CN108123763A - Frequency difference compensation method and device - Google Patents

Abstract

The present invention provides a frequency difference compensation method and device, which are applied to mobile terminals with millimeter wave communication functions. The method includes: step A: collecting the current local oscillator frequency signal of the mobile terminal and the current transmission frequency signal of the fixed-point device in real time, and performing Perform difference calculation between the current local oscillator frequency signal and the current transmit frequency signal to obtain the current intermediate frequency signal, and perform spectrum analysis on the current intermediate frequency signal to obtain the frequency difference; step B: determine whether the frequency difference exceeds the frequency difference tolerance, and if so, perform step C , if no, return to step A; step C: adjust the current local oscillator frequency of the mobile terminal according to the frequency difference so that the frequency difference does not exceed the frequency difference tolerance, and return to step A. Based on the method of the present invention, by monitoring and compensating the frequency difference in real time, it can ensure that the frequency difference is always within a certain tolerance, and solve the Doppler frequency deviation problem and the individual frequency difference problem existing in millimeter wave communication in the high-speed rail environment. The method and device of the application do not produce the Doppler expansion problem and the car body radiation problem.

Description

Frequency difference compensation method and device

technical field

The invention relates to the communication field, in particular to a frequency difference compensation method and device.

Background technique

At present, high-speed rail transit and the Internet have become an important part of life. The speed of high-speed rail is getting faster and faster, and the demand for mobile data transmission is growing explosively. Millimeter wave communication technology is considered by the industry to be the best solution for wireless broadband access in high-speed rail. Good way, it has the advantages of broadband communication, and at the same time, millimeter wave communication is connected through the vehicle-ground broadband, so there is no problem of excessive electromagnetic radiation caused by the unstable signal of the existing LTE communication. However, millimeter wave communication also faces problems in the high-speed rail environment, including:

One: Doppler frequency deviation (frequency difference) caused by high-speed movement.

Two: frequency difference caused by individual differences and time aging.

For the above-mentioned Doppler frequency deviation (frequency deviation) problem caused by high-speed movement, the existing processing methods include: (1) directly perform frequency deviation compensation through the baseband digital signal; or (2) rely on its own anti-Doppler characteristics, improve SNR and sacrifice BER. However, the problem with method (1) is that the compensation is inaccurate because the sampling frequency in the sampling process is too high, and at the same time, this processing method cannot solve the problem of Doppler spread caused by multipath in traditional communication. The problem with method (2) is that this processing method does not compensate Doppler, and it is also an inefficient processing method from the perspective of performance.

For the frequency difference problem caused by individual differences and time aging, the satellite frequency calibration method is usually used to achieve the same frequency of the base station, and sometimes the manual frequency calibration method is also used; Tunnels, etc., the entire network coverage cannot be achieved through satellite frequency calibration, and the cost of manual frequency calibration is too high.

For the above problems, no effective solution has been proposed yet.

Contents of the invention

The present invention provides a frequency difference compensation method and device to solve the problems existing in millimeter wave communication in the high-speed rail environment at present, including the Doppler frequency deviation problem and the individual frequency difference problem.

The present invention provides a frequency difference compensation method, which is applied to a mobile terminal with a millimeter wave communication function; the method includes:

Step A: Collect the current local oscillator frequency signal of the mobile terminal and the current transmission frequency signal of the fixed-point device in real time, perform difference calculation on the current local oscillator frequency signal and the current transmission frequency signal to obtain the current intermediate frequency signal, and perform spectrum analysis on the current intermediate frequency signal to obtain frequency difference;

Step B: Determine whether the frequency difference exceeds the frequency difference tolerance, if yes, perform step C, if not, return to step A;

Step C: Adjust the current local oscillator frequency of the mobile terminal according to the frequency difference so that the frequency difference does not exceed the frequency difference tolerance, and return to step A.

The present invention also provides a frequency difference compensation device, which is applied to a mobile terminal with a millimeter wave communication function; the device includes: a frequency difference acquisition and analysis module, a judgment module and an adjustment module.

Frequency difference acquisition and analysis module: collect the current local oscillator frequency signal of the mobile terminal and the current transmission frequency signal of the fixed-point device in real time, and perform difference calculations on the current local oscillator frequency signal and the current transmission frequency signal to obtain the current intermediate frequency signal. Do spectrum analysis to get the frequency difference.

