Air-ground downlink channel modeling method

Abstract

The invention belongs to the technical field of communication channel modeling and simulation, and particularly relates to an air-ground downlink channel modeling method, which comprises the following steps of: S1, multiplying a complex Gaussian white noise signal by an amplitude attenuation factor, and performing FFT (Fast Fourier Transform) to obtain a first frequency domain signal; s2, constructing an FIR Doppler filter by taking the speed and position information of the aircraft as parameters; s3, applying the first frequency domain signal to the FIR Doppler filter to output a second frequency domain signal, performing inverse fast Fourier transform (IFFT) on the second frequency domain signal to form a time domain signal, and generating a Rayleigh fading channel based on the time domain signal; s4, adding a constant signal on the basis of the Rayleigh fading channel to generate a Rician fading channel; and S5, adding the Rician fading channel and the Rayleigh fading channel to obtain the air-ground channel. The efficiency of the Doppler filter model is greatly improved, and the design of the filter is simplified.

Description

Technical Field

[0001] The present application belongs to the technical field of communication channel modeling and simulation, and in particular, relates to a method for modeling an air-to-ground downlink channel. Background Art

[0002] Air-to-ground communication refers to the information transmission process between ground base stations and aerial vehicles. For air-to-ground communication system applications, the channel quality between the transmitter and the receiver has a significant impact on the performance of the communication system, which in turn directly affects the quality of the received signal. When the aircraft is at a low elevation angle of the ground base station antenna, the reflection of ground obstacles will form a multipath effect with a large multipath delay spread; and the relative speed between the aircraft and the ground station is large, and the Doppler effect generated is also strong. In this case, it is very difficult to perform highly reliable transmission. Therefore, for air-to-ground communication, combining channel parameters, analyzing and modeling the propagation characteristics of the air-to-ground channel is the basis and prerequisite for achieving stable air-to-ground communication. Summary of the invention

[0003] In order to solve the above problems, a method for modeling an air-to-ground downlink channel is provided, wherein the air-to-ground downlink channel is an air-to-ground channel established between an aircraft and a ground base station, characterized in that the air-to-ground channel is divided into a main path channel caused by a refracted wave and a scattered wave and a secondary path channel caused by the superposition of a far-end reflected wave, the main path channel is a Rice fading channel, the secondary path channel is a Rayleigh fading channel, and a wide-sense stationary non-correlated scattering function is used to simulate the air-to-ground channel coefficient; the air-to-ground downlink channel modeling method comprises:

[0004] Step S1: multiplying the complex Gaussian white noise signal by the amplitude attenuation factor and then performing FFT transformation to obtain a first frequency domain signal;

[0005] Step S2: constructing a FIR Doppler filter with the aircraft speed and position information as parameters;

[0006] Step S3: applying the first frequency domain signal to the FIR Doppler filter to output a second frequency domain signal, performing an IFFT inverse transform on the second frequency domain signal to form a time domain signal, and generating a Rayleigh fading channel based on the time domain signal;

[0007] Step S4: Adding a constant signal to generate a Rice fading channel based on the Rayleigh fading channel;

[0008] Step S5: Add the Rice fading channel and the Rayleigh fading channel to obtain the air-ground channel.

[0009] Preferably, the complex Gaussian white noise signal is generated by superimposing two unrelated Gaussian noise sources.

[0010] Preferably, the model h of the Rice fading channel is1 for:

[0011] h 1 =a 1 s 1 δ(t-τ 1 );

[0012] Among them, a 1 is the amplitude attenuation factor of the main path channel, s 1 is the time-varying factor caused by the Doppler effect of the main path channel, τ 1 is the path delay of the main path channel.

[0013] Preferably, the model h of the Rice fading channel is 2 for:

[0014] h 2 =a 2 s 2 δ(t-τ 2 );

[0015] Among them, a 2 is the amplitude attenuation factor of the subpath channel, s 2 is the time-varying factor caused by the Doppler effect of the subpath channel, τ 2 is the path delay of the secondary channel.

