I/O Interfaces & Buses
Why does forward error correction (FEC) become more important with PAM4 than with NRZ?
- APAM4 makes clock recovery impossible, so every bit is wrong
- BFitting four levels into the same swing shrinks the spacing to about one third, so noise causes more bit errors
- CPAM4 signals are encrypted and decryption errors must be fixed
Show answer and explanation
Answer: B. Fitting four levels into the same swing shrinks the spacing to about one third, so noise causes more bit errors
With the same voltage swing, two-level NRZ has the full swing between 0 and 1, but four-level PAM4 leaves only about one third of it between adjacent levels. The noise margin drops by roughly 9.5 dB, so bit errors become more likely. PCI Express 6.0 therefore adds a lightweight error-correcting code so the receiver can fix small errors before use.
- A. Clocks can be recovered with PAM4 too.
- B. Correct. The noise margin shrinks.
- C. It has nothing to do with encryption.