Judging module: judging whether the frequency difference exceeds the frequency difference tolerance, if yes, execute the adjustment module, if not, return to the frequency difference collection and analysis module.

Adjustment module: adjust the current local oscillator frequency of the mobile terminal according to the frequency difference so that the frequency difference does not exceed the frequency difference tolerance, and return to the frequency difference collection and analysis module.

The present invention monitors and compensates the frequency difference in real time, constructs a closed-loop self-adaptive frequency difference compensation system, and ensures that the frequency difference is always within a certain tolerance. The method and device of the present invention can solve the Doppler frequency offset problem and the individual frequency difference problem existing in millimeter wave communication in the high-speed rail environment, and at the same time, the method and device of the present application will not cause the Doppler expansion problem and the car body radiation problem .

Description of drawings

Fig. 1 is the flow chart of frequency difference compensation method of the present invention;

Fig. 2 is the embodiment of Fig. 1 step A of the present invention;

Fig. 3 is a structural diagram of the frequency difference compensation circuit of the present invention;

Fig. 4 is a structural diagram of the frequency offset compensation device of the present invention.

Detailed ways

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

In the present invention, the millimeter wave communication is connected to the train-ground broadband to solve the problem of Internet access. The mobile terminal in the train can fully enjoy the service of the Internet by accessing the WIFI in the train body, and communicate with the outside world by using a chat tool. , such as VOIP, WeChat, etc.

In addition to the above-mentioned WIFI Internet, the access services carried by millimeter wave communication also include 2G voice services; the data transmission volume of 2G voice services is small, which is inconsistent with the IP mechanism of WIFI Internet, and the 2G signal is stable, so there will be no car body radiation problems , mmWave communication can carry such access services.

Since the Internet access of LTE (Long Term Evolution) and WIFI Internet access are based on the IP mechanism, the two are the same access service, and the millimeter wave communication of the present invention does not access LTE under the premise of accessing WIFI Internet. .

The present invention utilizes the wideband communication characteristic of the millimeter wave communication to carry the access service of the traditional voice communication, and can improve the electromagnetic environment in the vehicle body; the narrow beam transmission of the millimeter wave communication can reduce the influence of the multipath effect.

Based on the limitation of the access service of the millimeter wave communication in the present invention, only the Doppler frequency deviation and the individual frequency difference need to be considered in the millimeter wave communication process of the present invention, and there is no need to pay attention to the Doppler spread problem and the electromagnetic radiation problem.

The frequency difference compensation method of the present invention, as shown in FIG. 1 , is applied to a mobile terminal with a millimeter-wave communication function, and the millimeter-wave communication mechanism adopts zero-IF modulation and demodulation.

The method includes the following steps:

Step A (S101): Collect the current local oscillator frequency signal of the mobile terminal and the current transmission frequency signal of the fixed-point device in real time, perform a difference operation on the current local oscillator frequency signal and the current transmission frequency signal to obtain the current intermediate frequency signal, and perform a calculation on the current intermediate frequency signal Spectrum analysis to get the frequency difference;

Step B (S102): judge whether the frequency difference exceeds the frequency difference tolerance, if yes, then perform step C, if not, return to step A;

Step C (S103): According to the frequency difference, adjust the current local oscillator frequency of the mobile terminal so that the frequency difference does not exceed the frequency difference tolerance, and return to step A.

In Figure 1, the fixed-point device is the ground access point RAU (Route Area Update), and the reference crystal oscillator frequency of the fixed-point device is:

fg _ref = fg+Kgvco*Vgtune (1)

Among them: Kgvco represents the tuning parameters of the fixed-point device reference crystal oscillator

Vgtune indicates the tuning level of the fixed-point device reference crystal oscillator

Local oscillator frequency of fixed-point equipment: LOg＝N*fg _ref (2)

The reference crystal oscillator frequency of the mobile terminal is:

fm _ref =fm+Kmvco*Vmtun (3)

Among them: Kmvco represents the mobile terminal reference crystal oscillator tuning parameters

Vmtune indicates the tuning level of the mobile terminal reference crystal oscillator

The local oscillator frequency of the mobile terminal: LOm＝N*fm _ref (4)

Let the frequency difference be Δf, then Δf=LOg-LOm (5)

According to the calculation formula of the local oscillator frequency of the mobile terminal, it can be known that by adjusting the tuning level of the reference crystal oscillator of the mobile terminal, the local oscillator frequency of the mobile terminal can be changed, thereby adjusting the frequency difference.

