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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 23 — Nov. 5, 2012
  • pp: 25369–25377
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SC-FDE for MMF short reach optical interconnects using directly modulated 850 nm VCSELs

Victor S. C. Teichmann, Andre N. Barreto, Tien-Thang Pham, Roberto Rodes, Idelfonso T. Monroy, and Darli A. A. Mello  »View Author Affiliations


Optics Express, Vol. 20, Issue 23, pp. 25369-25377 (2012)
http://dx.doi.org/10.1364/OE.20.025369


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Abstract

We propose the use of single-carrier frequency-domain equalization (SC-FDE) for the compensation of modal dispersion in short distance optical links using multimode fibers and 850 nm VCSELs. By post-processing of experimental data, we demonstrate, at 7.9% overhead, the error-free transmission (over a 4 Mbit sequence) of OOK-modulated 5 Gbps over 2443 meters of OM3 fiber (with a nominal 3300 MHz×km bandwidth). The proposed solution may be applied as a low cost alternative for data center and supercomputer interconnects.

© 2012 OSA

1. Introduction

This paper is organized as follows: Section 2 describes the SC-FDE system, including discussions on channel estimation and frame synchronization. Section 3 presents the experimental results, including a comparison between zero-forcing and minimum mean square error (MMSE) equalization. Lastly, Section 4 concludes the paper.

2. Single-carrier frequency-domain equalization

2.1. Concept

The block diagram of the SC-FDE system is shown in Fig. 1. The concept is very similar to that of OFDM with the difference that, in SC-FDE, the IFFT block is moved from the transmitter to the receiver. Thus, SC-FDE and OFDM have equivalent complexity, although the transmitter in the first case is simpler and the receiver is more complex. At the transmitter, a binary (in this paper, we consider OOK transmission) data sequence of length Nb is generated. This sequence is then split into Ns blocks, and a cyclic prefix (CP) is added to each block, in such a way that the last Ncp bits of the block are repeated at the beginning. Each block with cyclic prefix forms a so-called SC-FDE block. Thus, each SC-FDE block has size S=NbNs+Ncp bits, and the transmitted sequence X has size S × Ns. To avoid interblock interference between SC-FDE blocks, the condition Tcp > Tch must be satisfied, where Tcp = NcpTb is the temporal duration of the cyclic prefix, Tch is the maximum delay spread of the channel and Tb is the bit duration. Data blocks are alternated with pilot and synchronization blocks, composing a frame of length S × Ns + K, where K is the overhead length, including pilot and frame synchronization sequences. The periodicity of a frame is chosen as to optimize the trade-off between overhead and channel tracking. The SC-FDE blocks are then converted to the analog domain by the modulator (in our case, a directly modulated VCSEL). At the receiver site, the PIN photodetector makes the optical-to-electrical conversion, and the resulting electrical signal is sampled by the analog-to-digital converter, forming sequence Y.

Fig. 1 The SC-FDE block diagram. Upper boxes represent procedures that are made block by block, whereas the side boxes represent procedures performed on pilot blocks only.

2.2. Frame synchronization

Fig. 2 Correlation between the original and the expected synchronization block versus the window position for (a) back-to-back and (b) fiber transmission.

2.3. Equalization

3. Experimental setup and results

3.1. Experimental setup

3.2. Experimental results

Figure 3(a) shows the estimated channel frequency response for a 5 Gbps transmission. The peaks and dips indicate that the estimated frequency response is strongly affected by noise. Thus, some form of smoothing is necessary to have a satisfactory estimate. To reduce the noise impact on the channel estimation, we made an impulse response restriction. This approach is commonly used when the channel impulse response in known to be bounded to a certain duration. This way, we locate the highest point from the impulse response, and considered only the neighboring impulses in the specified window, forcing the other values to zero. If the window is designed correctly, the dominant impulses will remain, and the channel estimation will not lose accuracy. However, a poorly specified window can yield equally poor channel estimates. The results are shown in Figs. 3(b) and 3(d). In our experiment, we used a window of size 10 samples. This way, the spurious peaks were removed from the estimated channel transfer function, which became smoother. It can also be noted from Fig. 3(b) that the amplitude response of the channel remained stable between the first and last pilots stored for offline processing (10 μs time interval).

