Four-dimensional antenna array radio frequency stealth multi-beam forming method based on joint modulation

Abstract

The invention discloses a four-dimensional antenna array radio frequency stealth multi-beam forming method based on joint modulation. A basic system is composed of an array and radio frequency cables, wherein the array is composed of a baseband modulator, a sequential control system, a local oscillator source, a frequency mixer, a power divider, a modulation network, a band-pass filter, a power amplifier and N antenna units, and the radio frequency cables are used for connecting all the parts. By combining the degree of freedom of baseband modulation and time modulation, the method can improve the signal uncertainty in the non-target direction to the greatest extent under the condition of not increasing extra hardware requirements. And moreover, the time-frequency characteristic of the sidelobe can be disturbed, the radiation power of the sidelobe can be effectively reduced, and signals in multiple target directions are ensured not to be interfered.

Description

Technical Field

[0001] This invention belongs to the field of wireless transmission. Specifically, it is based on four-dimensional antenna array technology and fully combines the time modulation degree of freedom and the digital baseband degree of freedom. It proposes a simple, efficient and flexible radio frequency stealth multi-beamforming method that can be used in multi-target radio frequency stealth systems. Background Technology

[0002] Currently, the electromagnetic environment is becoming increasingly complex, with various wireless electronic devices present in space, in which radar systems play a crucial role. For radar systems, if their detection signals are intercepted by an adversary, they are easily jammed or even rendered inoperable. Radio frequency stealth design enables radar systems to counter intercepting aircraft.

[0003] Traditional radio frequency (RF) stealth performance can be improved through waveform design, such as using broadband signals. In RF stealth systems, frequently used broadband waveforms include frequency-modulated (FM) continuous waves and phase-modulated (PM) continuous waves. It is worth noting that RF stealth performance based on broadband signals is not always effective and is easily intercepted by high-performance interceptors. This is because the spectral structure of a signal radiated by an antenna or antenna array does not fundamentally change as it propagates in wireless space. By introducing "time" as a new design parameter, four-dimensional antenna arrays can simultaneously control radiation in the spatial, temporal, and frequency domains, effectively achieving RF stealth design.

[0004] A patent with publication number CN 102857280 B discloses a secure communication system and method based on a four-dimensional antenna array. This method achieves distortion-free signal transmission in a specified direction by setting the multi-harmonic radiation intensity pointed to by the target to zero, and controls the radiation intensity of harmonics in areas not pointed to by the target to achieve aliasing of broadband signals, further improving signal security. However, this method cannot achieve simultaneous transmission of multiple beams.

[0005] In patent publication CN 118842490 B, a radio frequency stealth method for suppressing sidelobe power in a subarray-level four-dimensional antenna array is proposed. This method combines different elements into subarrays and randomly controls the number of active elements in each subarray, significantly increasing the uncertainty of the radiated signal of the four-dimensional array, thereby improving its anti-interception capability. However, this method cannot achieve multi-beam transmission, which limits its application in radio frequency stealth systems.

[0006] In patent publication CN 120567251 A, a method for multi-beamforming a four-dimensional antenna array using combined amplitude and phase baseband modulation is proposed. This method maps multiple signals to antenna elements through amplitude and phase modulation combined with digital baseband modulation, and has the potential to achieve radio frequency stealth performance. Although this method has better radiation capability than using amplitude modulation alone, it has high requirements for the performance of the modulation switch.

[0007] To address the above application requirements, this invention discloses a four-dimensional antenna array radio frequency stealth multi-beamforming method based on joint modulation. By combining the degrees of freedom of baseband modulation and time modulation, this method can maximize the signal uncertainty in non-target directions without increasing additional hardware requirements, while ensuring that signals in multiple target directions are not interfered with. Summary of the Invention

[0008] This invention is made in view of the above-mentioned technical background, and aims to provide a multi-beamforming method for radio frequency stealth based on joint modulation of a four-dimensional antenna array. With simple hardware complexity, it achieves radio frequency stealth capability against multiple targets through joint baseband modulation and time modulation. The specific technology is as follows:

[0009] Consider a four-dimensional antenna array with N elements; its far-field time-domain radiation field can be expressed as:

[0010]

[0011] Where f0 is the carrier frequency, E(θ,t) is the time-domain array factor of the antenna array, and U k (t) is the timing function of the k-th element, β is the free space wavenumber, d is the spacing between adjacent elements, N represents the total number of antennas, θ is the spatial angular direction, t represents time, e is the natural base, and j is the imaginary unit. U k The modulation period of (t) is T s That is, satisfying U k (t)=U k (t+T s ), f s =1 / T s That is the corresponding modulation frequency.

