data is then digitized using a real-time sampling scope (Tektronix DPO 72004) and synchronized. Synchronization is realized by sending two subsequent OFDM symbols carrying identically known data (training symbols) at the transmitter [8

8. S. L. Jansen, I. Morita, T. C. W. Schenk, N. Takeda, H. Tanaka, and Tanaka, “Coherent Optical 25.8-Gb/s OFDM Transmission Over 4160-km SSMF,” J. Lightwave Technol. 26, 6–15 (2008) [CrossRef]

]. The two symbols are correlated at the receiver and the OFDM symbol boundaries are defined. In addition, the training sequence is utilized to define the rotation angle of each sub-carrier due to chromatic dispersion.

Fig. 2. OFDM receiver configuration. Inset1: electrical spectrum. Inset2: equalized constellation at OSNR=15dB

The signal is then down-converted to baseband and passed through a low-pass filter. Following this, the cyclic extensions are removed and the data is fed to a 2048 FFT block. Finally, the zero padding is removed and the data is equalized and then decoded.

4. Loop experiment

The experimental setup used in this work is shown in Fig. 3. An 11.1Gbit/s QAM-4 OFDM signal was generated at 1554.94nm. Because of the memory capacity of the eDCO card, the maximum bit sequence length that could be used was a 214-1 PRBS. The OFDM symbol period was 2048×45ps=92.16ns. The recirculating loop consisted of a span of 80km of standard SMF and no optical dispersion compensation was deployed. Two transmission distances were investigated: 12 and 20 recirculation (for a total of 960 and 1600km respectively). Cyclic extensions of 1.395ns and 2.295ns were added to the OFDM data for 960 and 1600km transmission respectively. The guard interval values were calculated using the equation given in [3

3. W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett. 42, 587–589 (2006). [CrossRef]

]. The signal was noise loaded at the receiver to obtain plots of BER versus received OSNR. An EDFA pre-amplifier followed by a 50GHz wavelength demultiplexer were deployed before the photodiode. The data was captured by the 50Gs/s Tektronix DPO 72004 real-time oscilloscope before being resampled at 22.2Gs/s and processed off-line. 16 PRBS cycles were processed at a time to calculate BER values down to 3.8×10-6.

Fig. 3. Re-circulating loop setup. OFDM Tx and Rx are illustrated in Fig. 1 and Fig. 2 respectively.

5. Results and discussion

Figure 4 shows the BER versus OSNR after 960km for launch power values ranging from -10 to 0dBm plotted with 2dB resolution. Monte Carlo simulations lead to a value of the required OSNR for a BER=10-3, using our setup and a perfect demultiplexer (a brick wall 50GHz filter) at the receiver, of ~10dB (using 0.1nm resolution bandwidth). However, we observed a 1.5dB penalty when using a second-order Gaussian filter (OSNR becomes 11.5dB). In the back-to-back experimental results it can be observed that the OSNR value increases to 12.2dB. This penalty is due to the imperfect nature of the optical filter at the receiver (demux) and also to the fact that the frequency response of the transmitter is not flat over the 5.55-11.1GHz band (it exhibits amplitude and phase distortion). With launch power of -10dBm, we observed an OSNR penalty of ~1.5dB compared with back-to-back, the OSNR sensitivity increasing to 13.8dB. The required OSNR increases gradually with increasing launch power to reach 14.4dB at -2dBm. However at 0dBm, where the SPM effect is the strongest, a significant increase to 16.3dB occurred. We detected all transmitted bits without error for launch powers -6, -4 and -2dBm at 19, 20.5 and 21dB OSNR respectively.

