Electronic equalization for enabling communications at OC-192 rates using OC-48 components G. S. Kanter, A. K. Samal, O. Coskun and A. Gandhi Santel Networks, 39899 Balentine Drive, Suite 350, Newark, California 94560
[email protected]
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 OCIS codes: (060.2330) Fiber optics communications; (060.2340) Fiber optics components; (230.0250); (230.5160) Photodetectors
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). 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). 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). 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). 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). 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
1. Introduction Over the last several years it has become economically advantageous to increase the data rate of systems from 2.5Gb/s to 10Gb/s. While 10Gb/s systems generally require more expensive components, such as wide bandwidth optical modulators and photo-receivers, the net costper-bit is still lower than transmission at 2.5Gb/s since only one-fourth as many channels are required at the higher data rate. Unfortunately, 10Gb/s systems are more susceptible to optical impairments such as polarization mode dispersion (PMD) and chromatic dispersion (CD). In many cases, the cost of optically compensating these effects is prohibitively expensive, as #2665 - $15.00 US
(C) 2003 OSA
Received July 01, 2003; Revised August 18, 2003
25 August 2003 / Vol. 11, No. 17 / OPTICS EXPRESS 2019
well as overly bulky. Optical impairments such as CD and PMD limit the transmission distance for a specified power budget. Alternatively, for a specified link length, these impairments reduce the power margin. Electronic processing in the receiver has been used to compensate for optical impairments such as PMD, CD, and modal dispersion [1-6]. Since micro-chip sized electronic equalizers can be mass produced using standard semiconductor processing, they are very likely to become indispensable parts of next-generation optical communication systems. Moreover, electronic processing can also compensate for distortions occurring in the electrical domain. We propose that such equalization technology could also be used to relax the specifications of optical modulators and receivers down to levels traditionally used in 2.5Gb/s systems, thereby producing significant cost savings. Additional benefits such as lower driving voltages, reduced spectral bandwidth, and increased sensitivity may also be obtained. In this paper we numerically model the performance of 10Gb/s optical links using various types of low-bandwidth transmitters and receivers. We predict that acceptably low penalties, and in some cases significant improvements, may be obtained by substituting low-bandwidth components and subsequent processing with an electronic equalizer, also known as an electronic dispersion compensator (EDC). We also show that the effect of receiver bandwidth on transmission quality, which is usually limited by balancing inter-symbol interference (ISI) on the low bandwidth side and added noise on the high bandwidth side, is modified by the addition of an EDC. This can be understood by noting that the EDC works to partially cancel ISI. Therefore, lower receiver bandwidths can be used with minimal added penalty or, in some cases, improved performance. Additionally, we experimentally demonstrate the use of Mach-Zehnder interferometer (MZI) and electro-absorption (EA) modulators designed for OC-48 systems, operating at OC-192 rates. A proto-type EDC is used to drastically reduce the penalty normally incurred by using such low-bandwidth transmitters. We also show that adding a 2.5GHz bandwidth filter after our photo-receiver causes minimal penalty after electronic equalization. 2. Results and discussion 2.1 Low bandwidth receivers We first simulate the performance of optical noise limited systems which are dominated by amplified spontaneous emission (ASE) noise from optical amplifiers. A 223 -1 pseudo-random bit sequence drives the NRZ modulator at 10Gb/s. The system is modeled using the commercial program OPTSIM. For simplicity, we assume that the same 25GHz optical bandpass filter is used in every link. For a given transmitter and link length, the bandwidth of the photo-receiver (4th order Bessel filter) is varied. After detection the signal is either passed through or by-passed around an EDC. A 6 tap (two taps per symbol) feed-forward equalizer (FFE) is modeled with a 1 tap decision feedback equalizer (DFE) [1]. The Q-function is then determined by evaluating the noise on the ones and zeros of the digital signal. The Q is quoted in dB where a Q of 15.56dB represents a 10-9 BER. We use the definition QdB = 20*log10(Qlinear). We checked the accuracy of the Q-function estimation by performing several simulations directly counting errors to find the bit error rate. The two methods typically differ by 0.1 to 0.4dB, due in part to error propagation effects in the DFE [7]. The Q estimation technique is thus reasonably accurate and will be used throughout this work. Least-mean square (LMS) algorithm is a widely used technique in order to acquire and track the system parameters. LMS algorithm [8-9] 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.
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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.
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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
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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 powerlimited links. Without EDC, the ISI added by moving to low (