In actual application, the transmission frequency of the fixed-point device is LOg±BW, where LOg is the local oscillator frequency of the fixed-point device, and the bandwidth of the BW modulation signal. The receiving local oscillator frequency of the mobile terminal is LOm, and the mobile terminal receives the mixed intermediate frequency signal IF=(LOg-LOm)±BW=Δf±BW; where Δf is the frequency difference.

In step A of the present application, the implementation is to extract the frequency difference Δf from the IF signal, perform spectrum analysis on the IF signal, and find out the frequency corresponding to the maximum amplitude value, which is the frequency difference Δf.

Let the frequency difference tolerance be δf. Assuming that the current LOg remains unchanged, when Δf>δf, by adjusting the tuning level Vmtune of the reference crystal oscillator of the mobile terminal, the output frequency of the reference crystal oscillator is changed, thereby causing LOm to change. After LOm is changed, Δf is updated, and the updated Δf Satisfy Δf≤δf.

Further, as shown in Figure 2, a part of the current intermediate frequency signal can be selected for calculating the frequency difference, based on this step A can include:

Step A-1 (S201): The mixer collects the current local oscillator frequency of the mobile terminal and the current transmission frequency of the fixed-point device in real time, and outputs the current intermediate frequency signal after performing a difference operation on the current local oscillator frequency signal and the current transmission frequency signal;

Step A-2 (S202): Inputting the output current intermediate frequency signal into an active low-pass filter circuit, and the active low-pass filter circuit outputs a low-frequency analog signal, and the low-frequency analog signal is a signal below the cut-off frequency of the current intermediate frequency signal;

Step A-3 (S203): performing frequency spectrum analysis on the low-frequency signal to obtain the frequency difference.

Optionally, step A-3 can be realized by a DSP chip. The low-frequency analog signal is input to the DSP chip, and the DSP chip converts the low-frequency analog signal into a low-frequency digital signal, and then uses FFT transformation to convert the low-frequency digital signal into a frequency domain signal, and extracts the frequency corresponding to the maximum amplitude value in the frequency domain signal as frequency Poor, output frequency difference.

The low-frequency signal may also be processed by other existing technologies to obtain the frequency difference, which is limited in this application.

In Figure 2, the cutoff frequency of the active low-pass filter circuit is fH, fH=2(Θf+fd), where Θf is the maximum individual frequency difference of the crystal oscillator, and fd is the maximum Doppler frequency at the highest speed. The calculation formula of fH is to achieve the most appropriate cut-off frequency, which not only ensures that the cut-off data contains frequency difference information, but also ensures that the amount of data after cut-off is the smallest, so that the calculation resources occupied by data processing and analysis after cut-off are less. Calculations are faster.

In step A-2 of this application, an active low-pass filter circuit is used to separate the mixing signal. This circuit is in a high impedance state for the intermediate frequency signal, which can ensure that the original intermediate frequency signal is not affected or can be affected during the signal separation process. neglect.

The power supply of the operational amplifier of the active low-pass filter circuit is preferably a high-performance linear regulated power supply, or in order to improve the stability of the power supply, a polar capacitor can be added to the power supply end of the operational amplifier.

For example, the linear regulated power supply can choose TPS79330.

According to the calculation formula of the local oscillator frequency of the mobile terminal and the reference crystal oscillator frequency, it can be known that the local oscillator frequency of the mobile terminal can be changed by adjusting the tuning level of the reference crystal oscillator. Generally, on the circuit board of the mobile terminal, there is a dedicated circuit or module for generating the local oscillator frequency. Variety

For example, when the circuit or module generating the local oscillator frequency is a phase-locked loop circuit, the current local oscillator frequency output by the phase-locked loop circuit is changed by adjusting the current input tuning level of the reference crystal oscillator of the phase-locked loop circuit.

In step C, make the frequency difference not exceed the frequency difference tolerance, and optionally, adjust the frequency difference to a preset value within the frequency difference tolerance. For example, the target frequency difference (preset value) may be set as half of the frequency difference tolerance.

