As the most painful hardware engineer, in addition to the difficulties encountered in research and development, it is to master a bunch of seemingly powerful theorems, but others can not understand what you are talking about, even the daily thoughts and family spit can not talk about. Now Xiaobian teaches you how to complicate the complex theorem interpretation, not only allows you to better understand these complex terminology, but also allows you to communicate with non-technical personnel. RF RF RF Radio Frequency Analogy: If people want to swim in the air, they can use the aircraft as a carrier. The condition for the aircraft to take off is that there must be a certain speed, and the speed can be lifted through a certain length of the airport runway. Information is transmitted in the air, and radio waves must be used as carriers. However, when the frequency of radio waves is lower than 100 KHz, the waves are absorbed by the ground objects, and the receiving device is also very complicated. Only radio waves that reach a certain frequency can be transmitted over long distances in the air, and it is easy to receive information. The radio frequency is a high-frequency alternating electric wave that can be emitted, and the frequency ranges from 300 kHz to 30 GHz. A transmission cable capable of transmitting a radio frequency signal is an RF line, such as a feeder used in engineering. The modulated high frequency radio waves transmitted in the radio frequency line are called radio frequency cable transmission. The RF line is connected to the antenna, and the RF signal is transmitted or received through the antenna to the air. noise Noise What is the noise? People talk on busy streets, and they can't communicate properly when they are a little farther away. At this time, it is affected by the car humming (noise outside the human being), and the noise of the crowd (noise in humans) is the noise that affects people's normal communication. These noises vary in size and size depending on the environment. We cannot predict the size of a specific noise at a specific moment, but it has a statistical probability law. In the process of radio wave signal processing and propagation, there are also interference signals that cannot be accurately predicted but have statistical probabilities. This signal is different from the mutual interference between radio waves of a specific frequency, called noise. Noise is divided into noise inside the system and noise outside the system. The noise inside the system includes thermal noise related to the ambient temperature, noise generated when the tube is working, intermodulation products between the signal and the noise, and the like. The noise outside the system comes from the noise generated by lightning storms, the ignition noise of cars, and the noise generated by other electrical equipment. Phase noise Phase Noise Analogy: After the flight from Beijing to Shanghai is arranged, it will take off and land at a fixed time every day. However, due to the weather, the flight could not take off and land normally. Many flights were delayed compared to normal time. Phase noise refers to the random variation of the phase of the output signal of the system caused by various noises in the system (such as various RF devices). The three elements describing the radio wave are amplitude, frequency, and phase. Frequency and phase interact with each other. Ideally, the fixed-frequency wireless signal fluctuation period is fixed, and the takeoff time is fixed just like a normal flight of an airplane. In the frequency domain, a pulse signal (spectrum width close to 0) is a sine wave of a certain frequency in the time domain. However, the actual situation is that the signal always has a certain spectral width, and due to the influence of noise, the power of the signal is also far from the center frequency, just like a flight with more than one hour delay. Signals far away from the center frequency are called sideband signals, and sideband signals may be squeezed into adjacent frequencies, just as delayed flights may squeeze into other flights and affect them. So this sideband signal is called phase noise. How does phase noise describe its size? The ratio of the power in the unit bandwidth to the total signal power in a certain range of the offset center frequency, in units of dBc/Hz. Just as to assess the impact of weather on a particular day, you can define the proportion of flights and flights that are more than one hour late. The smaller the ratio, the better. Thermal noise within the RF device system can cause phase noise. The phase noise size can measure the pros and cons of RF devices. The smaller the phase noise, the better the RF device. SNR Signal to Noise Ratio analogy: Goku asks the eight precepts: "What kind of girlfriend are you looking for?" Ba Jie replied: "Of course, the more beautiful, the better." Wukong asked: "Let you chase for a lifetime, do you want to?" Ba Jie said: "Don't dare." Wukong asked Sha Fei: "What kind of speed do you want to go online?" Shaying replied: "Of course, the sooner the better." Wukong asked: "One bit wants you two dollars, can you still get on?" Sha Fei said: "Don't dare to go." Wukong asked Tang Yan: "What kind of mount do you want?" Tang Yan replied: "The faster the better, the better the fuel, the safer the better." Wukong asked: "I want you to sell a house in Beijing to buy a face, a brand-name car, you still can't buy it?" Tang