Fig. 4. BER versus OSNR (0.1nm resolution bandwidth) after 960km for various launch powers

Figure 5 shows the BER versus received OSNR curves for the same power values after 1600km. It can be observed that the required OSNR for levels less than -10dBm is 13.8dB. This is the same as 960km and shows that there are negligible additional penalties when extending the transmission distance. This demonstrates that OFDM is only noise limited, for low launch powers, albeit with extra overheads. The required OSNR increased gradually up to a launch power of -6dBm where it reaches 14.1dB and significantly increases for higher values, reaching 16.5dB at -2dBm. We could not achieve a BER=10-3 at 0dBm even at a maximum OSNR of 23dB. Figures 4 and 5 demonstrate how the effects of nonlinearity increase with increasing distance. Moreover, the optimum launch power (that achieves the highest OSNR margin) in the 960km transmission distance is about -2dBm (<1dB penalty wrt -10dBm curve) whereas it reduces to approximately -4dBm for 1600km (~1.6dB penalty).

Fig. 5. BER versus OSNR (0.1nm resolution bandwidth) after 1600km for various launch powers

Fig. 6. Comparison of the required OSNR penalty (at BER=10-3) versus launch power for OFDM and EPD after 1600km at 11.1Gbit/s.

6. Conclusion

In this work we realized an 11.1Gbit/s single-sideband direct-detection OFDM system using an optical filter at the transmitter. The back-to-back required OSNR for a BER=10-3 was 12dB, and 13.8dB after transmission over 960km and 1600km of standard SMF. The launch power was incrementally increased from -10 to 0dBm to investigate the effects of SPM. For the 960km transmission distance, the optimum power was -2dBm with less than 1dB penalty due to nonlinearity. In the 1600km case, the optimum launch power was around -4dBm with less than 1.5dB penalty. Finally we compared the impact of SPM on OFDM and EPD and we found comparable performance between the two techniques.

Acknowledgments

The authors would like to thank Kim Roberts and Doug McGhan of Nortel, for donating the eDCO card and for the useful discussions. We also wish to thank David Krauss (Nortel) for assistance with the eDCO card. We acknowledge Prof. Polina Bayvel for useful discussions and feedback. Funding support from EPSRC is gratefully acknowledged.

References and links

1.

A. J. Lowery, B. D. Liang, and J. Armstrong, “Performance of Optical OFDM in Ultralong-Haul WDM Lightwave Systems,” J. Lightwave Technol. 25, 131–138 (2007). [CrossRef]

2.

B. J. C. Schmidt, A. J. Lowery, and J. Armstrong, “Experimental Demonstrations of Electronic Dispersion Compensation for Long-Haul Transmission using Direct-Detection Optical OFDM,” J. Lightwave Technol. 26, 196–203 (2008). [CrossRef]

3.

W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett. 42, 587–589 (2006). [CrossRef]

4.

S. L. Jansen, I. Morita, T. C. W. Schenk, D. van den Borne, and H. Tanaka, “Optical OFDM - A Candidate for Future Long-Haul Optical Transmission Systems,” Proc. Optical Fiber Commun., paper OMU3 (2008).

5.

T. Jiang and Y. Wu, “An Overview: Peak-to-Average Power Ratio Reduction Techniques for OFDM Signals,” IEEE Transactions on Broadcasting , 54, 257–268 (2008). [CrossRef]

6.

K. Roberts, C. Li, L. Strawczynski, M. O’Sullivan, and I. Hardcastle, “Electronic precompensation of optical nonlinearity,” Photon. Technol. Lett. 18, 403–405 (2006). [CrossRef]

7.

O. Bulakci, M. Schuster, C.-A. Bunge, and B. Spinnler, “Precoding based Peak-to-Average Power Ratio Reduction for Optical OFDM demonstrated on Compatible Single-Sideband Modulation with Direct Detection,” Proc. Opt. Fiber Commun Paper JThA56 (OFC) (2008)

8.

S. L. Jansen, I. Morita, T. C. W. Schenk, N. Takeda, H. Tanaka, and Tanaka, “Coherent Optical 25.8-Gb/s OFDM Transmission Over 4160-km SSMF,” J. Lightwave Technol. 26, 6–15 (2008) [CrossRef]

9.

Y. Benlachtar, S. J. Savory, P. Bayvel, and R. I. Killey, “Investigation of the use of Electronic Pre-Distortion and MLSE Equalization in Long-Haul Transmission,” Proc. Europ. Conference on Optical Commun. (ECOC), paper 9.1.4 (2007)

10.

