Non-Return-to-Zero (NRZ)

What is Non-Return-to-Zero (NRZ)?

Non-return-to-zero (NRZ) is a binary digital signal modulation method applied in optical modules. NRZ utilizes two different signal levels — represented by 0 and 1. Each of the signal levels represents one bit of logical information. The binary signal levels are transmitted by specific direct current (DC) voltages, and the amplitude of the carrier signal is constant during each bit of time. In the common positive-logic NRZ, the lower state signal level (0) corresponds to a more negative voltage, and the higher state signal level (1) corresponds to a more positive voltage.

What Are the Differences Between NRZ and PAM4?

Bit Rate

Since each symbol of NRZ coding contains only one bit of logical information while each of the PAM4 symbols represents two bits, the bit rate of NRZ equals the baud rate of NRZ and the bit of PAM4 is the doubled number of the baud rate of PAM4. That is:

50Gbaud NRZ = 50Gbps, 50Gbaud PAM4 = 100Gbps.

One example of the application is the high-speed 100G single Lambda module, such as QSFP-100G-DR-S and QSFP-100G-FR-S, which can convert 4x25Gbps NRZ signals to 1x50Gbaud PAM4l signal.

SNR (Signal-to-Noise) and Bit Error Rate (BER)

In terms of Power Spectrum Density, PAM4 generates three distinct eye patterns, whereas NRZ features a single large eye. The eye diagram of PAM4 reveals its inherent limitations regarding signal-to-noise ratio (SNR) and Bit Error Rate (BER). Due to the signal amplitude (eye height) of PAM4 being only one-third that of NRZ, the voltage levels are closer together, making it more vulnerable to noise interference. This increased the SNR of PAM4 by -9.54 dB and higher BER.

Advanced equalization techniques are needed in PAM4 transceivers to mitigate noise issues. This added complexity makes the design of PAM4 optics more intricate and increases power consumption compared to other solutions.

Signal Loss

PAM4 enables the transmission of data twice as much as NRZ in each symbol period. Hence, the baud rate is only half that of NRZ at the same bit rate. The doubled transmission efficiency in PAM4 signaling significantly decreases signal loss caused by transmission channels. The advantage of PAM4 is that it allows for a higher data rate using the existing channels while not increasing the baud rate and channel signal loss.

Power Consumption

To lower BER in PAM4 signaling, equalization in the RX end and re-compensation in the TX end are required, both of which are more power-consuming than NRZ. However, cutting-edge state-of-the-art silicon photonics (SiPh) technology applied to silicon photonic optical modules can solve this problem.

What are the Advantages of NRZ?

The main pros of NRZ compared to PAM4 are better tolerance to noise, higher error correction capability, and lower power consumption. Besides, since the signal quality of NRZ is relatively better than PAM4, the mechanical design of NRZ optics is simple, not requiring complex signal processing units such as not requiring a Clock Recovery circuit. The cost of an NRZ modulized device is often cheaper than that of a PAM4.