Simulating PAM-4 Modulation with pSim: Applying Photonic IC Simulation Tools for High-Quality Electro-Optical Design

Introduction

With the insatiable need for more compute power comes the need to increase interconnect bandwidth while decreasing costs. PAM4 (Pulse Amplitude Modulation – 4 Level) is one of the more common modulation formats used for data communications. PAM4 encodes two bits of information into four intensity levels (00, 01, 10, or 11), thus doubling the information transferred per clock cycle as compared to NRZ (non-return-to-zero), which carries only 1 bit per symbol (either a 0 or a 1). This enables engineers to use lower-cost components while also transferring twice the amount of information as compared to traditional NRZ modulation.

There are other modulation schemes such as coherent modulation that can carry greater amounts of information over longer distances than NRZ and PAM, but the transmitter and receiver designs become complex and add to the system cost. Since PAM4 is an intensity modulation method, a direct-detection receiver can detect the received PAM4 signal without adding complexity and latency of coherent receivers. As a result, PAM4 has attracted immense amount of interest for high-speed optical interconnect applications such as PCIe6 [1].

Two of the most common performance metrics in NRZ transmitter design are extinction ratio and the transmitter dispersion penalty (TDP) at maximum dispersion. In PAM4 transmitters, the equivalent quantities are optical modulation amplitude (OMA) and transmitter dispersion eye closure penalty quaternary (TDECQ) [2].

The pSim photonic IC simulation tool is part of the PIC Studio tool suite that enables engineers to quickly and accurately generate and verify complex electro-optical chip designs. pSim offers a familiar design automation environment for photonic IC link design with advanced capabilities such as electro-optical co-simulation using pSim’s optical and electrical solvers. The platform increases quality of results and reduces human errors, providing fast and accurate predictions of electro-optical system performance. Designers can also optimize photonic IC link performance using pSim's mature photonic IP design kits.

Link Construction Guide

To perform PAM4 photonic IC simulation with pSim, engineers can follow these steps:

1. Create a new project in pSim

2. Select required devices like lasers, modulators, fibers etc from the photonic component library

3. Connect these devices using the drag-and-drop interface

4. Configure parameters for each device

5. Add electrical circuit models like drivers, TIA etc

6. Run simulation and analyze results

For details on link theory, working principles, and device parameter settings, please refer to Latitude’s tutorial slides: “SiPh IC Design with pSim”. pSim provides an intuitive graphical user interface for visualizing the photonic IC link topology to generate key performance metrics like eye diagrams, spectra, waveforms etc. pSim also offers powerful post-simulation analysis in combination with the PhotoCAD layout tool. Figure 1 shows the simulation result of a PAM-4 driven Mach-Zehnder modulator in pSim's intuitive graphical interface.

Figure 1. Simulation result of a PAM-4 driven Mach-Zehnder modulator in pSim

The eye diagram in Figure 1 shows an open and clear eye for the PAM4 signal, indicating good signal quality. As pSim continues to evolve, advanced analysis features like TDECQ will be added to enable deeper performance characterization of PAM4 transmitters.

Conclusion

pSim provides a powerful yet accessible photonic IC link simulation environment for designing and analyzing high-speed optical interconnects. Its co-simulation capabilities, extensive device libraries, and integration with mature process design kits make pSim well-suited for today's complex PIC designs. As a mature commercial tool, pSim can improve engineer efficiency and design quality.

References

[1] Dana Neustadter, Gary Ruggles, and Priyank Shukla, “What is new in PCIe 6.0 Specification: Bandwidth and Security,” What’s New in the PCIe 6.0 Specification: Bandwidth & Security | Synopsys Blog, April 26, 2022.

[2] Jonathan King, “TDEC for PAM4 (TDECQ) – Changes to clause 123, to replace TDP with TDECQ, rev 1a,” TDEC for PAM4 (‘TDECQ’) (ieee.org), May 26, 2016