NRZ and PAM4: Explore the Difference

NRZ and PAM4 are crucial coding and modulation technologies in modern communications and data transmission. They are widely used in digital signal transmission. The rise of trends such as cloud computing and big data has driven the exponential growth of traffic and the rise of 400G Ethernet. This article will give you a detailed understanding of the two modulation technologies, NRZ and PAM4, and compare them to let you know which is better for the growing rate requirements.

Definition of NRZ and PAM4

What is NRZ

NRZ is a traditional digital signal encoding method, mainly used for binary data transmission. It uses two signal levels to represent 0 and 1 of digital signals. Logic 0 is a negative voltage and logic 1 is a positive voltage. One bit of logical information can be sent or received in each clock cycle. The absence of a "return to zero" state is a significant feature of NRZ, which means that between consecutive 1 and 0, the signal will not return to the zero level, which allows NRZ to perform efficient data transmission at lower frequencies.

What is PAM4

PAM4 is an emerging modulation technology that uses four different levels to represent data. It can transmit two bits of binary data in the same symbol period, i.e. 00, 01, 10, or 11. Therefore, under the same bandwidth conditions, the transmission rate of PAM4 is twice that of NRZ. This also makes it popular in high-speed applications, such as data centers, cloud computing, etc. It is more popular because of its high efficiency and good bandwidth utilization.

Comparing NRZ and PAM4

Bit Rate

As we all know, one symbol can carry one bit. NRZ can only transmit one bit of binary data in the same symbol period, while PAM4 can transmit two bits. So under the same bandwidth conditions, when NRZ can transmit 28Gbps, PAM4 can reach 56Gbps. The bit rate of 28Gbps is equivalent to the baud rate of 28GBdps. Since each PAM4 symbol can carry 2 bits, the line transmission speed of 56Gbps PAM4 is 28GBdps. So under the same baud rate, PAM4 increases the bit rate to twice that of NRZ, which can bring higher transmission rates. It can use fewer channels to implement a 400Gbps Ethernet interface, while NRZ requires more channels. PAM4 can use 8 channels at 50Gbps or 4 channels at 100Gbps. NRZ requires 16 or 8 channels under the same bandwidth.

Signal transmission integrity

Since PAM4 uses four different levels to transmit information, while NRZ only uses two, PAM4 is more sensitive to noise and signal distortion and is more susceptible to interference. Although it can transmit more data in the same bandwidth, it has higher requirements for signal integrity. At the same time, as the distance increases, signal attenuation and distortion will increase, resulting in a higher loss rate. NRZ, on the other hand, uses only two signal levels to transmit information, which makes it more resistant to interference and can be transmitted over longer distances. QT's 400G OSFP module has an FEC function, which helps to ensure the integrity of signal transmission and can support longer distances.

Signal-to-noise ratio and bit error rate

As shown in the figure below, the eye height of NRZ is only one-third of that of PAM4, which only means that PAM4 increases the signal-to-noise ratio by -9.54 dB (link budget loss). This has a significant impact on signal quality and will introduce additional restrictions in high-speed signal transmission. Since the vertical eye opening is reduced by 33%, this will also make the PAM4 signal more sensitive to the noise environment, resulting in a higher bit error rate. However, since PAM4 has FEC, it can help the link system achieve the required BER, which makes the application of PAM4 possible.

Complexity

NRZ encoding and decoding are simpler than PAM4, making it suitable for most traditional communication systems. PAM4 involves more levels, and its encoding and decoding process is more complicated, it requires higher-performance hardware support. Therefore, its technical and deployment complexity is higher than NRZ, and its deployment cost will also be higher.

Power consumption

Although the baud rate of PAM4 is half of that of NRZ at the same bit rate, since PAM4 uses 4 levels for information transmission and needs to use FEC to control BER within a certain range, it has higher power consumption, which means that PAM4 transceivers will generate higher heat and bring more operation and maintenance costs. However, QT's 400G transceiver uses Broadcom and Macom's DSP chips, which have better processes and designs and can effectively reduce overall power consumption, while ensuring the efficiency and reliability of high-speed data transmission to the greatest extent.

Applications

NRZ

In 25G Ethernet, the bit rate of NRZ is 25Gbps, which can usually be used for 100G to 4x 25G applications. This not only enables better traffic management and load balancing, but also helps optimize network performance, reduce bottlenecks, and improve overall efficiency.

PAM4

PAM4 can achieve 400G to 4x 100G applications and can support high-speed connections between a large number of servers and storage devices to meet the needs of big data processing and cloud computing. It is designed to improve the efficiency and spectrum efficiency of data transmission to support the implementation of high-speed applications. This can also help the network achieve higher reliability and fault tolerance through redundant connections and load sharing to avoid network interruptions.

Conclusion

NRZ and PAM4 are two different modulation methods in optical modules. They play their advantages in different applications. Although NRZ has lower transmission efficiency than PAM4, its transmission is more stable and more suitable for long-distance transmission. PAM4 supports higher transmission rates in short distances. If you have any questions about NRZ and PAM4, please feel free to contact QSFPTEK's CCIE/HCIE engineers at [email protected].