said: "Don't buy it." Wukong concluded: "When you get the benefits, you will definitely pay the price. What you have to consider is whether the benefits you get and the price paid are appropriate, that is, the price/performance problem. Not the more benefits, the better, but The higher the price, the better." The signal-to-noise ratio is simply the ratio of useful signal to interference noise. In the process of transmission, the useful signal will inevitably introduce various kinds of noise, and at least there is thermal noise. An RF device such as an amplifier amplifies the useful signal power and inevitably amplifies the corresponding noise. Signal-to-noise ratio (Signal/Noise), usually expressed in terms of SNR. The signal-to-noise ratio expressed in terms of power under the same RF condition is the square of the signal-to-noise ratio expressed by voltage. In engineering, it generally refers to the ratio of power. If expressed in decibels (dB), the signal-to-noise ratio expressed in terms of power is twice the signal-to-noise ratio expressed in terms of voltage. The larger the signal to noise ratio, the better. Application: Speakers and MP3s with signal-to-noise ratio (voltage) below 80dB are not recommended. Miss Gao's price/performance ratio is reduced - noise figure NF Noise Factor analogy: After saying that Ba Jie and Miss Gao were married a few years later, Wukong asked the Eight Precepts: "How, small days are good!" Ba Jie looked bitter and said: "Don't mention, Miss Gao has a much lower price/performance ratio. The temper is a lot worse, life is lazy, and I continue to have higher living expenses." Miss Gao's pre-marriage cost-effective ratio is many times higher than the cost of marriage, this multiple can be called the marriage magic box coefficient, can describe The quality of marriage. The RF device itself adds noise, and the signal-to-noise ratio at the input is higher than the signal-to-noise ratio at the output. The ratio of the signal-to-noise ratio at the input to the signal-to-noise ratio at the output is the noise figure of the RF device. NF=10lg (input signal to noise ratio / output signal to noise ratio) The noise figure measures the radio frequency (RF) performance of the receiver and amplifier, indicating the loss of useful power and amplification of the noise power after passing through the RF device. The noise figure of the base station is about 3~5dB, and the noise figure of the user mobile station is about 7~9dB. 涓涓 涓涓 converge into a river - additive noise Additive noise Analogy: The Wanli Yellow River is formed by the tidal stream formed by alpine snow water. The more important sources are three: one is Zhaqu, the other is about Guzong column, and the third is Ka Riqu. When the Zhaqu is dry, Kažqu has plenty of water. Additive noise is applied to the useful signal by direct superposition of power. Its existence is independent of the useful signal. Regardless of the useful signal, additive noise always exists in the RF device, which affects the quality of normal communication. In the general communication, the random additive noise is regarded as the background noise of the system; from the source, the additive noise can be divided into radio noise, industrial electrical noise, natural noise, and internal thermal noise of the radio frequency device. The interference frequency of the radio is fixed and can be avoided as much as possible by strengthening the management of the radio frequency. Industrial electrical noise comes from a variety of electrical equipment, but the interference spectrum is concentrated in the lower frequency range, and the choice of higher power frequency operation can prevent interference. Natural noise comes from lightning, sunspots and cosmic rays. This type of noise is hard to avoid. The internal thermal noise is caused by the irregular thermal motion of the electronic device, which can be mathematically described by a random process, which can also be called random noise. Your family's apple looks great - distortion Distortion Analogy: Everyone is more familiar with the story of the emperor's new clothes. Adults praise the emperor's clothes beautifully, and only the children say: in fact, he did not wear anything. When the child just talks, he sees the neighbor's apple, wants to eat, crying and eating apples, and truly expressing his intentions. When I was six or seven years old, I still wanted to eat the apples of my neighbors, but said: "Your apple is very good." When I grow up, I want to eat apples from my neighbors, in order to show that I don't lack apples. Dodge said: "I don't eat, I really don't eat." The child tells the truth that he sees the situation or tells his true thoughts. This is called childlikeness; and the adult conceals his true vision and true thoughts. Called to lose innocence (reality), or distortion. The so-called distortion is the loss of reality, or the fact that the real thing is distorted. When the signal passes through the RF transceiver channel, due to the introduction of additive noise and multiplicative noise, there will be some degree of distortion of the transmitted signal. This is the distortion of the wireless signal. The distortion of wireless signals can be divided into linear distortion and nonlinear distortion. Searchlights on the site - stray radiation Spurious Emission Phenomenon analogy: There is a construction site next to our living quarter, which is completely bright and