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Optics Express

  • Editor: Michael Duncan
  • Vol. 11, Iss. 17 — Aug. 25, 2003
  • pp: 2019–2029
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Electronic equalization for enabling communications at OC-192 rates using OC-48 components

G. S. Kanter, A. K. Samal, O. Coskun, and A. Gandhi  »View Author Affiliations


Optics Express, Vol. 11, Issue 17, pp. 2019-2029 (2003)
http://dx.doi.org/10.1364/OE.11.002019


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Abstract

We propose using electronic equalization technology to allow components typically used in 2.5Gb/s systems to be used at 10Gb/s. We simulate the performance of links exploiting this concept and study the effect of receiver bandwidth on equalized systems in general. Links utilizing transmitters designed for 2.5Gb/s rates are experimentally demonstrated. Experiments also show that photo-receivers with 2.5 GHz bandwidths add minimal penalty when post-detection electronic equalization is employed.

© 2003 Optical Society of America

1. Introduction

2. Results and discussion

2.1 Low bandwidth receivers

Least-mean square (LMS) algorithm is a widely used technique in order to acquire and track the system parameters. LMS algorithm [8

8. W. A. Sethares, I. M. Y. Mareels, B. D. O. Anderson, and C. R. Johnson, “Excitation conditions for signed regressor least mean squares adaptation,” IEEE Trans. Circuits Syst. ,. 35, 613–624 (1988). [CrossRef]

9

9. A. Shoval, D. A. Johns, and W. M. Snelgrove, “Comparison of dc offset effects in four LMS adaptive algorithm,” IEEE Tran. Cir. Sys. II: An. Dig. Si. Pi ,. 42, (1995).

] can be seen as the adaptive implementation of the Wiener solution for a set of unknown parameters. Specifically it searches for the set of coefficients where the projection of the estimation error is orthogonal to the excitation vector. In our case the error is the difference between the detected digital data and the equalizer output while the excitation vector is the receive data.

Figure 1(a) shows the effect of varying the receiver bandwidth on the Q-function. For a Mach-Zehnder interferometer (MZI) modulator designed for OC-192 rates, the ideal receiver bandwidth is about 7GHz. The performance begins to drop quickly once the bandwidth is decreased below 5GHz. In contrast, the same modulator coupled with EDC shows much less sensitivity to receiver bandwidth. The performance is nearly constant between 2 and 12 GHz. These results show that it is not desirable to use OC-48 receiver components (which typically have bandwidths between 1.7 and 3 GHz) for 10Gb/s data rates unless combined with electronic equalization.

Fig. 1. Simulated results for transmission using an MZI modulator specified for 10Gb/s data rates (a) Top -Effect of receiver bandwidth on an ASE-noise limited system (b) Center - Effect of filter bandwidth after photo-detection using a wide-band receiver in a power-limited system (c) Bottom - Effect of receiver bandwidth when its noise properties are adjusted to give a constant sensitivity (1E-9) at about 3000 photons per bit (see text).

Figure 1(b) shows a similar plot but with electronic noise dominating the system. In Fig. 1(b), a wide-bandwidth (40GHz) photo-detector is assumed and the bandwidth of a Bessel filter after photodetection is varied. In Fig. 1(c), we change both the bandwidth and sensitivity of the receiver together so as to factor out the sensitivity parameter, making the assumption that the number of photons required per bit remain constant (about 3000 photons/bit). For instance, we assume that a 7.5GHz bandwidth detector has a sensitivity of -24dBm at 10Gb/s and a 3.75GHz bandwidth detector has a sensitivity of -27dBm at 5Gb/s. The OPTSIM program back calculates the noise properties of the receivers from the knowledge of their sensitivities at a specified bit rate. The electrical noise is assumed to come from a white-noise current. The receivers are evaluated at 10Gb/s data rates. Figure 1(c) shows that the ideal receiver bandwidth is reduced by adding EDC. Additionally, performance at the ideal bandwidth is significantly increased. By comparing Fig. 1(b) and 1(c), we can see that this increase in performance is due to the assumed increase in sensitivity as the bandwidth is reduced (a characteristic often seen in practice). This characteristic may make low bandwidth receivers, when used with an EDC, particularly well suited for power-limited links. Without EDC, the ISI added by moving to low (<3GHz) bandwidths outweighs the improved noise performance and causes significant power penalties.