Fig. 3 On the left: channel frequency response (top) and impulse response (bottom) estimations before the impulse response restriction. On the right: channel frequency response (top) and impulse response (bottom) estimations after the impulse response restriction.

We also evaluated the difference in performance between zero-forcing and MMSE equalizers. The design of a proper MMSE equalizer requires knowledge of the received signal SNR, as indicated in Eq. (3). Since in the experimental setup it is relatively difficult to estimate the precise noise variance, we varied the SNR over a wide range and calculated the bit error rate for each case. The results are shown in Fig. 4. It can be seen that, for all investigated transmission rates, the BER is minimized when the assumed SNR−1 is equal to zero, effectively turning the MMSE equalizer into a zero-forcing equalizer. Therefore, we expect both equalizers to exhibit similar performances. This conclusion is consistent with the results obtained by [15

15. M. Wolf, L. Grobe, M. R. Rieche, A. Koher, and J. Vucic, “Block transmission with linear frequency domain equalization for dispersive optical channels with direct detection,” in Proceedings of International Conference on Transparent Optical Networks (2010), pp. 1–8. [CrossRef]

].

Fig. 4 MMSE equalizer performance. The SNR−1 parameter of Eq. (3) was varied from zero to one, and the BER calculated for several transmission rates. The received signal power was kept constant at −9 dBm. An assumed SNR−1 equal to zero turns the MMSE equalizer into a zero-forcing equalizer.

We considered transmission rates from 5 to 10 Gbps. Thus, the bit duration is at least Tb = 100 ps. As we transmitted the SC-FDE signal over an OM3 fiber of 2443 m, with 3300 MHz×km bandwidth, the channel duration is around 740 ps, or approximately 8 bits, at 10 Gbps, and 4 bits at 5 Gbps. Hence, the minimum size of the cyclic prefix is 8 bits in order to avoid interblock interference at the worst case of a transmission rate of 10 Gbps. For short blocks, this required overhead can become excessively large. For this reason, we choose to transmit SC-FDE blocks of size 1024 bits. Therefore, the overhead caused by the cyclic prefix addition does not become significant. Also, the block does not become so long as to require a high capacity buffering at the receiver. To compare the performance of SC-FDE to a simple OOK transmission, we calculated the BER for different transmission rates considering both systems, as seen in Fig. 5. As expected, the SC-FDE system outperformed the OOK in all cases, presenting an error-free transmission (over a 4 Mbit sequence) for transmission rates inferior to 5.5 Gbps. The results show that SC-FDE modulation can improve the performance of current short distance systems, which are mostly based on OOK modulation.

Fig. 5 Bit error rate versus transmission rate for the zero-forcing equalizer. The received signal power was kept constant at −9 dBm.

4. Conclusion

In this work we proposed and experimentally demonstrated the use of SC-FDE with OOK modulation for mitigating intermodal dispersion in short-reach optical systems. We were able to obtain error-free transmission (over a 4 Mbit sequence) for 5 Gbps over 2443 meters of OM3 fiber. Compared with OFDM, SC-FDE simplifies the transmitter avoiding the generation of complicated waveforms, while the total computational complexity is maintained. Therefore, we believe that SC-FDE is a promising alternative to enable cost-effective high-capacity MMF transmission using 850 nm VCSELs, OOK modulation format and direct detection. Future works may also include forward error correction in the analysis.

Acknowledgments

This work was supported by the CPqD Foundation.

References and links

1.

A. V. Rylyakov, C. L. Schow, F. E. Doany, B. G. Lee, C. Jahnes, Y. Kwark, C. Baks, D. M. Kuchta, and J. A. Kash, “A 24-channel 300 Gb/s 8.2 pJ/bit full-duplex fiber-coupled optical transceiver module based on a single holey CMOS IC,” in Optical Fiber Communication Conference (Optical Society of America, 2010), pp. 1–3.

2.

A. Vahdat, Hong Liu, Xiaoxue Zhao, and C. Johnson, “The emerging optical data center,” in Optical Fiber Communication Conference (Optical Society of America, 2011), pp. 1–3.

3.

L. Aronson and L. Buckman, “Guide to HP Labs ROFL/OFL fiber measurements from 12/15/97–12/19/97 IEEE 802.3 10 GbE Study Group,” http://www.ieee802.org/3/z/mbi/index/html.

4.

