Multipath propagation is the propagation of radio waves (signals) transmitted from the transmitter antenna to the receiver antenna along two or more paths.
In FM radio broadcasting, there are two or more transmission paths between the transmitter and receiver.Multipath propagation effect is caused by reflected signals from large buildings or mountains.The receiving antenna will receive the direct signal and the reflected signal with delay.Multipath effect will produce distortion. When watching TV programs, multi-path propagation effect will make images appear "ghosting".
Multipath effect: refers to the interference caused by the superposition of each component field according to their respective phases after the electromagnetic wave propagates through different paths, which causes the original signal distortion or error.
Chinese name
Multipath propagation
Foreign name
Multipath propagation
Features
Phenomenon of signal propagation along two or more paths
Multipath propagation is the propagation of radio waves (signals) transmitted from the transmitter antenna to the receiver antenna along two or more paths.
Radio wave is a kind of electromagnetic wave, and its main mode of propagation is space wave, namelyDirect beam、reflected wave、Refracted wave、Diffracted waveAnd their synthetic waves.When radio waves encounter objects, they will generate reflection, refraction and scattering, while in the process of radio wave propagation, they will encounter different objects, which will generate different reflection, refraction and scattering. Therefore, at any receiving point, they may receive homologous electromagnetic waves from different paths, which is called multi-path propagation.
The signal received by the receiver is the combination of direct wave and multiple reflected waves.Since the atmospheric refraction changes with time, the propagation path difference will also change with time and terrain features.If multi-path signals are in phase, they will be added;If the phase is reversed, cancel it.The resulting amplitude change of the signal at the receiving end is called fading.Since this fading is caused by multipath, it is calledmultipath fading 。
In FM radio broadcasting, there are two or more transmission paths between the transmitter and receiver.Multipath propagation effect is caused by reflected signals from large buildings or mountains.The receiving antenna will receive the direct signal and the reflected signal with delay.Multipath effect will produce distortion. When watching TV programs, multi-path propagation effect will make images appear "ghosting".
Research status at home and abroad
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Foreign research
In radar tracking, the understanding of multipath propagation is almost as early as that of the discipline itself.In the past few decades, many researchers abroad have done a lot of research on multipath problems, and have made many scientific achievements.As early as the 1950s, people began to pay attention to radarMultipath effect。From the 1960s to the mid-1970s, the research on low altitude radar reached a climax.During this period, people established specular reflection model and diffuse reflection model of multi-path effect, and proposed a variety of methods to suppress multi-path effects.Since the mid seventies, the diffuse reflection model has been further improved.
Beckman Spizzichin, as an early researcher, proposed the classical theory applicable to diffuse reflection of far-field rough surfaces, assuming that the surface height fluctuation obeys the Gaussian distribution with the mean value of zero, and that the height distributionAutocorrelation functionIt also obeys the Gaussian distribution and is statistically independent.On such a surface, they put forward the theory of scintillating surface, thinking that the reflected wave in the scintillating surface can reach the radar antenna, without considering the shielding effect between small panels.
Barton. D. K. mainly analyzes the radar measurement error caused by diffuse reflection of rough reflector when tracking low altitude targets.He pointed out that at this time, the target echo is not unique in the radar measurement coordinates, but is distributed according to a certain power distribution function (usually Rice distribution). Accordingly, the radar measurement is also a random value with a certain probability distribution in each coordinate.When the angle between the target and the mirror relative to the radar and the amplitude of the reflection coefficient reach certain conditions, the component of the mirror reflection in the total reflection increases, resulting in the power multipath fading of the received target signal. This factor needs to be considered separately.
Under the monopulse amplitude comparison angle measurement system, how to suppressMultipath effect, early WD. White proposed the maximum likelihood method to estimate the pitch angle measurement for multipath interference caused by specular reflection.In the early 1990s, Bruder pointed out that there was a fixed deviation of radar due to specular reflection on a flat reflector, Bar Shalom. Y, Sinha. A and DaeipourE has systematically studied the multipath effect of amplitude comparison under single pulseMaximum likelihood methodAnd interactive multimode method to suppress multipath interference and achieve target tracking in low elevation.And DaeipourE A deviation compensation algorithm is proposed for the fixed deviation caused by mirror reflection in the monopulse amplitude comparison angle measurement system pointed out by Bruder.
