PAM4

What is PAM4?

Pulse Amplitude Modulation 4-level (PAM4) is a digital modulation technology that takes four different signal levels (symbols) for signal transmission. Each symbol can encode two bits of data, that is, 00, 01, 10, and 11. Compared to NRZ, also known as the two-level binary modulation format (which uses two signal levels with each carrying only one bit of logical information - 0 or 1), PAM4 modulation doubles the signal levels. This feature led to the bit rate of PAM4 being twice that of NRZ under the same baud rate. This difference is reflected in the performance of doubling the transmission efficiency.

Eye pattern and bit pattern of NRZ vs PAM4

Why Do We Need PAM4?

In the big data world, higher bandwidth is always a continuing trend. NRZ has low signal transmission efficiency and drawbacks in meeting the ever-increasing high-speed data networks, which is where the PAM4 comes in. 

One way to enhance the transmission rate is to increase the number of signal transmission channels; the other is to improve the single-channel rate. The latter is a more cost-effective solution since the former method requires extra construction expense. 

PAM4 boosts single-channel efficiency by increasing signal levels and doubling network interface rates without adding channels or changing optical devices. Upgrading the optical module's internal chips enables this, and high-end equipment (e.g., X86 chips, SerDes interfaces) can handle PAM4 signals.

The IEEE 802.3bj standard 2014 defines 100G backplane signal modes: NRZ at 4 x 25.78 Gbit/s and PAM4 at 4 x 13.6 Gbit/s. While NRZ quickly became commercially viable, PAM4 was initially limited by technology and cost; however, with increasing data rate requirements for 200G/400G interfaces, PAM4 now stands as the optimal solution.

What Are the Pros and Cons of PAM4?

Pros:

• Double Transmission Efficiency: PAM4 coding transmits two bits per symbol, so for the same baud rate, PAM4 achieves double the throughput of NRZ. For example, 28 GBaud PAM4 equals 56 Gbps, while 28 GBaud NRZ equals 28 Gbps.

• Save System Cost: The PAM4 multi-level signal has twice the bit rate of NRZ in the same symbol period, which reduces the required optical components and saves the cost per bit.

Cons:

• Vulnerable to Extral Interfaces: From the eye diagrams of signals, we can see the PAM4 signal has smaller eyes (the eye height of PAM4 is about ⅓ that of NRZ), which means there is narrower space between different voltage levels of PAM4 signals and causes PAM4 is susceptible to interference from the environment, especially noise. This is why transceivers using PAM4 modulation have higher Bit Error Rate (BER) and Signal-to-noise Ratio (SNR).

• Transmission Limitation: The PAM4 signal is vulnerable to noise and other interference factors. Therefore, the signal quality is insatiable over long-distance transmission.

• Require Signal Correctness Devices: PAM4 signal over long-distance transmission requires extra devices to maintain the signal quality, such as amplifiers and FEC to help enhance the single and radiators for better heat dissipation. 

• Higher Power Consumption: The enhanced auxiliary equipment required for the long-distance transmission of PAM4 signals results in higher power consumption, which is higher overall than that of NRZ signals.

What are the PAM4 Applications?

PAM4 is widely used in high-speed data transmission, particularly in data centers and telecommunications. Its main applications include:

Single-Lambda Approach

The single-lambda approach uses the four-level pulse amplitude modulation, which transmits twice the data of the previous NRZ method without needing faster optical components. This means existing optical technology capable of 50G with NRZ can handle 100G with PAM4. Some of the typical single-channel optical modules that use PAM4 are 100GBASE-FR QSFP28,  100GBASE-DR QSFP28, 100GBASE-LR QSFP28, 100GBASE-ERL QSFP28, 100G DWDM QSFP28.

100G/200G/400G/800G Ethernet

PAM4 enhances the transmission rates of optical fibers in data centers, enabling speeds of 100G, 200G, 400G, and 800G beyond by modulating 1x 100G PAM-4, 4x 50G PAM-4, 4x 100G PAM-4 or 8x 50G PAM-4, 8X 100G PAM-4.

Data Center Interconnect (DCI)

DCI efficiency depends on data transfer efficiency between servers, storage systems, and network devices. High-performance optical modules using PAM4 coding improve data transmission, converting NRZ to PAM4 signals. This increases data capacity, optimizes space, and supports the first three OSI layers, enhancing cross-regional operations, user access, and disaster recovery.

5G Mobile Transport Network

5G needs more bandwidth to address the demands for services like eMBB, URLLC, and mMTC. It has 3-5 times better spectrum efficiency than 4G and starts at 100MHz, five times more than early 4G. Sub-6GHz bandwidth is 15-25 times greater, and high-frequency 5G can exceed 800MHz. NGMN estimates 5G transport will move to 50GE/200GE in Sub-6GHz and 100GE/200GE/400GE in high-frequency. PAM4 technology will help support these high-bandwidth demands.

Metropolitan Networks

Metro networks link citywide data, voice, and video services. PAM4 expands core and aggregation interfaces, enhancing network efficiency and user experience. Due to HD, 4K, 8K, and VR/AR advancements, current 10GE and 40GE interfaces will upgrade to 50GE, 200GE, and 400GE.