Shared with The Gufo
Abstract
The most common modulation scheme for 100GE today is NRZ modulation. 100GE is achieved by using 4 lanes of 25 gigabit per second (Gb/s) NRZ modulated signals. Theoretically, reaching 400GE speeds with NRZ is possible by applying these same concepts using 8 lanes of 56 Gb/s signaling. However, as speeds of NRZ designs increase above 28 Gb/s, channel loss of the transmission medium becomes a limiting factor. PAM4 signals use 4 amplitude levels with logical bits 00, 01, 10 and 11 to represent a symbol. However, PAM4 designs are far more susceptible to noise since four signal levels are packed into an amplitude swing of two. Therefore, the signal to noise ratio (SNR) is lower and analyzing noise from transceiver designs needs to account for channel return loss, as well as noise from the test instrumentation. PAM4 will use forward error correction (FEC) to account for this.
