PAM4 vs NRZ

PAM4 and NRZ are two important signaling formats used in high-speed electrical and optical communication. NRZ represents binary data with two signal levels, while PAM4 uses four signal levels to encode two bits per symbol.

The move from NRZ to PAM4 has enabled higher data rates without requiring a proportional increase in symbol rate. At the same time, PAM4 introduces smaller voltage or optical level spacing, making signal integrity, noise, equalization, and error performance more challenging.

1. What Is NRZ?

NRZ stands for Non-Return-to-Zero. In a conventional binary NRZ signal, two signal levels are used to represent two logical states, commonly referred to as 0 and 1.

One symbol carries one bit of information. This makes NRZ relatively simple to generate, transmit, receive, and recover compared with multi-level signaling schemes.

2. What Is PAM4?

PAM4 stands for Pulse Amplitude Modulation with 4 levels. Instead of two amplitude levels, PAM4 uses four distinct signal levels.

Because four levels can represent four possible symbol states, each PAM4 symbol carries two bits of information.

3. PAM4 vs NRZ at a Glance

4. The Fundamental Difference

The main difference is the number of signal levels.

NRZ has two levels and carries one bit per symbol. PAM4 has four levels and carries two bits per symbol.

This allows PAM4 to transmit more bits at the same symbol rate, or to achieve the same bit rate at a lower symbol rate.

5. Why PAM4 Carries Two Bits per Symbol

Four signal levels provide four possible states:

00, 01, 10, 11

Each state corresponds to a different amplitude level. Therefore, one PAM4 symbol can represent two binary bits.

6. NRZ Carries One Bit per Symbol

With two signal levels, NRZ provides two possible states:

0 and 1

One symbol therefore represents one bit. This simpler mapping is one of the reasons NRZ has traditionally been easier to implement.

7. Symbol Rate vs Data Rate

Data rate and symbol rate are related but not identical.

For binary NRZ, one symbol carries one bit, so the symbol rate is approximately equal to the bit rate before considering coding and overhead.

For PAM4, one symbol carries two bits, so the symbol rate is approximately half the raw bit rate.

8. Example: 100 Gb/s Signaling

To carry approximately 100 Gb/s on a single lane, an NRZ system would require a symbol rate close to 100 GBd before overhead.

A PAM4 system can carry the same binary rate at approximately 50 GBd because each symbol carries two bits.

9. Why Lower Baud Rate Matters

Reducing the symbol rate can reduce some of the bandwidth requirements of the electrical and optical channel.

This can make it possible to transmit higher aggregate data rates through existing or practical channel bandwidths. However, lower baud rate does not make PAM4 automatically easier because the four signal levels introduce a much smaller amplitude margin.

10. PAM4 Eye Diagram

An NRZ eye diagram normally contains one major eye opening because the signal has two levels.

PAM4 creates three vertically stacked eye openings because four amplitude levels produce three decision boundaries.

These smaller eye openings make PAM4 more sensitive to noise, distortion, crosstalk, and nonlinear effects.

11. PAM4 Has Smaller Level Spacing

For a given overall signal amplitude, dividing the waveform into four levels produces smaller spacing between adjacent levels than a two-level NRZ signal.

As a result, the same amount of voltage or optical noise can represent a larger fraction of the available decision margin.

12. Noise Tolerance

NRZ generally provides greater vertical noise margin because it has only two signal levels.

PAM4 has four levels and therefore smaller distances between adjacent decision thresholds. This makes accurate detection more difficult in the presence of noise.

13. BER Performance

Bit Error Rate (BER) is an important consideration for both modulation formats.

Because PAM4 has tighter signal-level spacing and more decision thresholds, maintaining a low error rate is more challenging than with conventional NRZ signaling under comparable signal conditions.

14. Why FEC Is Important for PAM4

Forward Error Correction (FEC) is widely used in modern high-speed PAM4 systems to improve link-level error performance.

The optical or electrical channel can operate with a higher pre-FEC error rate while the FEC mechanism corrects a defined portion of errors and reduces the resulting post-FEC error rate.

