Pulse Amplitude Modulation (PAM)

Pulse amplitude modulation is a multi-level signaling scheme used in digital communications at the physical layer that allows transmitting multiple bits per clock cycle by varying the amplitude of voltage impulses. The result is an increased data rate. PAM4 uses four voltage levels to transmit 2 bits per clock signal, doubling the data rate compared to traditional non-return to zero (NRZ) signaling.

PAM4 doubles the data rate by encoding two bits per clock cycle without requiring additional bandwidth in the channel. Multilevel signaling is more bandwidth- and spectral-efficient, getting the most data without increasing the speed at which the signal transmits (the baud rate).

PAM4 signals are more sensitive to noise and distortion due to the decreased noise thresholds. As the voltage levels increase, the thresholds between them (defining the bits) become smaller. Maintaining proper signal integrity across noisy channels becomes even more critical as signal speed and signaling levels increase, necessitating further equalization and error correction.

PAM4 is currently used in IEEE 802.3 and OIF-CEI Ethernet electrical and optical data center networking standards, as well as PCIe 6.0 for faster serial peripheral communications. It is also used in other technologies, such as wireless base stations and in-vehicle networks (automotive Ethernet).

For now, 400GE, 800GE, and 1.6T Ethernet will continue to use PAM4 signaling (112 GBd, 224 Gb/s lane speeds). However, other technologies use different PAM-N levels, including USB4 Version 2.0, which uses PAM3 to achieve 80 Gbps transmissions. PAM6, PAM8, and PAM16 are considered for future data standards, while coherent optical uses QAM (quadrature amplitude modulation), which modulates the signal in two dimensions.

Due to the high error rates resulting from increased bandwidth and sensitivity to noise, PAM4 systems are often complimented by Forward Error Correction (FEC) to keep bit error ratios (BER) at acceptable levels. FEC adds redundant information to transmitted data, which allows the receiver to detect and correct any errors that may have occurred. Combining PAM4 with FEC allows high-speed digital systems to maintain high bandwidth while keeping the BER at acceptable levels.