Equalization Enhancement Approaches for PAM4 Signaling for Next Generation Speeds

Four-level pulse-amplitude modulation signal (PAM4) is gaining more attention as an alternate coding scheme to binary modulation NRZ (Non Return to Zero) in high speed systems as the signal rate is reduced by half. Transmitter and receiver equalizations continue to play an important role with PAM4 encoding. Even though de-emphasis is a well standardized approach for equalization in NRZ, two enhancement approaches for PAM4 de-emphasis are proposed in the paper. The first enhancement approach is to compensate voltage levels in the de-emphasis process and the second one is to equalize the signal by bit rather than by symbol. Several test cases are presented to show the improvement in eye opening compared to the old approach. The limitations of these two enhancement approaches are also discussed.