15. PAM4 Is Not the Same as FEC

PAM4 is a modulation format, while FEC is an error-correction technique.

They solve different problems. PAM4 increases the number of bits carried per symbol, while FEC adds redundancy to detect and correct transmission errors.

16. NRZ and FEC

NRZ systems can also use FEC, especially at higher data rates and in applications where link performance requirements are demanding.

Therefore, FEC should not be described as a feature that belongs exclusively to PAM4. The use of FEC depends on the transmission standard and system architecture.

17. Bandwidth Efficiency

PAM4 provides higher bit efficiency per symbol because it carries two bits per symbol.

This allows a system to double the binary data rate without doubling the baud rate. That characteristic is one of the main reasons PAM4 became important as network data rates moved beyond the practical range of traditional NRZ implementations.

18. Electrical Channel Requirements

Although PAM4 reduces the required baud rate for a given bit rate, it does not eliminate high-speed electrical channel challenges.

PCB loss, connector loss, crosstalk, return loss, impedance discontinuities, package parasitics, and reflections remain critical design parameters.

19. Equalization

Equalization is commonly used in high-speed PAM4 systems to compensate for channel distortion.

Transmitter equalization, receiver equalization, and other signal-conditioning techniques can help restore signal integrity after transmission through a lossy electrical channel.

20. PAM4 and DSP

Many high-speed PAM4 optical transceivers use Digital Signal Processing (DSP) to manage electrical and optical signal impairments.

A DSP can perform functions such as equalization, clock recovery, signal conditioning, monitoring, and other processing required by the specific optical architecture.

21. NRZ Implementation Complexity

NRZ generally requires fewer signal levels and simpler decision logic.

As a result, its transmitter and receiver architecture can be simpler than a comparable PAM4 system, especially in lower-speed applications.

22. PAM4 Implementation Complexity

PAM4 requires accurate generation and detection of four signal levels.

The transmitter must maintain appropriate level spacing, while the receiver must distinguish multiple thresholds in the presence of noise, distortion, and timing errors. These requirements increase implementation complexity.

23. Timing Margin

PAM4 introduces not only vertical decision challenges but also tight timing requirements.

Because the signal must be correctly sampled at multiple amplitude levels, jitter and timing distortion can reduce the available eye opening and degrade overall link performance.

24. Optical PAM4

PAM4 can be implemented in optical communication by using an optical transmitter and receiver architecture capable of preserving the four-level intensity signal.

In modern data center optics, PAM4 is commonly associated with intensity-modulation and direct-detection architectures.

25. NRZ Optical Transmission

NRZ has been widely used in optical transceivers for many generations of Ethernet and data communications.

Its two-level structure made it practical for applications such as 10G and 25G optical links, as well as earlier generations of higher-speed parallel optical systems.

26. 100G NRZ

100G optical systems can be implemented using multiple NRZ lanes.

For example, an architecture using four 25G-class NRZ lanes can achieve an aggregate 100G electrical or optical data rate, subject to the applicable coding and overhead.

27. 100G PAM4

100G can also be implemented using PAM4 technology in some architectures.

Instead of using four approximately 25G NRZ lanes, a 100G implementation can use fewer higher-speed PAM4 lanes because each PAM4 symbol carries two bits.

28. 200G PAM4

PAM4 became increasingly important with 200G networking because a small number of higher-speed PAM4 lanes can provide high aggregate bandwidth.

This reduces the need to continually increase the number of physical lanes as network speeds scale.

29. 400G PAM4

400G optical networking heavily relies on PAM4-based electrical signaling.

Different 400G optical architectures can use four or eight high-speed electrical lanes depending on the host interface and optical implementation.

This provides the bandwidth needed for high-density data center and AI networking.

30. 800G PAM4

800G systems further increase lane speeds while continuing to use PAM4 technology.

112G-class electrical signaling became an important building block for 800G systems, with eight approximately 100Gb/s-class lanes providing an aggregate 800G host interface.

