This article comes from the WeChat public account: fresh date class (ID: xzclasscom) , author: Jun jujube, title figure from: vision China
Hello everyone, this is Xiaozaojun. Today we come to talk about baseband and radio frequency.
Speaking of baseband and radio frequency, I believe everyone is familiar with it. They are two common concepts in the communications industry and often appear in front of us.
However, the more common the concept, the more confusing the online information and the more errors. These mistakes have caused trouble for many beginners, and even formed a long-term misunderstanding. Therefore, I think it is necessary to write an article to give a basic introduction to baseband and radio frequency.
“End-to-end” is now popular. Let’s take a mobile phone call as an example and observe the entire process of the signal from the mobile phone to the base station to see what the baseband and RF are used for.
When the phone is connected, the voice of the person will be picked up by the microphone of the phone and become an electrical signal. This electrical signal is an analog signal, and we can also call it the original signal.
Sound waves (mechanical waves) are converted into electrical signals
At this time, our first protagonist, baseband, began to debut.
Baseband, English called Baseband, basic frequency band.
The basic frequency band refers to a special frequency bandwidth, that is, the frequency range is near the zero frequency (from DC to several hundred KHz) Segment bandwidth. For signals in this frequency band, we become baseband signals. The baseband signal is the most “basic” signal.
Baseband we often refer to in real life, more refers to the baseband chip, circuit of the mobile phone, or the baseband processing unit of the base station (that is, we often Said BBU) .
Return to the voice analog signal we just mentioned.
These signals will be sampled, quantized, and encoded by the AD digital-to-analog conversion circuit in the baseband to become digital signals. The specific process is as follows:
The coding in the picture above is called source coding.
Source coding, to put it plainly, is to change the sound and picture into 0 and 1. In the conversion process, the source code also needs to be compressed as much as possible in order to reduce the “volume”.
For audio signals, we commonly use PCM coding (Pulse code modulation, aboveYes) and MP3 encoding etc. In the mobile communication system, taking 3G WCDMA as an example, AMR speech coding is used.
For video signals, MPEG-4 encoding (MP4) is commonly used, as well as H.264 and H.265 encoding. Everyone should be familiar with it.
In addition to source coding, channel coding is also required for baseband.
Code is divided into source code and channel code
Channel coding is completely different from source coding. Source coding is to reduce “volume”. The opposite of channel coding is to increase the “volume”.
Channel coding improves link performance by adding redundant information to combat interference and attenuation in the channel.
For example, channel coding is like stuffing protective foam around the cargo. If bumps or collisions occur on the road, the probability of damage to the cargo will be reduced.
The Turbo code, Polar code, LDPC code, and the more famous convolutional code mentioned in the Lenovo voting event last year are all channel coding.
In addition to encoding, the baseband also encrypts the signal.
The next job is still the baseband, that is modulation.
Modulation, in simple terms, is to make “waves” better represent 0 and 1.
The most basic modulation method is frequency modulation (FM) , amplitude modulation (AM) , phase modulation (PM) . As shown in the figure below, different waveforms are used, representing 0 and 1.
Modern digital communication technology is very developed. Based on the above, a variety of modulation methods have been developed. For example, amplitude shift keying (ASK) , frequency shift keying (FSK) , phase shift keying (PSK) , and quadrature amplitude modulation, which is the famous QAM (pronounced” quam “) .
In order to express various modulation methods intuitively, we will use a tool called constellation diagram. Points in the constellation diagram can indicate the possible states of the amplitude and phase of the modulated signal.
constellation diagram
16QAM schematic diagram (1 symbol represents 4 bits)
After modulation, the amount of information that a single symbol can carry is greatly increased. 256QAM, which is commonly used in 5G, can use one symbol to represent 8-bit data.
256QAM
Okay, the baseband work is finally done. What should I do next?
It’s RF turn.
Radio frequency, the English name is Radio Frequency, which is the familiar RF. Literally speaking, Radio Frequency means radio frequency. Strictly speaking, radio frequency refers to high-frequency electromagnetic waves in the frequency range of 300KHz ~ 300GHz.
Everyone knows that when a current passes through a conductor, a magnetic field is formed. When an alternating current passes through a conductor, an electromagnetic field is formed, which generates electromagnetic waves.
Electromagnetic waves with a frequency lower than 100kHz will be absorbed by the ground and cannot form an effective transmission. Electromagnetic waves with a frequency higher than 100kHz can be propagated in the air and reflected by the ionosphere at the outer edge of the atmosphere, forming a long-distance transmission capability.
This high-frequency electromagnetic wave with long-distance transmission capability is called RF (signal) .
Like baseband, we usually refer to a series of things that produce radio frequency signals, such as radio frequency circuits, radio frequency chips, radio frequency modules, radio frequency components, etc., generally referred to as radio frequency.
So, we often hear people say: “XX mobile phone’s baseband is bad”, “XX company can’t make baseband”, “XX equipment’s RF performance is very good”, “XX’s RF is very expensive” … Or something.
The frequency of the signal sent from the baseband is very low. The radio frequency thing to do is to continue to modulate the signal from low frequency to the specified high frequency band. For example, 900MHz GSM band, 1.9GHz 4G LTE band, 3.5GHz 5G band.
The role of radio frequency, just like dispatcher
The reason why RF radio frequency does this kind of modulation is, on the one hand, as mentioned earlier, the baseband signal is not conducive to long-distance transmission.
On the other hand, wireless spectrum resources are tight, and low-frequency bands are generally occupied by other uses. The high frequency band resources are relatively abundant, and it is easier to achieve large bandwidth.
No matter what, you must also modulate to the specified frequency band, otherwise it will be illegal to interfere with others.
In terms of engineering implementation, low frequencies are not suitable.
According to the antenna theory, when the length of the antenna is 1/4 of the wavelength of the radio signal, the transmission and reception conversion efficiency of the antenna is the highest. The wavelength and frequency of electromagnetic waves are inversely proportional to (speed of light = wavelength × frequency) , if low-frequency signals are used, the size of mobile phone and base station antennas will be larger Increase the difficulty of engineering realizationdegree. Especially on the mobile phone side, large antenna sizes cannot be tolerated and will occupy valuable space.
After the signal is modulated by RF, the power is small, so it needs to be amplified by the power amplifier to get enough RF power before it is sent to the antenna.
After the signal reaches the antenna, it is filtered by the filter (to eliminate interference clutter) , and finally transmitted through the antenna element.
Propagation of electromagnetic waves
After the base station antenna receives the wireless signal, it takes the reverse process of the previous process—filtering, amplifying, demodulating, and decoding. The processed data will be sent to the core network through the bearer network to complete the subsequent data transmission and processing.
The above is the general change process of the signal. Note that this is a rough process, but the actual process is still very complicated. There are still some intermediate frequencies that are not described in detail.
I draw a simple schematic diagram of the general process as follows:
How about, is it equivalent to revisiting our “Principles of Communication”? In fact, you will find that the reality and the contents of our books are still very different.