[0016] According to the formula, step S4 is specifically: h2 is the Rayleigh channel, h2(t) is the fading coefficient of the channel, and the direct-line path component A is introduced, then the Rice channel coefficient h1(t) can be expressed as h1(t)=h2(t)+A. The present invention uses a wide-sense stationary non-correlated scattering function to simulate the air-to-ground channel coefficient, so h1(t) is replaced by a1s1; h2(t) is replaced by a2s2, so the relationship between the Rice channel coefficient and the Rayleigh channel coefficient can also be expressed as a1s1=a2s2;

[0017] Preferably, the time-varying factor brought by the Doppler effect is obtained by the Doppler frequency shift, and the frequency f of the received signal is c The formula is as follows:

[0018]

[0019] in, is the Doppler frequency shift f d The maximum value, f d Satisfies the following formula:

[0020]

[0021] Where v is the flight speed of the aircraft, f c is the center frequency, and θ is the incident angle of the signal.

[0022] Preferably, the air-to-ground channel adopts a multi-tap delay line model.

[0023] Preferably, the air-to-ground channel:

[0024] h(t,τ)=a 2 s 2 δ(t-τ 2 )+a 1 s 1 δ(t-τ 1 ).

[0025] The advantages of the present application include: abstracting the fading characteristics of the aviation air-to-ground channel into the delayed power spectrum, Doppler power spectrum and envelope of the reference model. The FFT-based Doppler filter modeling method proposed in the present invention greatly improves the efficiency of the Doppler filter model and simplifies the filter design. The present invention provides a theoretical basis for the design of an air-to-ground wireless communication system by establishing an efficient air-to-ground channel simulation modeling. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of low elevation aviation channel of this application;

[0027] Figure 2 A schematic diagram of a multi-tap delay line model;

[0028] Figure 3 Schematic diagram of filter channel modeling in this application. DETAILED DESCRIPTION

[0029] In order to make the technical solution and advantages of the present application clearer, the technical solution of the present application will be further described in detail in detail and in detail in conjunction with the accompanying drawings. It can be understood that the specific embodiments described here are only partial embodiments of the present application, which are only used to explain the present application, not to limit the present application. It should be noted that, for the convenience of description, only the parts related to the present application are shown in the accompanying drawings, and other related parts can refer to the general design. In the absence of conflict, the embodiments in the present application and the technical features in the embodiments can be combined with each other to obtain new embodiments.

[0030] In addition, it should be noted that, unless otherwise clearly specified and limited, the words "installed", "connected", "connected" and similar terms used in the description of this application should be understood in a broad sense. For example, the connection can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, an indirect connection through an intermediate medium, or the internal connection of two components. Technical personnel in the field can understand their specific meanings in this application according to the specific circumstances.

[0031] The present invention first analyzes and studies the transmission characteristics of an air-to-ground downlink channel, and then proposes a method for modeling an air-to-ground communication channel.

[0032] 1. Channel Characteristics Analysis

[0033] The present invention mainly focuses on the situation of small elevation angle. In the low elevation angle area, during the communication between the ground base station and the aircraft, affected by the geographical environment of the ground and the refraction of the air, especially the reflected waves of the ground and the scattering of the receiver itself, multiple radio signals with different delays arrive at the receiver from different directions, which will cause a very serious multipath effect. When a pulse signal is sent from the ground base station, the received signal of the aircraft contains not only the pulse, but also the pulses of each delay. This phenomenon of pulse width expansion in the received signal caused by the multipath effect is delay expansion. In digital communication, delay expansion is easy to cause inter-code interference, which seriously affects the transmission quality of digital signals. In addition, the Doppler effect caused by the relative motion between the aircraft and the ground station causes the receiving frequency to change. The Doppler frequency shift of the incoming wave direction of the signals with different delays is different, and the Doppler expansion phenomenon occurs, which will cause frequency dispersion and cause the time-varying characteristics of the channel.

