Introduction
PAM4 is a four-level signal modulation technology, used for data transmission. Its high efficiency makes it widely used in optical fiber communication. With the increasing demands of data transmission, The technique will continue to play a vital role in the future. Thus, this article will provide an accessible introduction to PAM4, covering its concept, principle, analytical tool, advantages and challenges.
Concept
PAM4, or Pulse Amplitude Modulation 4-level, is an advanced modulation technique. Traditional NRZ (Non-Return-to-Zero) can be considered as PAM2, for each signal transmits one bit (0 or 1). However, PAM4 increases the level of signal variation, by using 4 distinct voltage levels, allowing each signal to transmit 2 bits of data.
Working Principle
If we have an electrical signal, the signal can have different voltage values at different times. PAM4 uses 4 distinct voltage levels to represent 4 different data combinations: 00, 01, 10, 11.
Taking the comparison between NRZ and PAM4 as an example, we can draw the conclusion from the figure below: when the two transmit the sequence of 1101011000101100, PAM4 takes only half the time compared to NRZ. Therefore, within the same signal cycle, compared with NRZ, PAM4 can carry twice the amount of data, significantly improving the efficiency of data transmission.
Eye Diagram: A Tool for Signal Quality Analysis
Below is the eye diagram for PAM4, where the four voltage levels form three “eyes”. The upper and lower edges represent the voltage fluctuations of the signal, while the horizontal axis typically represents time.
- The wider the “eye”opening, the better the signal quality and the lower the bit error rate (BER). Conversely, if the “eye” opening becomes narrower or even closed, it indicates a stronger signal interference and a higher BER.
- The width of the time period also reflects the precision of data synchronization. A larger width means that the receiver has more time to evaluate the signal. If the width is too narrow, the receiver can only sample the signal during a very short time period, leading to an inaccurate signal reception, then subsequently resulting in bit errors.
Why Choose PAM4
Higher Transmission Speed & Bandwidth Efficiency. PAM4 can transmit more data within the same bandwidth, crucial for high-speed networks and big data applications.
Lower Infrastructure Costs. PAM4 can significantly reduce the number of fiber channels required, lowering the Costs of Transmission Devices. Additionally, It transmits more data in one signal period, meaning that the number of optical modules and switching devices can be reduced, thereby decreasing the complexity and cost of the equipment.
Challenges
Signal Interference. PAM4 uses 4 voltage levels, with small signal differences, susceptible to interference, resulting in high BER and short transmission distance. To address this, It employs Forward Error Correction (FEC) and equalization techniques to reduce the BER and improve signal quality.
Signal-to-Noise Ration (SNR) Requirements. Due to the high SNR demands, PAM4 adopts adaptive equalization, advanced signal modulation and high-precision receiver designs, enhancing its SNR, while ensuring the stability and reliability of data transmission.
Conclusion
As an efficient modulation technique, PAM4 increases its data transmission speed and bandwidth efficiency through four voltage levels, especially in high-speed networks and big data applications. With the progress of hardware and signal processing technology, PAM4 will play a greater role in high-speed data transmission, offering more stable and efficient solutions.
