Decision-feedback equalizer (DFE)

A decision-feedback equalizer is the receiver's clever trick of cleaning up a bit by subtracting the known damage left by the bits it just decided. After the sampler commits to '1' or '0', the DFE remembers that decision, multiplies it by how much that bit is expected to bleed into the next few bits (the post-cursor ISI), and subtracts that exact amount from the incoming signal before the next decision. It's like cancelling a known echo: you heard yourself say 'hello', so you subtract the delayed 'hello' that bounces back a moment later.

The DFE's superpower is that, because it works on already-decided digital bits rather than the noisy analog waveform, it removes ISI without amplifying noise — unlike a CTLE or FFE. Its catch is two-fold: it can only cancel post-cursor (trailing) ISI, not pre-cursor, and if it makes one wrong decision it feeds the wrong correction forward, briefly causing error propagation. At PAM4 and 100+ Gb/s, the first DFE tap must settle within a single unit interval, which is one of the hardest timing problems in the whole chip.