[0034] In general, direct waves and scattered waves are inseparable multipaths, and they and the far-end reflected waves are separable multipaths. Therefore, the air-to-ground communication channel can be compared to a two-path Rice channel, where the main path is caused by the refracted wave and the scattered wave, and obeys the Rice distribution, and the other path is caused by the superposition of the far-end reflected waves, and obeys the Rayleigh distribution. Before modeling the channel, the statistical channel coefficients of the air-to-ground communication channel must be analyzed and determined first.

[0035] (1) Determination of delay characteristics;

[0036] The present invention uses a statistical model to describe the delay characteristics of the channel. The path parameters of different time delays in the air-ground channel are uncorrelated, so the non-correlated scattering characteristics are satisfied. Therefore, the delay spread characteristics of the air-ground channel obey the wide-sense stationary characteristics and can be described by a wide-sense stationary non-correlated scattering (WSSUS) random process. Here, τ represents the path delay to be determined.

[0037] (2) Determine the Doppler shift;

[0038] Due to the influence of multipath effect, the Doppler spectrum of the received signal is It varies within a certain range and obeys Jakes distribution. is the maximum Doppler shift, defined by the Doppler shift formula

[0039]

[0040] Where v is the flight speed of the aircraft, f dis the center frequency, θ is the incident angle of the signal. According to the Doppler frequency shift formula, the determination of this parameter requires the determination of the aircraft's flight speed, the center frequency, and the incident angle of the signal.

[0041] 2. Statistical modeling;

[0042] After determining the relevant channel coefficients, the present invention uses statistical modeling to model the air-to-ground channel.

[0043] Referring to the multi-tap delay line model (TDL) in the channel impulse response modeling, the channel model in this scenario is simulated by configuring the power delay spectrum parameters. In this model, the delay channel parameters generated by different paths are regarded as a tap delay line. If the delay extension of a certain path is greater than the symbol period, this path is considered to be a resolvable path. The present invention represents the resolvable path with a tap delay filter. The channel parameters are set to specific values ​​in specific cases, and finally the received signals generated by all paths are superimposed to obtain the signal after the transmitted signal passes through the communication channel.

[0044]

[0045] Among them, h k (t) is the channel coefficient of each resolvable path. In general, the channel coefficient will change with the specific environment and time of each path. In order to facilitate simulation and consider the channel characteristics, the present invention adopts the wide-sense stationary uncorrelated scattering function (WSSUS) to simulate the air-to-ground channel coefficient. Wide-sense stationary means that the channel coefficient does not change with time, and its average characteristics remain stable. Over a period of time, the average power of the received signal will not fluctuate irregularly. Uncorrelated scattering means that the signal components generated by different scattering paths are uncorrelated, that is, in this channel coefficient, each weighting coefficient is uncorrelated and independent, which is specifically determined by the channel delay and Doppler power spectrum. Therefore, the above formula can be specifically as follows:

[0046]

[0047] a k is the amplitude attenuation factor, s k is the time-varying factor brought by the Doppler effect. In the air-to-ground channel, the k value is 1 and 2, where the main path is the line-of-sight component and obeys the Rice distribution; the secondary path is the non-line-of-sight component and obeys the Rayleigh distribution.

[0048] 3. Channel Model Implementation

[0049] Through the above process, a preliminary model of the air-to-ground channel is constructed. The present invention adopts a FIR (Finite Length Unit Impulse Response) Doppler filter method based on FFT (Fast Fourier Transform) to implement the above channel model.

[0050] The specific steps are as follows:

[0051] a) Add two uncorrelated random Gaussian noise sources as in-phase and quadrature components to form a complex Gaussian white noise signal;

[0052] b) multiplying the complex Gaussian white noise signal by the amplitude attenuation factor to normalize the amplitude of each path in the multipath;

[0053] c) A FIR Doppler filter is constructed with the aircraft speed and position information as parameters. The complex Gaussian white noise result is converted into a frequency domain signal through FFT transformation and applied to the FIR Doppler filter. The output frequency domain signal is inversely transformed into a time domain signal through IFFT to generate a Rayleigh fading channel, i.e., h 1 =a 1 s 1 δ(t-τ 1 )

[0054] d) Based on the Rayleigh signal, add a constant signal to simulate the Rice fading channel, that is,

[0055] h 2 =a 2 s 2 δ(t-τ 2 ).

