Understanding wireless communications starts with understanding these 20 professional terms[Copy link]
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Shannon's Theorem Analogy: What does the speed of a car on a city road depend on? It depends on the width of the road, the power of the car, and other interference 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 link bandwidth, 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 limit of link speed (bits per second (bps)) and link signal-to-noise ratio and bandwidth. Shannon's theorem can explain the difference in the maximum single-carrier throughput supported by various 3G standards due to different bandwidths. Skin effect Analogy: After a heavy rain, the dirt road in the countryside is full of water in the middle, and people have to queue up along the roadside to pass. The effective passing 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 has a greater obstruction effect on alternating current than the surface, when alternating current passes through the conductor, the current density of each part is uneven, and the current density on the surface of the conductor is large (reducing the cross-sectional area and increasing the loss). This phenomenon is called skin effect. The higher the frequency of alternating current, the more significant the skin effect. When the frequency is high to a certain extent, it can be considered that the current flows completely through the surface of the conductor. Practical application: hollow wires replace solid wires to save materials; in high-frequency circuits, multiple strands of mutually insulated thin wires are woven into bundles to weaken the skin effect. Coherence time Analogy: Twin brothers wearing the same clothes and looking similar appear side by side at the same time, and it is difficult for ordinary people to tell them apart. If they take a photo side by side with the same action, it seems that one person has double images in the photo, and the viewer thinks that they are dazzled. Coherence time is the maximum time difference range within which the channel remains constant. The same signal from the transmitter reaches the receiver within the coherence time, and the fading characteristics of the signals are completely similar, so the receiver considers it to be one signal. If the autocorrelation of the signal is not good, it may also introduce interference, similar to the double images in the photo that dazzle people. From the perspective of transmit diversity: time diversity requires that the time between two transmissions must 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 two transmitted signals will experience the same fading, and the diversity anti-fading effect will not exist. The time length of each TD-SCDMA chip is 0.78us, that is, the coherence time between code chips is 0.78us. If the code chips of the same signal reaching the receiver through different paths exceed this time, there will be a multipath diversity effect; otherwise, self-interference will be formed. Coherence bandwidth (1/coherence time) Analogy: On a busy traffic artery in a city, half of a section of the road is being repaired. As the road becomes narrower, the speed of vehicles passing by needs to slow down. Some vehicles are squeezed onto the bicycle lane, and some vehicles simply take a detour. Coherence bandwidth is an important parameter that characterizes the characteristics of a multipath channel. It refers to a specific frequency range in which any two frequency components have a strong amplitude correlation, that is, within the coherence bandwidth, the multipath channel has a constant gain and 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 will experience a flat fading process, and the spectral characteristics of the transmitted signal can still remain unchanged in the receiver. If the bandwidth of the signal is greater than the coherence bandwidth of the channel, the received signal will experience frequency selective fading. At this time, some frequencies of the received signal have a greater gain than other components, causing distortion of the received signal, thereby causing inter-symbol interference. Power Control Analogy: When you want to call your friend Zhang Hua who is walking in front of you, you call out his name: "Hey, Zhang Hua!" When you find that he doesn't hear you, you will raise your voice and call out his name again. If Zhang Hua has heard your voice and tells you: "Keep your voice down, you're scaring others," you will lower your voice and talk to him. Power control can ensure that the power transmitted by each user reaches the base station at a minimum, which can meet the minimum communication requirements while avoiding unnecessary interference to other users' signals and maximizing system capacity. When a mobile phone moves in a cell, its transmission power needs to change. When it is closer to the base station, it needs to reduce the transmission power to reduce interference to other users. When it is farther away from the base station, it should increase the power to overcome the increased path loss. Maxwell's Equations Interesting fact: Maxwell's later life was full of troubles. No one understood his theories, and his wife was ill for a long time. This double misfortune made him exhausted. In order to take care of his wife, he once did not sleep in bed for three weeks. Despite this, his lectures and laboratory work never stopped. 