Shulou(Shulou.com)06/01 Report--

Shannon's theorem

Analogy: what does the speed of cars on urban roads have to do with? It has something to do with the width of the road, the power of your car, and other disturbing 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 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 of the maximum throughput of single carrier supported by different 3G systems due to different bandwidths.

Skin effect

Analogy: after the heavy rain, the dirt road in the countryside was filled with water, so everyone had to line up along the roadside. The effective passing area of the road is reduced due to stagnant water, which affects people's travel efficiency.

Because the inductance inside the conductor hinders the alternating current more than the surface, when the alternating current passes through the conductor, the current density of each part is uneven, and the surface current density 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 is, the more significant the skin effect is, and the frequency is high to a certain extent, so 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 insulated thin wires are woven into bundles to weaken the skin effect.

Coherent time

Analogy: twin brothers who wear the same clothes and look similar appear side by side at the same time, it is difficult for ordinary people to distinguish. If they take pictures side by side with the same action, it seems that a person has a double image, and the viewer thinks he is dazzled.

The coherent time is the maximum time difference range in which the channel remains constant. The same signal of the transmitter arrives at the receiver within the coherent time. The fading characteristics of the signal are completely similar, and the receiver considers it to be a signal. If the autocorrelation of the signal is not good, interference may also be introduced, which is dazzling like ghosting in photography. From the point of view of transmit diversity, time diversity requires that the time of two transmissions is greater than the coherent time of the channel, that is, if the transmitting time is less than the coherent time of the channel, the signals transmitted two times will experience the same fading. The anti-fading effect of diversity does not exist. Each chip of TD-SCDMA is 0.78us, that is, the coherence time between chips is 0.78us. If the chip of the same signal reaches the receiver through different paths beyond this time, it will have the effect of multipath diversity; otherwise, it will form self-interference.

Coherent bandwidth (1 / coherent time)

Analogy: on the busy trunk lines of the city, half of a section of the road is under renovation. As the road changes from wide to thin, the speed of traffic needs to slow down, and some cars are squeezed into bike lanes, while others make a detour.

Coherent bandwidth is an important parameter to characterize the characteristics of multipath channels. it refers to a specific frequency range in which any two frequency components have a strong amplitude correlation, that is, in the coherent bandwidth range, multipath channels have constant gain and linear phase. In a wireless communication system, if the bandwidth of the signal is less than the coherent bandwidth of the channel, the received signal will undergo a flat fading process, and the spectral characteristics of the transmitted signal can remain unchanged in the receiver. If the bandwidth of the signal is greater than the coherent bandwidth of the channel, the received signal will experience frequency selective fading, and some frequencies of the received signal will gain more than other components, resulting in distortion of the received signal, resulting in inter-symbol interference.

Power control

Analogy: when you want to stop your friend Zhang Hua who is walking in front of you, you shout his name: "Hello, Zhang Hua!" If you find that he is not listening, you will raise your voice and shout his name again. If Zhang Hua has heard your voice, he tells you, "keep your voice down and scare others." You will lower your voice and talk to him.

Power control can ensure that the transmitted power of each user to the base station is kept to a minimum, which can not only meet the lowest communication requirements, but also avoid unnecessary interference to other user signals and maximize the system capacity. When the mobile phone moves in the cell, its transmission power needs to be changed. When it is close to the base station, it needs to reduce the transmission power and reduce the interference to other users, and when it is far away from the base station, it should increase the power to overcome the increased path attenuation.

Maxwell equations

Interesting news: 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 exhausted him. He didn't sleep in bed for three whole weeks in order to take care of his wife. In spite of this, his speech and his laboratory work were never interrupted. In 1879, the last year of his life, Maxwell still relentlessly promoted the theory of electromagnetism. At this time, there were only two people in his lecture. One is a graduate student from the United States, and the other is Fleming, who later invented the electronic tube. In the empty ladder classroom, there are only two students sitting in the front row. Maxwell walked firmly to the podium with his handout under his arm, his face thin, serious and solemn. It was as if he was not explaining his theory to two listeners, but to the whole world. On November 5, 1879, Maxwell died at the age of 49. His exploits were not taken seriously when he was alive. It was only after Hertz proved the existence of electromagnetic waves that he was recognized as "the greatest mathematical physicist in the world since Newton".

Maxwell equations Maxwell's equations describes four basic equations of electric field and magnetic field, among which:

No.1 equation: describes the properties of the electric field. In general, the electric field can be either a Coulomb electric field or an induced electric field excited by a changing magnetic field, while the induced electric field is a vortex field whose potential shift line is closed and has no contribution to the flux of the closed surface.

No.2 equation: 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 the magnetic induction lines are closed lines, which have no contribution to the flux of the closed surface.

No.3 equation: describes the law of the electric field excited by a changing magnetic field.

No.4 equation: describes the law of magnetic field excited by a changing electric field.

electromagnetic wave

Electromagnetic wave (should be the first wireless word spoken)

Anecdote: there were 24 million "domestic" sparrows in Britain. These sparrows make their nests in the attic of the house and play in the gardens every day. In recent years, however, the British sparrow population has suddenly declined sharply. British scientists are puzzled by this. Some people think that the cat ate the sparrow, some people think that unleaded gasoline affects the survival of the worm, and the sparrow depends on this bug to feed the young sparrow, and some people think that the building attic is closed, making it impossible for the sparrow to build a nest. Recently, British scientists and zoologists have pointed out that electromagnetic waves emitted by mobile phones are the main culprit for the disappearance of sparrows. The British have been using a lot of mobile phones since 1994. It was during these years that the British sparrow began to decline greatly. Studies have shown that electromagnetic waves affect the sparrow's sense of direction. The sparrow relies on the earth's magnetic field to find its direction. The electromagnetic wave will interfere with the sparrow's ability to find its 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 wave is a kind of motion form of electromagnetic field. Electricity and magnetism can be said to be one and two sides, the current will produce a magnetic field, and the changing magnetic field will produce an electric 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 magnetoelectricity and electricity is relatively slow, and almost all the energy returns to the original circuit without energy radiation. In the high-frequency electrical oscillation, the magnetoelectric interaction is very fast, and the energy can not all return to the original oscillation circuit, so the electric energy and magnetic energy propagate to the space in the form of electromagnetic wave with the periodic change of electric field and magnetic field, and can transfer energy to the outside without a medium. This is a kind of radiation. Electromagnetic wave is a kind of energy. Any object above absolute zero will emit electromagnetic wave. Apart from light waves, people can't see electromagnetic waves everywhere.

