【Comic Science】What is PAM4
Latest update time：2021-09-01 13:25
Reads：
Xiao Ming and Xiao Bai are classmates and also good friends.
Xiao Ming is very smart and has good grades, while Xiao Bai is more playful and has poor grades.
Mr. Wang, the head teacher, arranges exams every day. Xiao Ming always gets full marks, while Xiao Bai always gets zero marks.
So, Xiaobai asked Xiaoming:
In order to help his good friend, Xiao Ming thought hard and finally came up with a plan -
Mr. Wang’s test papers only test true or false questions, and the answers are either 1 (correct) or 0 (incorrect).
So, he made an agreement with Xiaobai: during the test, he would blow bubbles, and blowing big bubbles represented 1 (correct) and blowing small bubbles represented 0 (wrong).
Sure enough, after adopting this method, Xiao Ming could accurately pass the answers to Xiao Bai in every exam, which greatly improved Xiao Bai's test scores.
Proud Xiaobai
However, not long after, the course entered a new phase, and the difficulty increased significantly. Teacher Wang decided to change all the judgment questions in the test paper into
single-choice questions
, with a total of four options: A/B/C/D.
Now Xiaobai is panicking. There are four options. What should he do?
So, Xiao Ming and Xiao Bai had a new conversation:
Xiao Ming told Xiao Bai that there was no need to blow bubbles in the shape of letters, as that would easily be discovered by the teacher.
"After years of practice, I can now precisely control the size of the bubbles I blow," Xiao Ming said. "When I blow a large bubble, it's A; a medium bubble, it's B; a small bubble, it's C; and an extra-small bubble, it's D."
Sure enough, with the help of the new method, Xiao Ming can still accurately convey the answers to Xiao Bai and continue to help Xiao Bai get good grades.
Later, Xiao Ming was admitted to Peking University, and Xiao Bai entered a department store. They both had bright futures. . .
—— End of story ——
Okay, did everyone understand the story above?
First of all, I want to solemnly declare that cheating is wrong. Under no circumstances should you cheat. Please do not imitate me.
Then, let’s analyze the cheating techniques in the story.
In essence, the two cheating methods used by Xiao Ming and Xiao Bai are two very important basic technologies in the field of optical communications, namely NRZ and PAM4.
NRZ is the abbreviation of Non-Return-to-Zero, which literally means "non-return to zero", that is, non-return to zero coding.
A signal using NRZ encoding is a digital logic signal that uses high and low signal levels to represent transmitted information.
NRZ has unipolar non-return-to-zero code and bipolar non-return-to-zero code.
Unipolar non-return-to-zero code, "1" and "0" correspond to positive level and zero level, or negative level and zero level respectively.
Unipolar Non-Return-to-Zero
Bipolar non-return-to-zero code, "1" and "0" correspond to positive level and equivalent negative level respectively.
Bipolar Non-Return-to-Zero
The so-called "non-return to zero" does not mean that there is no "0", but that the signal does not need to return to zero level after each bit of data is transmitted. (Obviously, NRZ saves bandwidth compared to RZ.)
In optical module modulation, we use the power of the laser to control 0 and 1.
In simple terms, if the actual emitted light power is greater than a certain threshold value, it is 1. If it is less than a certain threshold value, it is 0.
Transmitting 011011 is like this:
NRZ modulation
Next, let’s take a look at Xiao Ming and Xiao Bai’s second cheating method, which is PAM4.
As bandwidth demand continues to increase, we need to find ways to increase the amount of logical information transmitted per unit time.
PAM4, which stands for 4-Level Pulse Amplitude Modulation, is a modulation technology that uses 4 different signal levels for signal transmission.
Still transmitting 011011, it becomes like this:
PAM4 modulation
In this way, the logical information represented by a single symbol period doubles from 1 bit in NRZ to 2 bits.
NRZ VS PAM4 (eye diagram on the right)
So the question is, why hasn't such a cool technology been used before? If 4-level can be doubled, why don't we have 8-level, 16-level, 32-level? Wouldn't it be great to double the speed?
The main reason is still the technical process of lasers (
Xiao Ming's eloquence
). To achieve PAM4, the laser needs to be able to accurately control the power.
4 levels, 4 sizes of bubbles, Xiao Ming must control it and not make mistakes, and
Xiao Bai must also see it clearly and not make mistakes.
If the sound is louder and louder, is it B or C?
If it is not well controlled, it will cause a high bit error rate and the signal can only be re-blown, affecting the signal transmission efficiency.
PAM4 is more sensitive to noise. If the noise is too loud, PAM4 modulation will obviously not work properly.
With the transmission of optical fiber communication, the speed of blowing bubbles is very fast.
Let's take a single-channel 25G baud rate as an example. The so-called baud rate refers to how many complete pulses can be sent in one second.
For example, a 25G EML chip sends about
25
×
10
9
pulses per second (25 billion bubbles per second). If NRZ is used, the bit rate is 25Gbps. If PAM4 modulation technology is used, the bit rate is doubled to 50Gbps. Therefore, after a 25G EML chip is modulated with PAM4, it can be made into a single-channel 50G PAM4 optical module.
Nowadays, many large-capacity optical modules are
doubled
in this way
.
Let’s take another example of a 400G DML optical transceiver module based on PAM4 modulation.
When sending unit signals, 16 25G NRZ electrical signals are input from the electrical interface unit. After pre-processing and PAM4 modulation by the DSP processor, 8 25G PAM4 electrical signals are output and loaded onto the driver chip. The high-speed electrical signals are converted into 8 50Gbps high-speed optical signals through 8 lasers. After being combined through the wavelength division multiplexer, one 400G high-speed optical signal is synthesized for output.
When receiving the unit signal, the received 400G high-speed optical signal is input through the optical interface unit, converted into 8 50Gbps high-speed optical signals through the demultiplexer, and the input optical signal is received by the optical receiver. After the received optical signal is converted into an electrical signal, the electrical signal is clock recovered, amplified, equalized, and PAM4 demodulated by the DSP processing chip, and then converted into 16 25G NRZ electrical signals.
Well, the above is a simple introduction to NRZ and PAM4. Do you understand?
Featured Posts
京公网安备 11010802033920号