bright. The main purpose of installing the searchlight is to facilitate inspections to avoid the loss of various materials on the site (radiation within the working bandwidth). However, the searchlights are too bright, radiating to our community (stray radiation), affecting the rest of many people in our community (stray radiation will inevitably bring interference). The RF transmitter should have sent a wireless signal within the specified frequency range, ie, transmitting the in-band signal; just as the searchlight should primarily illuminate the site. Since the internal components of the RF transmitter are not ideal devices, there is more or less non-linearity, and many signals in the unspecified frequency range are generated in the process of transmitting the wireless signal, that is, spurious radiation occurs; just like the searchlight Go to the living quarter next to you. When the transmitter transmits a signal that is not in its own frequency range, it may cause interference to other communication systems, just as the searchlight on the site affects the rest of the residents in the neighborhood. The stray radiation may be harmonic components, intermodulation signals, etc. generated by some nonlinear components. In order to prevent the stray radiation of one system from causing interference to other wireless communication systems, it is necessary to improve the electromagnetic compatibility of the system. The maximum stray radiation level for different out-of-band frequency ranges of this system is generally specified in the protocol. The general form of regulation is the maximum allowable spurious radiation of a certain bandwidth in a frequency range. If the protocol specifies WCDMA transmitters, the spurious emissions per 10 kHz bandwidth in the range of 150 kHz to 30 MHz cannot exceed -36 dBm. Shannon's theorem Analogy: What is the relationship between the speed of a car on a city road and what? It has a relationship with the width of the road, and it has something to do with the power of the car. It also has other disturbance factors (such as the number of cars and the number of red lights). Shannon's theorem is the most basic principle of all communication systems. C=Blog2(1+S/N): where C is the available link speed, B is the bandwidth of the link, S is the average signal power, N is the average noise power, and S/N is the signal to noise ratio. Shannon's theorem gives the relationship between the upper link speed (bits per second (bps)) and the link signal-to-noise ratio and bandwidth. Shannon's theorem can explain the difference in the maximum throughput of single carriers supported by 3G systems due to different bandwidths. Skin effect Analogy: After the heavy rain, the middle of the rural dirt road was full of water, and everyone had to line up along the road. The effective passage area of ​​the road is reduced due to the accumulation of water, which affects people's travel efficiency. Since the inductive reactance inside the conductor is more resistant to alternating current than the surface, when the alternating current passes through the conductor, the current density of each part is not uniform, and the current density of the conductor surface is large (the cross-sectional area is reduced, and the loss is increased). For the skin effect. The higher the frequency of the alternating current, the more significant the skin effect, the frequency is high to a certain extent, it can be considered that the current flows completely from the surface of the conductor. Practical application: hollow wire instead of solid wire, saving material; in high frequency circuit The skin effect is weakened by braiding a plurality of mutually insulated fine wires. Coherent time Analogy: Twin brothers with the same appearance and similar appearance appear side by side at the same time, which is difficult for ordinary people to distinguish. If they take pictures of the same movement side by side, it seems that a person has a heavy picture, and the person who looks at it thinks that he is blind. The coherence time is the maximum time difference range in which the channel remains constant. The same signal at the transmitting end reaches the receiving end within the coherent time. The fading characteristics of the signal are completely similar, and the receiving end considers it to be a signal. If the autocorrelation of the signal is not good, it may introduce interference, which is dazzling like a photo shoot. From the perspective of transmit diversity, time diversity requires that the time of two transmissions be greater than the coherence time of the channel, that is, if the transmission time is less than the coherence time of the channel, the signals transmitted twice will experience the same fading, and the effect of diversity anti-fading It doesn't exist anymore. The length of each chip of TD-SCDMA is 0.78us, that is, the coherence time between chips is 0.78us. If the same signal reaches the receiving end through different paths, the multipath diversity effect will occur; otherwise, Form self-interference. Coherent bandwidth (1/coherence time) Analogy: Half of the road is being refurbished on the busy traffic lines of the city. As the roads become wider and narrower, the speed of the passing vehicles needs to slow down. Some cars are pushed onto the bicycle lanes, and some cars are detoured. Coherence bandwidth is an important parameter to characterize multipath channel. It refers to