Doppler effect

Doppler effect

Example: when the alarm sound of the police car and the engine approaches us at a certain speed, the sound will be harsher than usual. The sound softens when you are away from us; by the same token, you can hear a shrill change as the train passes by, indicating the existence of the Doppler effect.

The Doppler effect indicates that the receiving frequency of the wave becomes higher when the wave source moves to the observer and 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 away from the base station, the frequency becomes lower. Astronomer Hubble used the Doppler effect to conclude that the universe is expanding. In medicine, Doppler effect is used to judge the oxygen supply in the process of blood circulation, atherosclerosis and so on.

Multipath effect

Analogy: when we were young, we all played with dirt, poured water at the top of a small mound, and the water flowed away from everywhere, and a lot of water seeped into the soil or was lost in different directions. some of the water flows through different paths and at different times to a low-lying place.

The multipath effect of radio waves means that there are often many transmission paths with different delay and loss from the transmitter to the receiver, which can be direct, reflection or diffract. the superposition of the same signals with 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, a "buzzing" sound may be heard

White noise refers to the noise whose power spectral density is uniformly distributed in the whole frequency domain. Random noise with the same energy at all frequencies is called white noise. It sounds very bright "sizzling" from the frequency response of our ears. White noise is a kind of random signal or random process with constant power spectrum density. The power of this signal is the same in all frequency bands, and the ideal white noise has infinite bandwidth, so its energy is infinite, which is impossible in the real world, but it makes us more convenient in mathematical analysis. For one thing, as long as the spectrum width of a noise process is much larger than the bandwidth of the system in which it operates, and its spectrum density can basically be considered as a constant, it can be treated as white noise. Thermal noise can be thought of 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 whose amplitude distribution obeys Gaussian distribution and its power spectral density is uniformly distributed, it is called Gaussian white noise. The envelope of the sum of two orthogonal Gaussian noise signals obeys Rayleigh distribution. The amplitude obeys the Gaussian distribution is that the amplitude probability density distribution takes the mean as the axisymmetric, the maximum at the mean, and the inflection point of the curve at a variance. The linear combination of Gaussian noise is still Gaussian noise. When summing the noise generated by independent noise sources, it can be added directly according to the power.

Hertz

Interlude: seven years before the Hertz experiment, a man named David also received an electromagnetic wave signal, and he immediately reported it to Stokes, president of the British × × Association, but Stokes thought it was just an ordinary electromagnetic induction phenomenon. David was too superstitious in authority and paid no attention to the opportunity given by this day, so that the discovery was buried.

Hertz, a German physicist, Hertz's greatest contribution to mankind is to prove the existence of electromagnetic waves by experiments. In January 1888, Hertz summarized his research results in the article "on the Propagation Speed of electrokinetic effect". After the Hertz experiment was announced, it caused a sensation in the scientific community all over the world. The electromagnetic theory pioneered by Faraday and summed up by Maxwell won 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 periodic repetitions per second.

Diffraction

Analogy: see "Direct Wave"

When the wireless path between the receiver and the transmitter is blocked by sharp edges, the phenomenon that radio waves travel around obstacles is called diffraction. During diffraction, the path of the wave changes or bends. The secondary waves generated by the barrier surface are scattered in space, even on the back of the barrier. Diffraction loss is the loss caused by various obstacles to the transmission of radio waves.

Direct wave

Direct Wave

Analogy: in billiards, many laws are very similar to the laws of electromagnetic waves. If the ball hits the center of the ball directly, if there is no hindrance, the ball will move in a straight line; if the ball hits the edge of the table, it will operate according to the law of incident angle such as the angle of reflection; if the cue ball is tangent to another ball, according to its strength and direction, it can bypass the ball within the line of sight, much like diffraction Assuming that many balls in a range are no more than one ball apart from each other, when the cue ball hits the middle of these balls, it will arouse many balls to move in different directions, much like scattering.

Perception: the most fundamental laws of many things in nature are the same. That's why Dao can do it. However, we always feel that there is some deficiency in the law we speak out, and it is also "extraordinary Tao". The most fundamental Tao can only be realized.

The radio wave from the transmitting antenna to the receiving point in a straight line is called direct wave. Radio wave propagation in free space is the propagation of radio waves in vacuum and is an ideal propagation condition. When the radio wave propagates in free space, it can be regarded as direct wave propagation, and its energy will not be absorbed by obstacles, nor will it be reflected or scattered.

Reflected wave

Reflected wave Reflection wave

Analogy: see "Direct Wave"

Application: when selecting stations with wireless coverage in high-speed railway, we should pay attention to the incident angle of radio waves. The alternative site should not be too far away, otherwise the angle of incidence is too large and the refraction capacity entering the carriage will be reduced. Generally choose the station site about 100 meters away from the railway (there are other factors to be considered, later).

The wireless signal is reflected to the receiving point through the ground or other obstacles, which is called reflected wave. Reflection occurs on the surface of the earth, buildings and walls. The reflected wave occurs only in the interface of two kinds of media with different density. the greater the density difference of the interface medium is, the greater the reflected amount of the wave is and the smaller the refractive amount is. The smaller the incident angle of the wave is, the smaller the amount of reflection is and the greater the amount of refraction is. Direct and reflected waves are called space waves.

