What's the difference between RF and baseband?

"Baseband" and "radio frequency" are two common concepts in communication industry, and everyone may have different understanding of these two concepts, which is caused by insufficient understanding of them. ?

What are baseband and RF used for? Take the cell phone call as an example, observe the whole process of signal from the cell phone to the base station, and the role of baseband and RF.

First, sound waves (mechanical waves) are converted into electrical signals through the pickup of a microphone. When the mobile phone is connected, the sound made by people will be picked up by the microphone of the mobile phone and become an electrical signal (this is the original analog signal). ?

Secondly, the original analog signal of sound is converted into digital signal by baseband modulation. Baseband refers to a signal with a very narrow frequency range, that is, a bandwidth with a frequency range close to zero (from DC to several hundred KHz). The signal in this frequency band, which we call baseband signal, is the most "basic" signal without carrier modulation. In real life, we often mention the baseband chips and circuits of mobile phones, or the baseband processing unit (BBU) of base stations. ?

At this time, it is difficult for us to understand what carrier modulation is, and it is easier for me to understand the process of digital carrier modulation through carrier modulation of analog signals. Modulation is the process of changing one or more characteristics of a carrier signal. Changing the characteristics is nothing more than changing the amplitude or phase of the carrier signal. The modulated signal usually contains the information to be transmitted. ?

The purpose of analog modulation is to transmit analog baseband (or low-pass) signals on analog frequency band communication channels with different frequencies. The purpose of digital modulation is to transmit digital bit streams through analog communication channels. These original analog signals will be sampled, quantized and encoded by the digital-to-analog (A/D) conversion circuit in the baseband chip and become digital signals. ?

The process shown above is called source coding, which is to change the sound and picture into "0" and "1". The purpose is to reduce the redundancy of information sources and transmit them more effectively and economically. The most common application form is compression to reduce "volume". ?

In addition to source coding, baseband should also do channel coding. Channel coding is completely different from source coding. The source code is to reduce the "volume". Channel coding is just the opposite, it is to increase the "volume". Channel coding can counter the interference and attenuation in the channel and improve the link performance by adding redundant information (such as check code). Channel coding is like filling a protective bubble at the edge of a commodity. In this way, the probability of damage to the goods in transit will be reduced.

In addition to encoding, baseband also encrypts the signal. The most basic modulation methods are frequency modulation (FM), amplitude modulation (AM) and phase modulation (PM). As shown below, different waveforms are used to represent 0 and 1. ?

Modern digital communication technology is very developed. Based on the above, various modulation methods have been developed. Such as: ASK (amplitude shift keying), FSK (frequency shift keying), PSK (phase shift keying), and now the common QAM (quadrature amplitude modulation). We intuitively express various modulation methods through the constellation diagram, as shown below.

Points in the constellation can indicate the possible states of the amplitude and phase of the modulated signal. ?

For example, 16QAM, four bits of data can be represented by 1 symbol. ?

256QAM, which is widely used in 5G, can represent 8-8bit data with 1 symbol. ?

After modulation, the amount of information that a single symbol can carry is greatly improved.

Third, at this time, the baseband has completed its work, and it is the turn of Radio frequency (radio)

Frequency (RF for short) refers to high-frequency electromagnetic waves with frequency ranging from 300 kHz to 300 GHz. Electromagnetic waves with frequencies below 100kHz will be absorbed by the surface and cannot be effectively transmitted. Electromagnetic waves with frequencies higher than 100kHz can propagate in the air and be reflected by the ionosphere at the outer edge of the atmosphere, thus forming a long-distance transmission capability.

High-frequency electromagnetic waves with long-distance transmission ability are called radio frequency (signal). Electromagnetic waves are generated by alternating current passing through a conductor, which will form an electromagnetic field and generate electromagnetic waves. ?

In real life, we usually refer to RF circuits, RF chips, RF modules, RF components and so on. Generating a radio frequency signal as a radio frequency. For example, some people say that "the baseband of XX mobile phone is not good", "XX company can't make baseband", "the RF performance of XX equipment is very good" and "the RF of XX is very expensive" ... The signal frequency sent by baseband is very low. What RF needs to do is to continuously modulate the signal from low frequency to the specified high frequency band. Such as: 900MHz GSM band, 1.9GHz 4G.

LTE band, 3.5GHz 5G band. ?

Why is the radio frequency modulated like this? Wireless spectrum resources are scarce, and laws and regulations clearly indicate the corresponding uses of frequency bands and do not interfere with each other. The low frequency band is generally used for other purposes, while the high frequency band is relatively rich in resources, making it easier to achieve large bandwidth. Baseband signals are not conducive to long-distance transmission;

Low frequency band is not conducive to engineering realization; When the antenna length is 1/4 of the wavelength of the radio signal, the transmission and reception conversion efficiency of the antenna is the highest. The wavelength of electromagnetic wave is inversely proportional to the frequency (light speed = wavelength × frequency). If low-frequency signals are used, the size of mobile phones and base station antennas will be larger, which will increase the difficulty of engineering implementation. Especially in the mobile phone, the large antenna size is intolerable and will occupy valuable space. ?

After RF modulation, the power of the signal is very small, and it needs to be amplified by a power amplifier to get enough RF power, and then it is sent to the antenna. After the signal reaches the antenna, it is filtered by the filter (eliminating interference clutter) and finally emitted in the form of electromagnetic waves through the antenna vibrator.

Four. Receiving and Conversion of Wireless Signals After receiving the wireless signal, the base station antenna will filter, amplify, demodulate and decode the signal, and then send it to the core network through the bearer network, and then the other mobile phone base station and mobile phone will complete the subsequent data transmission and processing. This process is the reverse of the above reception. ?

The above is the general change process of signal from mobile phone to mobile phone, and the actual process will still be much more complicated.