a specific frequency range, and any two frequency components in the frequency range have strong amplitude correlation, that is, within the coherent bandwidth. The multipath channel has a constant gain and a linear phase. In a wireless communication system, if the bandwidth of the signal is less than the coherence bandwidth of the channel, the received signal experiences a flat fading process, at which time the spectral characteristics of the transmitted signal remain unchanged in the receiver. If the bandwidth of the signal is greater than the coherence bandwidth of the channel, the received signal experiences frequency selective fading. At this time, some frequencies of the received signal obtain greater gain than other components, causing distortion of the received signal, causing intersymbol interference. . Power Control Analogy: When you want to stop Zhang Hua, who is walking in front of you, you shouted his name: "Hey, Zhang Hua!" He found that he didn't listen, and you will raise his voice to call his name again. If Zhang Hua has heard your voice, he tells you: "You whisper, scare others.", you will lower your voice and talk to him. The power control can ensure that the power transmitted by each user reaches the base station to keep the minimum, which can meet the minimum communication requirements, and avoid unnecessary interference to other user signals, so as to maximize the system capacity. When the mobile phone moves within the cell, its transmit power needs to be changed. When it is closer to the base station, it needs to reduce the transmit power and reduce the interference to other users. When it is far away from the base station, it should increase the power and overcome Increased path loss. Maxwell's equations Anecdote: Maxwell's later life is full of troubles. No one understands his theory, and his wife has been ill for a long time. This double misfortune made him exhausted. In order to care for his wife, he had not slept in bed for three weeks. Despite this, his speech, his laboratory work, has never been interrupted. 1879 was the last year of Maxwell's life, and he continued to publicize electromagnetic theory. At this time, his lectures only had two audiences. One is a graduate student from the United States, and the other is Fleming who later invented the tube. In the empty lecture hall, there are only two students sitting in the front row. Maxwell held the handouts and walked firmly to the podium. His face was thin and his face was serious and solemn. As if he was not explaining to the two audiences, but explaining his theory to the world. On November 5, 1879, Maxwell died of cancer, only 49 years old. His merits did not get people's attention when he was alive. It was recognized after Hertz proved the existence of electromagnetic waves that he was "the greatest mathematical physicist in the world after Newton." Maxwell's equations describe the four basic equations of electric and magnetic fields, where: Equation No. 1: Describes the nature of the electric field. In general, the electric field can be a Coulomb electric field or an induced electric field excited by a changing magnetic field, and the induced electric field is a vortex field whose electrical displacement line is closed and does not contribute to the flux of the closed surface. Equation No. 2: Describes the nature of the magnetic field. The magnetic field can be excited by the conduction current, or by the displacement current of the changing electric field. Their magnetic fields are all vortex fields, and the magnetic induction lines are closed lines, which do not contribute to the flux of the closed surface. Equation No. 3: Describes the law of the excited electric field excited by an electric field. Equation No. 4: Describes the law of a changing electric field that excites a magnetic field. Electromagnetic wave Electromagnetic wave (should be the first wireless vocabulary) Anecdote: There were 24 million "domestic" sparrows in the UK. These sparrows are nesting in the attic of the house, playing in the gardens every day and becoming a landscape in the UK. However, in recent years, the number of sparrows in the UK has suddenly decreased dramatically. British scientists are puzzling. Some people think that cats eat sparrows. Some people think that unleaded gasoline affects the survival of insects. The sparrows rely on this kind of insects to feed small sparrows. Others think that the building loft is closed, making the sparrows unable to make nests. Recently, British scientists and zoologists pointed out that the electromagnetic waves emitted by mobile phones are the main culprit in the disappearance of the sparrow. The British have used mobile phones extensively since 1994. It is during these years that the British sparrows began to decline in large numbers. Studies have shown that electromagnetic waves affect the sense of direction of the sparrow. The sparrow relies on the Earth's magnetic field to identify the direction. And electromagnetic waves can interfere with the ability of the sparrow to find a way, thus making it lose its way. Studies have also shown that electromagnetic waves can also affect the sperm count and ovulation function of animals. Electromagnetic waves are a form of motion of an electromagnetic field. Electricity and magnetism can be said to be two sides, the current will generate a magnetic field, and the changing magnetic field will generate a current. The changing