Scattered wave

Scattered Wave

Analogy: not long ago saw a car accident, a lot of vehicles are driving, the distance between each other is not enough to pass another car. But there is a car behind the car without any deceleration from behind into the middle of a large number of vehicles, the current situation is terrible.

Scattering occurs when there are objects smaller than wavelengths in the medium through which radio waves travel, and the number of barriers per unit volume is very large; scattered waves are produced on rough surfaces, small objects or other irregular objects. In the actual communication system, leaves, street signs and lamp posts will cause scattering.

NLOS transmission

NLOS,Non Line of Sight

Interesting fact: when I was a student at the engineering university, there were very few girls, and everyone was very mysterious about the life of women. Fortunately, at right angles to our boys' dormitory is a girls' dormitory, and the water room is at the end of the boys' building. In summer, the sound of water can only be heard, but not seen. A classmate said, "Oh, it's a pity that it's non-line-of-sight transmission." After a while, I found that the student creatively installed a mirror on the wall not far away. He looked at it with binoculars for half an hour every day. Finally, the girl found out.

The wireless signal is blocked by obstacles from the transmitting point to the receiving end and cannot be propagated 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 that of line-of-sight transmission.

Fresnel zone

Fresnel Zone

Analogy: sometimes, I feel that the most effective range of vision of the human eye is also an ellipsoid. Things outside the ellipsoid can also be seen, but they are no longer particularly clear. For a trained shooter, his effective vision must be concentrated in the ellipsoid with a very small radius of his and the target.

Application: in the wireless site survey, be sure to pay attention to whether the coverage area is larger than the Fresnel radius. Especially big billboards, tall buildings and other obstacles.

The Fresnel region is an ellipsoid, and the transceiver antenna is located on the two focal points of the ellipsoid. The radius of this ellipsoid is the first Fresnel radius. In the free space, the electromagnetic energy radiated from the emitting point to the receiving point is mainly transmitted through the first Fresnel zone. As long as the first Fresnel region is not blocked, the propagation conditions 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 such that the obstacles between them should not exceed 20% of their Fresnel zone as much as possible, otherwise the multipath propagation of electromagnetic waves will have an adverse effect, resulting in a decline in communication quality and even interruption of communication.

Free space propagation model

Free space propagation Model

Feeling: Laozi said: the difficult things in the world must be done in the easy; the great things in the world must be done in detail. In the process of research and modeling of many physical phenomena, we first consider the most essential and simple laws of complex phenomena, and then consider some non-essential 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 propagated in free space. In this way, when estimating the propagation loss, it can be very simple.

Anecdote: I was walking down the street in Beijing with a colleague who joked: "after being wireless for a long time, I can feel how big the TD signal is in this place I'm walking. The signal here is-78dBm." We took a look at the size of the signal on the test phone, which is-77.5dBm. I said, "you're almost a test phone!"

Radio waves propagate unhindered in free space and do not produce reflection, refraction, diffraction, scattering and absorption. However, when the radio wave travels through a path, the energy will still be attenuated, which is caused by the diffusion of radiant energy.

The free space propagation loss is the ratio of the energy falling on the effective receiving area of the antenna to the total energy transmitted by the wireless signal of the transmitting point which spreads uniformly in the whole sphere and spreads to the receiving antenna.

Finally, the propagation formula in free space is derived.

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 frequency; when the frequency of the signal is given, it is only related to the distance. In the actual propagation environment, if the environmental factor n is also considered, the formula is simplified to L=38.45+10*n*log (dm). N is generally recommended between 2 and 5 according to the environment. The previous brother knows the power of the antenna mouth, uses the above simplified propagation model, estimates his distance from the TD antenna to 100m, and then calculates the radio wave intensity of his location.

(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 talk about it.)

When understanding the simplified formula of radio wave propagation in 2000MHz, we should pay attention to:

1. The loss is 38.45dB at 1 meter and 58.45dB at 10 meters.

2. When the distance is doubled, the loss is increased by 6dB (many students mistakenly think it is 3dB)

3. The loss in free space does not increase linearly with distance, but exponentially. Some students ask what is the propagation loss per 100 meters of free space. The question itself is wrong. Because the wireless signal through the first 100 meters and the second 100 meters loss is not the same.

Ultra-high frequency UHF

Ultra High Frequency

UHF: decimeter band, refers to the frequency of 300~3000MHz UHF radio waves.

Radio waves are distributed between 3Hz and 3000GHz and are divided into 12 bands in this spectrum. The frequency propagation characteristics are different in different frequency bands. The smaller the frequency is, the smaller the propagation loss is, the farther the coverage distance is, the stronger the diffraction ability is. However, the low-frequency band frequency resources are tight, and the system capacity is limited. The high frequency band is rich in frequency resources 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 to achieve, and the higher the cost of the system.

The frequency band used in the selection of mobile communication system should comprehensively consider the coverage effect and capacity. Compared with other frequency bands, UHF band has a better tradeoff between coverage effect and capacity, so it is widely used in the field of mobile communication.

Reference: long wave communication, radio communication with a wavelength of 10000m / 1000m (frequency of 30mm / 300kHz). Long-wave communication is mainly used in military, such as submarine communication, underground communication and navigation. In a certain range, long-wave communication is dominated by ground wave propagation, and when the communication distance is greater than the maximum propagation distance of ground wave, the signal is propagated by sky wave. The advantages of long-wave communication are: the communication distance is long, it can pass through the mountain and sea water to a certain depth, and the communication is relatively stable and reliable. Its disadvantages are: due to the ultra-long wavelength, large transceiver equipment and antenna system, high cost; narrow passband, not suitable for multi-channel and fast communication; vulnerable to astronomical interference. no, no, no.