1879 was the last year of Maxwell's life, and he still persisted in promoting electromagnetic theory. At this time, there were only two listeners to his lectures. One was a graduate student from the United States, and the other was Fleming, who later invented the electron tube. In the empty lecture hall, there were only two students sitting in the front row. Maxwell walked up to the podium with his handouts, his face was thin, and his expression was serious and solemn. It seemed that he was not explaining his theory to two listeners, but to the whole world. On November 5, 1879, Maxwell died of cancer at the age of 49. His achievements were not taken seriously when he was alive. It was not until Hertz proved the existence of electromagnetic waves that he was recognized as "the greatest mathematical physicist in the world after Newton." Maxwell's equations Maxwell's equations describe the four basic equations of electric and magnetic fields, among which: No.1 equation: describes the properties 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. The induced electric field is a vortex field, and its electric displacement lines are closed and do not contribute to the flux of the closed surface. Equation No.2: describes the properties 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 their magnetic induction lines are all closed lines, which do not contribute to the flux of the closed surface. Equation No.3: describes the law of the electric field excited by the changing magnetic field. Equation No.4: describes the law of the magnetic field excited by the changing electric field. Electromagnetic wave (should be the first wireless word to be mentioned) Interesting fact: There were 24 million "housed" sparrows in the UK. These sparrows built their nests in the attics of houses and played in the gardens of each family every day, becoming a landscape in the UK. However, in recent years, the number of sparrows in the UK has suddenly decreased sharply. British scientists are puzzled by this. Some people think that cats ate sparrows, some think that unleaded gasoline affected the survival of insects, which sparrows rely on to feed their young, and some think that the attics of buildings were closed, making it impossible for sparrows to build nests. Recently, British scientists and zoologists pointed out that electromagnetic waves emitted by mobile phones are the culprit for the disappearance of sparrows. British people began to use mobile phones in large numbers in 1994. It was during these years that the number of sparrows in Britain began to decrease significantly. Studies have shown that electromagnetic waves affect the sense of direction of sparrows. Sparrows rely on the earth's magnetic field to distinguish directions. Electromagnetic waves interfere with the ability of sparrows to find their way, causing them to lose their way. Studies have also shown that electromagnetic waves can also affect the number of sperm and ovulation function of animals. Electromagnetic waves are a form of motion of the electromagnetic field. Electricity and magnetism can be said to be two sides of the same coin. Electric current will produce a magnetic field, and a changing magnetic field will produce electric current. The changing electric field and the changing magnetic field constitute an inseparable unified field. In low-frequency electrical oscillations, the mutual change between magnetism and electricity is relatively slow, and almost all of its energy returns to the original circuit without energy radiation; in high-frequency electrical oscillations, the mutual change between magnetism and electricity is very fast, and it is impossible for all energy to return to the original oscillation circuit. Therefore, electric energy and magnetic energy propagate into space in the form of electromagnetic waves with the periodic change of electric and magnetic fields. Energy can be transmitted to the outside without a medium, which is a kind of radiation. Electromagnetic waves are a kind of energy. Any object above absolute zero will release electromagnetic waves. In addition to light waves, people cannot see the ubiquitous electromagnetic waves. Doppler effect Example: When the sirens of police cars and the engines of racing cars approach us at a certain speed, the sound will be more harsh than usual.When the sound is far away from us, it will be softer; in the same way, you can hear the change of harsh sound when the train passes by, which shows the existence of Doppler effect. The Doppler effect states that the receiving frequency of the wave becomes higher when the wave source moves toward the observer, and the receiving frequency becomes lower when the wave source moves away from the observer. In mobile communication, when the mobile station moves toward the base station, the frequency becomes higher, and when it moves away from the base station, the frequency becomes lower. Astronomer Hubble used the Doppler effect to conclude that the universe is expanding. In medicine, the Doppler effect is used to judge the oxygen supply during blood circulation, atherosclerosis and other conditions. Multipath effect Analogy: When we were young, we played with mud. When we poured water on the top of a small pile of mud, the water flowed away from all directions. A lot of water seeped into the soil or flowed in different directions and was lost. Some of the water flowed through different paths and at different times to a low-lying place. The multipath effect of radio waves refers to the phenomenon that the signal from the transmitter to the receiver often has many transmission paths with different delays and losses, which can be direct, reflected or diffracted. The superposition of the same signal on different paths at the receiving end will increase or decrease the energy of the received signal. White noise Analogy: When an old electrical device such as a radio is turned on, you may hear a "buzzing" sound. White noise refers to noise with a power spectrum density that is uniformly distributed throughout the frequency domain. Random noise with the same energy at all frequencies is called white noise. From the frequency