electric field and the changing magnetic field constitute an inseparable unified field. In the low-frequency electrical oscillation, the mutual change between the magnetoelectricity is relatively slow, and almost all of its energy returns to the original circuit without energy radiation. In the high-frequency electrical oscillation, the magnetoelectric interaction becomes very fast, and the energy cannot be returned completely. The original oscillating circuit, then the electric energy and the magnetic energy propagate into the space in the form of electromagnetic waves with the periodic change of the electric field and the magnetic field, and the energy can be transmitted outward without the medium. This is a type of radiation. Electromagnetic waves are a kind of energy. Any object above absolute zero will emit electromagnetic waves. Except for light waves, people can't see the ubiquitous electromagnetic waves. Doppler effect Doppler effect Example: When the alarm sound of the police car and the engine of the car approach us at a certain speed, the sound will be more harsh than usual. When we are away from us, the sound will be moderated; the same reason, you can listen when the train passes by. The squeaky changes indicate the existence of the Doppler effect. The Doppler effect indicates that the receiving frequency becomes higher when the wave source moves toward the observer, and the receiving frequency becomes lower when the wave source is far away from the observer. In mobile communication, when the mobile station moves to the base station, the frequency becomes higher, and when it is far away from the base station, the frequency becomes lower. The astronomer Hubble applied the Doppler effect to conclude that the universe is expanding. Medical application of the Doppler effect to determine the oxygen supply during the blood circulation, vascular atherosclerosis and other conditions. Multipath effect Analogy: When you were young, you played clay. You poured water on the top of a small mound. The water flowed away from it. Many of the water leaked into the soil or flowed in different directions. Some of the water flowed through different paths and different times. Remitted to a low-lying place. The multipath effect of radio waves means that the signal often has many transmission paths with different delays and different losses from the transmitting end to the receiving end. It can be direct, reflected or diffracted. The same signal of different paths is superimposed at the receiving end. Increasing or decreasing the energy of the received signal White Noise Analogy: When the old electrical equipment, such as a radio, is turned on, you may hear a "beep" sound; White noise refers to noise whose power spectral density is evenly distributed throughout the frequency domain. Random noise with the same energy at all frequencies is called white noise. It sounds very bright "å’" from the frequency response of our ears. White noise is a random signal or random process with a constant power spectral density. The power of this signal is the same in each frequency band. The ideal white noise has an infinite bandwidth, so its energy is infinite, which is impossible in the real world, but it makes us more convenient in mathematical analysis. In general, as long as a noise process has a spectral width that is much larger than the bandwidth of the system in which it operates, and its spectral density can be considered as a constant in this bandwidth, it can be treated as white noise. Thermal noise can be considered as white noise. Gaussian white noise (and Rayleigh distribution) Analogy: Thermal noise and shot noise are Gaussian white noise. Gaussian white noise: If a noise, its amplitude distribution obeys a Gaussian distribution, and its power spectral density is evenly distributed, it is called Gaussian white noise. The envelope of the sum of two orthogonal Gaussian noise signals obeys the Rayleigh distribution. The amplitude obeys the Gaussian distribution, which is the amplitude probability density distribution with the mean axis symmetry, the largest at the mean, and the curve inflection point at one variance. The linear combination of Gaussian noise is still Gaussian noise. When the noise generated by independent noise sources is summed, they can be directly added by power. hertz Interlude: Seven years before the Hertz experiment, a person named David also received an electromagnetic wave signal. He then reported to the Royal Society President Stokes, but Stokes thought it was just an ordinary electromagnetic induction phenomenon. David is too superstitious and authoritative, and he has not given the opportunity to pay attention to this day, so that the discovery is buried. Hertz, a German physicist, Hertz's greatest contribution to mankind is the experimental confirmation of the existence of electromagnetic waves. In January 1888, Hertz summarized his research results in the article "On the speed of the propagation of electrokinetic effects." After the publication of the Hertz experiment, it sensationalized the scientific community around the world. Created by Faraday, Maxwell's summary of the electromagnetic theory, has only achieved a decisive victory. To commemorate Hertz, the unit of frequency in the International System of Units is defined as Hertz, which is a measure of the number of repetitions of periodic variations in each second. Diffraction Analogy: See "Direct Waves" When the