PAM4

What Is PAM4?

Traditional binary signalling — NRZ — sends one bit per symbol: light on or light off, high or low. PAM4 uses four amplitude levels instead of two, so each symbol carries two bits. At the same symbol rate you get twice the data.

This is how the industry got from 25G to 50G and then to 100G per lane without doubling the electrical bandwidth each time. Every modern high-speed optical interface — 400G, 800G, 1.6T — is PAM4. It is not an option a vendor chose; it is the only way the bandwidth arrived on schedule.

What It Costs

Squeezing four levels into the same amplitude range means the gaps between them are a third of what they were. The eye diagram that used to be one large opening becomes three small ones stacked vertically. Theoretically that costs about 9.5 dB of signal-to-noise ratio, and in practice rather more once transmitter nonlinearity is counted — the four levels are not perfectly evenly spaced, because laser and modulator response is not perfectly linear.

Nothing about PAM4 is free. It buys bandwidth with noise margin, and the margin has to be bought back somewhere. That somewhere is forward error correction, which is why every PAM4 standard mandates it and why the raw error rate on a working PAM4 link would have been considered a hard fault in the NRZ era.

What Changes Operationally

The single most useful thing to internalise: on a PAM4 link, pre-FEC errors are normal. A 400G link with a pre-FEC bit error rate of one in a million and zero post-FEC errors is working exactly as designed. Treating that number as a fault is how healthy modules get replaced.

What matters is the margin to the FEC's correction limit and the direction the number is moving. A link sitting at 1e-6 has plenty of headroom; one drifting toward 1e-4 is approaching the cliff, and when it goes it goes suddenly. Trending pre-FEC BER is the single best early-warning signal available for contamination and marginal optics.

Compliance testing changed too. With three stacked eyes and a mandatory receiver equaliser, the old eye-mask and extinction-ratio tests stopped being meaningful, and TDECQ replaced them with one figure that rolls up everything degrading the eye.

Why Not Something Denser?

The obvious question is why stop at four levels. PAM8 would carry three bits per symbol. The answer is that the SNR penalty grows faster than the benefit: each doubling of levels costs roughly another 6 dB, and direct-detect optical links do not have that margin to give.

Above PAM4 the industry goes to coherent detection instead, which recovers phase and polarisation as well as amplitude and therefore has far more dimensions to encode into. That is a different and much more expensive class of optics, which is why it lives in transport rather than inside the data hall.

Related Terms

NRZ, FEC, SerDes, TDECQ and BER.