As a popular signal transmission technology for high-speed signal interconnection in Next-Generation Data Center (NGDC), PAM4 technology is widely used for electrical or optical signal transmission on 200G/400G interfaces. This article will take you through the full understanding of PAM4 technology.
What Is PAM4
PAM4, short for 4-Level Pulse Amplitude Modulation, is a kind of PAM technology that uses 4 different signal levels for signal transmission. Each symbol period can represent 2 bits of logic information (0, 1, 2, 3), that is, four levels per unit time.
When it comes to PAM4 signal transmission technology, it is necessary to mention NRZ (Non-Return-to-Zero). NRZ is the most traditional digital signal, that is, using high and low signal levels to represent the 1/0 information of the digital logic signal can transmit 1 bit of logic information per signal symbol period.
The figure below is a comparison of the typical PAM4 and NRZ signal waveforms and eye diagrams.
Compared to NRZ, PAM4 has four digital amplitude levels, each of which contains two information bits, and at the same baud rate, the throughput is twice that of NRZ.
Why PAM4?
The rapid development of the network era has brought about a higher demand for network transmission rates.
Generally, there are three ways to increase the optical communication transmission rate:
- Increase the modulation rate.
- Increase the number of WDM channels
- Increase the number of levels
PAM4 technology can effectively improve the bandwidth utilization efficiency. At the same time, PAM4 adopts high-order modulation format, which is an effective way to reduce the number of optical devices used, reduce the performance requirements of optical devices and performance, cost and work in different applications. A balance is reached between consumption and density.
The arrival of big data and cloud computing, the growth of traffic, the urgent need for a more complex modulation method, PAM4 is a more efficient modulation technology.
In the development of the new generation of 200G/400G interface standards, the general appeal is that the data rate on each pair of differential lines should be increased to more than 50Gbps. If NRZ technology is still used, the use of PAM4 technology is almost an inevitable trend because the time margin for transceiver chips and transmission links is more demanding because each symbol period is less than 20ps.
The Basis of PAM4
Generally, the technologies for implementing PAM4 are classified into two types, namely, a DSP-based digital DAC implementation method or a simulation-based combine method.
The mainstream analog mode is based on MSB & LSB combiner to implement PAM4 signal, and there are two NRZ signals for adding operation.
The mainstream digital approach is based on a high-speed DAC for fast output at 0/1/2/3 levels.
At the same time, PAM4 compared with NRZ, the challenges it faces in the design and testing process can not be ignored.
The Challenges of PAM4
- The PAM4 signal has 16 switching states, the middle eye is symmetrical, and the upper and lower eye patterns are asymmetrical. In an optical eye diagram, the linearity of the optical device is very poor, or the linearity of the driving of the laser is very poor. The upper and lower eye diagrams of the PAM4 are easily deformed, and the deformation is easily misjudged. The parameters of the test, such as jitter, eye width, eye height is not accurate.
- Although the PAM4 signal reduces the symbol rate of the signal, the channel loss of more than 10dB still makes the eye diagram of the receiving end completely closed. Therefore, for the PAM4 signal, the pre-emphasis of the transmitting end and the signal equalization of the receiving end are also important.
- Compared to NRZ, the Signal-to-Noise Ratio (SNR) of PAM4 is reduced by 9.6dB, so in most cases, the PAM4 system (without FEC) will not operate without errors: BER=ERFC(SQRT(SNR)).
- Digital PAM4 Products—DSP Solutions. The DSP can support a variety of complex and efficient modulation methods. The electric port has strong adaptability and good photoelectric performance.
- Analog PAM4 Products—Analog CDR Solutions with low power consumption and low cost.
In most PAM4 scenarios, there is no possibility that the system will run without errors. IEEE 802.3 specifies the pre-FEC BER indicator for the PAM4 system.
At present, the product of the DSP scheme is BER<1E-7 before the actual FEC. The current product of the simulation scheme is BER<1E-6 before the actual FEC. In fact, the requirement to ensure that no large block error occurs is prior to the BER parameter.
The Gigalight PAM4 Solutions
For the challenges above, Gigalight has a first-class R&D team in the industry and has overcome the signal-integrity-design challenges of PAM4. The Gigalight PAM4 series 200G/400G products include 200G QSFP56 SR4, 200G QSFP56 AOC, 200G QSFP56 FR4, 400G QSFP56-DD SR8, 400G QSFP56-DD AOC, etc. .
All of the PAM4 products from Gigalight can be divided into two categories from the solution:
Gigalight always adheres to the concept of innovation, innovative technology, and overcomes difficulties. It invests a lot of human resources and material resources in the research and development of next-generation data center products, and aims to provide effective solutions for the development of next-generation data centers.
Originally published at https://sunmorph.wixsite.com/website/blog/understanding-the-pam4-technology-for-ngdc-in-one-minute