How are "Baseband" and "Radio Frequency" related? - Understanding

btty038

How are "Baseband" and "Radio Frequency" related? - Understanding [Copy link]

Speaking of baseband and radio frequency, I believe everyone is familiar with them. 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 information on the Internet is, and the more errors there are. These errors have caused trouble for many beginners and even formed long-term misconceptions.

Therefore, I think it is necessary to write an article to give a basic introduction to baseband and RF.

"End-to-end" is popular nowadays. Let's take mobile phone calls 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 radio frequency are used for.

When a mobile phone call is connected, the person's voice will be picked up by the mobile phone microphone and converted into an electrical signal. This electrical signal is an analog signal, which we can also call the original signal.

Converting sound waves (mechanical waves) into electrical signals

At this point, our first protagonist, the baseband, begins to appear.

Baseband, called Baseband in English, basic frequency band.

The basic frequency band refers to a special frequency bandwidth, that is, the bandwidth of the frequency range near zero frequency (from DC to several hundred KHz). The signal in this frequency band is called the baseband signal. The baseband signal is the most "basic" signal.

The baseband we often mention in real life refers more to the baseband chip and circuit of the mobile phone, or the baseband processing unit of the base station (which we often call BBU).

Let’s go back to the voice analog signal we just talked about.

These signals will be sampled, quantized, and encoded through the AD conversion circuit in the baseband and converted into digital signals. The specific process is as follows:

The encoding in the above figure is called source encoding.

Source coding, to put it simply, is to convert sound and images into 0 and 1. During the conversion process, source coding also needs to be compressed as much as possible to reduce the "volume".

For audio signals, we often use PCM coding (Pulse Code Modulation, as shown in the figure above) and MP3 coding. In mobile communication systems, 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, which should be familiar to everyone.

In addition to source coding, the baseband also needs to perform channel coding.

Coding is divided into source coding and channel coding

Channel coding is completely different from source coding. Source coding reduces the "volume". Channel coding is just the opposite, it increases 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 putting protective foam around the side of the cargo. If there are bumps on the road or collisions, 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 incident last year all belong to channel coding.

In addition to encoding, the baseband also encrypts the signal.

The next task is still the responsibility of the baseband, which is modulation.

Modulation, to put it simply, is to make the "wave" better represent 0 and 1.

The most basic modulation methods are frequency modulation (FM), amplitude modulation (AM), and phase modulation (PM). As shown in the figure below, different waveforms are used to represent 0 and 1.

Modern digital communication technology is very advanced, and based on the above, a variety of modulation methods have been developed, such as amplitude shift keying (ASK), frequency shift keying (FSK), phase shift keying (PSK), and quadrature amplitude modulation, also known as QAM (pronounced "quam").

In order to intuitively express various modulation methods, we will use a tool called a constellation diagram. The points in the constellation diagram can indicate the possible states of the amplitude and phase of the modulated signal.

Constellation map

16QAM diagram

(1 symbol represents 4 bits)

After the signal is modulated, the amount of information that can be carried by a single symbol is greatly increased. The 256QAM currently used in 5G can represent 8 bits of data with one symbol.

256QAM

OK, the baseband work is finally done. What should we do next?

Now it’s time for radio frequency to come on stage.

Radio Frequency, also known as RF, is a term that we are all familiar with. Literally, Radio Frequency means radio frequency. Strictly speaking, radio frequency refers to high-frequency electromagnetic waves in the frequency range of 300KHz~300GHz.

As we all know, when electric current passes through a conductor, a magnetic field is formed. When alternating current passes through a conductor, an electromagnetic field is formed, generating electromagnetic waves.

Electromagnetic waves with frequencies below 100kHz will be absorbed by the ground and cannot form effective transmission. Electromagnetic waves with frequencies above 100kHz can propagate in the air and be reflected by the ionosphere at the outer edge of the atmosphere, forming long-distance transmission capabilities.

This high-frequency electromagnetic wave with the ability to transmit over long distances is called radio frequency (signal).

Like the baseband, we usually refer to the series of things that generate RF signals, such as RF circuits, RF chips, RF modules, RF components, etc., as RF.

Therefore, we often hear people say: "XX mobile phone's baseband is terrible", "XX company cannot produce baseband", "XX device has very good RF performance", "XX's RF is very expensive" and so on.

The signal frequency sent by the baseband is very low. What the radio needs to do is to continue to modulate the signal from the low frequency to the specified high frequency band, such as the 900MHz GSM band, the 1.9GHz 4G LTE band, and the 3.5GHz 5G band.

The role of radio frequency is like a dispatcher

The reason why RF radio frequency needs to be modulated in this way is that, as mentioned earlier, baseband signals are not conducive to long-distance transmission.

On the other hand, wireless spectrum resources are scarce, and low-frequency bands are generally occupied by other uses. High-frequency band resources are relatively abundant, making it easier to achieve large bandwidth.

In addition, you must also modulate to the designated frequency band, otherwise you will interfere with others and it will be illegal.

In terms of engineering implementation, low frequency is not suitable either.

According to antenna theory, the antenna's transmission and reception conversion efficiency is highest when the antenna length is 1/4 of the radio signal wavelength. The wavelength of electromagnetic waves is proportional to the frequency (speed of light = wavelength × frequency). If low-frequency signals are used, the size of mobile phone and base station antennas will be relatively large, increasing the difficulty of engineering implementation. Especially on the mobile phone side, large antenna size is intolerable and will take up valuable space.

After the signal is modulated by RF, its power is relatively low. Therefore, it needs to be amplified by a power amplifier to obtain sufficient RF power before it is sent to the antenna.

After the signal reaches the antenna, it is filtered by the filter (to eliminate interference and 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, amplification, demodulation, 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 process of signal change. Please note that it is a general process, the actual process is still very complicated, and some intermediate frequencies are not introduced in detail.

I draw a simple diagram of the general process as follows:

How about it? Is it equivalent to reviewing our "Principles of Communication"? In fact, you will find that the situation in reality is quite different from the content in our books.

Questions for consideration:

1. Is the signal sent by the RF module to the antenna a digital signal or an analog signal?

2. Is the signal sent by the mobile phone baseband to the radio frequency a baseband signal?

3. After the signal reaches the base station, in addition to channel decoding, is it necessary to perform source decoding?

The article comes from Xianzao Classroom, author Xiaozaojun

RF-刘海石

Simply put, RF is spectrum movement

btty038

RF-Liu Haishi published on 2020-6-13 09:11 Simply put, radio frequency is spectrum migration

I rarely see you. Are you busy with the project?

RF-刘海石

btty038 posted on 2020-6-13 12:19 I rarely see you. Are you busy with the project?

Yes, I am working on my own now, which is different from working at a job. At work, I can take some time off, but working on my own has too much to deal with. I don't have much time to come to the forum.

btty038

RF-刘海石 posted on 2020-6-13 16:27 Yes, I am working on my own now, which is different from working at a job. At work, I can take a break from my busy schedule, but working on my own requires too much to deal with. I don’t have much time to come to the forum.

It is really hard to run your own business. You have to watch and follow everything. Being busy is also fulfilling.