1. Overview of communication principle

1.1, what is communication

1.1.1 Communication in a broad sense

1. The definition of communication in a broad sense: No matter what method or transmission medium is used, as long as the information is transmitted from one place to another, it can be called communication.
   For example, Fei Ge Chuan Shu, Fiberhome Communication, etc., as shown in the figure below.
   
   

1.1.2, narrow communication

1. The definition of narrow communication: narrow communication only includes telecommunications and radio and television.
2. What is telecommunications?
   Telecommunications refers to the use of "electricity" to transmit information, such as telegraph and telephone communication.
   The transmitter in the telegraph communication and the telephone in the telephone communication are shown in the figure below.
   
   
3. What is radio and television?
   Broadcasting means that listeners use the radio to listen to the sound programs of radio stations. The radio is shown in the figure below.
   
   Television means that viewers use the television to watch the video programs of the television station. The TV set is shown in the figure below.
   

1.2 What is a communication system

1. Definition of communication system: All technical equipment and transmission media required to realize information transmission are collectively referred to as communication system.
2. The development history of the communication system: from simple to complex, from wired to wireless, from analog to digital.

1.2.1. Wired analog communication system

1. The invention of the telephone: In 1875, Bell discovered that the strength of the electric current can simulate the change of the sound level, so he thought of using electric current to transmit sound and invented the telephone. The simplest wired telephone communication system mainly consists of a microphone, a receiver and a telephone line between the two, as shown in the figure below.
   
   The microphone in the picture above is also called a microphone or a microphone, and is responsible for converting changes in sound into changes in current. The earpiece is also called a speaker, horn, or receiver, which is responsible for converting changes in current into changes in sound.
   The microphone that has been widely used in telephone communication systems is a carbon particle microphone, as shown in the figure below.
   
   
   The working principle of the carbon particle microphone in the picture above is: when sound waves act on the vibrating diaphragm, the carbon particles are squeezed and become compact, the resistance decreases, and the current increases; when the sound becomes smaller, the carbon particles become loose , The resistance increases and the current decreases. The current is transmitted on the wire, and the change of sound can be controlled by the carbon microphone to change the current. After the current is transmitted to the receiving end, the original sound can be restored by the change of the current.
   The most used earpiece is the moving coil earpiece, as shown in the figure below.
   
   
   The working principle of the moving coil earpiece in the above picture: There is a coil in the earpiece, embedded in the gap of the ring magnet. When an audio current passes through, it generates a magnetic field that changes with the current law. Under the joint action of the ring magnet , The coil drives the paper cone to vibrate and make a sound.
2. Disadvantages of wired telephone communication: The letter requires a long telephone line, which is very inconvenient to deploy.

1.2.2, wireless analog communication system

1. The invention of radio communication: Hertz confirmed the existence of electromagnetic waves through experiments in 1887. Inspired by Hertz's electromagnetic wave experiment, Marconi began radio communication experiments in 1894, invented wireless telegraphy in 1896, and completed the first international radio call between Britain and France in 1899.
2. The composition of the wireless analog telephone communication system: It is mainly composed of a microphone, a modulator, a transmitting antenna, a receiving antenna, a demodulator and a receiver, as shown in the figure below.
   
   The microphone in the picture above is used to receive sound information and convert it into an analog electrical signal, modulate the analog electrical signal into a modulated signal, and then transmit the modulated signal with a high-frequency carrier through the transmitting antenna. , The modulated signal is received by the receiving antenna, and then restored to an analog electric signal through an analog demodulator and delivered to the microphone. Finally, the microphone converts the analog electric signal into sound information.
   Since it is an analog signal that is transmitted, it is called an analog communication system.
3. Disadvantages of analog signals: poor anti-interference ability, easy to be distorted due to interference during transmission ( see Note 1 for details ).
   For example, the analog electrical signal emitted by the microphone at the beginning is shown in the figure below.
   
   After the transmission reaches the earpiece, distortion occurs, as shown in the figure below.
   
4. Note 1: What is distortion?
   In radio technology, the output signal is not consistent with the input signal. Such as sound quality changes, image distortion, etc.

