PAM4 vs NRZ: Modulation Behind 100G, 400G and 800G Optics

Why does a 400G module need forward error correction when a 10G module does not? The answer is the shift from NRZ to PAM4 modulation. NRZ (two levels) carries one bit per symbol, while PAM4 uses four amplitude levels to encode two bits per symbol at the same baud rate - doubling throughput but squeezing the signal eye to a third of the height and adding about 9.5 dB of SNR penalty. This article explains PAM4 vs NRZ in practical terms: the trade-offs, the FEC requirement, and which IEEE standards (802.3bs, 802.3cd, 802.3ck) put PAM4 inside your optics.

The Core Difference: Bits per Symbol

NRZ is really 2-level pulse amplitude modulation (PAM2): a positive level means 1, a negative level means 0, one bit per symbol, baud rate equals bit rate. PAM4 uses four levels (-3, -1, +1, +3), so each symbol carries two bits and a 53.125 GBaud lane delivers 100 Gbps instead of ~53 Gbps. That is how 400G interfaces are built from only four or eight lanes instead of sixteen 25G NRZ lanes.

Why 400G Could Not Stay on NRZ

The first 400G proposals used 16 x 25G NRZ lanes (400GBASE-SR16), but 16-fiber ribbon cabling and connector bulk made it impractical. IEEE 802.3bs (December 2017) standardized PAM4 for 200G/400G Ethernet - 400GBASE-DR4 (4 x 100G PAM4 over parallel single-mode fiber, 500 m) and 400GBASE-FR8 (8 x 50G PAM4 via WDM, 2 km). By 2022, 802.3ck extended PAM4 to 100 Gbps per lane at 53.125 GBaud, enabling single-lane 100G and 800G modules.

The SNR Penalty and the FEC Requirement

PAM4's four levels sit closer together: the vertical eye opening shrinks to 1/3 of NRZ, reducing the SNR budget by about 9.5 dB and pushing the raw bit error rate from ~1e-12 (NRZ) up to 1e-4 ~ 1e-6. That is why PAM4 links mandate Reed-Solomon FEC - RS(544,514) in 802.3bs - plus better equalization (FFE/DFE) and DSP in the module. A useful rule of thumb when troubleshooting: above 50 Gbps per lane you are in a PAM4 + FEC world, and a link that fails to come up is often a FEC mode mismatch between the switch port and the optic.

Practical Comparison Table

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原文链接：https://fs.com/uk/blog/nrz-vs-pam4-modulation-techniques-2471.html