Shadow effect

Shadowing Effect

Analogy: when the genial sun shines on the earth, trees and houses have shadows. This shadow is not completely dark, but a kind of light with much weaker intensity.

In the propagation path, when radio waves are blocked by obstacles such as uneven terrain, buildings of different heights and tall trees, a shadow area with weak field strength of radio waves will be formed behind the barriers. 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 violently, the change of the 5-week line is relatively slow.

In the process of radio wave propagation, the median value of the signal strength curve changes slowly, which is called slow fading. Slow fading reflects the median after the instantaneous weighted average, and reflects the mean change of the receiving level of hundreds of wavelengths in the medium range, which generally obeys the lognormal distribution.

The causes of slow decline:

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 sharply, much like a fast decline.

Fast fading is the phenomenon of instantaneous rapid fluctuation and rapid change of the field strength of the received signal. Fast fading is due to the superposition of multipath signals caused by various topography, ground objects and moving objects at the receiving point, and the amplitude of the superimposed signal fluctuates violently due to the different phase, frequency and amplitude of the received multipath signals. When the mobile station is running at high speed, the carrier frequency range of the received wireless signal changes continuously with time, which can also cause a sharp change in the amplitude of the superimposed signal. In other words, multipath effect and Doppler effect can cause fast fading.

General fast fading can be subdivided into:

1) the spatial selective fading is caused by multipath effect, that is, the fading characteristics are different in different locations and different transmission paths.

2) the change of carrier frequency causes the range of carrier width to exceed the range of coherent bandwidth, which causes signal distortion, which is called frequency selective fading.

3) Doppler effect or multipath effect can cause different signals to arrive at the receiving point of different time difference, beyond the coherent time, the signal distortion caused by time selective fading.

Time dispersion

Time Dispersion

Analogy: a girl first has a handsome guy like, after a while, there is an equally handsome boy likes her, she does not know how to choose.

In wireless communication, the problem of co-frequency interference caused by the difference between the main signal arriving at the receiver and other multipath signals in space. Time dispersion can cause wireless signals reflected from objects far away from the receiving antenna to arrive at the receiver several symbols slower than the direct signal, which may lead to inter-symbol interference. For example, "1" affects "0" and causes the receiver to decode errors.

Propagation loss

Propagation Loss

Analogy: people who do long-distance vegetable trafficking know that if the cabbage bought from farmers is 10 cents per jin, plus the transportation fees, booth fees, taxes, packaging fees, and so on, the final consumers will get at least 50 cents per jin. In the end, the vegetable seller needs to subtract all the loss of profits from the total turnover.

In the wireless system with given frequency, the wireless propagation loss is mainly the path loss (Path Loss) that varies with the distance. The three most basic propagation mechanisms that affect the path loss are reflection, diffraction and scattering, namely reflection loss (Reflection Loss), diffraction loss (Scattered Loss) and ground loss (Clutter Loss). If the electromagnetic wave passes through walls, cars, trees and other obstacles, it is also necessary to consider the penetration loss (Penetration Loss). If the mobile phone is used close to the human body, it is also necessary to consider human body wear (Body Loss) and so on.

The environmental factor coefficient n of path loss generally varies with the propagation environment, which is 4: 5 in dense urban areas, 3-4 in ordinary urban areas and 2.5-3 in suburbs. In the actual wireless environment, the height of the antenna can affect the path loss. Generally, if the height of transmitting antenna or receiving antenna is doubled, the propagation loss of 6dB can be compensated.

The reflection loss varies with the reflection surface, the reflection loss of water surface is 0~1dB, the reflection loss of wheat field is 2~4dB, and the reflection loss of city and mountain is 14dB~20dB.

The diffracted wave spreads around the diffraction point and spreads to all directions except obstacles, and the loss varies greatly in different cases. The loss of ground objects is mainly caused by surface scattering, and the size of the loss depends on the specific situation.

The penetration loss is closely related to the material of the building and the incident angle of the electromagnetic wave. In general, the partition blocks 5~20dB, the floor blocks each 20dB, the thick glass 6~10dB, the penetration loss of train cars is 15~30dB, and the penetration loss of elevators is about 30dB.

Human body loss generally takes three dB, that is, radio waves pass through the human body, and half of the energy is absorbed by the human body.

Propagation model Propagation Model

Funny analogy: a private company boss often stresses to everyone: "I want results, you give me results, I do not want the process." One day a mathematical modeler asks the boss to promote his universal mathematical model, which is characterized by the ability to give the results of any problem, and you don't have to care about the process, but only if you enter no more than three sets of data as required. The company uses this model to forecast sales, manpower demand, cost reduction, and so on, and the results prove to be very correct. So private enterprise bosses want to use this model to make a judgment about what kind of person they are and what kind of development they are. The universal mathematical model first requires him to input the data of the salary he has offered to his employees in the past year, again asks him to enter the attendance records of his employees to and from work, and finally asks him to enter the number of lovers, after half an hour of calculation, the model gives the calculation results: please don't make fun of the iron rooster who can't lay eggs.

In the actual wireless environment, it is impossible to have such ideal wireless propagation conditions as free space. Under the influence of different reflection, diffraction and scattering conditions, the variation law of the median of radio field strength is very complex, so it is difficult to calculate with a simple mathematical expression. The mathematical expression of radio wave propagation loss established by theory or measurement is called propagation model. There are two ways to study the propagation model: one is to analyze the mathematical law of the propagation loss of all electromagnetic waves from the transmitting point to the receiving point based on the wireless propagation theory; the other is to statistically analyze the mathematical law of propagation loss on the basis of a large number of test data.

