Analyzing Transmitter and Dispersion Eye Closure for PAM4 (TDECQ) with pSim Plus

Introduction

Transmitter and Dispersion Eye Closure for PAM4 (TDECQ) is a critical measurement for evaluating the performance of 400 Gigabit Ethernet optical transmitters. TDECQ quantifies the vertical eye closure of an optical transmitter when transmitted through a worst-case optical channel and measured through a reference receiver. This tutorial will explore the concept of TDECQ and demonstrate how to analyze it using the pSim Plus system-level simulator from Latitude Design Systems.

Understanding TDECQ

TDECQ is a measure of an optical transmitter's vertical eye closure when transmitted through a worst-case optical channel, as measured through an optical-to-electrical converter (O/E) and oscilloscope with a combined frequency response of 19.34 GHz. The signal is then equalized with a reference equalizer, as described in the IEEE 802.3bs standard.

The TDECQ measurement process involves the following steps:

1. Transmit a specified test pattern repetitively through the optical lane under test.

2. Capture the complete test pattern using an oscilloscope.

3. Apply the reference equalizer to the captured waveform.

4. Construct an eye diagram from the equalized waveform.

5. Measure vertical histograms at specific time intervals within the eye diagram.

6. Convolve the histograms with Gaussian noise distributions to estimate the partial symbol error ratio (SER) for each sub-eye.

7. Optimize the equalizer tap coefficients to minimize the maximum of the left and right SERs.

8. Calculate TDECQ from the optimized SER and the Outer Optical Modulation Amplitude (OMA).

The TDECQ value represents the RMS noise that could be added by a receiver while still meeting the target symbol error ratio of 4.8 × 10^-4 for Gray-coded PAM4 signals.

Analyzing TDECQ with pSim Plus

pSim Plus is a powerful system-level simulator that supports various optical communication link models, including single-mode, multi-mode, BER/DSP encoding/decoding, and TDECQ analysis. It provides a comprehensive simulation environment for evaluating the performance of optical transmitters and receivers.

Table 1. Transmitter compliance channel specifications

Figure 1 from the IEEE 802.3bs standard illustrates the TDECQ conformance test block diagram, which can be implemented in pSim Plus using the appropriate components and configurations.

Figure 1. TDECQ conformance test block diagram

Example TDECQ Analysis in pSim Plus

To further understand the TDECQ analysis process, let's consider an example using pSim Plus. Figure 2 demonstrates a TDECQ analysis setup in pSim Plus.

In this example, an optical source and a reference source are loaded, and their signals are converted to electrical signals by a photodiode (PD). The electrical signals are then equalized by an electrical equalizer, and finally, the TDECQ analysis is performed.

The TDECQ example shows the calculated values for various parameters, including the coefficient C_eq, average optical power (P_ave_lo and P_ave_hi), Outer OMA, sub-eye threshold levels (P_threshold1, P_threshold2, and P_threshold3), sigma_g, and the final TDECQ value of 1.747613159.

Figure 2. TDECQ function demonstration in pSim Plus simulator

Conclusion

TDECQ is a crucial measurement for evaluating the performance of PAM4 optical transmitters in high-speed Ethernet applications. pSim Plus, with its comprehensive system-level simulation capabilities, provides a powerful platform for analyzing TDECQ and optimizing transmitter designs. By following the steps outlined in this tutorial and leveraging the examples provided, engineers can gain a deeper understanding of TDECQ and utilize pSim Plus to ensure compliance with industry standards and achieve optimal performance in their optical communication systems.

Reference

[1] Draft Amendment to IEEE Std 802.3-2015 IEEE Draft P802.3bs/D3.1 IEEE P802.3bs 200 Gb/s and 400 Gb/s Ethernet Task Force