[0012] To achieve combined beam control using digital baseband and analog time modulation, the digital baseband modulation can be written as:

[0013]

[0014] In the formula, Q represents the number of beams to be generated, and when p ≤ Q-1, s p(t) indicates that the p-th beam needs to be transmitted to θ. p The p-th original baseband signal in the direction, h p (t) is related to the p-th signal s p (t) is the corresponding baseband pre-modulation signal. When p=Q, s Q (t) As a jamming signal, it can interfere with the illegal interceptor's ability to correctly intercept the signal that needs to be transmitted. Q (t) is related to the Qth signal s Q (t) corresponds to the baseband pre-modulation signal. Digital weighting can be achieved by designing h. p This is achieved using (t), and its expression can be written as:

[0015]

[0016] in and Let represent the digital amplitude and phase weighting coefficients of the k-th element in the p-th beam, respectively. Thus, the radiation field of the four-dimensional array carrying the baseband signal s(t) can be expressed as:

[0017]

[0018] The timing sequence must satisfy:

[0019]

[0020] In the formula, C is a constant related to the modulation switch type. At the transmitting end, a passband frequency of [f0-f...] is used. s / 2,f0+f s When using a bandpass filter with a / 2], E can be eliminated. t The harmonic components of (θ,t) affect s p Due to the influence of (t), the filtered radiation field F(θ,t) of the four-dimensional antenna array can finally be expressed as:

[0021]

[0022] As can be seen from the above formula, the original radar signal, after being jointly modulated by digital baseband and analog time, is radiated by multiple weighted beams. In this way, without introducing additional hardware complexity, the flexibility of radiation control of the four-dimensional antenna array is improved by combining digital baseband and analog time modulation.

[0023] By optimizing the formula The effect. To achieve radio frequency stealth performance, through optimization and To achieve control over the p-th beam, the beams are divided into two categories. When p ≤ Q-1, the radiation pattern characteristics of the p-th signal are as follows:

[0024] (1) The direction of beamforming is θ p This is used to ensure the correct transmission of signals in a specified direction;

[0025] (2) The radiation pattern at θ1, θ2, ..., θ p-1 θ p+1 θ p+2 , …, θ Q-1 The direction forms a zero depth to reduce the interference of this signal on other signals in the corresponding transmission direction;

[0026] (3) The beam exhibits a low sidelobe effect, at which point the radiation power in the non-target area is low, which can reduce the probability of the signal in the sidelobe area being detected.

[0027] The radiation pattern characteristics of the Qth signal are:

[0028] (1) Forming a beam without a clear direction can ensure that the interference signal can be distributed relatively evenly throughout the space;

[0029] (2) The radiation pattern at θ1, θ2, ..., θ Q-1 The direction forms a zero depth, ensuring that the interference signal will not interfere with the transmission direction of multiple targets, and guaranteeing that s is true when p≤Q-1. p (t) at θ p High-quality transmission;

[0030] (3) The beam exhibits a low sidelobe effect, which can reduce the probability of the signal in the sidelobe region being detected.

[0031] Based on the above principles, this invention uses a traditional four-dimensional antenna array as a transmission system as an example for illustration. Figure 1As shown, the system consists of a baseband modulator, a timing control system, a local oscillator, a mixer, a power divider, a modulation network, a bandpass filter, a power amplifier, an array of N antenna elements, and RF cables connecting the various parts. The baseband modulator is connected to the timing control system on one side and to the mixer via the local oscillator on the other. The mixed RF signal is transmitted to each channel via the power divider. Each channel is connected to the modulation network, which is connected to both the timing control system and the bandpass filter. The bandpass filter is connected to the power amplifier, which is ultimately connected to the antenna elements. The timing control system regulates the modulation network according to the optimized timing sequence. The modulation network can use any type of switch, such as an on-off absorptive switch or a 0° / 180° phase switch. Based on the timing sequence generated by the timing control system, Q signals to be transmitted are pre-modulated simultaneously, according to the formula... The final baseband signal s(t) is obtained and generated by the baseband modulator. This signal, along with the local oscillator signal generated by the local oscillator source, is then mixed with the signal to form an RF signal. A power divider then evenly distributes the RF signal across the transmission paths. Finally, a modulation network achieves the desired multi-beam effect, with a passband frequency of [f0-f...]. s / 2,f0+f s The bandpass filter of / 2] can filter out harmonics generated by the modulation network and prevent signal distortion due to aliasing. The signal amplitude is then increased by the power amplifier and finally radiated by the antenna.