A. J. Lowery, S. Wang, and M. Premaratne, “Calculation of power limit due to fiber nonlinearity in optical OFDM systems,” Opt. Express 15, 13282–13287 (2007). http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-20-13282 [CrossRef] [PubMed]

11.

L. B. Y. Du and A. J. Lowery, “Fiber nonlinearity precompensation for long-haul links using directdetection optical OFDM,” Optics Express , 16, 6209–6215 (2008). http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-9-6209 [CrossRef] [PubMed]

OCIS Codes
(060.4080) Fiber optics and optical communications : Modulation
(060.4510) Fiber optics and optical communications : Optical communications

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: August 1, 2008
Revised Manuscript: September 8, 2008
Manuscript Accepted: September 11, 2008
Published: September 16, 2008

Citation
Yannis Benlachtar, Giancarlo Gavioli, Vitaly Mikhailov, and Robert I. Killey, "Experimental investigation of SPM in long-haul direct-detection OFDM systems," Opt. Express 16, 15477-15482 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-20-15477


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References

  1. A. J. Lowery, B. D. Liang, and J. Armstrong, "Performance of Optical OFDM in Ultralong-Haul WDM Lightwave Systems," J. Lightwave Technol. 25, 131 - 138 (2007). [CrossRef]
  2. B. J. C. Schmidt, A. J. Lowery, and J. Armstrong, "Experimental Demonstrations of Electronic Dispersion Compensation for Long-Haul Transmission using Direct-Detection Optical OFDM," J. Lightwave Technol. 26, 196 - 203 (2008). [CrossRef]
  3. W. Shieh and C. Athaudage, "Coherent optical orthogonal frequency division multiplexing," Electron. Lett. 42, 587 - 589 (2006). [CrossRef]
  4. S. L. Jansen, I. Morita, T. C. W. Schenk, D. van den Borne, and H. Tanaka, "Optical OFDM - A Candidate for Future Long-Haul Optical Transmission Systems," Proc. Optical Fiber Commun., paper OMU3 (2008).
  5. T. Jiang and Y. Wu, "An Overview: Peak-to-Average Power Ratio Reduction Techniques for OFDM Signals," IEEE Transactions on Broadcasting,  54, 257 - 268 (2008). [CrossRef]
  6. K. Roberts, C. Li, L. Strawczynski, M. O'Sullivan, and I. Hardcastle, "Electronic precompensation of optical nonlinearity," Photon. Technol. Lett. 18, 403 - 405 (2006). [CrossRef]
  7. O. Bulakci, M. Schuster, C.-A. Bunge, and B. Spinnler, "Precoding based Peak-to-Average Power Ratio Reduction for Optical OFDM demonstrated on Compatible Single-Sideband Modulation with Direct Detection," Proc. Opt. Fiber Commun Paper JThA56 (OFC) (2008)
  8. S. L. Jansen, I. Morita, T. C. W. Schenk, N. Takeda, H. Tanaka, "Coherent Optical 25.8-Gb/s OFDM Transmission Over 4160-km SSMF," J. Lightwave Technol. 26, 6 - 15 (2008) [CrossRef]
  9. Y. Benlachtar, S. J. Savory, P. Bayvel, and R. I. Killey, "Investigation of the use of Electronic Pre-Distortion and MLSE Equalization in Long-Haul Transmission," Proc. Europ. Conference on Optical Commun. (ECOC), paper 9.1.4 (2007)
  10. A. J. Lowery, S. Wang, and M. Premaratne, "Calculation of power limit due to fiber nonlinearity in optical OFDM systems," Opt. Express 15, 13282-13287 (2007). http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-20-13282 [CrossRef] [PubMed]
  11. L. B. Y. Du and A. J. Lowery, "Fiber nonlinearity precompensation for long-haul links using direct-detection optical OFDM," Opt. Express,  16, 6209-6215 (2008). http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-9-6209 [CrossRef] [PubMed]

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