1.2.3, wired digital communication system

1. Compared with analog signals, what are the advantages of digital signals: strong anti-interference ability, easy to multiplex transmission, easy to use time slot exchange to realize data exchange between users, easy to encrypt and decrypt, easy to store, and the advantages of digital circuits are redundant Analog circuit. Examples of
   strong anti-interference ability of digital signals : Take the most common binary digital signal as an example. It uses high level and low level to represent binary digits 0 and 1, respectively. The receiving end only needs to pay attention to the level value at the sampling time, and can distinguish high level and low level. It does not need to care too much about the waveform of the received signal, so the signal waveform distortion has little effect on the digital signal. For example, the sender sends out a string of binary numbers 010101..., the waveform is shown in the figure below.
   
   The signal in the above figure is distorted after being transmitted to the receiving end, as shown in the figure below.
   
   If the transmission line is relatively short, the signal attenuation is relatively small, the signal waveform distortion is not too serious, and the binary number 010101... is easy to be correctly restored at the receiving end. If the transmission line is long, you can use a repeater to forward the signal after a certain amount of distortion. This ensures that the final signal will not be too severely distorted. Therefore, digital signals are more suitable for long-distance transmission than analog signals. The wired digital communication model with repeater is shown in the figure below.
   
   If the analog signal is processed using the similar idea in the above figure, that is, can the analog signal be transmitted over a long distance by amplifying the analog signal through an amplifier during the long-distance transmission process? The specific model is shown in the figure below.
   
   Although this seems feasible, while amplifying the analog signal, the noise on the analog signal will also be amplified. The more times of amplification, the worse the quality of the signal. This is not the case for digital signals. When the digital signal is restored through the repeater, the noise will not be superimposed. Therefore, the digital signal is suitable for long-distance transmission and the analog signal is not suitable.
   Digital signal is convenient to realize the multiplexing transmission of multiple signals. Take the parallel transmission of 4 signals as an example. As long as the 4 signals are staggered in time and occupy the transmission line in turn, the multiplex transmission of 4 signals can be realized, as follows As shown in the figure.
   
   Digital signal facilitates time slot exchangeFor example: suppose that the data of user A is transmitted in the 1# time slot of the A line, and the data of the B user is transmitted in the 3# time slot of the H line. Through the time slot exchange, it is easy to exchange the content in the A line 1# time slot to the H line 3# time slot, as shown in the figure below.
   
   Supplementary note on time slot exchange: In digital program-controlled exchanges, voice signals from different users or analog trunk lines are first converted into digital signals, and multiplexed to different PCM multiplex lines, and then connected to the internal digital switching network . In order to realize the conversation between different users, the digital switching network must complete the exchange of different time slots between different multiplexing lines, that is, the content of a certain time slot on a certain input multiplexing line of the digital switching network is exchanged to the designated output multiplexing line The designated time slot. Examples of
   easy encryption and decryption of digital signals :
   Definition of encryption: The sender processes the plaintext and encryption key together with a special encryption algorithm to make it a complex encrypted ciphertext and send it out.
   Definition of decryption: After receiving the ciphertext, the receiver uses the same key and the inverse algorithm of the same algorithm to decrypt the ciphertext and restore the plaintext.
   Assuming that the plaintext is 101101011011 and the key is 01010101001, perform an XOR operation on the two to obtain the ciphertext 110111110010, which completes the encryption; as long as the same key is used for the XOR operation with the ciphertext, the plaintext can be obtained and decryption is completed , The XOR operation here is both an encryption algorithm and a decryption algorithm, as shown in the figure below. Examples of
   
   easy storage of digital signals: Digital signals can be easily stored in VCD/DVD discs, U disks, hard disks, or network disks. A small U disk can easily store hundreds of songs. Relatively speaking, the storage of analog signals is not so convenient. In the past, common audio tapes and video tapes were used to store analog signals. Generally, an audio tape or video tape can only store an hour's analog audio signal or analog video signal.
   The comparison between digital circuit and analog circuit is shown in the figure below.
   
   It is precisely because of the above-mentioned advantages of digital signals that digital signals have begun to be used in communication systems, and analog communication systems have gradually evolved into digital communication systems.
2. The model of the wired digital communication system is shown in the figure below.
   
