With the rapid increase in data transmission demand, to improve the transmission efficiency and rate, there are different modulation methods. Among these modulation methods, NRZ and PAM4 are the two most widely used coding methods. This article will discuss the NRZ and PAM4 modulation modes in optical modules, including their definitions and comparisons.
Definition of NRZ
NRZ (Non-Return to Zero) is a traditional digital signal encoding method. The binary “0” and “1” are represented by two different levels of voltage. In NRZ encoding, the time of each symbol remains constant, which means that the signal level stays high or low and does not return to zero. This makes it efficient to transmit more stable data with limited bandwidth.
Definition of PAM4
PAM4 (Pulse Amplitude Modulation with 4 levels) is an advanced modulation technique. It uses four different levels to represent data, PAM4 transmits two symbols per symbol period and uses four levels to represent “00”, “01”, “10” and “11” respectively. Compared to NRZ, PAM4 doubles the data carried in one symbol, thereby effectively increasing the data transfer rate.
With the same high data rate requirement, PAM4 is significantly more bandwidth efficient than NRZ, making it suitable for higher-capacity fiber optic transmission applications.
Comparison between NRZ and PAM4
Bandwidth Efficiency
PAM4 significantly improves the bandwidth efficiency of data transmission. Since PAM4 is capable of carrying two bits per symbol, while NRZ carries only one bit per symbol. At the same bandwidth, PAM4 typically delivers twice the transmission rate of NRZ, reducing the number of channels required for data transmission. Therefore, PAM4 becomes a more attractive choice in high-speed applications such as data centers and cloud computing.
Complexity
NRZ its encoding method does not require much level control, relatively simple. It is more suitable for lower-rate applications, such as traditional communication systems. In contrast, PAM4 involves four-level controls and a more complex modem design. Its coding method and decoding are more complex, requiring higher precision level detection and higher performance hardware support.
BER
PAM4 adopts multi-level modulation, which is more sensitive to noise and signal attenuation, so the BER is usually higher. NRZ, on the other hand, has simpler signals at lower rates and lower BER.
Applicable Scenarios
NRZ: It is used in lower-rate application scenarios such as 10G and 40G. It can realize better traffic management in shorter-distance transmission and occupies an important position in optical modules for low-speed communication.
PAM4: Used in 400G and above large data volume, high-speed transmission scenarios, supporting high-speed connection between devices.PAM4 is very suitable for data centers, cloud computing, and other high-speed network optical communication, transmitting more data while improving network reliability and fault tolerance.
Conclusion
Both NRZ and PAM4 modulation methods have their own advantages and limitations. The choice between the two depends on the actual application requirements, including factors such as transmission rate, distance, system complexity, and anti-interference capability.