Fig.2. Schematic of the general test system. The low-pass filter (LPF) and FFE (equalizer) may or may not be inserted in the system depending on the particular test. The clock input to the BERT is selected accordingly.
Fig. 3. Five tap FFE structure used in experiments with tap spacing at the symbol rate. Equalization and de-multiplexing are simultaneously performed.

In order to support these simulations, experiments were performed using a prototype 5 tap (1 tap per symbol) FFE equalizer with self-adaptive control algorithm [10

10. J. G. Proakis and M. Salehi, Communication Systems Engineering (Prentice Hall, 1994), 570–589

]. The tap weights are digitally controlled with a 7-bit resolution. Unlike the simulations, no DFE stage is used. Figure 2 shows the schematic of the general test-bed. First we evaluate the effect of low-pass filtering after photo-detection. A MZI modulator designed for OC-192 rates is used. It is driven with a 223 -1 pseudo-random bit sequence. A 7.4 GHz PIN/TIA converts the optical signal into an electrical signal which is further amplified by an automatic gain control (AGC) post-amplifier with an estimated bandwidth of 8Ghz. The AGC has a differential output. One output is either sent directly to a bit-error ratio (BER) tester, or sent to the EDC for processing. The performance of the system is evaluated both with and without a 2.5 GHz low-pass filter (LPF) after the AGC. The LPF is approximately a fourth-order Bessel-Thompson type. The EDC de-multiplexes the input data stream into four OC-48 streams (see Fig. 3); one of which is sent to the BER tester for evaluation. When measuring the de-multiplexed data BER, a 2.5GHz clock generated in the FFE block is used to trigger the BERT. The other AGC output is sent (unfiltered) into a clock-recovery (CR) module. Because this CR module is designed for low ISI (typical) systems, it will not properly recover the clock after the LPF. However, we believe that a CR circuit designed to recover signals with high ISI would be able to function properly.

Fig. 4. Experimental data showing the performance with the 2.5GHz LPF inserted in the system before the FFE (solid lines). For comparison, the performance without the filter/FFE is also shown. Additionally, a data set with the filter and without the EDC is shown at a data rate of 8.5Gb/s - black dashed line (all other data is at 9.953 Gb/s). This demonstrates the poor performance the LPF causes without compensation.

Figure 5(a) shows eye diagrams after the band-limiting filter both before and after equalization. The eye before equalization is nearly closed due to the large amount of ISI added by the LPF. After equalization, the eye is clearly open. Figure 5(b) shows that, even at fairly high input powers, the eye after the LPF becomes fully closed when the data rate is increased to 10.664Gb/s. This rate is chosen because it is compatible with standard forward-error correction (FEC). Because the eye is fully closed, FEC would not be effective in this situation and could not be used instead of equalization.

Fig. 5. (a) Left - Eye diagram after the 2.5GHz LPF before (top) and after (bottom) the FFE. The received power is -19.4dBm. (b) Right - Eye diagrams after the 2.5GHz filter with -16dBm input power at 9.953 Gb/s (top) and 10.664 Gb/s (bottom). The fully closed eye at the FEC compatible rate suggests that FEC would not be an alternative to equalization.

2.2 Low bandwidth transmitters

Fig. 6. Simulations showing the sensitivity results for various modulators at a BER of 1e-12. SMF fiber has a dispersion coefficient of 17ps/nm.km.

Table 1 (a):. Parameters used for various transmitters

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Table 1 (b):. Parameters used for various receivers (without post-compensation)

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Figure 7(a) shows the performance of the system using a 2.5Gb/s or a 10Gb/s MZI. The 10Gb/s MZI has a -20.3 dBm sensitivity at a BER of 10-9, which is within 0.1dB of the sensitivity of the photo-receiver measured at the manufacturer. Adding the EDC improves the performance by an additional 1dB due to mitigation of residual ISI (not shown in Fig.). The 2.5Gb/s MZI has a back to back sensitivity of -16.9dBm without equalization and -20.6dBm with equalization. Impressively, adding 75km of SMF fiber degrades the sensitivity after equalization by only 0.4dB. Similar to the simulations, the data suggests that replacing a 10G MZI with a 2.5G MZI and electronic equalization can actually improve system performance, especially for longer links. In fact, the experiments show an improvement even in the back-to-back case. This is most likely due to additional distortions occurring in the AGC amplifier which are partially compensated by the EDC.