   It can be seen from the above figure that compared to the wired analog telephone communication system: an analog/digital converter is added at the transmitting end to convert analog voice signals into digital signals; a digital/analog converter is added at the receiving end to convert digital The signal is converted back to an analog voice signal.

1.2.4, wireless digital communication system

1. The model of the wireless digital communication system is shown in the figure below.
   
   It can be seen from the above figure that compared to the wireless analog telephone communication system: an analog/digital converter is added at the transmitting end to convert the analog voice signal into a digital signal, and the analog modulator is changed to a digital modulator at the same time; The tuner is changed to a digital demodulator, and a digital/analog converter is added to convert the digital signal back to an analog voice signal.

1.3, communication system model

1. The basic one-way communication system model is shown in the figure below.
   
   It can be seen from the above figure that the process of communication is the process of sending and receiving information through the channel between the source and the sink . Common one-way communication systems consist of radio, television, etc.
2. The basic two-way communication system model is shown in the figure below.
   
   The model in the figure above is suitable for commercial mobile communications.

1.3.1, source and sink

1. Overview of the source: it is located at the sending end and is responsible for converting the original information into electrical signals.
2. Overview of the sink: It is located at the receiving end and is responsible for converting electrical signals back to the original information.
3. The source and sink in the wireless microphone.
   The model of the wireless microphone is shown in the figure below.
   
   The microphone in the picture above is used as a source to convert sound into a sound signal and sent out; the speaker acts as a sink to convert the received sound signal back to sound.
4. Source and sink in video surveillance.
   The approximate model of video surveillance is shown in the figure below.
   
   The camera in the picture above is used as the source to convert images into image signals and sent out; the monitor acts as a sink to convert the received image signals back to images.
5. The source and sink in telegraph communication.
   
   The transmitter in the picture above is used as the source, which converts the Morse code carrying text information ( see Note 1 for details ) into a pulse signal and sends it out; the receiver acts as a sink, and converts the received pulse signal back to Morse. code.
6. Note 1: What is Morse code?
   Morse code is a system composed of "dots" and "strokes" invented in 1838 by the American painter and telegraph inventor, Mr. Morse. It is determined by the different arrangement sequence of "dots" and "strokes" intervals. Express different English letters, numbers and punctuation marks. In 1844, with the financial support of the U.S. Congress, Mr. Morse opened the first telegraph line from Baltimore, Maryland to Washington, D.C., using "Morse code" communications.

1.3.2. Overview of the channel

1. Definition of channel: Channel refers to the transmission channel of information.
   Information must be converted into a signal that meets the channel requirements before it can be transmitted in the channel. The signal will be attenuated when it is transmitted through the channel, and interference and noise on the channel will also affect the signal, resulting in signal distortion. When the signal is severely distorted, it will cause bit errors. To achieve error-free information transmission, error detection and correction processing must be considered when designing a communication system.
   For example, send a signal as shown in the figure below.
   
   After the transmission of the channel, the signal has attenuated and distorted, and the received signal is shown in the figure below.
   

1.3.3, sender and receiver

1. Overview of the transmitter: After the transmitter performs necessary error detection and error correction coding on the information sent by the source, it converts it into a signal suitable for transmission on the channel and sends it to the channel. The position of the transmitter in the communication system is shown in the figure below.
   
2. Overview of the receiver: The receiver is responsible for receiving signals from the channel, performing error detection and correction processing, and then recovering the information and sending it to the sink. The position of the receiver in the communication system is shown in the figure below.
   

1.4, Shindo

1. From the perspective of traditional media, the classification of channels: The characteristics of the channel determine the form of information transmission on the channel, and the characteristics of the channel depend on the transmission medium. According to the different transmission media, communication channels are divided into wired channels and wireless channels.

1.4.1, wired channel

1. The traditional media for wired channels are: telephone lines, network cables, and optical fibers.
   The telephone line is used for telephone communication, and the telephone communication model is shown in the figure below.
   
   The network cable is usually used as the transmission medium for communication between computers. The computers are connected by an Ethernet switch, and the transmission medium between the computer and the Ethernet switch is generally a network cable, as shown in the figure below.
   
