NRZ is dead, but not everywhere | EDN–but-not-everywhere

PAM4 modulation is everywhere.

“Five years ago, PAM4 was a concept”

“PAM4 is being deployed everywhere except in short-reach links.”

“DSP is enabling PAM4 optical channels”

PAM4 has won the battle in fiber-optic and other medium and long-reach applications.


  1. Tomi Engdahl says:

    The fundamentals of PAM4

    As our society’s hunger for data grows—not only more data, but more data delivered faster—older modulation schemes based on NRZ-type encoding grow increasingly inadequate.

    For quite some time, NRZ-type encoding has been the mainstay modulation scheme for data transmission

    We need a way to double the bit rate in the channel without doubling the required bandwidth, and that’s where PAM4 enters the picture. PAM4 takes the L (Least Significant Bit) signal, divides it in half, and adds it to the M (Most Significant Bit) signal. The result is four signal levels instead of two, with each signal level corresponding to a two-bit symbol.

    The PAM4 signal looks like trace M+L/2

    We have, in effect, traded off SNR for bandwidth. Many serial links are bandwidth-constrained, as it’s difficult to move much more than 28 Gb/s over any length of copper. But when you have some SNR headroom, it may well pay off to consider a PAM4 modulation scheme.

  2. Tomi Engdahl says:

    Home> Test-and-measurement Design Center > How To Article
    PAM4: A new measurement science–A-new-measurement-science


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