Understanding PAM4 Modulation: A Beginner Guide

NADDOD Jason Data Center Architect Dec 23, 2022

As a high-order modulation technology, PAM4 can be considered as an effective method to accelerate the development of optical technology by using electrical technology. Here are some common questions and answers of PAM4.

What is PAM4?

PAM4 is a type of PAM (Pulse Amplitude Modulation) technology. PAM signal is a popular signal transmission technology following NRZ (Non-Return-to-Zero), and it is also a representative of multi-level modulation technology. It has been widely used in the field of high-speed signal interconnection.

What is the difference between NRZ and PAM4 signaling?

● The NRZ signal uses two signal levels, high and low, to represent 1 and 0 of the digital logic signal, and can transmit 1 bit of logic information within each clock cycle.
● PAM4 signals use 4 different signal levels for signal transmission, namely 00, 01, 10, and 11, and can transmit 2-bit logical information within each clock cycle.
Therefore, under the same baud rate conditions, the bit rate of the PAM4 signal is twice that of the NRZ signal, and the transmission efficiency is doubled.

Difference between PAM4 and NRZ

Why is PAM4 technology needed?

Because the current NRZ technology is difficult to break through the single-channel 56G transmission rate. The emergence of PAM4 technology overcomes this problem, doubles the bit rate without increasing the bandwidth, and greatly reduces the loss caused by the transmission channel. Lay the technical foundation for the research and development of optical modules above 400G.

What types of optical modules apply PAM4 technology?

Currently, there are 50G (1*50G PAM4)/ 200G (4*50G PAM4)/ 400G (8*50G/ 4*100G PAM4) optical modules.

What are the differences between NRZ and PAM4 module solutions?

The main differences are in the laser driver chip, TIA chip and data processing chip:
● Because there are 4 levels of logic in the PAM4 code, the Driver and TIA chips inside the module both have linear output functions, while the limiter output solution is enough for the NRZ module.
● The PAM4 module needs to use DSP to complete the conversion of 50G PAM4 and 2*25G NRZ signals, while the NRZ module only needs to use the traditional CDR chip to complete the data transmission.

What are the application scenarios of PAM4 optical modules?

● 5G bearer network
● Metropolitan area network
● Data center interconnection (DCI) or data center network (DCN)