The American Institute of Aeronautics and Astronautics (AIAA), D. Bucco, Yiding Hu and others summarized various existing multipath signal models on the premise of radar guided missiles, compared different models, and gave the applicability of models in different actual situations.
Zhang Yu, Li Lingling, et al. from the China Radio Wave Research Institute in China, gave theMultipath effectA representative model of, and consideration ofSurface roughness, masking effect, etc.Beijing University of Aeronautics and AstronauticsDuan Shizhong, Zhou Mengqing, et al. proposed a time domain complex envelope waveform digital simulation model based on multi-path echo of multi reflection point complex target for flat and undulating ground or sea surface.Shi Guining and Chen Junwen from the Second Institute of China Aerospace Science and Industry Corporation gave a complete simulation model for the low altitude multipath effect of semi-active Doppler missiles, which includes three parts: radar antenna model, ground scattering model and target model,Through their geometric relationship and scattering relationship.Li Yanzuo and Jin Yaqiu of Fudan University used a two scale approximation model of sea surface fluctuation to carry out numerical simulation of multipath sea clutter spectrum, simulated time-varying random rough sea surface through spectrum fitting, calculated the effective multipath scattering area of the sea surface under the given conditions of receiving and transmitting height and spacing, and calculated the electric field strength, radar bistatic scattering coefficient and receiving pointpower spectral density Sea clutter spectrum knife.In addition, Dang Qun and others of Beihang calculated elevation error by using closed-loop tracking method, and calculated diffuse reflection of electromagnetic wave on undulating sea surface by using improved small plane method. The actual calculation shows that elevation tracking error of seeker in tracking low altitude target can be realistically simulated, and the calculation time is greatly reduced.Yang Shihai of the National Defense University of Science and Technology has studied the multipath elimination technology in phased array tracking system in detail. According to the actual situation, he proposed improved methods for the classical multipath suppression algorithm of monopulse angle measurement, such as complex angle method, complex monopulse ratio and dynamic deviation compensation method of joint relative phase.[1]
Multipath effect
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Multipath effect: refers to the interference caused by the superposition of each component field according to their respective phases after the electromagnetic wave propagates through different paths, which causes the original signal distortion or error.For example, if the electromagnetic wave propagates along two different paths, and the length of the two paths is exactly half a wavelength different, then the two signals just cancel each other when they reach the end point (the wave crest coincides with the wave trough).This phenomenon is often encountered in the process of watching analog signal TV in the past. If the signal is poor when watching TV, you will see ghost on the screen. This is because when the electronic gun on the TV scans from left to right, the signal after use forms a virtual image on the right.Therefore, multipath effect is an important cause of fading.Multipath effect has a very serious impact on digital communication, radar optimal detection, etc.
Multipath effect is not only the frequent cause of fading, but also one of the fundamental factors limiting transmission bandwidth or rate.stayShortwave communicationIn order to ensure the maximum and minimum delay of the circuit in multi-path transmissionDelay differenceNot more than a specified value, and the operating frequency is required to be not less than a certain maximum usable frequency of the circuitpercentage。This percentage, called multipath reduction factor, is one of the important bases for determining the lowest available frequency of the circuit.Anti multipath measures in tropospheric propagation channels usually include suppressing ground reflection, using narrow antenna beam andDiversity receptionEtc.
Multipath effect will cause flat fading, time selective fading andfrequency selective fading , mainly frequency selective fading.
Assuming that the signal symbol length is T, and the difference between the signal delay and the average signal delay of the ith transmission path is △ t, the different combination of the two can produce three different fading phenomena.
〔1〕 When the signal symbol length T is small and △ t<<T, it will cause "flat fading";
〔2〕 When the signal symbol length T is long and △ t<<T, it will cause "time selective fading";
〔3〕 When the signal symbol length T is relatively small, but △ t is relatively large, and does not meet △ t<<T, it will cause "frequency selective fading" (this is the reflection of time diffusion in the frequency domain).Because multi-path synthetic waveform may fall in the following symbolstime intervalInter symbol interference is caused. Therefore,frequency selective fading It is the most harmful for high-speed data transmission.[2]
