Comparative Analysis of PAM4 and NRZ Modulation Technologies

PAM4 and NRZ are two mainstream digital signal modulation technologies in optical communication and high-speed data transmission. Their fundamental differences lie in the number of signal levels, transmission efficiency, and anti-interference capabilities. The following analysis will examine them from three dimensions: principles, characteristics, and application scenarios.

I. NRZ ( Non- Return-to-Zero Encoding)

1. Definition and Principle

NRZ (Non-Return-to-Zero) is a binary encoding method that uses high and low voltage levels to represent "1" and "0" in a digital signal, respectively. The signal remains at a constant level throughout each clock cycle and does not return to zero, hence the name "non-return-to-zero encoding".

2. Features

Simple to Implement: the encoding method is intuitive, the hardware overhead is small, and the bandwidth utilization efficiency is high.

Limited Noise Immunity: It has only two level states with a large level interval. It has some noise immunity at medium speeds, but it is insufficient in high-speed or complex electromagnetic environments.

There is a DC component: The signal spectrum contains a DC component, which may cause baseline drift and signal attenuation during transmission.

Requires independent clock synchronization: The signal itself does not carry clock information and requires an additional clock line or a clock data recovery (CDR) mechanism.

3. Application Scenarios

NRZ is widely used in low-to-medium speed, cost-sensitive, and high-reliability scenarios, such as low-speed Ethernet, chip-to-chip interfaces, PCIe Gen1–Gen4, and serial communication interfaces such as UART, RS-232/422/485. In the field of optical communication, NRZ is commonly used in optical modules with speeds below 100Gbps.

II. PAM4 (Four-Level Pulse Amplitude Modulation)

1. Definition and Principle

PAM4 (4-Level Pulse Amplitude Modulation) is a four-level pulse amplitude modulation technique that uses four different signal levels to represent the symbols "00", "01", "10", and "11". It can transmit 2 bits of information per clock cycle, significantly improving transmission efficiency compared to NRZ.

2. Features

High Transmission Efficiency: At the same symbol rate, the data transmission rate is twice that of NRZ.

Low Resistance to Interference: the interval between the four levels is small, making it more sensitive to noise and interference. The bit error rate (BER) is theoretically higher than NRZ, and it has higher requirements for channel quality.

Reliance on Complex Signal Processing: To ensure reliability, PAM4 usually needs to be combined with digital signal processing techniques such as forward error correction (FEC) and equalization, which increases the system complexity and power consumption.

Suitable for High-Speed and Long-Distance Transmission: Despite the reduced noise immunity, PAM4 has become the mainstream choice in high-speed, long-distance scenarios due to its high transmission efficiency and the combination of advanced optoelectronic devices and algorithms.

3. Application Scenarios

PAM4 has become a core technology in fields such as high-speed Ethernet, data center interconnect, 5G mobile bearer, computing and storage interfaces, and vehicle networks.

High-speed Ethernet and data centers: This is the core modulation method for 400G/800G Ethernet and 200G/400G optical modules.

5G bearer network: High-speed optical modules used in 5G fronthaul, midhaul and backhaul networks, supporting diverse scenarios such as eMBB , uRLLC and mMTC .

Compute and storage interface: PCIe 6.0 uses PAM4 signaling, with a single link rate of up to 64 GT/s.

In-vehicle Ethernet: Multi-gigabit in-vehicle Ethernet based on PAM4 achieves transmission rates of 2.5Gbps, 5Gbps and 10Gbps.

III. Summary

The choice between NRZ and PAM4 is essentially a trade-off between transmission efficiency and signal-to-noise ratio.

NRZ offers advantages in terms of lower complexity and higher noise tolerance, making it suitable for low-to-medium speed systems that are sensitive to cost and latency.

PAM4 achieves higher data throughput with the same bandwidth, but requires complex signal processing to compensate for the reduced noise immunity, making it the mainstream technology in current high-speed interconnection scenarios.

As data centers, AI computing clusters, and next-generation interfaces evolve towards 800G and 1.6T, PAM4 will continue to play an increasingly important role in optical modules and high-speed copper cable interconnects, while NRZ will continue to maintain its key value in low-to-medium speed, low-power, and low-latency on-board interconnects and control plane links.