[0032] Based on the above principles, this invention discloses a method for radio frequency stealth multi-beamforming based on joint modulation of a four-dimensional antenna array, comprising the following steps:

[0033] (1) Based on the actual modulation network, the formula is... The required timing sequence is obtained through optimization, and the required control timing sequence is generated through a timing control system.

[0034] (2) Based on the required sidelobe level, the radiation pattern characteristics of the first Q-1 signals are optimized to obtain the desired sidelobe level. and (p≤Q-1);

[0035] (3) Based on the required sidelobe level, the radiation pattern characteristics of the Qth signal, i.e., the interference signal, are optimized to obtain the desired sidelobe level. and ;

[0036] (4) Based on the Q group digital amplitude and phase weighting coefficients and The baseband pre-modulated signal is obtained;

[0037] (5) The baseband signal s(t) generated by the baseband modulator is obtained from the baseband premodulation signal and the original baseband signal;

[0038] (6) After s(t) is mixed with the local oscillator, it passes through the power divider, the modulation network controlled by the timing control system, the bandpass filter, and the power amplifier in sequence, and is finally radiated by the antenna array.

[0039] The most significant innovation of this invention lies in its combination of time modulation and baseband modulation to achieve a high-performance radio frequency stealth system for multiple targets, with each target carrying its corresponding information via a separate beam. Compared to existing four-dimensional antenna array multi-beam technology, this invention offers the following advantages:

[0040] (1) The excitation amplitude and phase of each signal radiation pattern can be directly controlled by digital weighting, breaking through the limitation that the excitation amplitude of the existing architecture can only be controlled by analog time modulation. This greatly simplifies the hardware complexity and fully improves the hardware utilization.

[0041] (2) Compared with existing four-dimensional antenna array radio frequency stealth systems, the present invention can realize the transmission of the required signals in multiple directions at any given time, and there is basically no interference between the signals;

[0042] (3) This method achieves the specified distribution of multiple radar signals and interference signals in space using only one array, which is equivalent to the effect of Q sets of high-precision excitation traditional antenna arrays working simultaneously.

[0043] The basic method we provide is a principle explanation; in practical applications, this basic solution can be improved according to specific circumstances.

[0044] (1) Since this method directly controls the excitation amplitude and phase of each signal in each antenna element through digital weighting, the radiation pattern of each signal is flexible and can be optimized and modified as needed.

[0045] (2) This method is applicable to arrays with arbitrary distribution, such as planar arrays, heterogeneous arrays, etc., and will achieve radio frequency stealth effect in the pitch angle as well. Attached Figure Description

[0046] Figure 1 This is a schematic diagram of a four-dimensional antenna array radio frequency stealth multi-beamforming system based on joint modulation. The system consists of a baseband modulator, a timing control system, a local oscillator, a mixer, a power divider, a modulation network, a bandpass filter, a power amplifier, antenna elements, and radio frequency cables connecting the various parts.

[0047] Figure 2The following is a set of multi-beam RF stealth timing sequences obtained by phase modulation optimization based on 0° / 180° when N=16. Black represents state "180°" and gray represents state "0°".

[0048] Figure 3 The time-frequency distribution of four radar signals is given, where s1(t) is the first signal that needs to be transmitted to -20°, characterized by: Figure 3 (a); s2(t) is the second signal that needs to be transmitted to 0°, characterized by: Figure 3 (b); s3(t) is the third signal that needs to be transmitted to 40°, characterized by: Figure 3 (c); s4(t) is the interference signal, characterized by: Figure 3 (d);

[0049] Figure 4 The 16-element digital amplitude weighting coefficients corresponding to the radiation patterns of the four signals. The gray solid line “△” symbol represents the weighting coefficient of s1(t), the gray dashed line “◇” symbol represents the weighting coefficient of s2(t), the gray solid line “○” symbol represents the weighting coefficient of s3(t), and the black dashed line “□” symbol represents the weighting coefficient of s4(t).