wireless path between the receiver and the transmitter is blocked by sharp edges, the phenomenon that radio waves propagate around the obstacle is called diffraction. When diffracted, the path of the wave changes or bends. The secondary waves generated by the blocking surface are scattered in the space, even on the back side of the barrier. The diffraction loss is the loss caused by various obstacles to the transmission of radio waves. Direct wave Direct Wave Analogy: In the sport of billiards, many laws are very similar to the laws of electromagnetic waves. If you hit the center of the ball directly, if there is no block, the ball will run in a straight line; if the hit ball hits the edge of the table, it will follow the angle of incidence such as the angle of reflection; if the ball and the other ball Cut, according to the force and direction, it can bypass the ball in the line of sight, much like a diffraction; assume that a lot of balls in a range are not more than one ball apart, when the cue ball hits the middle of the ball, it will arouse a lot The ball moves in different directions, much like scattering. Feeling: The most fundamental laws of many things in nature are connected. This is the reason for the road. However, the laws we have revealed always feel that they are lacking, and they are "very bad." The most fundamental way can only be realized. A radio wave that arrives at a receiving point in a straight line by a transmitting antenna is called a direct wave. Free space radio wave propagation is the propagation of electric waves in a vacuum and is an ideal propagation condition. When a wave propagates in free space, it can be considered as a direct wave propagation, and its energy is neither absorbed by the obstacle nor reflected or scattered. Reflected wave Reflection wave Analogy: See "Direct Waves" Application: When selecting high-speed railway wireless coverage stations, pay attention to the incident angle of radio waves. The alternative site should not be too far away, otherwise the angle of incidence will be too large and the ability to enter the compartment will be reduced. Generally, the site is about 100 meters away from the railway (other factors need to be considered, I will talk about it later). Wireless signals are reflected by the ground or other obstacles to the receiving point, called reflected waves. Reflections occur on the surface of the earth, on buildings and on walls. The reflected wave occurs in the interface of two different mediums with different densities. The greater the difference in density of the interface medium, the larger the amount of reflection of the wave and the smaller the amount of refraction. The smaller the incident angle of the wave, the smaller the amount of reflection and the larger the amount of refraction. The direct wave and the reflected wave are collectively referred to as a spatial wave. Scattered wave Scattered Wave Analogy: I saw a car accident not long ago. Many vehicles are driving, and the distance between them is not enough to pass through one car. However, there is a car behind without any slowdown from the back to the middle of many vehicles, the situation is terrible. When there is an object smaller than the wavelength in the medium through which the radio wave travels, and the number of the blocking body per unit volume is very large, scattering occurs; the scattered wave is generated on a rough surface, a small object or other irregular object. In actual communication systems, leaves, street signs, and lamp posts can cause scattering. Non-line-of-sight transmission nLOS,Non Line of Sight Fun fact: When studying at Engineering University, there are very few girls, and everyone is very mysterious about women's lives. Fortunately, at the right angle of our male dormitory is a female dormitory, and the water room is close to the male building. In the summer, you can only hear the sound of water, but you can't see it. One classmate said, "Oh, unfortunately it is non-line-of-sight transmission." After a while, I found that the classmate was very creative in installing a mirror on the wall not far away. Eventually discovered by girls. The wireless signal is blocked by obstacles from the transmitting point to the receiving end, and cannot be transmitted along a straight line, which is called non-line-of-sight transmission. The wireless propagation loss of non-line-of-sight transmission is much higher than the line-of-sight transmission. Fresnel District Fresnel Zone Analogy: Sometimes I feel that the most effective range of vision for a human eye is also an ellipsoid. Things outside the ellipsoid can be seen, but they are not particularly clear. A well-trained shooter whose effective range of vision must be concentrated in the ellipsoid with a very small radius of his target. Application: When surveying wireless sites, be sure to pay attention to whether the coverage has a barrier larger than the Fresnel radius. Especially big billboards, high-rise buildings and other obstacles. The Fresnel zone is an ellipsoid with the transmit and receive antennas at the two focal points of the ellipsoid. The radius of this ellipsoid is the first Fresnel radius. In free space, the electromagnetic energy radiated from the emission point to the receiving point is mainly propagated through the first Fresnel zone, and as long as the first Fresnel zone is not blocked, the propagation condition of the approximate free space can be