Introduction to NRZ and PAM4 modulation techniques

      ⦁ Background.
      Leading trends such as cloud computing and big data are driving exponential traffic growth, data center networks are facing greater bandwidth demands, and the infrastructure needs innovative technologies to meet the changing needs for next-generation Ethernet with two different signal modulation techniques: non-return-to-zero (NRZ) and 4-stage pulse-amplitude modulation (PAM4), and this article takes a look at and compares these two modulation techniques.
      ⦁ Introduction to NRZ and PAM4 Modulation Techniques
      NRZ is a modulation technique that uses two signal levels to represent the 1/0 information of a digital logic signal, where logic 0 is a low level and logic 1 is a high level, and 1bit of logic information can be sent or received in each clock cycle, as shown in the following figure:

Figure 1  NRZ signal and the corresponding waveforms

      PAM4 is a technique that uses four different signal levels for signaling, where each symbol period represents 2 bits of logic information and the waveform has 4 different levels carrying 2 bits: 00, 01, 10 or 11, as shown below:

Figure  2 PAM4 signals and corresponding waveforms

      ⦁ Comparison of NRZ and PAM4 techniques
      Comparing these two technologies, the NRZ signal uses two signal levels, high and low, to represent the 1 and 0 of the digital logic signal, and can transmit 1bit of logic information per cycle, while the signal of PAM4 uses four different signal levels for signal transmission, and can transmit 2bit of logic information per clock cycle, i.e., 00, 01, 10, and 11.
      Therefore, under the same baud rate conditions, the PAM4 signal bit rate is 2 times that of the NRZ signal, doubling the transmission efficiency, that is to say, PAM4 can increase the network bandwidth without adding new optical fiber on the basis of the effective enhancement of bandwidth utilization.
      Currently, 25G baud rate and 50G baud rate PAM4 are widely used in high-speed modules, and the following is a list of several:

Table 1 Application of PAM4 in 400G optical modules

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