Non-Return to Zero (NRZ)

• NRZ is a binary (two levels) modulation represented by logic 0 and logic 1
• NRZ transmits one bit per symbol
• NRZ modulation served as the base of modern high-speed interfaces until 2012
• NRZ is PAM2 with 1 bit/symbol

Pulsed-Amplitude Modulation 4 levels (PAM4)

• PAM4 encodes four levels, represented by four combinations of
  two bits logic: 00, 01, 10 and 11.
• The PAM4 eye diagram presents three eye openings and four levels.
• PAM4 transmits 2 bits per symbol, therefore the Baud rate is
  twice the bits per second

PAM4 vs NRZ

• PAM4 doubled the density of data vs NRZ in same time period
  • 28Gbps NRZ vs 28GBd PAM4 --> 28GBd PAM4 = 28G * 2 bits = 56 Gbps
  • 56Gbps NRZ vs 56GBd PAM4 --> 56GBd PAM4 = 56G * 2 bits = 112 Gbps
• PAM4 reduces the # of transmission lanes & # of fibers compared to NRZ

Note 1:
400Gbps is obtain with:
8x I/O Lanes of 28GBd (53 Gbps/lane)
4x I/O Lanes of 56GBd (112 Gbps/lane)
Customers will have to specify which one
they are interested in (if not both).

Note 2:
The SNR is divided by a factor of 3 with PAM4. FEC required to maintain optical budget.

Symbol transition NRZ vs PAM4

NRZ

2 symbol’ transitions with none
return to zero when 2 consecutive 1s

PAM4

16 symbol’ transitions 6 different
rise and fall times

PAM2: 1 bit/symbol

PAM4: 2 bit/symbol

Challenges introduced by PAM4

• Analyzing and debugging 4-level systems is more complex (16 symbol transitions!)
• PAM4 signal is more susceptible to noise:
  • Signal to Noise divided by 3 compared to NRZ
• PAM4-based units consume higher power than a transceiver supporting NRZ because of the need for more advanced equalization

400G interface rate vs lane vs Baud rate

BER tester: Typical applications

Optical transceivers leverage NRZ and PAM4 technology to generate traffic.

• NRZ-based pluggables: From 10G to 100G
• PAM4-based pluggables: From 50G or 100G to 400G

NRZ units still relevant because of 5G rates (25G & 50G)