RSoft Application Gallery Note: Electronic Dispersion Compensation

Tools Used: ModeSYS

This example demonstrates the application of electronic dispersion compensation (EDC) in a multimode link. Upgrade of existing Ethernet links from 1 to 10 Gb/s with keeping the 300-m transmission reach faces the challenge of overcoming performance degradation due to the higher modal dispersion penalties. One of the solutions for this problem is being implemented in IEEE standard for 10 Gigabit Ethernet 10GBASE-LRM [1] and is based on applying electronic dispersion compensation to the signal at the receiver. The link configuration under study is:

application of electronic dispersion compensation

The OptSim example depicted above demonstrates a 10 Gigabit multimode link. The multimode fiber is a 300-m graded-index fiber with a 50-micron core diameter. The fiber refractive index profile has a dip defect in the center of the core ñ one of the typical profile distortions due to manufacturing imperfections. The fiber profile used is:

Refractive Index Test Plot

The deviations of index profile from an ideal graded-index may cause increase in differential modal delay, and as a result the performance degradation.

The output signal from the transmitter has a single-mode Gaussian pulse profile with 7 micron FWHM. The connector between transmitter and fiber adds 10 micron offset from the fiber core center. As a result, the received eye diagram at the PIN/TIA receiver is distorted and the computed link BER for this eye diagram is as high as 3.1x10-3.

Eye Diagram - Before EDC

To improve the eye diagram and BER of the link, we apply EDC to electrical signal after the first receiver in the form of equalization filter implemented as an electrical finite-impulse response (FIR) filter. This equalization scheme is also known as linear or forward-feedback equalization (LE or FFE). The FIR filter model used applies the following transformation to the electrical signal:

Equation

where x(t) is incoming signal, y(t) the outgoing signal, (2N+1) the number of taps, Ci- weight coefficients or taps, and DT the taps delay.

Eye Diagram - After EDC

For the eye diagram before EDC, we found that a FFE parameters setting with 3 taps, tap delay of ?TB, and taps coefficients {-0.3, 1.0, -0.3} is sufficient to improve the eye diagram quality, and hence, the BER. The eye diagram after applying EDC shows a significant improvement. The corresponding computed BER is equal to 3.7x10-13, i.e. satisfies the link BER requirement of 1x10-12 or better.

The electrical signal waveforms before and after the EDC:

electrical signal waveforms before and after the EDC

The transfer function for the applied FIR filter:

transfer function for the applied FIR filter

In this example the parameters of equalization filter are optimized for the given offset of laser-fiber alignment. To demonstrate that we can perform the parameters scan simulation for different values of offsets. Figure 7 shows simulated link BER and Q-factor before and after EDC for different values of offset, ranging from 0 to 20 microns. One can see that the biggest gain from EDC is observed at the 10-micron offset ñ i.e. the one we use in the example. Thus, if one wants to use different offset values, then the FFE parameters have to be modified (optimized) for the best performance.

simulated link BER and Q-factor before and after EDC

References

[1] IEEE P802.3aq 10GBASE-LRM Task Force, http://www.ieee802.org/3/aq/public/index.html