L. Raddatz, I. H. White, D. G. Cunningham, and M. C. Nowell, “An experimental and theoretical study of the offset launch technique for the enhancement of the bandwidth of multimode fiber links,” J. Lightwave Technol. 16, 324–331 (1998). [CrossRef]

5.

L. A. Buckman, B. E. Lemoff, A. J. Schmit, R. P. Tella, and W. Gong, “Demonstration of a small-form-factor WWDM transceiver module for 10-Gb/s local area networks,” IEEE Photon. Technol. Lett. 14, 702–704 (2002). [CrossRef]

6.

C. F. Lam, Hong Liu, B. Koley, Xiaoxue Zhao, V. Kamalov, and V. Gill, “Fiber optic communication technologies: what’s needed for datacenter network operations,” IEEE Commun. Mag. 48, 32–39 (2010). [CrossRef]

7.

D. J. F. Barros and J. M. Kahn, “Comparison of orthogonal frequency-division multiplexing and on-off keying in direct-detection multimode fiber links,” J. Lightwave Technol. 29, 2299–2309 (2011). [CrossRef]

8.

J. M. Tang, P. M. Lane, and K. A. Shore, “Transmission performance of adaptively modulated optical OFDM signals in multimode fiber links,” IEEE Photon. Technol. Lett. 18, 205–207 (2006). [CrossRef]

9.

X. Q. Jin, J. M. Tang, K. Qiu, and P. S. Spencer, “Statistical investigations of the transmission performance of adaptively modulated optical OFDM signals in multimode fiber links,” J. Lightwave Technol. 26, 3216–3224 (2008). [CrossRef]

10.

Y. Benlachtar, R. Bouziane, R. I. Killey, C. R. Berger, P. Milder, R. Koutsoyannis, J. C. Hoe, M. Pusc, and M. Glick, “Optical OFDM for the data center,” in Proceedings of International Conference on Transparent Optical Networks (2010), pp. 1–4. [CrossRef]

11.

J. Armstrong, “OFDM for optical communications,” J. Lightwave Technol. 27, 189–204 (2009). [CrossRef]

12.

N. Benvenuto and S. Tomasin, “On the comparison between OFDM and single carrier modulation with a DFE using a frequency-domain feedforward filter,” IEEE Trans. Commun. 50, 947–955 (2002). [CrossRef]

13.

A. Gusmao, R. Dinis, J. Conceicao, and N. Esteves, “Comparison of two modulation choices for broadband wireless communications,” in Proceedings of IEEE Vehicular Technology Conference (2000), pp. 1300–1305.

14.

A. Czylwik, “Comparison between adaptive OFDM and single carrier modulation with frequency domain equalization,” in Proceedings of IEEE Vehicular Technology Conference (1997), pp. 865–869.

15.

M. Wolf, L. Grobe, M. R. Rieche, A. Koher, and J. Vucic, “Block transmission with linear frequency domain equalization for dispersive optical channels with direct detection,” in Proceedings of International Conference on Transparent Optical Networks (2010), pp. 1–8. [CrossRef]

16.

M. Wolf and L. Grobe, “Block transmission with frequency domain equalization in the presence of colored noise,” in Proceedings of International Conference on Transparent Optical Networks (2011), pp. 1–4. [CrossRef]

17.

D. Falconer, S. L. Ariyavisitakul, A. Benyamin-Seeyar, and B. Eidson, “Frequency domain equalization for single-carrier broadband wireless systems,” IEEE Commun. Mag. 40, 58–66 (2002). [CrossRef]

OCIS Codes
(060.2360) Fiber optics and optical communications : Fiber optics links and subsystems
(060.4510) Fiber optics and optical communications : Optical communications

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: July 2, 2012
Revised Manuscript: September 6, 2012
Manuscript Accepted: September 22, 2012
Published: October 24, 2012

Citation
Victor S. C. Teichmann, Andre N. Barreto, Tien-Thang Pham, Roberto Rodes, Idelfonso T. Monroy, and Darli A. A. Mello, "SC-FDE for MMF short reach optical interconnects using directly modulated 850 nm VCSELs," Opt. Express 20, 25369-25377 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-23-25369


