Non-Return-to-Zero (NRZ)

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

Non-return-to-zero (NRZ) is a binary modulation method used in digital signal transmission, commonly applied in optical modules. In NRZ, data is represented using two distinct signal levels — 0 and 1. Each level corresponds to one bit of information, with the signal levels being transmitted via specific direct current (DC) voltages. The amplitude of the signal remains constant for each bit. In positive-logic NRZ, the signal level for 0 corresponds to a lower voltage, while the signal for 1 corresponds to a higher voltage.

How NRZ Compares to PAM4

Bit Rate

NRZ and PAM4 differ significantly in how they encode information. NRZ encodes one bit per symbol, while PAM4 encodes two bits per symbol. Therefore, for a given baud rate, the bit rate for PAM4 is twice that of NRZ. For example, a 50Gbaud NRZ signal results in a 50Gbps data rate, whereas a 50Gbaud PAM4 signal translates to 100Gbps. This is why high-speed 100G modules, like the QSFP-100G-DR-S and QSFP-100G-FR-S, use PAM4 to convert four 25Gbps NRZ signals into one 50Gbaud PAM4 signal.

Signal-to-Noise Ratio (SNR) & Bit Error Rate (BER)

PAM4’s signal amplitude is one-third that of NRZ, making it more susceptible to noise. This results in a lower eye height in PAM4, which increases the SNR degradation by about -9.54 dB and raises the BER. As a result, PAM4 transceivers require advanced equalization techniques to reduce noise and minimize errors, leading to increased power consumption and more complex design compared to NRZ.

Signal Loss

PAM4 offers higher transmission efficiency by encoding twice the data in each symbol period compared to NRZ. While the baud rate in PAM4 is half that of NRZ for the same bit rate, its increased transmission efficiency reduces signal loss over the transmission medium. This allows PAM4 to achieve higher data rates without increasing the baud rate and exacerbating channel signal loss.

Power Consumption

To improve BER, PAM4 signaling requires both equalization at the receiver (RX) end and re-compensation at the transmitter (TX) end, which results in higher power consumption compared to NRZ. However, innovations in silicon photonics (SiPh) technology have made it possible to mitigate these power challenges in PAM4-based optical modules.

Advantages of NRZ

NRZ offers several advantages over PAM4, including better noise tolerance, higher error correction capability, and lower power consumption. Its simpler signal quality also means NRZ optical designs are less complex, with no need for advanced signal processing components like clock recovery circuits. Additionally, NRZ modulators typically cost less than their PAM4 counterparts, making them a more cost-effective choice for certain applications.