obtained. In order to ensure the normal communication of the system, the height of the transceiver antenna should be set so that the obstacle between them should not exceed 20% of its Fresnel zone. Otherwise, the multipath propagation of electromagnetic waves will have adverse effects, resulting in a decline in communication quality and even Interrupt communication Free space propagation model Free space propagation Model Feeling: Lao Tzu said: The world is difficult to do in the easy; the world's major things must be done in detail. In the research and modeling process of many physical phenomena, we first consider the most essential and simplest laws of the complicated phenomenon, and then consider some non-essential factors. Application: In the actual wireless environment, wireless signals can be considered to propagate in free space as long as they are not blocked in the first Fresnel zone. This makes it very easy to estimate the propagation loss. Anecdote: I walked with a colleague on the streets of Beijing. He joked with me and said, "I have been doing wireless for a long time. I can feel how big the TD signal is in this place. The signal here is -78dBm." We looked at the signal size on the test phone, which is -77.5dBm. I said, "You are almost ready to test your phone!" Radio waves propagate unblocked in free space without reflection, refraction, diffraction, scattering, and absorption. However, when the electric wave propagates through a path, the energy is still attenuated due to the diffusion of the radiant energy. The free space propagation loss is the ratio of the energy of the transmitting point's wireless signal spreading out uniformly throughout the sphere to the receiving antenna, the energy falling on the effective receiving area of ​​the antenna and the total energy emitted. The final free space propagation formula is L=32.45+20log(dkm)+20log(fMHz)(dB) When f = 2000MHz, the formula can be simplified to L = 38.45 + 20 log (dm). The free-space propagation model is the simplest model of radio wave propagation. The loss of radio waves is only related to the propagation distance and the frequency of the radio wave; when given the frequency of the signal, it is only related to the distance. In the actual propagation environment, the environmental factor n is also considered, and the formula is simplified to L=38.45+10*n*log(dm). n is generally between 2 and 5 depending on the environment. The former brother knows the power of the antenna port. Using the simplified propagation model described above, it is estimated that he is 100 meters away from the TD antenna, and then the radio wave intensity port at the location is calculated. (In the daily vocabulary, I try to explain the formula as little as possible, but this formula is more important to the practitioners, so I must speak) Pay attention to the simplified formula for understanding the propagation of radio waves at 2000 MHz: 1. The loss at 1 meter is 38.45dB, and the loss at 10 meters is 58.45dB; 2, the distance is doubled, the loss increase is 6dB (many students mistakenly think it is 3dB); 3. The loss in free space does not increase linearly with distance, but increases exponentially. (Some students ask about the loss of free space per 100 meters. The problem itself is wrong. Because the first 100 meters and the second 100 meters of the wireless signal are not the same. Ultra High Frequency UHF Ultra High Frequency Ultra-high frequency: Decimeter band, refers to UHF radio waves with a frequency of 300~3000MHz. The radio waves are distributed between 3 Hz and 3000 GHz and are divided into 12 bands in this spectrum. The frequency propagation characteristics are different in different frequency bands. The smaller the frequency, the smaller the propagation loss, and the farther the coverage distance is, the stronger the diffraction ability is. However, the frequency resources in the low frequency band are tight and the system capacity is limited. The high frequency band has abundant frequency resources and large system capacity; however, the higher the frequency, the larger the propagation loss, the smaller the coverage distance, the weaker the diffraction capability, the greater the technical difficulty realized, and the corresponding system cost. The frequency band selected by the mobile communication system should take into account the coverage effect and capacity. Compared with other frequency bands, the UHF band has a better compromise between coverage effect and capacity, and is widely used in the field of mobile communications. Reference: Long-wave communication, radio communication with a wavelength of 10,000 to 1000 meters (frequency of 30 to 300 kHz). Long-wave communications are mainly used in military applications such as submarine communications, underground communications, and navigation. In a certain range, long-wave communication is mainly based on ground wave propagation. When the communication distance is greater than the maximum propagation distance of the ground wave, the signal is transmitted by the sky wave. The advantage of long-wave communication is that the communication distance is long, and the communication can be stable and reliable through a certain depth of the mountain and sea water. The disadvantages are: due to the long wavelength, the transceiver device and the antenna system are large, the cost is high; the passband is narrow, and it is not suitable for multi-channel and fast communication; and