Introduction to PAM4 Technology

Before understanding PAM4 technology, what is modulation technology? Modulation technology is the technique of converting baseband signals (raw electrical signals) into transmission signals. In order to ensure communication effectiveness and overcome problems in long-distance signal transmission, it is necessary to transfer the signal spectrum to a high-frequency channel through modulation for transmission.

PAM4 is a fourth order pulse amplitude modulation (PAM) modulation technique.

PAM signal is a popular signal transmission technology after NRZ (Non Return to Zero).

The NRZ signal uses two signal levels, high and low, to represent the 1 and 0 of the digital logic signal, and can transmit 1 bit of logic information per clock cycle.

PAM4 signal uses 4 different signal levels for signal transmission, and each clock cycle can transmit 2 bits of logic information, namely 00, 01, 10, and 11.
Therefore, under the same baud rate conditions, the bit rate of PAM4 signal is twice that of NRZ signal, which doubles the transmission efficiency and effectively reduces costs.

PAM4 technology has been widely used in the field of high-speed signal interconnection. At present, there are 400G optical transceiver module based on PAM4 modulation technology for data center and 50G optical transceiver module based on PAM4 modulation technology for 5G interconnection network.

The implementation process of the 400G DML optical transceiver module based on PAM4 modulation is as follows: when transmitting unit signals, the received 16 channels of 25G NRZ electrical signals are input from the electrical interface unit, preprocessed by the DSP processor, PAM4 modulated, and output 8 channels of 25G PAM4 electrical signals, which are loaded onto the driver chip. The high-speed electrical signals are converted into 8 channels of 50Gbps high-speed optical signals through 8 channels of lasers, combined by a wavelength division multiplexer, and synthesized into 1 channel of 400G high-speed optical signal output. When receiving unit signals, the received 1-channel 400G high-speed optical signal is input through the optical interface unit, converted into 8-channel 50Gbps high-speed optical signal through a demultiplexer, received by an optical receiver, and converted into an electrical signal. After clock recovery, amplification, equalization, and PAM4 demodulation by a DSP processing chip, the electrical signal is converted into 16 channels of 25G NRZ electrical signal.

Apply PAM4 modulation technology to 400Gb/s optical modules. The 400Gb/s optical module based on PAM4 modulation can reduce the number of required lasers at the transmitting end and correspondingly reduce the number of required receivers at the receiving end due to the use of higher-order modulation techniques compared to NRZ. PAM4 modulation reduces the number of optical components in the optical module, which can bring advantages such as lower assembly costs, reduced power consumption, and smaller packaging size.

There is a demand for 50Gbit/s optical modules in 5G transmission and backhaul networks, and a solution based on 25G optical devices and supplemented by PAM4 pulse amplitude modulation format is adopted to achieve low-cost and high bandwidth requirements.

When describing PAM-4 signals, it is important to pay attention to the difference between baud rate and bit rate. For traditional NRZ signals, since one symbol transmits one bit of data, the bit rate and baud rate are the same. For example, in 100G Ethernet, using four 25.78125GBaud signals for transmission, the bit rate on each signal is also 25.78125Gbps, and the four signals achieve 100Gbps signal transmission; For PAM-4 signals, since one symbol transmits 2 bits of data, the bit rate that can be transmitted is twice the baud rate. For example, using 4 channels of 26.5625GBaud signals for transmission in 200G Ethernet, the bit rate on each channel is 53.125Gbps, and 4 channels of signals can achieve 200Gbps signal transmission. For 400G Ethernet, it can be achieved with 8 channels of 26.5625GBaud signals.