Perception: human beings always want to use mathematical means to build models for numerous and complicated social and natural phenomena, so as to get some mathematical laws to guide our work and life. But unfortunately, any mathematical model is an approximate expression of the general law of the development and change of things, and can not fully accord with the reality. If the economic model works, the financial crisis will not break out; if the management model works, no company will fail; if the wireless communication model is absolutely accurate, wireless networks will not have weak coverage.

Ray tracing model

Ray Tracing Model

Analogy: thousands of people travel from Beijing to all parts of the country every day. If you want to know how many passengers leave from Beijing to Shanghai every day. In theory, all we have to do is add up all the possible transport kits from Beijing to Shanghai every day, including people who can be transported by planes, trains and cars. But you may think less about some people, they may run to Shanghai, or take a train to Tianjin, and then take a ship to Shanghai. But after all, there are a small number of such people, which have little impact on the calculation results.

The basic principle of ray tracing model analyzes all possible propagation paths of radio waves from transmitting point to receiving point in a certain scene, including direct transmission, transmission, diffraction, etc., through the superposition of signal vectors at the receiving point, the field strength of the received signal is calculated.

Volcano model, WaveSight model and WinProp model are typical ray tracing models.

The X-ray model needs a high-precision 3D digital map with a precision of at least 5m and a better precision of 1m. Because of the high accuracy of the map, it is expensive to model the wireless environment with this method, and it can only be used in dense urban areas. The accuracy of the model prediction is closely related to the accuracy of the digital map and the accuracy of the station parameters such as antenna position, antenna height, azimuth, dip angle and so on. At the same time, ray tracking models generally do not consider the impact of moving vehicles on wireless signal propagation, and ignore higher-order reflected / diffracted waves, ground reflected waves, electromagnetic waves passing under buildings, transmitted waves, diffuse reflected waves and so on.

Okumura model

Analogy: an American sociologist has studied the relationship between people's education and their annual income after work. Through the analysis of a large number of academic qualifications on resumes and the income data of current jobs, it is found that the annual income of doctoral students is $XXXX more than that of master's degree students, the annual income of master's degree students is $XXXX more than that of undergraduate students, and the annual income of undergraduate students is $XXXX more than those who do not go to university. Although this sociologist pioneered the study of the relationship between education level and annual income, in many cases this relationship model based on statistical analysis of actual data is not valid. This relationship does not explain that the annual income of undergraduates who have worked for many years may be higher than that of masters, without taking into account the differences in treatment in different industries and jobs. As a result, the later sociologist modeling took into account the impact of length of service, industry and position on income, and further improved the relationship model between education and income, which became more complex. However, there is still a situation that does not apply. The income of many founders is not so directly related to their education and seniority. So later sociologists published papers in this direction, revising the above relationship and making it apply to the group of enterprise founders.

Explanation:

The most famous wireless propagation model based on test data statistics is the Okumura model, which is represented by a graph by Okumura on the basis of a large number of test data in Japan. But its scope of application is narrow, and its application is not very convenient.

On the basis of Okumura model, Hata uses mathematical regression analysis to fit the empirical formula of wireless propagation, that is, Okumura-Hata formula, which is convenient for computer calculation, which is suitable for the wireless propagation of frequency in 150~1500MHz, such as GSM900. The formula can be applied to the wireless environment of ordinary urban, suburban and rural areas with a radius of 1-20km under the condition of macro cell (large area).

But then there is DCS1800, and the working frequency of 3G is about 2000MHz, and the original Okumura-Hata formula is not applicable. COST 231-Hata extends the frequency range of Okumura-Hata model to 2000MHz, but it is still only applicable to macro-cellular conditions.

With the increasing demand for wireless communication, the original macro-cellular network can not meet the high requirements of wireless network quality in dense urban areas, so it needs to be covered by micro-cell, so there is a Walfisch formula suitable for micro-cell.

The demand for wireless communication is still growing, and the number of indoor wireless users is increasing day by day. Only outdoor macro-cell coverage indoor can not meet the high requirements of wireless network quality, so it is necessary to build indoor distribution system, so the indoor Keenan-Motley model is produced.

Lee's criterion

Lee's Criteria

Analogy: brother Yunge told Wu Da Pan Jinlian and Ximen Qing that he had an affair with Ximen Qing. Wuda went back home twice after selling cooking cakes without touching him. He said, "my wife is a decent woman. How could this happen?" Brother Yunge reminded him, "how could you do such a thing after you sold the cookies?" it's impossible to be in your home, and you have to come back a few times at the right time to touch it, and you have to go to see Wang's house. "

Yunge said in the language of communication that your sampling times should be enough and the sampling location should be correct.

How to test the field strength of wireless signal and fully reflect the characteristics of wireless environment. Dr. William Lee published a famous paper on wireless signal field strength sampling in 1985. Through strict mathematical derivation, the standard of wireless signal field strength sampling is 3650 points in 40 wavelengths. This standard has been widely used in wireless communication engineering.

Understanding: assuming that our wireless system uses a frequency of 2000MHz and the scanner can hit a maximum of 100 dots per second, what is the speed limit for wireless environment testing?

The radio wavelength of the 2000MHz is 0.15m. 40 wavelengths are 6m, which means there must be 50 points within a distance of 6m. The frequency sweeper can hit a maximum of 100 points per second, that is, a maximum of 12 meters per second, that is, the speed can not be higher than the 12m/s, so it is not enough to walk more sampling points.

SPM model

Standard Propagation Model

Analogy: in the pre-Qin period, the characters of various vassal states were not unified, and it was very inconvenient for people from different countries to communicate. Finally, Qin Shihuang told the world that the word he used was the standard word, and everyone used it uniformly.

There are many forms and scope of application of wireless propagation model. Due to the disunity in form, it is very inconvenient for wireless engineers to use it, so it is difficult to have a unified understanding of the same wireless environment.

