Introduction To NRZ And PAM4 Modulation Techniques

• Background

Major trends including cloud computing and big data are driving exponential growth in network traffic. Data center networks face increasing bandwidth demands, requiring innovative technologies to support next-generation Ethernet.

Two key signal modulation schemes are widely adopted: Non-Return-to-Zero (NRZ) and 4-level Pulse Amplitude Modulation (PAM4).

This article introduces and compares these two modulation techniques.

• Introduction to NRZ and PAM4 Modulation

NRZ is a modulation scheme that uses two signal levels to represent binary logic states: a low level for logic 0 and a high level for logic 1.

It transmits 1 bit of data per symbol (clock cycle), as shown below:

Figure 1 NRZ Signal and Corresponding Waveform

PAM4 uses four distinct signal amplitude levels for data transmission. Each symbol period carries 2 bits of information, corresponding to four combinations: 00, 01, 10, and 11.

Its waveform is shown below:

PAM4 signals and corresponding waveforms

Figure 2 PAM4 Signal and Corresponding Waveform

• Comparison Between NRZ and PAM4

NRZ uses two levels (high and low) to transmit 1 bit per cycle.

In contrast, PAM4 uses four amplitude levels and delivers 2 bits per symbol cycle.

At the same baud rate, PAM4 provides twice the bit rate of NRZ, effectively doubling transmission efficiency.

This allows PAM4 to significantly increase network bandwidth and spectral efficiency without deploying additional fiber infrastructure.

PAM4 at 25G Baud and 50G Baud is widely used in high-speed optical modules.

Typical applications in 400G optical modules are listed below:

Table 1 Application of PAM4 in 400G Optical Modules

Optical Module Type	Maximum transmission Distance	Fiber Type	Channel	Signal Transmission Technology	Baud Rate
400GBASE-SR8	100M	Multimode Fiber	8*50Gbps	PAM4	25GBd
400GBASE-DR4	500M	Singlemode Fiber	4*100Gbps	PAM4	50GBd
400GBASE-FE8	2KM	Singlemode Fiber	8*50Gbps	PAM4	25GBd
400GBASE-LR8	10KM	Singlemode Fiber	8*50Gbps	PAM4	25GBd
400GBASE-ER8	40KM	Singlemode Fiber	8*50Gbps	PAM4	25GBd
Note : 25GBd PAM4=50Gbps; 50GBd PAM4=100Gbps;

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Introduction To NRZ And PAM4 Modulation Techniques