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400GbE paves the way for PAM4 modulation scheme

Posted: 06 Jul 2015 ?? ?Print Version ?Bookmark and Share

Keywords:PAM4? modulation? 400GbE? silicon? IEEE?

The PAM4, a relatively new and complex modulation scheme, has been adopted for some aspects of the 400Gb/s Ethernet standard, which opens the path of innovative silicon development. Modulation formats have been hotly debated over the last 6-12 months in the IEEE P802.3bs group setting a standard for 400GbE.

Decisions to adopt PAM4 optical modulation schemes at the recent IEEE May meeting will have significant implications on the technology direction of the industry moving forward.

The 400GbE project includes electrical interfaces to address chip-to-chip and chip-to-module applications as well as optical interfaces for 100-metre multimode fibre (MMF), 500m single-mode fibre (SMF), 2km SMF and 10km SMF. Initially, the group was able to make fast decisions on some 16-wide interfaces for both electrical and optical MMF, adopting 16 x 25Gb/s with traditional NRZ modulation. These proposals strongly leverage the existing 25Gb/s signaling technology developed initially for the 100GbE market which is seeing wide development and implementation today.

Numerous proposals for interfaces narrower than the 16-lane approaches were submitted for consideration to respond to market pressure for cost effective interconnects. Such interfaces use fewer pins on chips or optical devices and therefore reduce power, lower cost, and increase use. Both 8-wide and 4-wide proposals were made to the group.

A 50Gbit/s NRZ approach was shown to be viable for 8-wide optical and electrical interfaces. Alternate modulation schemes were also proposed to further lower costs reasons or the baud rate to leverage existing technologies and know-how around 25Gbaud components. Alternate modulation schemes considered included PAM4 modulation for both electrical (8-wide) and optical (4- and 8-wide) and discrete-multitone (DMT) for 4-wide optical.

We saw how decisions about electrical and optical solutions could be made separately, however there was clearly strong perception that a common ecosystem could develop if one modulation scheme was adopted. After much debate, the electrical decision was made in March 2015 that a second 8-wide electrical interface would be defined based on 50Gb/s PAM4 modulation. This was widely supported by many of the major silicon suppliers with test results and simulations showing good performance with known connector and channels.

With the electrical decision out of the way, at the May 2015 meeting the 400GbE group focused on resolving the SMF optical interface decisions. After much consideration and deliberation, the group decided to support PAM4 modulation for at least two of the SMF solutions. An 8x50G PAM4 scheme was adopted for duplex 10km SMF and a 4x100G PAM4 scheme was adopted for 500m parallel fibre (4 fibres per direction).

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