PAM4 (Pulse Amplitude Modulation with 4 levels)
PAM4 (Pulse Amplitude Modulation with 4 levels) is a signaling method used in high-speed data transmission systems to increase bandwidth efficiency. Unlike traditional NRZ (Non-Return-to-Zero) signaling, which transmits one bit per clock cycle, PAM4 transmits two bits per clock cycle by encoding data into four distinct amplitude levels. Introduced widely in the 2010s for high-speed interconnects, PAM4 has become essential for advancing technologies like PCIe 6.0, Ethernet, and high-speed optical communications, enabling data rates to double without requiring higher frequencies.
https://en.wikipedia.org/wiki/Pulse-amplitude_modulation
One of the key advantages of PAM4 is its ability to deliver higher data throughput while maintaining compatibility with existing bandwidth limitations of physical channels. By encoding two bits per cycle, PAM4 effectively doubles the data rate compared to NRZ without the need for proportional increases in clock speed. However, the trade-off lies in the reduced signal-to-noise ratio (SNR), as the four distinct levels in PAM4 are closer together than the two levels in NRZ, making PAM4 more susceptible to noise and distortion.
https://www.keysight.com/us/en/assets/7018-03318/technical-overviews/5992-0412.pdf
To address these challenges, PAM4 implementations often incorporate error correction mechanisms like Forward Error Correction (FEC) and equalization techniques to ensure reliable data transmission. These enhancements make PAM4 a suitable choice for applications requiring extreme data speeds, such as 400G Ethernet and next-generation storage and computing interconnects. As data rates continue to scale, PAM4 is expected to remain a foundational technology in high-speed communication standards.
https://www.eetimes.com/understanding-pam4-and-its-role-in-next-generation-ethernet/