In this way, the adjustment value of the local oscillator frequency can be determined according to the target frequency difference value and the (current) frequency difference, and then the adjustment value of the tuning level can be determined according to formula (3) and formula (4).

Preferably, the application uses a tuning level chip to adjust the tuning level of the reference crystal oscillator. It specifically includes: inputting the frequency difference into the tuning level chip, and the tuning level chip adjusts the output tuning level according to the input frequency difference; and then inputting the tuning level into the reference crystal oscillator of the phase-locked loop circuit in the mobile terminal.

The above is the description of the frequency offset compensation method of the present application.

The method of this application enables the mobile terminal to self-adaptively pass signal separation (active low-pass filter circuit), AD sampling (analog-to-digital conversion of DSP chip) and FFT etc. complete the extraction of the difference frequency, combine formulas (3) and (4) to calculate the output tuning level, change the output frequency of the reference crystal oscillator by adjusting the tuning level of the reference crystal oscillator, and control the frequency difference within the tolerance in real time. Complete the adaptive compensation of the frequency difference.

According to the above description of the frequency difference compensation method of the present invention, it can be known that the method involves an active low-pass filter circuit, a DSP chip, a tuning level chip, a local oscillator frequency circuit, and a mixer, as shown in Figure 3, wherein a virtual frame 400 The local oscillator frequency circuit and the mixer are the existing modules of the mobile terminal, and the active low-pass filter circuit, DSP chip and tuning level chip in the virtual frame 300 are newly designed modules in order to realize the frequency difference compensation method.

Active low-pass filter circuit: input the mixed frequency signal (intermediate frequency signal) output by the mixer into the active low-pass filter circuit, and the active low-pass filter circuit outputs a low-frequency analog signal.

DSP chip: Input the low-frequency analog signal into the DSP chip, after the DSP chip converts the low-frequency analog signal into a low-frequency digital signal, and then uses FFT transformation to convert the low-frequency digital signal into a frequency domain signal, and extracts the maximum value of the frequency domain signal The corresponding frequency is the frequency difference, and the frequency difference is output.

Tuning level chip: input the frequency difference into the tuning level chip, and the tuning level chip judges whether the frequency difference is greater than the frequency difference tolerance, if yes, adjusts the output tuning level, if not, keeps the current tuning level unchanged .

Local oscillator frequency circuit: Input the tuning level into the reference crystal oscillator of the local oscillator frequency circuit, and the reference crystal oscillator outputs the crystal oscillator frequency. The local oscillator frequency is N times the crystal oscillator frequency, and the crystal oscillator frequency changes with the tuning level.

Mixer: The local oscillator frequency is input into the mixer, and the mixer also collects the transmission frequency of the fixed-point device in real time, and outputs a mixed frequency signal (intermediate frequency signal).

Further, in Fig. 3, the intermediate frequency signal that the mixer outputs comprises I road same direction intermediate frequency signal and Q road quadrature intermediate frequency signal; The low frequency analog signal that active low-pass filter circuit outputs also comprises I road same direction low frequency signal and Q channel quadrature low frequency signal. Selecting the I-way and Q-way signals can make the DSP chip obtain more reliable digital signals.

Examples

This embodiment illustrates that frequency difference compensation is performed on a 40 GHz communication system by using the frequency difference compensation method of the present application. The millimeter wave communication carries 2G voice communication services and WIFI, and its communication mechanism adopts zero-IF modulation and demodulation.

Assume that the frequency difference tolerance δf=1.2kHz, the maximum operating speed of the high-speed rail v=150m/s=540km/h, the individual (differential) frequency difference Θf=0.1ppm, the crystal oscillator frequency of the mobile terminal is 100MHz, and the local oscillator of the mobile terminal The frequency is 40GHz, the mobile terminal refers to the crystal oscillator tuning parameter Kmvco=0.1ppm/V, the dynamic range of the frequency amplitude of the intermediate frequency signal in the DSP chip is 50mV~500mV, and the frequency difference is Δf=2000Hz as an example to complete the self-adaptation of the frequency difference process of compensation.

The cut-off frequency fH of the active low-pass filter circuit, taking λ=7.5mm=0.0075m, v=150m/s, Θf=0.1ppm, as an example, is substituted into the following formula for calculation:

fH=2×(fd+Θf)=2×(150/0.0075+0.1ppm*40G)≈50kHz.