is susceptible to interference from the sky. . Shadow effect Shadowing Effect Analogy: When the sun shines on the earth, trees and houses have shadows. This shadow is not completely dark, it is a kind of light with a much weaker intensity. In the propagation path, when radio waves are blocked by obstacles such as buildings with uneven terrain, high and low heights, and tall trees, a shadow area with a weak field strength of the radio wave signal is formed behind the obstacle. This phenomenon is called the shadow effect. Slow fading Slow Fading Analogy: In the process of stock market decline, although the time-sharing curve fluctuates sharply, the 5-week line changes slowly. During the propagation of radio waves, the median value of the signal strength curve exhibits a slow change, called slow fading. Slow fading reflects the median value of the instantaneous weighted average, reflecting the mean change in the reception level of the hundreds of wavelengths in the medium range, generally following a lognormal distribution. Reasons for slow fading: 1) The main cause of slow fading is path loss; 2) Signal fading caused by shadow effect: Fast fading Fast Fading Analogy: During the decline of the stock market, the time-sharing instantaneous value of the stock price changes drastically, much like a fast decline. Fast fading is the phenomenon of instantaneous rapid fluctuations and rapid changes in the received signal field strength. The fast fading is caused by the multipath propagation signals caused by various terrains, features and moving objects superimposed at the receiving point. 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线信å·åœºå¼ºé‡‡æ ·çš„æ ‡å‡†ï¼šåœ¨40ä¸ªæ³¢é•¿å†…é‡‡æ ·36～50ä¸ªç‚¹ã€‚è¿™ä¸€æ ‡å‡†åœ¨æ— çº¿é€šä¿¡å·¥ç¨‹ä¸å¾—到了广泛应用。 ç†è§£ï¼šå‡è®¾æˆ‘ä»¬çš„æ— çº¿åˆ¶å¼ä½¿ç”¨çš„频率是2000MHz，扫频仪æ¯ç§’钟最多打100ä¸ªç‚¹ï¼Œé‚£ä¹ˆè¿›è¡Œæ— çº¿çŽ¯å¢ƒæµ‹è¯•çš„è½¦é€Ÿçš„ä¸Šé™æ˜¯å¤šå°‘？ 2000MHzçš„æ— çº¿ç”µæ³¢æ³¢é•¿æ˜¯0.15m，40个波长就是6m，也就是说6mçš„è·ç¦»å†…必须够50个点。扫频仪æ¯ç§’钟最多打100个点，也就是æ¯ç§’钟最多走12米，å³è½¦é€Ÿä¸èƒ½é«˜äºŽ12m/sï¼Œèµ°å¾—å¤šé‡‡æ ·ç‚¹å°±ä¸å¤Ÿäº†ã€‚ SPM 模型 Standard Propagation Model 类比：在先秦时代，å„诸侯国的文å—是ä¸ç»Ÿä¸€çš„，ä¸åŒå›½å®¶çš„人交æµèµ·æ¥å分ä¸æ–¹ä¾¿ã€‚最åŽç§¦å§‹çš‡å‘Šè¯‰å¤©ä¸‹äººï¼Œä»–用的å—å°±æ˜¯æ ‡å‡†å—，大家统一用这ç§æ–‡å—。 æ— çº¿ä¼ æ’模型有很多ç§å½¢å¼ï¼Œä¹Ÿæœ‰å¾ˆå¤šé€‚用范围，由于形å¼ä¸Šçš„ä¸ç»Ÿä¸€ï¼Œæ— 线工程师使用起æ¥å¾ˆä¸æ–¹ä¾¿ï¼Œå¯¹åŒä¸€æ— 线环境很难有比较统一的认识。 SPM模型的推出解决了这个问题。SPM模型适用于从150MHz到2GHz比较宽的频率范围，也适用于从密集城区ã€æ™®é€šåŸŽåŒºã€éƒŠåŒºã€å†œæ‘çš„å„ç§æ— 线环境。所以目å‰åº”用比较广泛。 Path Loss= K1+ K2log(d)+ K3log(Htxeff)+ K4Diffration+ K5log(d) log(Htxeff)+ K6(HRxeff ) +Kclutterf(clutter) among them: d：接收机与å‘射机之间的è·ç¦»(m)ï¼› HTxeff：å‘射天线的有效高度(m)ï¼› Diffraction loss：ç»è¿‡æœ‰éšœç¢è·¯å¾„引起的è¡å°„æŸè€—(dB)ï¼› HRxeff：接收天线的有效高度(m)ï¼› f(clutter): å› åœ°ç‰©æ‰€å¼•èµ·çš„å¹³å‡åŠ æƒæŸè€—ï¼› K1：常数(dB)ï¼›. K2：log(d)的系数； K3：log(HTxeff)的系数； K4：è¡å°„æŸè€—的系数； K5: log(HTxeff)log(d)的系数. K6: HRxeff的系数. Kclutter: f(clutter)的系数. åœ¨è‡ªç”±ç©ºé—´ä¼ æ’模型ä¸ï¼ŒK3ã€K4ã€K5ã€K6ã€Kclutter都是0，K1=38.45，K2=20。 åœ¨ä¸€èˆ¬çš„æ— çº¿çŽ¯å¢ƒä¸ï¼ŒK1å’ŒK1å–值也是éžå¸¸é‡è¦çš„，对整个结果的准确性影å“æ¯”è¾ƒå¤§ï¼Œå› ä¸ºæˆ‘ä»¬åœ¨åˆ©ç”¨ä¼ æ’模型计算的时候，主è¦å…³æ³¨çš„就是离å‘射机ä¸åŒä½ç½®çš„情况下，我的路æŸæ˜¯å¤šå°‘，å¯ä»¥å¾—到的信å·åœºå¼ºæ˜¯å¤šå°‘ã€‚è€Œå…¶ä»–å› ç´ å¦‚å¤©çº¿é«˜åº¦åœ¨ä¸€å®šæƒ…å†µä¸‹æˆ‘ä»¬è®¤ä¸ºä¸å˜åŒ– å³°å‡æ¯” PAR Peak-to-Average Ratio 类比： 一个æ‘å里é¢æœ‰æ¯”较富裕的人家，也有比较穷的人家，但大多数都是收入ä¸ç‰çš„普通人家，我们最有钱的人家的财富和æ‘å户平å‡è´¢å¯Œçš„比或者最穷人家的财富和户平å‡è´¢å¯Œçš„比，å¯ä»¥è¡¡é‡å‡ºæ‘å贫富两æžåˆ†åŒ–的程度。 但从全国æ¥çœ‹ï¼Œç”¨æŽ’在胡润排行榜上第一å的财富æ¥å’Œä¸å›½å®¶åºçš„å¹³å‡æ”¶å…¥æ¥æ¯”就显得ä¸é‚£ä¹ˆåˆé€‚，ä¸èƒ½å…¨é¢è¡¡é‡ä¸å›½çš„贫富差è·çŽ°è±¡ã€‚如果用1%çš„ä¸å›½å¯Œè£•é˜¶å±‚çš„å¹³å‡è´¢å¯Œå’Œä¸å›½å®¶åºçš„å¹³å‡æ”¶å…¥ï¼Œå°±å¯ä»¥è¯´æ˜Žä¸€äº›é—®é¢˜ã€‚å‡è‹¥ä¸å›½å®¶åºå¹³å‡å¹´æ”¶å…¥æ˜¯3万元，而最有钱的富豪家åºçš„年收入为30亿，30亿和3亿一比，就是10万å€ï¼Œå¦‚果用dB表示，就是50dB。 å¦‚æžœæˆ‘ä»¬ç ”ç©¶å…¨å›½å„自然æ‘富ç¿çš„财富的分布情况，以说明ä¸åŒçœä»½ç»æµŽå‘展水平，也å¯ä»¥ç”¨å³°å‡æ¯”的概念，å³æœ€æœ‰é’±çš„æ‘富ç¿çš„财富和所有æ‘富ç¿è´¢å¯Œçš„å¹³å‡å€¼ç›¸æ¯”。也就是说，峰å‡æ¯”一定è¦æŒ‡å‡ºæ˜¯ä»€ä¹ˆæ ·çš„峰值和å‡å€¼çš„比，å•ä½æ˜¯ç»å¯¹çš„比值还是dB值。 è§£é‡Šï¼šæ— çº¿ä¿¡å·ä»Žæ—¶åŸŸä¸Šè§‚测是幅度ä¸æ–å˜åŒ–çš„æ£å¼¦æ³¢ï¼Œå¹…度并ä¸æ’定，一个周期内的信å·å¹…度峰值和其他周期内的幅度峰值是ä¸ä¸€æ ·çš„ï¼Œå› æ¤æ¯ä¸ªå‘¨æœŸçš„å¹³å‡åŠŸçŽ‡å’Œå³°å€¼åŠŸçŽ‡æ˜¯ä¸ä¸€æ ·çš„。在一个较长的时间内，峰值功率是以æŸç§æ¦‚率出现的最大瞬æ€åŠŸçŽ‡ï¼Œé€šå¸¸æ¦‚率å–为0.01%。在这个概率下的峰值功率跟系统总的平å‡åŠŸçŽ‡çš„比就是峰å‡æ¯”。在概率为0.01%处的PARï¼Œä¸€èˆ¬ç§°ä¸ºå³°å€¼å› å（CF CREST Factor，CF）。 ç†è§£å³°å‡æ¯”的概念是需è¦æ³¨æ„ä»¥ä¸‹å‡ ç‚¹ï¼š 1.由于功率的峰å‡æ¯”是电压的峰å‡æ¯”的平方，PAR一般是指功率的峰å‡æ¯”，但也有书上把他当åšç”µåŽ‹çš„å³°å‡æ¯”æ¥ç”¨ã€‚ 2.如果功率幅值éšæ—¶é—´æ²¡æœ‰å˜åŒ–，å³â€œåŒ…络的最大值â€ä¸Žâ€œåŒ…络的平å‡å€¼â€å¤„处相ç‰ï¼Œå³â€œæ’包络â€ä¿¡å·çš„å³°å‡æ¯”为1或者是0dB。 3.如果åªè€ƒè™‘ä¸€ä¸ªå‘¨æœŸçš„æ— çº¿ä¿¡å·çº¯æ£å¼¦æ³¢ï¼ŒåŠŸçŽ‡å³°å‡æ¯”就是2，å³3dBï¼›è€Œå…¶ç”µåŽ‹çš„å³°å€¼å› åCF就是功率峰å‡æ¯”çš„å¹³æ–¹æ ¹1.414。但一般情况下，峰å‡æ¯”很少是指这ç§æƒ…况。 3.调制技术ã€å¤šè½½æ³¢æŠ€æœ¯éƒ½å¯èƒ½å¸¦æ¥è¾ƒå¤§çš„å³°å‡æ¯”，峰å‡æ¯”过大ä¸æ˜¯ä»€ä¹ˆå¥½äº‹ï¼Œä¼šå½±å“很多射频器件的应用效率。 Long Range Fiberglass Horn Speaker Long-range glass fiber reinforced plastic Horn Speaker, the box is made of glass fiber and anti-aging agent, durable, beautiful and environmentally friendly, built-in network decoding module, stereo power amplifier, high-fidelity speaker Long Range Fiberglass Horn Speakers,waterproof glass fiber horn speaker,Stadium Horn Speaker,full range horn speaker, Fiberglass Horn Speaker Taixing Minsheng Electronic Co.,Ltd. , https://www.ms-speakers.com
Remote FRP horn-type integrated network audio decoding speaker, used for audio decoding and sound restoration in broadcast partitions, especially suitable for open-air environments, such as sports venues, parks, exhibitions, etc.
