Doppler Shift means that when the mobile station moves along a certain direction at a constant rate, the phase and frequency will change due to the propagation path difference, which is usually called Doppler shift.It reveals the law that the properties of waves change in motion.
When moving in front of the wave source, the wave is compressed and the wavelength becomes shorter,frequencyBecome higher（blue shiftblue shift）；When moving behind the wave source, the opposite effect will be produced, the wavelength will become longer and the frequency will become lower（red shiftred shift）。
Doppler effect is in memory of Austrian physicist and mathematician Christian John·Doppler(Christian Johann Doppler) in 1842.The main contents are objectsradiationThe wavelength of is changed due to the relative motion of the wave source and the observer.In front of the moving wave source, the wave is compressed,wavelengthBecome shorter,frequencyBecome higher（blue shiftblue shift）；When behind the moving wave source, the opposite effect will occur.Wavelength becomes longer and frequency becomes lower（red shiftred shift）；The higher the velocity of the wave source, the greater the effect.According to the degree of blue (red) shift of the wave, the velocity of the wave source moving along the observation direction can be calculated.
Definition of Doppler shift
The difference between the transmitted and received frequencies caused by Doppler effect is called Doppler frequency shift.It reveals the law that the properties of waves change in motion.
This phenomenon also exists in other types of waves, such as light waves and electromagnetic waves.Scientists observed that the frequency of light waves coming from outer space is getting lower and lower, which is close to the red band with lower frequency. This is an example of the Doppler shift caused by light waves following the Doppler effect.For electromagnetic waves, when wireless communication is carried out on high-speed moving objects (such as high-speed railway), the signal quality will decline, which is an example of Doppler frequency shift in electromagnetic waves.[1]
principle
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If light waves are considered to be emitted at regular intervalspulseIt is conceivable that if you send a pulse every step, then every pulse before you is closer to yourself than when you stand still.The wave source behind you is a step further than when it was still.Or, before youPulse frequencyIt is higher than usual, and the pulse frequency after you is lower than usual.
so-calledDoppler effectThat is, when there is relative movement between the transmitting source and the receiver, the frequency of the transmitting information received by the receiver from the transmitting source is different from the frequency of the transmitting information from the transmitting source. The difference between the receiving frequency and the transmitting frequency is called Doppler frequency shift.Doppler effect also exists in the propagation of light. When there is relative movement between the light source and the receiver, the frequency of light wave received by the receiver
And light source frequency
Doppler shift exists
That is,
When the receiver and the light source are close to each other, the receiving frequency
Greater than transmission frequency
That is:
；
When the receiver and the light source are far away from each other, the receiving frequency
Less than transmission frequency
That is:
。
formula
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It can be proved that if the velocity between the receiver and the wave source is v and the wave velocity is c, then the Doppler frequency of the received wave of the receiver is,
f'=f·(c+-v1)/(c-+v2)
The frequency relationship between the receiver and the transmitter is:
Among them,
Is the received frequency,
Is the original transmission frequency from the medium,
Is the traveling speed of the wave in the medium;
Is the moving speed of the receiving end relative to the medium. If it is close to the transmission source, the front operation symbol is+, otherwise it is −;
Is the moving speed of the emission source relative to the medium. If it is close to the observer, the front operation symbol is −, otherwise it is+.[2]
Figure 2 Schematic Diagram of Doppler Effect
When the mobile station is at a constant speedvAt lengthd, endpoint isXandYReceived from remote source when moving on the path ofSThe signal sent is shown in Figure 3.
Radio wave slaveSDepart atXPoint andYThe path difference of the points received by the mobile station is:
Here Δ t is the time required for the mobile station to move from X to Y, and θ is the angle between X and Y and the incident wave.Since the source end is far away, it can be assumed that θ at X and Y are the same.Therefore, the phase change value of the received signal caused by the path difference is:
From this, we can get the frequency change value, namely Doppler frequency shift fdFor:
It can be seen from the above formula that the Doppler frequency shift is related to the included angle between the mobile station's moving speed and direction, and the incident direction of radio waves.If the mobile station moves towards the direction of the incident wave, the Doppler frequency shift is positive (that is, the receiving frequency rises);If the mobile station moves backward to the direction of the incident wave, the Doppler frequency shift is negative (that is, the receiving frequency decreases).The signal propagates in different directions, and its multipath components cause Doppler diffusion of the receiver signal, thus increasing the signal bandwidth[3]。