The DSP chip can choose STM32F407VET, the sampling frequency of each channel of STM32F407VET is as high as 2.4MSPS (Million Samples per Second), with 24 AD sampling channels, 1MB of Flash and 196KB of SRAM (Static Random Access Memory), the resources meet the needs; The FFT function library can quickly complete FFT, and comes with DMA (Direct Memory Access), which can quickly read and store data.

The DSP chip performs AD sampling on the input low-frequency analog signal, and converts the low-frequency model signal into a digital signal. At this time, the sampling frequency needs to be considered. Usually, when the sampling frequency is greater than twice the highest frequency in the signal, the digital signal after sampling completely retains the information in the original signal. The sampling frequency can be set to a fixed value, for example, the maximum value of the frequency difference Δf is 100kHz, then the fixed value of the sampling frequency can be set to 100*2.5=250kHz.

Further, considering that Δf is dynamically changing, the sampling frequency can also be dynamically set based on Δf. For example, when the device is turned on, the sampling frequency is set to 250kHz. When Δf<δf, the sampling frequency is 3kHz. Δf>δf In order to improve the frequency resolution rate, adjust the sampling frequency for different frequency differences, set the sampling frequency=2.5*Δf.

The DSP chip converts the low-frequency analog signal into a digital signal through AD, and uses FFT to convert the time-domain signal into a frequency-domain signal, and first detects whether the amplitude of the frequency-domain signal is within a reasonable range of the intermediate-frequency signal amplitude, that is, whether the detection amplitude is between 50mV and 500mV , if yes, extract the frequency corresponding to the maximum amplitude value as the frequency difference.

The DSP chip inputs the analyzed frequency difference into the tuning level chip, and the tuning level chip judges whether the frequency difference is greater than the frequency difference tolerance, if yes, adjusts the output tuning level, if not, keeps the current tuning level unchanged . Suppose the target frequency difference is δf/2(600Hz)

Then, the adjustment value of the local oscillator frequency is determined according to the target frequency difference value of the tuning level chip and the (current) frequency difference, and then the adjustment value of the tuning level is determined according to formula (3) and formula (4).

For example, when δf=1200Hz, the current frequency difference=2000Hz, the target frequency difference=600Hz, the crystal oscillator frequency of the mobile terminal is 100MHz, Kmvco=0.1ppm/V,

Then: Δfmref=2000-600=1400Hz=14ppm

ΔVmtune=14×0.1=1.4V, that is, the adjustment value of the tuning level is 1.4V.

If the tuning level increases, the difference frequency decreases, that is, the tuning level is opposite to the difference frequency, and the tuning level increases by 1.4V. If the tuning level increases and the difference frequency increases, the tuning level is in the same direction as the difference level, and the tuning level decreases by 1.4V. Determine the relationship between the frequency difference and the tuning level during the equipment test.

The minimum step of the tuning level chip is determined by the maximum value of the tuning level Vtune _max of the crystal oscillator. Taking Vtune _max = 3V as an example, the level accuracy of a 16-bit DAC is Vstep = Vtune _max /2 ¹⁶ =7.4×10 ^-4 V. Calculated from Vstep and formulas (3) and (4), the frequency modulation step fstep = Vstep × Kmvco × local oscillator frequency ≈ 3 Hz, 3 Hz is much smaller than 1200 Hz of δf, which can meet the fine control requirements.

As shown in FIG. 4 , the present invention also includes a frequency difference compensation device, which is applied to a mobile terminal with a millimeter wave communication function; the device includes: a frequency difference acquisition and analysis module, a judgment module and an adjustment module.

Frequency difference acquisition and analysis module: collect the current local oscillator frequency signal of the mobile terminal and the current transmission frequency signal of the fixed-point device in real time, and perform difference calculations on the current local oscillator frequency signal and the current transmission frequency signal to obtain the current intermediate frequency signal. Do spectrum analysis to get the frequency difference.

Judging module: judging whether the frequency difference exceeds the frequency difference tolerance, if yes, execute the adjustment module, if not, return to the frequency difference collection and analysis module.

Adjustment module: adjust the current local oscillator frequency of the mobile terminal according to the frequency difference so that the frequency difference does not exceed the frequency difference tolerance, and return to the frequency difference collection and analysis module.

Optionally, the frequency difference collection and analysis module includes: a mixer, a filter circuit and a frequency difference analysis module.

