What is PAM4 Signaling and How Does It Work?

Pulse Amplitude Modulation 4-Level (PAM4) is a multi-level signal modulation technique that doubles the data capacity of high-speed digital communications compared to traditional binary signaling. By utilizing four distinct voltage levels instead of two, PAM4 transmits two bits of data within every clock cycle (symbol), enabling higher bandwidth without requiring a corresponding increase in transmission frequency.

The Baseline: Binary Signaling (NRZ)

To understand PAM4, it helps to first look at standard binary signaling, commonly known as Non-Return-to-Zero (NRZ) or PAM2. In an NRZ system, an electrical or optical signal alternates between two distinct voltage levels:

• Low Voltage: Represents a logical 0
• High Voltage: Represents a logical 1

Because each voltage pulse (symbol) can only represent one of two states, an NRZ system transmits exactly one bit per symbol. If the system runs at 28 Gbaud (28 billion symbols per second), the resulting throughput is 28 Gbps.

How PAM4 Encodes Two Bits Per Symbol

PAM4 expands on binary signaling by using four distinct voltage amplitudes instead of two. Because four states can be represented mathematically as \(2^2\), each discrete voltage level maps directly to a unique two-bit binary combination:

• Level 0 (Lowest Voltage): Represents binary 00
• Level 1 (Lower-Mid Voltage): Represents binary 01
• Level 2 (Upper-Mid Voltage): Represents binary 11
• Level 3 (Highest Voltage): Represents binary 10

(Note: Gray coding is typically used for these assignments so that an adjacent amplitude error only results in a single-bit error rather than a two-bit error.)

By evaluating which of the four voltage thresholds the signal hits during a single symbol period, the receiver decodes two bits at once. Consequently, at the same symbol rate (baud rate) of 28 Gbaud, a PAM4 link achieves a throughput of 56 Gbps—effectively doubling the data rate of NRZ over the same channel bandwidth.

Key Trade-offs: Bandwidth vs. Signal-to-Noise Ratio

While PAM4 doubles spectral efficiency, it introduces design challenges:

• Eye Height and Noise Margin: In a standard eye diagram, NRZ has one large “eye” opening between its two levels. PAM4 divides that same total voltage range into three smaller stacked eyes. Each eye opening is roughly one-third the height of an NRZ eye, reducing the Signal-to-Noise Ratio (SNR) by approximately 9.5 dB.
• Error Correction: Due to the reduced margin for electrical noise, jitter, and crosstalk, PAM4 systems almost universally require Forward Error Correction (FEC) algorithms to detect and correct transmission errors at the receiver.

Summary

PAM4 achieves double the throughput of standard binary signaling by converting pairs of bits into four distinct amplitude levels. This allows high-speed communication standards—such as 400G/800G Ethernet and PCIe 6.0—to scale bandwidth dramatically without exceeding the physical frequency limits of existing copper traces and optical channels.