Figure 7(b) shows the performance with a 2.5G EAM modulator. This modulator was designed for use in long-haul (600km) OC-48 links. As might be expected, without equalization the EAM is unsuitable for OC-192 data rates. However, the EDC reduced the back-to-back penalty with respect to the 10G MZI to less than 3dB. This is fairly consistent with our simulations. We note that in this case the receiver bandwidth is wider than the transmission bandwidth, and further performance improvements might be made by optimizing the receiver bandwidth appropriately. Because the clock-recovery circuit did not function properly when using this transmitter, we used the clock directly from the PRBS generator. This technique limited us from taking consistent data after propagating through fiber due to a slow drift in propagation time through the fiber, probably due to thermal variations. However, given the back-to-back performance measured and our simulated results, we expect this transmitter to be able to reach well over 40km with a properly designed clock-recovery circuit.

Table 2. Eye diagrams of the various modulators using a fast photo-detector (14 GHz) at high input power.

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2.3 Low bandwidth transmitters and receivers

The 2.5Gb/s MZI and 2.5GHz APD can reach 120km with a BER <10 -12 at only -26.3dBm. If we assume 3dB allocation for connector losses and 0.2dB/km fiber loss, an MZI with 5dB insertion loss which is coupled to a 20mW laser will reach 120km with an over 4dB power margin. We note that the sensitivity of the 2.5G receiver at 10Gb/s when used with equalization is about 3dB worse than its defined sensitivity at 2.5Gb/s. This is consistent with our experimental results using the low-pass filter.

Fig. 7. (a) Top - Experimental data using OC-48 and OC-192 MZI modulators (b) Bottom - Data for the OC-48 EAM. The OC-192 MZI uncompensated case is shown as a reference.
Fig. 8. The effect of received power on performance (simulated) for transmission at 10Gb/s using (a) Top - 2.5G MZI and 2.5GHz receiver (b) Bottom - 2.5G EAM and 2.5GHz receiver. The unequalized system is inoperable.

3. Summary

Table 3:. Summary of penalties for various transmitter and receiver configurations

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References and links

1.

H. Bulow, F. Buchali, W. Baumert, R. Ballentin, and T. Wehren, “PMD mitigation at 10Gbit/s using linear and nonlinear integrated electronic equaliser circuits,” Electron. Lett. 36, 163–164 (2000). [CrossRef]

2.

J. C. Cartledge, R. G. McKay, and M. C. Nowell, “Performance of smart lightwave receivers with linear equalization,” J. Lightwave Technol. 10, 1105–1109 (1992). [CrossRef]

3.

X. Zhao and F.S Choa, “Demonstration of a 10Gb/s transmissions over a 1.5 km long multimode fiber using equalization techniques,” IEEE Photonics Letters 14, 1187–1189 (2002). [CrossRef]

4.

F. Buchali, H Bulow, W. Baumert, R. Ballentin, and T. Wehren, “Reduction of the chromatic dispersion penalty at 10Gbit/s by integerated electronic equalizers,” in OFC 2000 Vol 3 of OSA Proceedings Series (Optical Society of America, Washington, D.C., 2000), pp, 268–270.

5.

D. Schlump, B Wedding, and H. Bulow, “Electronic equalization of PMD and chromatic dispersion induced distortion after 100km standard fiber at 10Gb/s,” in 24th European Conference on Optical Communication, Vol 1 of OSA Proceedings Series (Optical Society of America, Washington, D.C., 1998), pp. 535–536.

6.

Moe Z. Win, Jack H. Winters, and Giorgio M. Vitetta, “Equalization Techniques for mitigating transmission impairments”, Optical Fiber Telecommunications 1V B, Academic Press2002, 965–997.