response of our ears, it sounds like a very bright "hissing" sound. White noise is a random signal or random process with a constant power spectrum density. The power of this signal is the same in each frequency band. The ideal white noise has infinite bandwidth, so its energy is infinite. This is impossible in the real world, but it makes it more convenient for us in mathematical analysis. Generally, as long as the spectrum width of a noise process is much larger than the bandwidth of the system it acts on, and its spectrum density in this bandwidth can basically be considered as a constant, 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 has a Gaussian distribution for its amplitude distribution and a uniform distribution for its power spectrum density, it is called Gaussian white noise. The envelope of the sum of two orthogonal Gaussian noise signals follows a Rayleigh distribution. The amplitude follows a Gaussian distribution, which means that its amplitude probability density distribution is symmetrical about the mean, is maximum at the mean, and has a curve inflection point at a variance. The linear combination of Gaussian noise is still Gaussian noise. When summing the noise generated by independent noise sources, the power can be directly added. Hertz Interlude: Seven years before Hertz's experiment, a man named David also received an electromagnetic wave signal. He immediately reported it to Stokes, the president of the Royal Society of the United Kingdom, but Stokes thought it was just an ordinary electromagnetic induction phenomenon. David was too superstitious about authority and did not pay attention to this godsend, so the discovery was buried. Hertz, a German physicist, made his greatest contribution to mankind by experimentally proving the existence of electromagnetic waves. In January 1888, Hertz summarized his research results in the article "On the Propagation Velocity of the Electrokinetic Effect". After the publication of Hertz's experiment, it caused a sensation in the scientific community around the world. The electromagnetic theory pioneered by Faraday and summarized by Maxwell had only achieved a decisive victory. In honor of 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 changes per second. Diffraction Analogy: See "Direct Wave" When the wireless path between the receiver and the transmitter is blocked by a sharp edge, the phenomenon of radio waves traveling around the obstacle is called diffraction. During diffraction, the path of the wave is changed or bent. The secondary waves generated by the blocking surface are scattered in space, even on the back of the blocking body. Diffraction loss is the loss caused by various obstacles to the transmission of radio waves. Direct Wave Analogy: In the sport of billiards, many laws are very similar to the laws of electromagnetic waves. If the ball hits the center of the ball directly and there is no obstruction when it is hit, the ball will move in a straight line; if the ball hits the edge of the table, it will move according to the law of the angle of incidence equal to the angle of reflection; if the mother ball is tangent to another ball, according to the force and direction, it can bypass the ball within the line of sight, which is very similar to diffraction; assuming that the distance between many balls in a range is no more than one ball, when the mother ball hits the middle of these balls, it will stimulate many balls to move in different directions, which is very similar to scattering. Insight: The most fundamental laws of many things in nature are interconnected. This is why the Tao can be spoken. But the laws we speak always feel a little lacking, and they are "not very Tao". The most fundamental Tao can only be understood. Radio waves that reach the receiving point in a straight line from the transmitting antenna are called direct waves. Free space radio wave propagation is the propagation of radio waves in a vacuum, which is an ideal propagation condition. When radio waves propagate in free space, they can be considered as direct wave propagation, and their energy will neither be absorbed by obstacles nor reflected or scattered. Reflection wave Analogy: See "direct wave" Application: When selecting a station for wireless coverage on a high-speed railway, attention should be paid to the angle of incidence of the radio wave. The candidate station site cannot be too far away, otherwise the angle of incidence will be too large and the refraction ability entering the carriage will be reduced. Generally, a station site about 100 meters away from the railway is selected (other factors need to be considered, which will be discussed later). The wireless signal is reflected by the ground or other obstacles to reach the receiving point, which is called a reflected wave. Reflection occurs on the surface of the earth, buildings and walls. Reflection waves only occur at the interface between two propagation media with different densities. The greater the density difference of the interface medium, the greater the reflection amount of the wave and the smaller the refraction amount. The smaller the angle of incidence of the wave, the smaller the reflection amount and the greater the refraction amount. Direct waves and reflected waves are collectively called space waves. Scattered Wave Analogy: I saw a car accident not long ago. Many vehicles were driving, and the distance between them was not enough for another car to pass through. However, a car behind the vehicle rushed into the middle of the other vehicles without any deceleration, and the situation was horrible. Scattering occurs when there are objects smaller than the wavelength in the medium through which the radio waves pass, and the number of obstructions per unit volume is very large; scattered waves are generated by rough surfaces, small objects or other irregular objects. 