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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]. Following is an overview of a PAM4 Photonic IC (PIC) transceiver design and the TDECQ measurement options in the Synopsys OptoCompiler™ design platform [3].
Synopsys OptoCompiler
To set some context, Synopsys OptoCompiler is the industry’s first unified electronic/photonic design automation platform that combines mature and dedicated photonic technology with Synopsys’ industry-proven electronic design tools to enable engineers to produce and verify complex PIC designs quickly and accurately (Figure 1).
Fig. 1: Synopsys OptoCompiler platform and tools for E-O co-design of PICs
Synopsys OptoCompiler offers a familiar design automation environment to IC designers for co-design of analog electronic and photonic ICs. OptoCompiler includes the normal automation features you would expect for analog, custom, or mixed-signal design and augments them with advanced capabilities for PIC design such as photonic-aware layout synthesis, dedicated photonic design rule and layout-vs-schematic checking using Synopsys IC Validator™, as well as a sophisticated capability for electro-optical circuit co-simulation using Synopsys OptSim™ and PrimeSim™ simulators. The platform increases quality of results and reduces chances of human errors providing fast and accurate predictions of how the electro-optical system will perform.
Designers can also optimize circuit performance by creating their own active and passive photonic devices using a tight integration between Synopsys Photonic Device Compiler and Synopsys’ world-class Sentaurus™ TCAD tools.
PAM4 PIC Design and TDECQ Measurement
A photonic PAM4 transmitter often includes a Mach-Zehnder modulator (MZM) driven by a digital-to-analog converter (DAC) that requires energy-inefficient electronics. Implementations with nested modulators and drivers also exist [4], but they typically have larger footprints. An alternative approach is to create a DAC-less design by using the inherent DAC capabilities of segmented phase-shifters [5]. Synopsys OptoCompiler enables users to implement different PAM4 transceiver configurations using OptSim library-based components. These configurations can then be mapped onto foundry PDK-based components to carry out complete E-O co-design, and assess performance, including measurement of TDECQ.
Figure 2 shows a Schematic of a high-speed PAM4 transmitter and multimode fiber (MMF)-based interconnect in OptoCompiler.
This example shows a Gray-encoded, 25GBd PAM4 signal generated from a pseudo-random binary sequence of degree 13 quaternary (PRBS13Q) [12] directly driving an O-band VCSEL. The modulated signal propagates through an MMF whose length is varied during simulation. A photodetector converts received optical signals into a current waveform that includes detection noise. The waveform then passes to a transimpedance amplifier that converts the current signal into voltage (with gain and additional receiver noise). Since Synopsys OptoCompiler and PrimeWave are ideally suited for native domain E-O cosimulation, we implemented a 4th-order Bessel filter in electrical domain using analogLib components.
Figure 2: Schematic of a high-speed PAM4 transmitter and multimode fiber (MMF)-based interconnect in OptoCompiler
Because of its lower system cost and less stringent tolerance requirements, an MMF is often preferred for short-distance interconnects over single mode fiber (SMF). However, an MMF supports multiple spatial modes, each traveling with different group velocity that results in modal dispersion. The modal dispersion manifests itself as inter-symbol interference (ISI) at the direct-detection receiver and adversely affects the bandwidth x distance product, which is a figure of merit for MMF-based systems.
For the specified value of symbol error rate (SER), the TDECQ algorithm seeks to determine the maximum amount of noise that can be added to the input signal while still meeting the target symbol error rate, and then compares this to the amount of noise that can be added if the signal were ideal. The TDECQ measurement functionality in Synopsys OptSim produces plots of several quantities of interest, in addition to the TDECQ, to help with a detailed assessment of the transmitter compliance and equalizer performance. These additional transmitter performance parameters include optimized decision shift, reference-equalizer noise enhancement, optical modulation amplitude (OMA), and noise characteristics of the incoming and ideal signals.
Read our white paper to learn more about TDECQ compliance testing [6] of high-speed PAM4 transmitters in Synopsys OptoCompiler and OptSim. We describe how TDECQ is calculated and show example plots that can be generated from the Synopsys design platform.
References
[1] Dana Neustadter, Gary Ruggles, and Priyank Shukla, “What is new in PCIe 6.0 Specification: Bandwidth and 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.
[3] Silicon Photonics Design Software – Synopsys OptoCompiler | Synopsys
[4] Luis Orbe, “Electro-Optical co-simulation of a PAM4 transmitter,” Electro-Optical Cosimulation of a PAM4 Transmitter | Synopsys, March 2, 2003.
[5] Jigesh Patel, “Designing PAM4 transmitter PICs for high bandwidth and energy savings in data centers,” Designing PAM-4 Transmitter PICs for Higher Bandwidth and Energy Savings in Data Centers – Optical and Photonic Solutions Blog (synopsys.com), February 25, 2022.
[6] Jigesh K. Patel, Pablo V. Mena, “TDECQ Compliance Testing of High-Speed PAM4 Transmitters in Synopsys OptoCompiler and OptSim,” https://www.synopsys.com/content/dam/synopsys/photonic-solutions/documents/whitepapers/tdecq-wp.pdf.