Mixer: The mixer collects the current local oscillator frequency of the mobile terminal and the current transmission frequency of the fixed-point device in real time, and outputs the current intermediate frequency signal after performing a difference operation between the current local oscillator frequency signal and the current transmission frequency signal;

Filter circuit: input the current intermediate frequency signal into the active low-pass filter circuit, and the active low-pass filter circuit outputs the low-frequency analog signal, which is the signal below the cut-off frequency of the current intermediate-frequency signal;

Frequency difference analysis module: perform frequency spectrum analysis on low-frequency signals to obtain frequency difference.

Optionally, the frequency difference analysis module includes:

The low-frequency analog signal is input into the DSP chip, and the DSP chip converts the low-frequency analog signal into a low-frequency digital signal, and then converts the low-frequency digital signal into a frequency domain signal by using FFT transformation, and extracts the frequency corresponding to the maximum amplitude value in the frequency domain signal is the frequency difference, and outputs the frequency difference.

Optionally, the cutoff frequency of the filter circuit is fH, fH=2(Θf+fd), where Θf is the maximum individual frequency difference of the crystal oscillator, and fd is the maximum Doppler frequency at the highest speed.

Optionally, in the adjusting module, the adjusting the current local oscillator frequency of the mobile terminal according to the frequency difference includes:

According to the frequency difference, the input tuning level of the reference crystal oscillator of the phase-locked loop circuit in the mobile terminal is adjusted, so that the current local oscillator frequency output by the phase-locked loop circuit changes.

Further, the frequency difference is input into the tuning level chip, and the tuning level chip adjusts the output tuning level according to the input frequency difference; the tuning level is input into the reference crystal oscillator of the phase-locked loop circuit in the mobile terminal.

Optionally, the making the frequency difference not exceed a frequency difference tolerance further includes: adjusting the frequency difference to a preset value within the frequency difference tolerance.

The present invention ensures that the frequency difference is always within a certain tolerance through real-time monitoring and compensation of the frequency difference, forming a closed-loop self-adaptive frequency difference compensation system. The method and device disclosed in the present invention can solve the Doppler frequency offset problem and the individual frequency difference problem existing in millimeter wave communication in the high-speed rail environment, and at the same time, the method and device of the present application will not cause the Doppler expansion problem and vehicle body radiation question.

It should be noted that the embodiment of the frequency offset compensation device of the present invention has the same principle as the embodiment of the frequency offset compensation method, and relevant parts can be referred to each other.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the technical solutions of the present invention are Should be included within the protection scope of the present invention.

Claims (14)