The introduction of SPM model solves this problem. The SPM model is suitable for a wide frequency range from 150MHz to 2GHz, and it is also suitable for a variety of wireless environments from dense urban areas, ordinary urban areas, suburbs and rural areas. Therefore, it is widely used at present.

Path Loss= K1 + K2log (d) + K3log (Htxeff) + K4Difficient + K5log (d) log (Htxeff) + K6 (HRxeff) + Kclutterf (clutter)

Where:

D: distance between receiver and transmitter (m)

HTxeff: effective height of the transmitting antenna (m)

Diffraction loss: diffraction loss caused by an obstacle path (dB)

HRxeff: effective height of the receiving antenna (m)

F (clutter): average weighted loss caused by ground features

K1: constant (dB);.

Coefficient of K2:log (d)

Coefficient of K3:log (HTxeff)

K4: coefficient of diffraction loss

K 5: the coefficient of log (HTxeff) log (d).

K6: coefficient of HRxeff.

Kclutter: the coefficient of f (clutter).

In the free-space propagation model, K3, K4, K5, K6 and Kclutter are all 0Magi K1, 38.45, K2, 20.

In the general wireless environment, the values of K1 and K1 are also very important, which have a great impact on the accuracy of the whole results, because when we use the propagation model to calculate, we are mainly concerned about the different positions away from the transmitter. What is my road loss and what is the signal field strength that can be obtained? While other factors, such as antenna height, do not change under certain circumstances.

Peak-to-average ratio

PAR Peak-to-Average Ratio

Analogy:

There are relatively rich and poor families in a village, but most of them are ordinary families with middle income. The ratio of the wealth of our richest families to the average wealth of village households or the ratio of the wealth of the poorest families to the average wealth of households can be used to measure the degree of polarization between the rich and the poor in the village.

But from a national point of view, it is not appropriate to compare the wealth at the top of the Hurun list with the average income of Chinese households, and it is not a comprehensive measure of the gap between rich and poor in China. If you use the average wealth of 1% of China's wealthy class and the average income of Chinese households, something can be said. If the average annual income of Chinese households is 30, 000 yuan, and the annual income of the richest households is 3 billion, 3 billion to 300 million, that is 100000 times, in terms of dB, it is 50dB.

If we study the wealth distribution of millionaires in natural villages across the country to show the level of economic development in different provinces, we can also use the concept of peak-to-average ratio, that is, the wealth of the richest village millionaires is compared with the average wealth of all village millionaires. In other words, the peak-to-average ratio must indicate what kind of peak to mean ratio, whether the unit is an absolute ratio or a DB value.

Explanation: the wireless signal is a sine wave whose amplitude is constantly changing in time domain, and the amplitude is not constant, and the peak amplitude of the signal in one cycle is different from that in other periods, so the average power and peak power of each cycle are different. Over a long period of time, the peak power is the maximum transient power that occurs with a certain probability, usually 0.01%. The ratio of the peak power under this probability to the total average power of the system is the peak-to-average ratio. The PAR at the probability of 0.01% is generally called the peak factor (CF CREST Factor,CF).

To understand the concept of PAPR, we need to pay attention to the following points:

1. Because the peak-to-average ratio of power is the square of the peak-to-average ratio of voltage, PAR generally refers to the peak-to-average ratio of power, but some books use it as the peak-to-average ratio of voltage.

two。 If the power amplitude does not change with time, that is, the "maximum envelope" is equal to the "average envelope" everywhere, that is, the peak-to-average ratio of the "constant envelope" signal is 1 or 0dB.

3. If only one period of the pure sine wave of the wireless signal is considered, the power peak-to-average power ratio is 2, that is, 3dB, and the peak factor of the voltage CF is the square root of the power peak-to-average power ratio 1.414. But in general, PAPR rarely refers to this situation.

3. Modulation technology and multicarrier technology may bring large PAPR. Too large PAPR is not a good thing, which will affect the application efficiency of many RF devices.

CW (Continuous Wave) test

Chairman Mao taught us: "if there is no investigation, there will be no right to speak." investigation and research is like a pregnancy in October, solving problems is like giving birth. " The acquisition of raw materials is the premise of correctly solving all problems. Similarly, the acquisition of raw data is the most important part of all mathematical modeling. Investigation, monitoring and testing are the means to obtain the original data.

Wireless propagation model is closely related to specific topographic and geomorphological factors. The process of determining the K values of SPM models (or other models) in various scenarios through a large number of tests is called model correction. CW test (continuous wave test) is an important step to obtain test data of radio wave propagation. The data obtained by CW test are the receiving level intensity of different positions, that is, the corresponding longitude and latitude information and field strength value. Can be used as a data source for model correction. The data obtained by the test is required to be typical and balanced, that is, the data can represent the wireless transmission characteristics of the area and can "proportionally" reflect the wireless transmission characteristics of different objects in the area. To do CW testing, it is necessary to avoid satellite occlusion or tall buildings at the proximal end of the transmitting antenna, so as not to affect the accuracy of latitude and longitude information.

Radio frequency RF

RF Radio Frequency

Analogy: if people want to travel in the air, they can use airplanes as carriers. The condition for a plane to lift off is that it must have a certain speed and pass through a certain length of airport runway in order to increase the speed.

When the information is transmitted in the air, there must be radio waves as the carrier, but when the frequency of the radio waves is less than 100 KHz, the radio waves will be absorbed by the ground objects, and the receiving device is also very complex. Only radio waves that reach a certain frequency can be transmitted over a long distance in the air, and it is easy to receive the information.

Radio frequency is a high-frequency alternating wave that can be emitted, ranging from 300KHz~30GHz to radio frequency.

The transmission cable that can transmit radio frequency signals is the radio frequency line, such as the feeder used in engineering. The transmission of modulated high-frequency radio waves in the radio frequency line is called radio frequency cable transmission. The radio frequency line is connected to the antenna, and the radio frequency signal is transmitted or received into the air through the antenna.

