Samtec Technology Frontier | Multiple reasons prompted PCIe® 6.0 to adopt PAM4

Abstract/Foreword

Samtec has been a proud member of the PCI-SIG® community for many years . Jignesh Shah, senior systems architect at Samtec, joined fellow PCI-SIG partners to discuss PAM4 encoding, a new feature of the PCIe® 6.0 specification.

PCIe® 6.0 new specification     

In early 2022, PCI-SIG® released the full version of the PCI Express® (PCIe®) 6.0 specification . Like previous versions, the latest version doubles the data rate  of the PCIe® 5.0 specification and maintains backward compatibility .

Key features of the latest PCIe® 6.0  specification include:

01 64 GT/s data rate, up to 256Gbps via x16 configuration;

02 PAM4 signaling utilizes the existing PAM4 in the industry;

03Lightweight FEC and CRC mitigate the increased bit error rate associated with PAM4 signaling;

04 Flit-based encoding supports PAM4 modulation and achieves >2 times bandwidth gain;

05 Updated packet layout for Flit mode to provide additional functionality and simplify processing;

06 Maintains backward compatibility with all previous generations of PCIe technology .

What is PAM4?

PAM stands for Pulse Amplitude Modulation , and the number 4 represents the number of levels . Most engineers are familiar with NRZ modulation, or non-return-to-zero modulation. NRZ uses a two-level signal. When Vcc ~ 0V, logic "0" is passed. When Vcc ~ 1.8/3.3/5V, logic "1" passes. Each unit interval sends a 0 or a 1, or a bit.   So NRZ is basically a PAM2 .

As an improved product, PAM4 is a multi-level technology that uses four-level signaling. Basically, there are four levels of signaling, corresponding to Vcc ~ 0V, Vcc/3, (2*Vcc)/3 and Vcc . Two bits are sent per unit interval instead of one. What it sends is 0-0, 0-1, 1-0 or 1-1 . The image below details these differences:

Generally speaking, NRZ modulation supports an eye diagram with an "eye" . For the PCIe® 5.0  specification, the "eye" has a specific eye height and voltage level , which depends on the defined lane length. PAM4 modulation eye diagram supports three "eyes" . For the PCIe® 6.0  specification, each "eye" also has a defined eye height and voltage level for that specific signal lane.

Why PCIe ®  6.0 Specification Adopts PAM4  

The previous PCIe specification was NRZ or PAM2. The PAM4 specification has been adopted by other network standards, starting with 56Gbps, 112Gbps, and 224Gbps in the future. There is precedent for PAM4 in other standards . 

PAM4 signaling doubles the data rate while using the same Nyquist frequency of the corresponding NRZ modulation principle . For example, PCIe ®  5.0 architecture uses a Nyquist frequency of 16GHz. Using PAM4 modulation, the PCIe® 6.0  architecture uses the same 16GHz Nyquist frequency but doubles the data rate to 64GT/s per link and achieves 256Gbps bidirectional bandwidth via x16 configuration .

What are the advantages of PAM4 compared with NRZ?  

PAM4 reduces channel loss because it runs at half the frequency of two bits per UI. This makes the PCIe® 6.0  specification's channel coverage  similar to that provided by the PCIe® 5.0 specification .  PAM4 modulation in the PCIe® 6.0 specification may even allow for less loss and extended coverage .

Looking at it another way, for the same signal frequency, you can double the data rate, or increase bandwidth or throughput for next-generation applications. 

What applications does the PCIe ®  6.0 specification support?   

PAM4 enables the PCIe® 6.0  specification to deliver the bandwidth required for high data rate applications such as data centers, artificial intelligence/machine learning, HPC (high performance computing), automotive, IoT and aerospace .

All of these applications require high data rates, so the doubling from the PCIe® 5.0 specification to the PCIe® 6.0 specification enables the specification to meet the needs of these markets.