4-level PAM (PAM4) multilevel signaling is an evolution from the traditional two state non-return-to-zero (NRZ) modulation. PAM4 effectively doubles the data rate for a link bandwidth at the expense of reduced signal to noise ratio (SNR).  PAM4 is used in 400GE, 800GE, and 1.6T Ethernet as well as PCIe 6.0® and other serial data standards. 

PAM4 Design and Simulation

Conventional signal impairments such as jitter, noise, channel loss, and intersymbol interference (ISI) behave differently in PAM4 designs. You need to understand the complex specifications of the latest data standards and simulate your design using device models and reference channels to analyze its performance and compliance to the standards before prototyping. We can help you save significant design time and minimize the effects of unexpected impairments.

PAM4 Transmitter and Receiver Test

PAM4 designs are far more susceptible to noise than NRZ designs, since four signal levels are packed into an amplitude swing of two. You must perform new transmitter (Tx) test measurements, such as signal to noise and distortion ratio (SNDR) and output jitter, to characterize your PAM4 transmitter devices. Forward error correction (FEC) is used to correct channel errors in PAM4 links and must be considered in PAM4 receiver (Rx) testing. We can help you address these new PAM4 physical layer test challenges to ensure your devices' compliance with the latest industry standards.

PAM4 Channel Characterization

This faster rise time of high data rate PAM4 signals creates larger reflections at impedance discontinuities and degrades the signal quality at the end of the channel. As a result, you need to account for signal degradation due to physical layer components such as printed circuit board traces, connectors, cables, and integrated circuit (IC) packages when testing the performance of your PAM4 devices. We can help you characterize PAM4 channel effects to ensure the signal integrity of your 800G, 1.6T, and PCIe 6.0 devices.

PAM4 channel image

Further Reading

Frequently Asked Questions - PAM4

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.

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