Noise

Noise

What is the noise? When people talk on busy streets, they can't communicate properly if they are a little farther away. At this time, it is affected by those, the beep of cars (noise outside human beings) and the noise of the crowd (noise within human beings) are all noises that affect people's normal communication. The size of these noises varies with the environment, and the degree of influence is also different. we can not predict the size of a specific noise at a specific time, but it has the law of probability.

In the process of radio signal processing and propagation, we will also encounter interference signals which can not be accurately predicted but have statistical probability. this kind of signal is different from the mutual interference between radio waves of specific frequency, which is called noise. Noise is divided into internal noise and external noise. The internal noise of the system includes the thermal noise related to the ambient temperature, the noise generated by the electron tube working, the Intermodulation products between the signal and noise, and so on. The noise outside the system comes from the noise produced by lightning wind and rain, the ignition noise of cars, and the noise generated by other electrical equipment.

Phase noise

Phase Noise

Analogy: after the flights from Beijing to Shanghai are arranged, they take off and land at a fixed time every day, over and over again. But one day, due to the weather, the flight could not take off and land normally, and many flights were delayed relative to the normal time.

Phase noise refers to the random change of the phase of the system output signal caused by all kinds of noise in the system (such as various radio frequency devices). The three elements that describe radio waves are amplitude, frequency and phase. Frequency and phase influence each other. Ideally, the fluctuation period of a fixed-frequency wireless signal is fixed, and the departure time is fixed just like the normal flight of an aircraft. In the frequency domain, a pulse signal (the spectrum width is close to 0) is a sine wave with a fixed frequency in the time domain.

But the actual situation is that the signal always has a certain spectrum width, and due to the influence of noise, the power of the signal is also far away from the central frequency, just like a flight with a delay of more than one hour. Signals far away from the central frequency are called sideband signals, which may be squeezed into adjacent frequencies, just as delayed flights may be squeezed into the time of other flights. So this sideband signal is called phase noise.

How to describe the magnitude of phase noise? Within a certain range of offset center frequency, the ratio of power per unit bandwidth to total signal power is measured in dBc/Hz. Just as you want to assess the impact of the weather on flights on a given day, you can define the proportion of flights to the total number of flights that are delayed by more than an hour, the smaller the better. The thermal noise in the RF device system may lead to phase noise. Phase noise can be used to measure the advantages and disadvantages of RF devices. The smaller the phase noise is, the better the RF device is.

SNR

Signal to Noise Ratio

Analogy:

Wukong asked Bajie, "what kind of girlfriend are you looking for?" Bajie replied, "of course, the more beautiful the better."

Wukong asked, "I'll let you chase it for the rest of your life. Do you still want it?" Bajie said, "I dare not take it."

Wukong asked Sha Wujing, "what kind of speed do you want to surf the Internet?" Sha Wujing replied, "of course, the sooner the better."

Wukong asked, "one bit costs you two yuan. Do you still want to pay?" Sha Wujing said, "I dare not go on it."

Wukong asked the Tang monk, "what kind of mount do you want?" The Tang monk replied, "the faster the better, the more fuel-efficient the better, and the safer the better."

Wukong asked, "if you are asked to sell the house in Beijing and buy a car with face and brand, will you still buy it?" The Tang monk said, "I dare not buy it."

Wukong concluded: "when you want to get a benefit, you will certainly pay the price. What you need to consider is whether the benefit you get is appropriate compared with the price you pay, that is, the question of performance-to-price ratio." it's not that the more benefits, the better, but the more cost-effective, the better. "

Signal-to-noise ratio is simply the ratio of useful signal to interference noise. In the process of useful signal transmission, it is bound to introduce all kinds of noise, at least thermal noise. When a radio frequency device such as an amplifier amplifies the power of a useful signal, it is bound to amplify the corresponding noise. Signal-to-noise ratio (Signal/Noise) is usually expressed by SNR. Under the same radio frequency condition, the signal-to-noise ratio expressed by power 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 power is twice the signal-to-noise ratio expressed in voltage. The greater the signal to noise ratio, the better.

Application: speakers with a signal-to-noise ratio (voltage) lower than 80dB and MP3 are not recommended.

The high performance-to-price ratio is reduced-- noise figure.

NF

Noise Factor

Analogy:

It is said that after Bajie and Gao × × married for a few years, Wukong asked Bajie, "what a nice little day!" Bajie looked bitter and said, "Don't mention it, the high performance-to-price ratio has decreased a lot." look a lot older, a lot of bad temper, a lot of lazy life, and I continue to ask for a higher cost of living. " The sex-to-price ratio before marriage is many times higher than that after marriage. This multiple can be called marriage magic box coefficient, which can describe the quality of marriage.

The RF device itself will add noise, and the signal-to-noise ratio of the input will be higher than that of the output. The ratio of input signal to noise ratio and output signal to noise ratio is the noise figure of RF devices.

NF=10lg (input signal to noise ratio / output signal to noise ratio)

The noise figure can measure the radio frequency (RF) performance of the receiver and amplifier, indicating the loss of useful power of the signal and the amplification of noise power after the RF device. The noise figure of the base station is about 3~5dB, while the noise figure of the user mobile station is about 7~9dB.

Trickle converges into a river-additive noise

Additive noise

Analogy: the Yellow River is gradually formed by a trickle of alpine snow water. There are three more important sources: first, Zaqu, second, Yoguzong Canal, and third, Kari qu. When Zhaqu dries up, Kari qu still has plenty of water.

Additive noise acts on the useful signal through the direct superposition of power, but its existence is independent of the useful signal. Whether there is a useful signal or not, additive noise always exists in the RF device, affecting the quality of normal communication.

