What the Heck Are “Modem SNR” and “Holistic Co-design”?
August 19, 2021
By Paul Momtahan
Director, Solution Marketing
Last month I presented a Lightwave webinar on the What, How, and Why of 800G Generation Optical Engine Performance, where I attempted to explain the key factors that drive 800G performance. In my first blog based on the content of this webinar, “Baud Rates and 800 Gb/s: Every Little Bit Helps (A Lot),” I explained the large impact even a relatively small increase in the baud rate can have on the reach of an 800 Gb/s wavelength. In this second blog, we will look at the next key driver of superior 800 Gb/s wavelength reach: modem signal-to-noise ratio (SNR), or the amount of noise and distortions generated inside the optical engine when it operates at very high baud rates and with high-order modulation (i.e., 64QAM). We will also discuss the factors that enable high modem SNR (i.e., low internal noise and distortions), including holistic co-design.
Figure 1: 800 Gb/s wavelengths: ICE6 reach vs. baud rate
In fact, modem SNR is the key reason for the surprising relationship between baud rate and 800G reach, shown again in Figure 1. With low-order modulation (i.e., 8QAM, QPSK), the SNR limit is relatively low, and optical noise and nonlinearities are the primary limitations on SNR and therefore reach. With the higher SNR limit of high-order modulation (i.e., 64QAM, 32QAM), modem SNR takes up a larger portion of the available noise limit, allowing for a much smaller amount of external noise (i.e., OSNR and nonlinearities). Low modem SNR (i.e., high internal noise and distortions) can therefore severely limit the reach of 800 Gb/s wavelengths. Modem SNR is also one of the key factors that determine the probabilistic constellation shaping (PCS) gain of a practical coherent transceiver, with both a long codeword and high modem SNR (i.e., low internal noise and distortions) required for good performance.
Figure 2: High modem SNR enabled by deep vertical integration
But how do you get high modem SNR? Well, the key factors, as shown in Figure 2, that can impact modem SNR include:
1. The Performance of Individual Components
The quality of the individual components – the digital ASIC/DSP, which includes the DAC and ADC, as well as the analog electronics and photonics – has a large influence on modem SNR.
For example, ICE6 leverages an indium phosphide single photonic integrated circuit (PIC), which includes a critical modulator function where indium phosphide enables the highly efficient electro-optic effect. Furthermore, ICE6 integrates multiple photonic functions including lasers, modulators, semiconductor optical amplifiers, photodetectors, and various passive functions into a single PIC. This improves modem SNR by minimizing coupling losses by connecting optical functions with waveguides inside the PIC, as opposed to coupling optics between discrete components.
Analog electronics also have a critical impact on modem SNR. Analog electronics include the drivers that convert lower voltages from the DSP/DAC to the higher voltage required by the modulator at the transmit end, and transimpedance amplifiers (TIAs) that convert current from the photodetectors to the voltages required by the ADC/DSP at the receive end. ICE6 leverages a single analog ASIC for two wavelengths, transmit and receive, with a total of eight drivers and eight TIAs. It is made with high-performance silicon germanium (SiGe), fabricated with a 180-nm bipolar CMOS process. The drivers leverage a two-stage amplifier design featuring built-in equalization, while the TIAs include automated gain control (AGC) amplifiers and built-in equalization.
2. Holistic Co-design
Modem SNR is also determined by holistic co-design, packaging, and the electrical/radio frequency (RF) interconnects between the ASIC/DSP and analog electronics as well as between the analog electronics and the photonics/PIC. Holistic co-design enables the design of each individual component, the RF interconnect, and packaging to be done with consideration for the impact on other components and overall optical engine performance, optimizing any trade-offs to maximize performance.
Figure 3: Holistic co-design of the critical RF interconnect
For example, the RF interconnects are critical to the performance of the optical engine, especially at ultra-high baud rates. As shown in Figure 3, optimal design of the RF interconnect between the digital ASIC/DSP and the analog electronics needs to consider multiple factors including RF crosstalk, reflections/echoes, P/N balance/skew (latency differences between positive and negative voltages), and RF loss/bandwidth response. Electronic mitigation can be located in the digital ASIC/DSP or the analog ASIC, so an optimized design considers the digital ASIC/DSP, analog ASIC, and RF interconnect, sharing the load of impairment mitigation.
Enabled by Deep Vertical Integration
The individual high-performance components and holistic co-design are themselves enabled by the deep vertical integration provided by the Infinera Optical Innovation Center (OIC). OIC disciplines include coherent ASIC/DSP design, photonic integrated circuit design and manufacturing, analog ASIC design, advanced packaging design and manufacturing, and holistic co-design, including the RF interconnect.
Nyquist Subcarriers and CD Compensation Noise
Figure 4: Nyquist subcarriers reduce chromatic dispersion
Finally, while higher baud rates can exponentially increase the effect of chromatic dispersion, ICE6 leverages Nyquist subcarriers, which digitally divide a single high-baud-rate carrier into multiple lower-baud-rate subcarriers, as shown in Figure 4. This dramatically reduces the amount of noise created inside the optical engine when compensating for chromatic dispersion.
To learn more about this important topic, download the Infinera white paper, “Maximizing the Capacity-Reach of 800G Generation Coherent: Baud Rates, Features, and Modem SNR.”