31. 1.6T PAM4

1.6T systems extend the concept further through 224G-class electrical signaling.

Eight approximately 200Gb/s-class electrical lanes can provide an aggregate 1.6Tb/s host interface, making PAM4 a key technology for scaling bandwidth density.

32. Why NRZ Becomes Difficult at Higher Speeds

As the target bit rate increases, maintaining NRZ transmission requires increasingly high symbol rates.

Very high baud rates place greater demands on PCB materials, connectors, packages, transmitters, receivers, and optical components. PAM4 provides a way to increase bit rate without requiring the same proportional increase in symbol rate.

33. PAM4 and Optical Power

PAM4 optical transmission requires careful management of the four optical levels.

The transmitter must maintain suitable level spacing while staying within the allowed average power, peak power, extinction, linearity, and other optical requirements. This is more demanding than a simple binary optical waveform.

34. PAM4 Linearity

Linearity becomes more important in PAM4 because the receiver must distinguish four amplitude levels rather than two.

Nonlinear distortion can move the signal levels closer together or make adjacent levels overlap, reducing the effective eye opening.

35. NRZ vs PAM4 Receiver Design

An NRZ receiver normally distinguishes between two principal signal levels.

A PAM4 receiver must distinguish among four levels and three decision boundaries. This increases requirements on the analog front end, equalization, clock recovery, and signal processing.

36. PAM4 and TDECQ

TDECQ, or Transmitter and Dispersion Eye Closure for PAM4, is an important measurement used for PAM4 optical transmitter performance.

It evaluates the effect of transmitter impairments and channel characteristics on the PAM4 eye and provides information that is more appropriate for multi-level signaling than traditional NRZ eye measurements alone.

37. PAM4 vs NRZ Reach

Modulation format alone does not determine optical transmission distance.

Reach depends on transmitter power, receiver sensitivity, optical loss, dispersion, fiber type, wavelength, optical components, FEC, and the complete link architecture.

Therefore, it is not correct to state that PAM4 is always shorter reach or that NRZ is always longer reach.

38. PAM4 vs NRZ Power Consumption

PAM4 can help reduce the number of electrical and optical lanes required for a given aggregate bandwidth, but the required DSP and signal-processing functions can increase power consumption.

NRZ can have simpler signal processing, but achieving the same aggregate bandwidth may require more parallel lanes.

39. PAM4 vs NRZ Cost

NRZ components and architectures can be simpler at lower speeds, while PAM4 requires more sophisticated components and signal processing.

At higher data rates, however, PAM4 can reduce the number of lanes and associated physical components, which can influence the total system cost.

40. PAM4 vs NRZ in Data Center Networks

41. PAM4 vs NRZ for AI Data Centers

AI and high-performance computing networks require very high aggregate bandwidth between switches, GPUs, NICs, and other processing equipment.

PAM4 allows higher data rates per electrical lane, which helps increase switch port bandwidth while limiting the number of physical lanes that would otherwise be required with binary signaling.

42. Choosing PAM4 or NRZ

The modulation format should be selected according to the target data rate, electrical channel capability, optical architecture, power budget, FEC requirements, transmission distance, and system cost.

NRZ remains practical for many established lower-speed applications, while PAM4 is increasingly important for high-speed interfaces where bandwidth density is a primary requirement.

43. PAM4 vs NRZ: Core Comparison

44. Conclusion

PAM4 and NRZ use fundamentally different signaling approaches. NRZ uses two signal levels and carries one bit per symbol, providing a relatively simple and robust signaling method. PAM4 uses four signal levels and carries two bits per symbol, allowing higher data rates without requiring the same increase in symbol rate.

The advantage of PAM4 comes with additional challenges. Smaller signal-level spacing makes PAM4 more sensitive to noise, distortion, jitter, and nonlinear effects, increasing the importance of equalization, signal integrity, optical performance, and FEC.

NRZ remains practical for many lower-speed and established optical applications, while PAM4 has become a key signaling technology for 200G, 400G, 800G, and emerging 1.6T data center networks.

45.PAM4 vs NRZ Q&A