In general communication, random additive noise is regarded as the background noise of the system; from the source, additive noise can be divided into radio noise, industrial electrical noise, natural noise and internal thermal noise of RF devices. The interference frequency of radio is fixed and can be avoided as much as possible by strengthening the management of radio frequency. Industrial electrical noise comes from a variety of electrical equipment, but the interference spectrum is concentrated in a lower frequency range, and the selection of higher power frequency can prevent interference. Natural noise comes from lightning, sunspots and cosmic rays. This kind of noise is hard to avoid. The internal thermal noise is caused by the irregular thermal motion of electronic devices, which can be described by random process in mathematics, which can also be called random noise.

Your apple is very good-looking-distortion

Distortion

Analogy: everyone is familiar with the story of the emperor's new clothes. Adults all praise the emperor's beautiful clothes, but only the children say: in fact, he is not wearing anything. When the child first knew how to speak, he saw the neighbor's apple and wanted to eat it. He cried and wanted to eat the apple, expressing his intention truly. When he was six or seven years old, he still wanted to eat the neighbor's apples, but said, "your apples are very beautiful." When he grew up, although he wanted to eat the apples next door, in order to show that he was not short of apples, he said, "I don't eat, I really don't eat." Children truly say what they see or what they really think, which is called innocence, while adults hide what they really see and think, which is called loss of innocence (innocence), or distortion.

The so-called distortion is the loss of truth, or the distortion of what is real. When the signal passes through the RF transceiver channel, due to the introduction of additive noise and multiplicative noise, the transmitted signal will be distorted to some extent, which is the distortion of the wireless signal. The distortion of wireless signal can be divided into linear distortion and nonlinear distortion.

Doppler effect

Analogy: when Zhong Duan (the alias of the terminal) first took part in the work, he was very afraid of the leader Ji Zhan (the pseudonym of the base station) to ask work-related questions. When Zhong Duan approached Ji Zhan at a certain speed, he felt the heartbeat speed up (the frequency deviation was positive). When he left Ji Zhan, his heartbeat gradually calmed down (no frequency offset). This process is similar to the Doppler shift effect.

Doppler effect means that the receiving frequency of radio waves increases when the wave source moves rapidly towards the observer, which is similar to the increase of his heartbeat frequency when the clock end approaches, while when the wave source is away from the observer, the receiving frequency becomes lower, as if the clock tip is far away from the leader. His heartbeat frequency gradually calms down.

When the alarm sound of the police car and the engine of the police car approach us at a certain speed, the sound will be more harsh than usual; when it is far away from us, the sound will be softened; by the same token, you can hear the change of the shrill sound as the train passes by, indicating the existence of the Doppler effect.

The range of effective visual acuity of human eyes-Fresnel zone

Fresnel Zone

Analogy: sometimes, I feel that the most effective range of vision of the human eye is also an ellipsoid. Things outside the ellipsoid can also be seen, but they are no longer particularly clear. For a trained shooter, the range of his effective vision must be concentrated in the ellipsoid with a very small radius between him and the target.

We know that there are direct, reflected and diffracted waves in the propagation path from the transmitting point to the receiving point of the electromagnetic wave. When there is little difference between the propagation path of the direct wave and the reflected wave, the electric field direction of the reflected wave is exactly opposite to that of the direct wave, and the phase difference is 180 degrees, so the reflected wave will weaken the signal strength of the direct wave and destroy the propagation effect.

This phenomenon is like the school propaganda of the main theme "knowledge is power" (understood as direct waves), while there is another kind of reflection in society: the theory of useless reading (which can be understood as reflected waves with completely opposite phases). If this wave of reflection exists within the school (similar to a Fresnel area), it will dampen students' enthusiasm for receiving knowledge (affecting the effect of dissemination).

From the above two formulas, it can be seen that the path difference and phase change between direct wave and reflected wave are related to the height and propagation distance of the antenna. When the height of the antenna is low and the distance is long, the path difference will become smaller, the phase change will also decrease, and the influence of the reflected wave on the direct wave will be increased. From this point of view, the higher the antenna height, the better, and the smaller the propagation range, the better. Therefore, in wireless engineering design, under the condition of cost permitting and interference controllable, the antenna of the base station is required to be as high as possible.

Application: in wireless site survey, we must pay attention to whether the coverage is larger than the Fresnel radius, especially to avoid large billboards, high-rise buildings and other obstacles.

Searchlight of construction site-stray radiation

Spurious Emission

Phenomenon analogy: there is a construction site next to our living community, which is thoroughly brightly lit. The main purpose of installing searchlights is to facilitate inspection so as to avoid the loss of all kinds of materials on the construction site (radiation within the working bandwidth). But the searchlight is too bright, radiation to our community (stray radiation), affecting the rest of many people in our community (stray radiation is bound to bring interference).

The RF transmitter is supposed to send wireless signals within the specified frequency range, that is, to transmit in-band signals, just as searchlights should mainly illuminate the site area. Because the internal components of the radio frequency transmitter are not ideal devices and have more or less nonlinearity, a lot of signals in the non-prescribed frequency range are generated in the process of transmitting wireless signals, that is, stray radiation occurs; it's like a searchlight shines on a nearby living area. If the transmitter transmits signals that are not in its own frequency range, it may interfere with other communication systems, just as the searchlights on the construction site affect the rest of the residents in the nearby community.

Stray radiation may be the harmonic components and Intermodulation signals generated by some nonlinear components. In order to prevent stray radiation from one system from interfering with other wireless communication systems, it is necessary to improve the electromagnetic compatibility of the system. The maximum stray radiation levels for different out-of-band frequency ranges of the system are generally specified in the protocol. The general form is the maximum allowable stray radiation of a certain bandwidth within a frequency range. DBm;. For example, it is stipulated in the protocol that the stray radiation per 10kHz bandwidth of the transmitter of WCDMA can not exceed-36dBm in the 150kHz~30MHz range.

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