[0050] Figure 5 The 16-element digital phase weighting coefficients corresponding to the radiation patterns of the four signals. The gray solid line “△” symbol represents the weighting coefficient of s1(t), the gray dashed line “◇” symbol represents the weighting coefficient of s2(t), the gray solid line “○” symbol represents the weighting coefficient of s3(t), and the black dashed line “□” symbol represents the weighting coefficient of s4(t).

[0051] Figure 6 According to Figure 2 The time-series multi-beam radiation patterns are shown as follows: the radar signal patterns pointing at -20°, 0°, and 40°, and the interference signals forming zero depth at these three angles. The solid gray line "△" represents s1(t) pointing at -20°, the dashed gray line "◇" represents s2(t) pointing at 0°, the solid gray line "○" represents s3(t) pointing at 40°, and the dashed black line "□" represents s4(t) forming zero depth. Due to sidelobe level control, the sidelobe level of each beam is -20dB.

[0052] Figure 7 To utilize Figure 4 The time-frequency distribution of the signal received by the beam shown in the -20° direction has the same characteristics as s1(t);

[0053] Figure 8 To utilize Figure 4The time-frequency distribution of the signal received by the beam shown in the -10° direction cannot yield any characteristics of s1(t) to s3(t);

[0054] Figure 9 To utilize Figure 4 The time-frequency distribution of the signal received by the beam in the 0° direction is exactly the same as that of s2(t);

[0055] Figure 10 To utilize Figure 4 The time-frequency distribution of the signal received by the beam shown in the 20° direction cannot yield any characteristics of s1(t) to s3(t);

[0056] Figure 11 To utilize Figure 4 The time-frequency distribution of the signal received by the beam shown in the 40° direction has the same characteristics as s3(t);

[0057] Figure 12 It shows the intensity distribution of received signals in the frequency dimension at different angles. It can be seen that the spectrum distribution corresponds to three radar signals only in the three target directions, while the other directions are chaotic. Detailed Implementation Plan

[0058] Taking a 16-element uniform array with a unit spacing of half a wavelength as an example, the radio frequency stealth performance of the proposed method is verified. The center frequency of the array is 2 GHz. Three different radar signals are transmitted from three different target directions, and a jamming signal is transmitted simultaneously to improve signal stealth capability as a specific case. The modulation network uses a 0° / 180° phase switch as a specific hardware example, with a modulation frequency f. p =10MHz. Assume that three target directions are arbitrarily chosen as θ1=-20°, θ2=0°, and θ3=40°, and the bandwidth of each of the four signals is 10MHz, and the signal duration is 4μs. Figure 1 A schematic diagram of the proposed four-dimensional antenna array RF stealth multibeamforming system based on joint modulation is presented.

[0059] The hardware specifically takes the form of a 0° / 180° phase switch. Let C=1, and use this hardware from the formula... The timing sequence is as follows Figure 2 As shown in the figure. In this case, three radar signals were used to transmit in designated directions, and the time-frequency distributions of these three radar signals are as follows. Figure 3 As shown in (a)~(c), it can be clearly observed that the frequency of s1(t) gradually increases with time, the frequency of s2(t) first increases and then decreases with time, and the frequency of s3(t) gradually decreases with time. The fourth signal is an interference signal, and its time-frequency distribution is as follows. Figure 3 As shown in (d), its characteristics are random distribution and no obvious frequency variation features.

[0060] To achieve radio frequency stealth performance, during the transmission of s1(t) to s3(t), a clear beam pointing direction is required in the corresponding target transmission direction, while a clear null depth is formed at the pointing directions of the other two radar signals to reduce signal interference. During the transmission of s4(t), its radiation pattern needs to form a null depth at the target transmission directions of the three radar signals, aiming to avoid interfering with signals transmitted in the target direction and only interfere with received signals in non-target directions. Furthermore, the radiation pattern characteristics of these four signals all exhibit low sidelobes, reducing the likelihood of signal detection. The sidelobe level is set to -20dB, and the required digital amplitude and phase weighting coefficients are optimized based on the four target radiation patterns. and Each as Figure 4 and Figure 5 As shown. Correspondingly, the radiation patterns of the four signals are as follows. Figure 6 As shown, it can be clearly seen that some radiation patterns exhibit a very obvious zero-depth effect at positions of -20°, 0°, and 40°.