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References

  1. A. V. Rylyakov, C. L. Schow, F. E. Doany, B. G. Lee, C. Jahnes, Y. Kwark, C. Baks, D. M. Kuchta, and J. A. Kash, “A 24-channel 300 Gb/s 8.2 pJ/bit full-duplex fiber-coupled optical transceiver module based on a single holey CMOS IC,” in Optical Fiber Communication Conference (Optical Society of America, 2010), pp. 1–3.
  2. A. Vahdat, Hong Liu, Xiaoxue Zhao, and C. Johnson, “The emerging optical data center,” in Optical Fiber Communication Conference (Optical Society of America, 2011), pp. 1–3.
  3. L. Aronson and L. Buckman, “Guide to HP Labs ROFL/OFL fiber measurements from 12/15/97–12/19/97 IEEE 802.3 10 GbE Study Group,” http://www.ieee802.org/3/z/mbi/index/html .
  4. L. Raddatz, I. H. White, D. G. Cunningham, and M. C. Nowell, “An experimental and theoretical study of the offset launch technique for the enhancement of the bandwidth of multimode fiber links,” J. Lightwave Technol.16, 324–331 (1998). [CrossRef]
  5. L. A. Buckman, B. E. Lemoff, A. J. Schmit, R. P. Tella, and W. Gong, “Demonstration of a small-form-factor WWDM transceiver module for 10-Gb/s local area networks,” IEEE Photon. Technol. Lett.14, 702–704 (2002). [CrossRef]
  6. C. F. Lam, Hong Liu, B. Koley, Xiaoxue Zhao, V. Kamalov, and V. Gill, “Fiber optic communication technologies: what’s needed for datacenter network operations,” IEEE Commun. Mag.48, 32–39 (2010). [CrossRef]
  7. D. J. F. Barros and J. M. Kahn, “Comparison of orthogonal frequency-division multiplexing and on-off keying in direct-detection multimode fiber links,” J. Lightwave Technol.29, 2299–2309 (2011). [CrossRef]
  8. J. M. Tang, P. M. Lane, and K. A. Shore, “Transmission performance of adaptively modulated optical OFDM signals in multimode fiber links,” IEEE Photon. Technol. Lett.18, 205–207 (2006). [CrossRef]
  9. X. Q. Jin, J. M. Tang, K. Qiu, and P. S. Spencer, “Statistical investigations of the transmission performance of adaptively modulated optical OFDM signals in multimode fiber links,” J. Lightwave Technol.26, 3216–3224 (2008). [CrossRef]
  10. Y. Benlachtar, R. Bouziane, R. I. Killey, C. R. Berger, P. Milder, R. Koutsoyannis, J. C. Hoe, M. Pusc, and M. Glick, “Optical OFDM for the data center,” in Proceedings of International Conference on Transparent Optical Networks (2010), pp. 1–4. [CrossRef]
  11. J. Armstrong, “OFDM for optical communications,” J. Lightwave Technol.27, 189–204 (2009). [CrossRef]
  12. N. Benvenuto and S. Tomasin, “On the comparison between OFDM and single carrier modulation with a DFE using a frequency-domain feedforward filter,” IEEE Trans. Commun.50, 947–955 (2002). [CrossRef]
  13. A. Gusmao, R. Dinis, J. Conceicao, and N. Esteves, “Comparison of two modulation choices for broadband wireless communications,” in Proceedings of IEEE Vehicular Technology Conference (2000), pp. 1300–1305.
  14. A. Czylwik, “Comparison between adaptive OFDM and single carrier modulation with frequency domain equalization,” in Proceedings of IEEE Vehicular Technology Conference (1997), pp. 865–869.
  15. M. Wolf, L. Grobe, M. R. Rieche, A. Koher, and J. Vucic, “Block transmission with linear frequency domain equalization for dispersive optical channels with direct detection,” in Proceedings of International Conference on Transparent Optical Networks (2010), pp. 1–8. [CrossRef]
  16. M. Wolf and L. Grobe, “Block transmission with frequency domain equalization in the presence of colored noise,” in Proceedings of International Conference on Transparent Optical Networks (2011), pp. 1–4. [CrossRef]
  17. D. Falconer, S. L. Ariyavisitakul, A. Benyamin-Seeyar, and B. Eidson, “Frequency domain equalization for single-carrier broadband wireless systems,” IEEE Commun. Mag.40, 58–66 (2002). [CrossRef]

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