   Optical fiber is usually used as the transmission medium between optical transmission equipment and communication equipment. The LTE base station and the core network ( see Note 1 for details ) equipment SGW are connected by transmission equipment, and the transmission medium used between the base station and the core network equipment SGW and the transmission equipment is generally optical fiber.
2. Note 1: What is the core network?
   The core network is one of the three major components of the communication network.
   The core network is the "management hub", responsible for managing data, sorting the data, and then telling it where to go. The processing and distribution of data is actually "routing and switching", which is the essence of the core network.

1.4.2, wireless channel

1. The transmission medium of the wireless channel is: electromagnetic wave.
2. Electromagnetic waves can be divided into: electric waves, light waves, X-rays, and gamma rays. The electromagnetic spectrum is shown in the figure below.
   
   The electromagnetic spectrum in the above figure can also be called the electromagnetic wave frequency band ( see Note 1 for details ). Electromagnetic waves mainly use radio waves and light waves in communications.
3. Note 1: Description of frequency bands.
   Frequency band is a term related to waves and communications. In the communications field, frequency band refers to the frequency range of electromagnetic waves, in Hz.
   The frequency band used in wireless communication is only a small part of the electromagnetic wave frequency band, which defines the frequency range of radio waves.
   In order to use spectrum resources rationally and ensure that various industries and services do not interfere with each other when they use spectrum resources, the International Telecommunication Union Radio Committee (ITU-R) promulgated international radio regulations to control the wireless use of various services and communication systems. The frequency bands have carried out a unified frequency range regulation.
   The frequency range of these frequency bands will be slightly different in actual applications in various countries and regions, but they must all be within these ranges specified internationally.
   According to international radio regulations, the existing radio communications are divided into more than 50 different services such as aeronautical communications, maritime communications, terrestrial communications, satellite communications, broadcasting, television, radio navigation, positioning, and telemetry, remote control, and space exploration. Each kind of business has stipulated certain frequency band.

1.5, signal conversion

1. The information-carrying signal needs to be processed in the transmitter before it can be transmitted in the channel. Similarly, the signal transmitted from the channel must undergo a certain processing before it can be restored to the original information-carrying signal.
   The signal processing performed by the transmitter includes: source coding, channel coding, interleaving, pulse shaping, and modulation.
   The signal processing performed by the receiver includes demodulation, sampling decision, de-interleaving, channel decoding and source decoding.
   The schematic diagram of signal transformation is shown in the figure below.
   

1.5.1, source coding

1. Source coding of analog sources: Generally, analog/digital conversion is performed first, the analog signal is digitized, and then compression coding is performed to eliminate redundant information as much as possible to reduce the occupation of transmission bandwidth, as shown in the figure below.
   

1.5.2, channel coding and interleaving

1. Channel coding: By adding redundant information, error correction processing can be performed at the receiving end to solve the problem of error codes caused by channel noise and interference. General channel decoding can only correct sporadic errors, and cannot do anything for continuous errors.
   For example: input sequence [11011], channel code output symbol [11 01 01 00 01], when it is transmitted through the channel, when it arrives at the receiving end, if there is an error [11 01 01 10 01], the channel can be decoded Detect and correct errors, as shown in the figure below.
   
   The figure above shows a situation where only one symbol is wrong.
   If there are errors in 3 consecutive symbols [11 01 01 11 11], the channel decoding will make an error, as shown in the figure below.
   
   The figure above shows that there are three consecutive symbols with errors. **Channel decoding cannot solve the problem of errors in consecutive symbols, it can only solve the problem of errors in sporadic symbols. ** Therefore, it is possible to reduce the probability of errors in consecutive symbols by shuffling the sequence of the original sequence of symbols, thus resulting in an interleaving operation.
   Interleaving: In order to solve the problem of continuous error codes, the data sequence after channel coding is shuffled according to a certain rule.
   De-interleaving: At the receiving end, the data sequence is restored before the channel decoding, so that the continuous error code will become sporadic errors at the receiving end, and the channel decoding can correct the error correctly, as shown in the figure below.

As can be seen from the above figure, after interleaving, 6 and 3 are consecutive error symbols. After de-interleaving returns to the normal order, 6 and 3 are separated and become sporadic errors, so that channel decoding can be processed Up.