7.

D. L. Duttweiler, J. E. Mazo, and D. G. Messerschmitt,“Error propagation in Decision-Feedback Equalizers,” IEEE Trans. Inform. Theory ,. IT-20, .490–497, (1974). [CrossRef]

8.

W. A. Sethares, I. M. Y. Mareels, B. D. O. Anderson, and C. R. Johnson, “Excitation conditions for signed regressor least mean squares adaptation,” IEEE Trans. Circuits Syst. ,. 35, 613–624 (1988). [CrossRef]

9.

A. Shoval, D. A. Johns, and W. M. Snelgrove, “Comparison of dc offset effects in four LMS adaptive algorithm,” IEEE Tran. Cir. Sys. II: An. Dig. Si. Pi ,. 42, (1995).

10.

J. G. Proakis and M. Salehi, Communication Systems Engineering (Prentice Hall, 1994), 570–589

OCIS Codes
(060.2330) Fiber optics and optical communications : Fiber optics communications
(060.2340) Fiber optics and optical communications : Fiber optics components
(230.0250) Optical devices : Optoelectronics
(230.5160) Optical devices : Photodetectors

ToC Category:
Research Papers

History
Original Manuscript: July 1, 2003
Revised Manuscript: August 8, 2003
Published: August 25, 2003

Citation
G. Kanter, A. Samal, O. Coskun, and Anil Gandhi, "Electronic equalization for enabling communications at OC-192 rates using OC-48 components," Opt. Express 11, 2019-2029 (2003)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-11-17-2019


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References

  1. H. Bulow, F. Buchali, W. Baumert, R. Ballentin, and T. Wehren, �??PMD mitigation at 10Gbit/s using linear and nonlinear integrated electronic equaliser circuits,�?? Electron. Lett. 36, 163-164 (2000). [CrossRef]
  2. J. C. Cartledge, R. G. McKay, and M. C. Nowell, �??Performance of smart lightwave receivers with linear equalization,�?? J. Lightwave Technol. 10, 1105-1109 (1992). [CrossRef]
  3. X. Zhao, F.S.Choa, �??Demonstration of a 10Gb/s transmissions over a 1.5 km long multimode fiber using equalization techniques,�?? IEEE Photonics Letters 14, 1187-1189 (2002). [CrossRef]
  4. F. Buchali, H.Bulow, W. Baumert, R. Ballentin, and T. Wehren, �??Reduction of the chromatic dispersion penalty at 10Gbit/s by integerated electronic equalizers,�?? in OFC 2000 Vol 3 of OSA Proceedings Series (Optical Society of America, Washington, D.C., 2000), pp, 268-270.
  5. D. Schlump, B Wedding and H. Bulow, �??Electronic equalization of PMD and chromatic dispersion induced distortion after 100km standard fiber at 10Gb/s,�?? in 24th European Conference on Optical Communication, Vol 1 of OSA Proceedings Series (Optical Society of America, Washington, D.C., 1998), pp. 535-536.
  6. Moe Z. Win, Jack H. Winters and Giorgio M. Vitetta, �??Equalization Techniques for mitigating transmission impairments�??, Optical Fiber Telecommunications 1V B, Academic Press 2002, 965-997.
  7. D. L Duttweiler,. J. E. Mazo , and D. G Messerschmitt,.�??Error propagation in Decision-Feedback Equalizers,�?? IEEE Trans. Inform. Theory,. IT-20, .490-497, (1974). [CrossRef]
  8. W. A. Sethares, I. M. Y. Mareels, B. D. O. Anderson, and C. R. Johnson, "Excitation conditions for signed regressor least mean squares adaptation," IEEE Trans. Circuits Syst.,. 35, 613�??624, (1988). [CrossRef]
  9. A. Shoval, D. A. Johns, and W. M. Snelgrove, "Comparison of dc offset effects in four LMS adaptive algorithm," IEEE Tran. Cir. Sys. II: An. Dig. Si. Pi,. 42, (1995).
  10. J. G. Proakis and M. Salehi, Communication Systems Engineering (Prentice Hall, 1994), 570-589

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