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 I was studying at an engineering university, there were very few girls, and everyone felt very mysterious about women's lives. Fortunately, there was a female dormitory building at right angles to our male dormitory building, and the water room was near the end of the male dormitory. In the summer, you could only hear the sound of water but not see it. A classmate said, "Oh, it's a pity that it's non-line-of-sight transmission." Not long after, it was discovered that the classmate had creatively installed a reflector on the wall not far away. This guy used a telescope to look for half an hour every day. Eventually, he was discovered by a girl. When the wireless signal is blocked from the transmitting point to the receiving end by obstacles and cannot be transmitted in a straight line, it is called non-line-of-sight transmission. The wireless propagation loss of non-line-of-sight transmission is much greater than that of line-of-sight transmission. Fresnel Zone Analogy: Sometimes, I feel that the most effective visual range of the human eye is also an ellipsoid. Although things outside the ellipsoid can be seen, they are no longer particularly clear. For a well-trained shooting athlete, his effective visual range must be concentrated in the ellipsoid with a very small radius between him and the target. Application: When surveying wireless sites, be sure to pay attention to whether there are obstacles greater than the Fresnel radius in the coverage area. Especially obstacles such as large billboards and tall buildings. The Fresnel zone is an ellipsoid, and the transmitting and receiving antennas are located at the two foci of the ellipsoid. The radius of this ellipsoid is the first Fresnel radius. In free space, the electromagnetic energy radiated from the transmitting point to the receiving point is mainly propagated through the first Fresnel zone. As long as the first Fresnel zone is not blocked, the propagation conditions close to free space can be obtained. To ensure normal communication of the system, the height of the transmitting and receiving antennas must be such that the obstacles between them do not exceed 20% of their Fresnel zone as much as possible, otherwise the multipath propagation of electromagnetic waves will have adverse effects, resulting in a decrease in communication quality or even interruption of communication. Free space propagation model Insight: Lao Tzu said: The most difficult things in the world must be done from the easy; the most important things in the world must be done from the details. In the process of researching and modeling many physical phenomena, we first consider the most essential and simplest laws in complex phenomena, and then consider some non-essential influencing factors. Application: In the actual wireless environment, as long as the wireless signal is not blocked in the first Fresnel zone, it can be considered to be propagating in free space. In this way, it is very simple to estimate the propagation loss. Interesting facts: I was walking on the streets of Beijing with a colleague, and he joked with me: "I have been working in wireless for a long time, and I can feel how strong the TD signal is in the place where I am walking. The signal here is -78dBm." We looked at the signal size on the test phone, which was -77.5dBm. I said, "You are almost a test phone!" Radio waves propagate unobstructed in free space, without reflection, refraction, diffraction, scattering and absorption. However, after the radio wave propagates through a path, the energy will still be attenuated, which is caused by the diffusion of radiation energy. Free space propagation loss is the ratio of the energy of the wireless signal at the transmitting point to the effective receiving area of the receiving antenna, which is evenly diffused outward on the entire sphere, and diffused to the receiving antenna, to the total energy transmitted. Finally, the free space propagation formula derived is: L=32.45+20log(dkm)+20log(fMHz)(dB) When f=2000MHz, the formula can be simplified to: L=38.45+20log(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 waves; when the frequency of the signal is given, it is only related to the distance. In the actual propagation environment, the environmental factor n must also be considered, and the formula is simplified to L=38.45+10*n*log(dm). n can generally be between 2 and 5 depending on the environment. The brother in front knows the power of the antenna port. Using the simplified propagation model mentioned above, he estimates that he is 100 meters away from the TD antenna, and then calculates the radio wave strength at his location. When understanding the simplified formula for radio wave propagation at 2000MHz, please note: 1. The loss at 1 meter is 38.45dB, and the loss at 10 meters is 58.45dB; 2. When the distance doubles, the loss increases by 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 asked what the free space propagation loss is per 100 meters. This question itself is wrong because the loss of the first 100 meters and the second 100 meters that the wireless signal travels are different. Ultra High Frequency (UHF) Ultra High Frequency: decimeter band, refers to ultra-high frequency radio waves with a frequency of 300~3000MHz. Radio waves are distributed between 3Hz and 3000GHz, and are divided into 12 bands within this spectrum. The frequency propagation characteristics in different frequency bands are different. The smaller the frequency, the smaller the propagation loss, the longer the coverage distance, and the stronger the diffraction ability. However, the frequency resources in the low-frequency band are tight and the system capacity is limited. The frequency resources in the high-frequency band are rich and the system capacity is large; but the higher the frequency, the greater the propagation loss, the smaller the coverage distance, the weaker the diffraction ability, the greater the technical difficulty of implementation, and the cost of the system is correspondingly increased. The mobile communication system should consider the coverage effect and capacity when selecting the frequency band to be used. 