[0061] Using the obtained timing sequence and digital amplitude and phase weighting coefficients, according to the formula Obtain the digital premodulated signal, and then according to the formula The baseband signal s(t) is obtained. The baseband signal s(t) is generated by the modulator and up-converted with the local oscillator signal to obtain the radio frequency signal. This signal then passes through a power divider, modulation network, bandpass filter, and power amplifier before being radiated by the antenna. The characteristics of the received signals at -20°, -10°, 0°, 20°, and 40° are examined to illustrate the radio frequency stealth performance. The time-frequency distributions of the received signals in the five directions are shown in [the diagram / image / image / etc.]. Figure 7 ~ Figure 11 In the -20° direction, its characteristics are exactly the same as the s1(t) signal; in the 0° direction, its characteristics are exactly the same as the s2(t) signal; and in the 40° direction, its characteristics are exactly the same as the s3(t) signal. This proves that the three required signals can be transmitted without distortion in the three target directions. The time-frequency distribution of the received signals in the -10° and 20° directions is very chaotic, and the characteristics of the s1(t) to s3(t) signals cannot be clearly obtained. This proves that the interceptor in the non-target direction cannot correctly intercept the characteristics of the transmitted signal.

[0062] To further illustrate the effectiveness of the proposed method, the spectral distribution across the entire space was plotted. For example... Figure 12 The diagram shows the power distribution along the angle-frequency dimension. It can be seen that, apart from the clearly defined signal characteristics in the target direction, the other non-target directions exhibit cluttered signals, thus significantly improving the system's radio frequency stealth performance.

[0063] In summary, the four-dimensional antenna array radio frequency stealth multi-beamforming method based on joint modulation proposed in this invention can maximize the signal uncertainty in non-target directions without increasing additional hardware requirements, while ensuring that signals in multiple target directions are not interfered with.

[0064] The above description is intended for those skilled in the art and describes the invention and its embodiments. This description should be considered illustrative rather than limiting. Those skilled in the art can implement the invention in detail according to the ideas in the claims, and can also make certain changes in form and detail without departing from the spirit and scope of the invention. These changes should all be considered within the scope of the invention.

Claims

1. A method for radio frequency stealth multi-beamforming based on joint modulation of a four-dimensional antenna array, characterized in that... Includes the following steps: S1. The required timing sequence is obtained by optimization based on the actual modulation network, and the required control timing sequence is generated by the timing control system. S2. Based on the required sidelobe level, the digital amplitude and phase weighting coefficients are obtained by optimizing the radiation pattern characteristics of the signal to be transmitted in the target direction; S3. Based on the required sidelobe level, the corresponding digital amplitude and phase weighting coefficients are obtained by optimizing the radiation pattern characteristics of the interference signal; S4. Based on all the digital amplitude and phase weighting coefficients, obtain the baseband pre-modulation signal; S5. The baseband signal s(t) generated by the baseband modulator is obtained from the baseband pre-modulation signal and the original baseband signal. After S6 and s(t) are mixed with the local oscillator, they pass sequentially through a power divider, a modulation network controlled by a timing control system, a bandpass filter, and a power amplifier, and are finally radiated by the antenna array.

2. The method for radio frequency stealth multibeamforming based on joint modulation of a four-dimensional antenna array according to claim 1, characterized in that: The radiation pattern of each signal can be directly controlled by digital weighting, without any restrictions on the type of modulation network.

3. The method for radio frequency stealth multi-beamforming based on joint modulation of a four-dimensional antenna array according to claim 1, characterized in that... Digital weighting can be achieved by designing h p This can be achieved using (t), and its expression can be written as: ； In the formula U k (t) is the timing function of the k-th unit, T p Let U be the modulation period of the timing function U(t). k (t) is the timing function of the k-th unit. and These represent the digital amplitude and phase weighting coefficients of the k-th element in the p-th beam, respectively.

4. The method for radio frequency stealth multibeamforming based on joint modulation of a four-dimensional antenna array according to claim 1, characterized in that: Additional interference signals increase the uncertainty of received signals from non-target directions; the characteristics of beam pattern with zero-depth effect reduce interference between multiple signals.

5. The method for radio frequency stealth multibeamforming based on joint modulation of a four-dimensional antenna array according to claim 1, characterized in that: By controlling the digital amplitude and phase weighting coefficients of each signal, each beam achieves a low sidelobe effect, reducing the probability of the signal in the sidelobe region being detected.

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