[0056] e) The sum of the Rayleigh fading channel and the Rice fading channel is the air-to-ground channel obtained by the present invention.

[0057] h(t,τ)=a 2 s 2 δ(t-τ 2 )+a 1 s 1 δ(t-τ 1 ).

[0058] One implementation example is to use the simulation filter method to establish an air-to-ground channel reference model. The channel parameters are: the communication frequency band is 5 GHz, the delay parameter is 8us, the Rice factor of the Rice channel is 10dB, the path losses of the two paths are 1dB and 0.9dB respectively, and the maximum Doppler frequency deviation is 3000Hz. In this way, the air-to-ground downlink can be statistically modeled to obtain the channel impulse response curve and the frequency response curve. Observe the frequency selective fading caused by the multipath effect to provide a theoretical basis for the selection of modulation methods for subsequent communication systems.

[0059] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application shall be based on the protection scope of the claims.

Claims

1. A method for modeling an air-to-ground downlink channel, wherein the air-to-ground downlink channel is an air-to-ground channel established between an aircraft and a ground base station, characterized in that: The air-to-ground channel is divided into a main path channel caused by a refracted wave and a scattered wave and a secondary path channel caused by the superposition of a far-end reflected wave, the main path channel is a Rice fading channel, the secondary path channel is a Rayleigh fading channel, and a wide-sense stationary non-correlated scattering function is used to simulate the air-to-ground channel coefficient; the air-to-ground downlink channel modeling method includes: Step S1: multiplying the complex Gaussian white noise signal by the amplitude attenuation factor and then performing FFT transformation to obtain a first frequency domain signal; Step S2: constructing a FIR Doppler filter with the aircraft speed and position information as parameters; Step S3: applying the first frequency domain signal to the FIR Doppler filter to output a second frequency domain signal, performing an IFFT inverse transform on the second frequency domain signal to form a time domain signal, and generating a Rayleigh fading channel based on the time domain signal; Step S4: Adding a constant signal to generate a Rice fading channel based on the Rayleigh fading channel; Step S5: Add the Rice fading channel and the Rayleigh fading channel to obtain the air-ground channel.

2. The air-to-ground downlink channel modeling method according to claim 1, characterized in that: The complex Gaussian white noise signal is generated by superimposing two unrelated Gaussian noise sources.

3. The air-to-ground downlink channel modeling method according to claim 1, characterized in that: The model h1 of the Rice fading channel is: h1=a1s1δ(t-τ1); Among them, a1 is the amplitude attenuation factor of the main path channel, s1 is the time-varying factor caused by the Doppler effect of the main path channel, and τ1 is the path delay of the main path channel.

4. The air-to-ground downlink channel modeling method as claimed in claim 3, characterized in that: The model h2 of the Rice fading channel is: h2=a2s2δ(t-τ2); Wherein, a2 is the amplitude attenuation factor of the subpath channel, s2 is the time-varying factor caused by the Doppler effect of the subpath channel, and τ2 is the path delay of the subpath channel.

5. The air-to-ground downlink channel modeling method as claimed in claim 3, characterized in that: The time-varying factor brought by the Doppler effect is obtained by the Doppler frequency shift, and the frequency f of the received signal c The formula is as follows: in, is the Doppler frequency shift f d The maximum value, f d Satisfies the following formula: Where v is the flight speed of the aircraft, f c is the center frequency, and θ is the incident angle of the signal.

6. The air-to-ground downlink channel modeling method according to claim 1, characterized in that: The air-to-ground channel adopts a multi-tap delay line model.

7. The air-to-ground downlink channel modeling method according to claim 1, characterized in that: The air-ground channel: h(t,τ)=a2s2δ(t-τ2)+a1s1δ(t-τ1).

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