1. A frequency difference compensation method, characterized in that, the method is applied to a mobile terminal with a millimeter wave communication function; The method comprises the steps of: Step A: collect the current local oscillator frequency signal of the mobile terminal and the current transmission frequency signal of the fixed-point device in real time, perform a difference operation on the current local oscillator frequency signal and the current transmission frequency signal to obtain the current intermediate frequency signal, and calculate the current The frequency difference is obtained by performing spectrum analysis on the intermediate frequency signal; Step B: judging whether the frequency difference exceeds the frequency difference tolerance, if yes, then execute step C, if not, return to step A; Step C: Adjust the current local oscillator frequency of the mobile terminal according to the frequency difference so that the frequency difference does not exceed the frequency difference tolerance, and return to step A. 2. The method according to claim 1, wherein said step A comprises: Step A-1: The mixer collects the current local oscillator frequency of the mobile terminal and the current transmission frequency of the fixed-point device in real time, and performs a difference operation on the current local oscillator frequency signal and the current transmission frequency signal to output the current intermediate frequency signal ; Step A-2: Input the current intermediate frequency signal into an active low-pass filter circuit, and the active low-pass filter circuit outputs a low-frequency analog signal, and the low-frequency analog signal is a signal below a cutoff frequency in the current intermediate frequency signal; Step A-3: performing frequency spectrum analysis on the low-frequency signal to obtain a frequency difference. 3. The method according to claim 2, wherein said step A-3 comprises: The low-frequency analog signal is input into a DSP chip, and after the DSP chip converts the low-frequency analog signal into a low-frequency digital signal, the low-frequency digital signal is converted into a frequency-domain signal by FFT transformation, and the frequency-domain signal is extracted. The frequency corresponding to the maximum value of the amplitude of is a frequency difference, and the frequency difference is output. 4. method according to claim 2, is characterized in that, described cut-off frequency is fH, fH=2(Θf+fd), and wherein, Θf is the maximum individual frequency difference of crystal oscillator, and fd is the maximum multiple under the highest speed Puller frequency. 5. The method according to claim 1, wherein in the step C, adjusting the current local oscillator frequency of the mobile terminal according to the frequency difference comprises: According to the frequency difference, adjust the input tuning level of the reference crystal oscillator of the phase-locked loop circuit in the mobile terminal, so that the current local oscillator frequency output by the phase-locked loop circuit changes. 6. The method according to claim 5, wherein, according to the frequency difference, adjusting the current input tuning level of the reference crystal oscillator of the phase-locked loop circuit in the mobile terminal comprises: Input the frequency difference into the tuning level chip, and the tuning level chip adjusts the output tuning level according to the input frequency difference; The tuning level is input to a reference crystal oscillator of a phase-locked loop circuit in the mobile terminal. 7. The method according to claim 1, wherein the making the frequency difference not exceed the frequency difference tolerance further comprises: adjusting the frequency difference to a preset value within the frequency difference tolerance. 8. A frequency difference compensation device, characterized in that the device is applied to a mobile terminal with a millimeter wave communication function; The devices include: Frequency difference acquisition and analysis module: collect the current local oscillator frequency signal of the mobile terminal and the current transmission frequency signal of the fixed-point device in real time, perform difference calculation on the current local oscillator frequency signal and the current transmission frequency signal to obtain the current intermediate frequency signal, performing frequency spectrum analysis on the current intermediate frequency signal to obtain a frequency difference; Judging module: judging whether the frequency difference exceeds the frequency difference tolerance, if yes, then execute the adjustment module, if not, return to the frequency difference collection and analysis module; An adjustment module: adjust the current local oscillator frequency of the mobile terminal according to the frequency difference so that the frequency difference does not exceed the frequency difference tolerance, and return to the frequency difference collection and analysis module. 9. The device according to claim 8, wherein the frequency difference acquisition and analysis module comprises: Mixer: The mixer collects the current local oscillator frequency of the mobile terminal and the current transmission frequency of the fixed-point device in real time, and outputs the current intermediate frequency signal after performing a difference operation on the current local oscillator frequency signal and the current transmission frequency signal; Filter circuit: input the current intermediate frequency signal into an active low-pass filter circuit, and the active low-pass filter circuit outputs a low-frequency analog signal, and the low-frequency analog signal is a signal below the cut-off frequency of the current intermediate-frequency signal; A frequency difference analysis module: performing frequency spectrum analysis on the low-frequency signal to obtain a frequency difference. 10. The device according to claim 9, wherein the frequency difference analysis module comprises: The low-frequency analog signal is input into a DSP chip, and after the DSP chip converts the low-frequency analog signal into a low-frequency digital signal, the low-frequency digital signal is converted into a frequency-domain signal by FFT transformation, and the frequency-domain signal is extracted. The frequency corresponding to the maximum value of the amplitude of is a frequency difference, and the frequency difference is output. 11. The device according to claim 9, wherein the cut-off frequency is fH, fH=2(Θf+fd), wherein, Θf is the maximum individual frequency difference of the crystal oscillator, and fd is the maximum frequency difference at the highest speed Puller frequency. 12. The device according to claim 9, wherein in the adjusting module, adjusting the current local oscillator frequency of the mobile terminal according to the frequency difference comprises: According to the frequency difference, adjust the input tuning level of the reference crystal oscillator of the phase-locked loop circuit in the mobile terminal, so that the current local oscillator frequency output by the phase-locked loop circuit changes. 13. The device according to claim 12, wherein, according to the frequency difference, adjusting the current input tuning level of the reference crystal oscillator of the phase-locked loop circuit in the mobile terminal comprises: Input the frequency difference into a tuning level chip, and the tuning level chip adjusts the output tuning level according to the input frequency difference; The tuning level is input to a reference crystal oscillator of a phase-locked loop circuit in the mobile terminal. 14. The device according to claim 8, wherein the making the frequency difference not exceed the frequency difference tolerance further comprises: adjusting the frequency difference to a preset value within the frequency difference tolerance.