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 1,000 meters (frequency of 30 to 300 kHz). Long-wave communication is mainly used in military applications, such as submarine communication, underground communication and navigation. Within 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 ground waves, sky waves are used to propagate signals. The advantages of long-wave communication are: long communication distance, ability to penetrate mountains and seawater to a certain depth, and relatively stable and reliable communication. Its disadvantages are: due to the ultra-long wavelength, the transceiver equipment and antenna system are huge and expensive; the passband is narrow, not suitable for multi-channel and fast communication; and susceptible to interference from sky waves. . 45dB; 2. When the distance doubles, the loss increases by 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 asked what the free space propagation loss is per 100 meters. This question itself is wrong because the loss of the first 100 meters and the second 100 meters that the wireless signal travels are different. Ultra High Frequency (UHF) Ultra High Frequency: decimeter band, refers to ultra-high frequency radio waves with a frequency of 300~3000MHz. Radio waves are distributed between 3Hz and 3000GHz, and are divided into 12 bands within this spectrum. The frequency propagation characteristics in different frequency bands are different. The smaller the frequency, the smaller the propagation loss, the longer the coverage distance, and the stronger the diffraction ability. However, the frequency resources in the low-frequency band are tight and the system capacity is limited. The frequency resources in the high-frequency band are rich and the system capacity is large; but the higher the frequency, the greater the propagation loss, the smaller the coverage distance, the weaker the diffraction ability, the greater the technical difficulty of implementation, and the cost of the system is correspondingly increased. The mobile communication system should consider the coverage effect and capacity when selecting the frequency band to be used. 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 1,000 meters (frequency of 30 to 300 kHz). Long-wave communication is mainly used in military applications, such as submarine communication, underground communication and navigation. Within 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 ground waves, sky waves are used to propagate signals. The advantages of long-wave communication are: long communication distance, ability to penetrate mountains and seawater to a certain depth, and relatively stable and reliable communication. Its disadvantages are: due to the ultra-long wavelength, the transceiver equipment and antenna system are huge and expensive; the passband is narrow, not suitable for multi-channel and fast communication; and susceptible to interference from sky waves. . 45dB; 2. When the distance doubles, the loss increases by 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 asked what the free space propagation loss is per 100 meters. This question itself is wrong because the loss of the first 100 meters and the second 100 meters that the wireless signal travels are different. Ultra High Frequency (UHF) Ultra High Frequency: decimeter band, refers to ultra-high frequency radio waves with a frequency of 300~3000MHz. Radio waves are distributed between 3Hz and 3000GHz, and are divided into 12 bands within this spectrum. The frequency propagation characteristics in different frequency bands are different. The smaller the frequency, the smaller the propagation loss, the longer the coverage distance, and the stronger the diffraction ability. However, the frequency resources in the low-frequency band are tight and the system capacity is limited. The frequency resources in the high-frequency band are rich and the system capacity is large; but the higher the frequency, the greater the propagation loss, the smaller the coverage distance, the weaker the diffraction ability, the greater the technical difficulty of implementation, and the cost of the system is correspondingly increased. The mobile communication system should consider the coverage effect and capacity when selecting the frequency band to be used. 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 1,000 meters (frequency of 30 to 300 kHz). Long-wave communication is mainly used in military applications, such as submarine communication, underground communication and navigation. Within 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 ground waves, sky waves are used to propagate signals. The advantages of long-wave communication are: long communication distance, ability to penetrate mountains and seawater to a certain depth, and relatively stable and reliable communication. Its disadvantages are: due to the ultra-long wavelength, the transceiver equipment and antenna system are huge and expensive; the passband is narrow, not suitable for multi-channel and fast communication; and susceptible to interference from sky waves. .
tiankai001 posted on 2018-12-24 08:33 No wonder I don’t know wireless communication. I have never heard of more than 15 of these 20 terms, let alone know what they mean
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