1.5.3, pulse shaping

1. Pulse shaping: In order for a digital signal to be transmitted in a channel, it must first be converted into a suitable pulse waveform. The easiest pulse waveform to think of is the rectangular pulse, as shown in the figure below.
   
   [+1] in the above figure represents the number 0, and [-1] represents the number 1. The digital signal [00010110] 0 corresponds to the continuous multiple rectangular pulse waveforms as shown in the figure below.
   

1.5.4, modulation

1. Modulation: The process of carrying information onto a high-frequency carrier signal that meets the channel requirements.
2. Why modulate the signal?
   For wireless communication, wireless communication transmits signals by means of spatial radiation. From the theory of electromagnetic waves, it can be known that the size of the antenna is one-tenth or more of the wavelength of the radiated signal, and the signal can be effectively radiated. If the single-tone signal is sent directly in space without modulation, the required antenna size is at least a few kilometers. Obviously, it is impossible to make such an antenna in fact. The signal spectrum is moved to a higher frequency range through modulation, so that the signal is easily radiated in the form of electromagnetic waves.
   For wired communication, similar to the principle of wireless communication, wired communication must move the signal spectrum to a suitable frequency range through modulation to meet the frequency requirements of the wired channel.

1.5.5, antenna technology

1. Electromagnetic wave: It is an electric field and magnetic field that oscillate in phase and are perpendicular to each other in the form of waves that move in space. Its propagation direction is perpendicular to the plane formed by the electric field and magnetic field. It can effectively transfer energy. In wireless communication systems, modulation is obtained by modulation. Signals must be converted into electromagnetic waves before they can be transmitted in space. The transmission of electromagnetic waves is shown in the figure below.
   
   The propagation speed of electromagnetic waves: The propagation speed v of electromagnetic waves is related to the transmission medium. The propagation speed of electromagnetic waves in a vacuum is equal to the speed of light, which is 300,000 kilometers per second. The propagation speed of electromagnetic waves in the air is slightly slower than the speed of light, but it is generally calculated at the speed of light.
   Wavelength of electromagnetic wave: Under the premise of a certain propagation speed, the wavelength of electromagnetic wave is inversely proportional to frequency. The formula is shown in the figure below.
   
   In the formula above, [v] is the propagation speed of electromagnetic waves, and [f] the frequency of electromagnetic waves is equal to the frequency of the modulated signal.
   Amplitude of electromagnetic waves: The amplitude of electromagnetic waves will attenuate as the propagation distance increases. The speed of attenuation is related to the frequency of electromagnetic waves. In the case of the same propagation distance, the higher the frequency, the faster the amplitude attenuation. In other words, the higher the frequency, the worse the coverage performance.
2. Transmitting antenna: Responsible for converting the electrical signal sent by the modulator into electromagnetic waves and emitting it. The model of the transmitting antenna is shown in the figure below.
   
3. Receiving antenna: Responsible for converting the received electromagnetic waves back into electrical signals and sending them to the demodulator. The model of the receiving antenna is shown in the figure below.
   

1.6, multiplexing and multiple access technology

1. Overview of multiplexing and multiple access technologies: To achieve simultaneous transmission of multiple data channels on one channel.

1.6.1, multiplexing technology

1. Multiplexing technology: refers to the technology of transmitting multiple data channels on one channel at the same time. The model is shown in the figure below.
   
   Common multiplexing technologies include TDM (Time Division Multiplexing), FDM (Frequency Division Multiplexing), and CDM (Code Division Multiplexing). Time division multiplexing is to decompose the data to be transmitted into a lot of small data, which is transmitted on the channel in a time-sharing stream, and finally the small data that reaches the receiving end is spliced ​​and restored. Frequency division multiplexing is to mount multiple channels of data on high-frequency carriers of different frequency bands for transmission during modulation. Since the modulated signals belong to different frequency bands, the receiving end can then distinguish information according to different frequencies.

1.6.2, multiple access technology

1. Multiple access technology: refers to a technology that simultaneously transmits multiple user data on one channel. The model is shown in the figure below.
   
   Common multiple access technologies are TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access), and CDMA (Code Division Multiple Access) are the three most common multiple access technologies. One user corresponds to one channel of data, and multiple users correspond to multiple channels of data. Therefore, multiple access technology is based on multiplexing technology.