PAM4 Optical Modulation: Meeting the Demands of Increasing Bandwidth

Have you ever wondered how the constant demand for bandwidth is met?

The demand from global end-users is insatiable, with innovations in cloud computing, 5G expansion, and AI pushing an operator’s switch, router, and server interconnect data rates past the 100G milestone into 400G, with 800G and 1.6T well on the horizon as we all saw at OFC ’23.

In the pluggable transceiver world, this continual growth in demand has been met with innovations in form-factor from GBIC to SFP, to QSFP and QSFP28 as we evolved from 1G to 100G data rates.

Throughout this evolution, the basic optical transmission format has remained the same, relying on non-return to zero (NRZ) modulation of the laser, with the binary ‘1’s and ‘0’s represented by two distinct light levels.

For a higher transmission rate, we just modulate faster. However, to get to the next level – 400G and beyond – NRZ just isn’t going to cut it. We need a more sophisticated way to modulate our optical signal beyond just turning it on and off faster and faster.

In this blog we explore four-level pulse amplitude modulation (PAM4) with direct-detect and its role in 400G, and our next blog will introduce you to the exciting world of coherent optical transmission.

What is PAM4?

To enable Ethernet speeds of 400G and beyond, PAM4 multilevel signaling is required, rather than NRZ modulation preferred for 100G applications. PAM4 modulation has transformed optical networking, but what exactly is it and how does it work?

Traditionally, network engineers have relied on NRZ modulation for 1G, 10G, and 25G. The pluggable industry achieved the transition from 10G to 40G and then 100G by parallelizing the 10G/25G NRZ modulations. Hence the “Q” (quad) in QSFP and QSFP28.

In NRZ modulation, the optical signal has two levels representing the digital “1” and “0”, and the laser is modulated between these two levels at the desired data rate.

However, NRZ-based technologies are no longer technically practical nor cost-effective when scaling up for speeds of 400G/800G and beyond. Consequently, the industry has turned to PAM4 modulation to realize ultra-high-bandwidth network architectures.

PAM4 is an optical modulation technique that allows for higher data rates and increased spectral efficiency compared to NRZ. In PAM4, each symbol represents multiple bits of information by varying the amplitude of the optical pulse to four distinct levels. These four levels now represent two bits of information in each signal period, doubling the effective bit rate.

At the receiver end, a decision-making algorithm determines the most likely transmitted amplitude level for each received symbol. This process involves comparing the received signal to predefined decision thresholds to determine the amplitude level.

So, What’s the Catch?

PAM4 is not a new modulation technique by any means, so why haven’t we been using it in our pluggables until now?

There are some tradeoffs that have to be considered vs NRZ. PAM4 is more susceptible to errors and noise due to the closer spacing between amplitude levels. As a result, advanced digital signal processing techniques and equalization methods are employed to mitigate these issues and ensure reliable data transmission. This means added complexity in the transmitter (TOSA) and receiver (ROSA) electronics.

As a result, optical transceivers capable of 400G will consume more power than their 100G and lower-rate counterparts. As the next generation switches and routers are deployed with 400G pluggable capabilities, keep an eye out for QSFP-DD equipping rules that take into account the higher power consumption of these pluggable modules.

PAM4 modulation introduces a penalty on the signal-to-noise ratio (SNR) versus NRZ which necessitates an increased use of forward error correction (FEC) to mitigate the loss of signal integrity. The additional signal processing, error correction, and higher-performing components brings in an added cost and complexity to 400G PAM4 links, as well as additional latency that needs to be considered by the network architects.

Meeting the Bandwidth Needs of the Future

Regardless of these trade-offs, the industry is steadfastly moving towards PAM4, setting its eyes firmly on the future. With the saturation point of NRZ reached, it’s clear that we must adopt more sophisticated techniques to navigate the bandwidth demands of tomorrow.

In our next blog post, we will explore another promising strategy to conquer the 400G frontier – coherent optical transmission.

Here at Integra Optics, we are wholeheartedly embracing rapid transition. As we voyage into uncharted territories of bandwidth, we remain steadfast in our commitment to delivering pioneering solutions that align with the evolving needs of our customers. Join us on this exciting journey and stay tuned for more updates on the fascinating world of optical networking.

Do you have questions or want to learn more about the shift to PAM4? Our experts at here to help. Feel free to reach out to us anytime. We’re always ready to guide you on your journey through the evolving landscape of optical transmission.