Radio frequency stealth method based on sidelobe power suppression of subarray-level four-dimensional antenna array

Abstract

The present invention discloses a radio frequency stealth method based on sidelobe power suppression of a subarray-level four-dimensional antenna array. The basic system of the method is composed of an antenna array, a radio frequency switch, a power divider, a signal source, a local oscillator source, a mixer, a power amplifier, a switch control system, a phase shifter, and an attenuator. Under the premise of considering the radiation pattern of the array antenna unit, the method is based on subarray division and array statistical performance, and achieves a low average radiation power effect in the sidelobe area through a low mean square value, so that the probability of signal detection is reduced. On this basis, the probability of the received signal being identified can be minimized in the non-desired direction by maximizing the relative variance. Compared with the existing confidentiality system, the physical structure of this method is simple and the calculation is convenient. The present invention is applied to wireless communication systems with high radio frequency stealth performance requirements.

Description

Technical Field

[0001] The present invention belongs to the field of wireless communications, and particularly relates to realizing a directional modulation function in a four-dimensional antenna array while achieving an effect of low average radiation power in the sidelobe region, and can be used in communication systems with higher radio frequency stealth requirements. Background Art

[0002] With the development of wireless communication technology, the RF stealth performance of wireless information systems has received increasing attention. Traditional transceiver antenna arrays can transmit or receive the same signal with different amplitudes in different directions, making it easy for unauthorized receivers to intercept the signal. Traditional antenna arrays use a low sidelobe array factor, which greatly reduces the signal level received in undesired directions. However, this does not fundamentally change the signal waveform or spectral structure, allowing unauthorized receivers to still receive weak signals using high-sensitivity receivers. Directional modulation is a method that modulates the signal in undesired directions without modulating it in the desired direction. Theoretically mature directional modulation of periodic four-dimensional antenna arrays can only transmit signals with a narrow bandwidth in the desired direction. Because the directional modulation effect is fixed, the RF stealth performance is poor.

[0003] Four-dimensional antenna arrays based on pseudo-random timing offer superior directional modulation performance. The bandwidth of signals transmitted in the desired direction is unrestricted. However, because the modulation effect on signals in undesired directions is random, unauthorized receivers can only receive chaotic signals in those directions. This superior directional modulation performance can be achieved simply by turning switches on and off, offering significant advantages.

[0004] Patent publication number CN105553641 B proposes a secure communication method and system based on chaos. This scheme pre-encrypts the signal using "chaos" before transmission. Only when the receiver knows information such as the working protocol can the information be correctly deciphered. However, this method only encrypts the signal itself; the signal remains the same regardless of direction, and there is no modulation effect on the propagation path. Therefore, strictly speaking, it is not directional modulation. If an unauthorized receiver knows the relevant protocol, the confidentiality performance will be lost.

[0005] Patent publication number CN 111092642 B proposes a method for vector synthesis in the desired direction using unit directional patterns. This method allows the QPSK signal to be synthesized using the corresponding complex vectors of certain conducting units. The greater the synthesis error, the more timing sequences can be selected. By randomly switching timing sequences that meet the requirements, low-distortion transmission of the signal in the desired direction is achieved, while the constellation diagram in the undesired direction becomes chaotic. However, there are also the following problems: when the array size is fixed, the number of complex vector states that can be synthesized is limited, and some signals cannot be synthesized in certain desired directions. When the array size is large, each timing combination needs to be calculated and error judgment performed, requiring enormous computing resources. Furthermore, the signal still has a high level in the sidelobe direction. Even if the signal cannot be correctly identified, the direction of the signal wave is exposed.

[0006] Publication No. CN 110890908 B proposes a secure communication system using a pre-modulated four-dimensional antenna array. This method utilizes periodic timing and normalizes the array factor amplitude at each moment, achieving distortion-free signal transmission in the desired direction and modulated signals in undesired directions. It also achieves a low average radiated power distribution in the array's sidelobes, significantly reducing the likelihood of signal detection and identification in undesired directions. However, this low sidelobe effect is achieved through high-precision amplitude excitation equivalent to periodic timing, making it still a periodic modulation method and subject to certain deficiencies in directional modulation performance.

[0007] In response to the above difficulties, the present invention discloses a radio frequency stealth method based on sidelobe power suppression of a subarray-level four-dimensional antenna array. This method can realize the design of any antenna array structure and the selection of timing, and by calculating the statistical performance of the array, the average radiation power, variance, and mean can be obtained, which provides guidance for describing the radio frequency stealth performance. By using partial discrete amplitude excitation, the effect of low average radiation power in the sidelobe area can be achieved, so that the probability of the signal being detected is reduced. At the same time, under the premise of low average radiation power in the sidelobe area, the probability of the received signal being recognized can be minimized by maximizing the relative variance. In addition, random switching based on switch equiprobability can achieve a high-performance radio frequency stealth effect with low detection and low recognition. Summary of the Invention

[0008] The present invention was developed in light of the aforementioned technical background. Its purpose is to provide a radio frequency stealth method based on subarray-level four-dimensional antenna array sidelobe power suppression, improving the physical layer radio frequency stealth capability by suppressing the signal level in the sidelobe region and modulating the signal direction. The specific technical solution is as follows:

[0009] Consider a four-dimensional antenna array with N elements. The expression of its time-domain far-zone radiation field is:

[0010]

[0011] Where s(t) is the signal to be transmitted, A k is the excitation amplitude of the kth unit, α k is the excitation phase of the kth unit, E k and P k are the amplitude and phase of the far-zone electric field of the kth unit, U k (t) is the switching function of the kth element, β is the free-space wave number (β = 2π / λ, where λ is the free-space wavelength), d is the spacing between elements, θ is the angular direction, t represents time, e is the natural base, and j is the imaginary unit. F(θ, t) is the output signal in the θ direction, and AF(θ, t) is the time factor of the antenna array, which represents the modulation effect.

[0012] When α k =-β(k-1)dsinθ0+P k (θ), the antenna unit will achieve coherent superposition at θ = θ0. k Satisfy the tapered distribution of the traditional phased array, and let ∑A k U k (t) does not change with time. Due to the differences between units, ∑E k A k U k (t) is approximately a constant value. The present invention proposes a method of using discrete excitation amplitude. Assuming that the amplitude excitation is B bit precision, then A k The possible value of is i / 2 B , i=1,2,…,2 B .

[0013] Define subarray m as: the amplitude A of all units in the subarray m All m / 2 B , assuming that there are x(m) elements in the sub-matrix m, the element number set corresponding to the sub-matrix is ​​y{m}={y(m,1),y(m,2),…,y(m,x(m))}.

[0014] Then the constraints satisfied by x(m) and y{m} are formula (2) and formula (3).

[0015]

[0016] If the number of conducting sub-arrays is n(m), then the total number of timings that can be satisfied at this time is By randomly and equally probabilistically selecting n(m) units in subarray m, different subarrays work together. The expression of the time domain array factor of subarray m can be expressed as:

[0017]

[0018] When switching the timing randomly, these random modulation effects have an average performance, which is expressed as the mean, defined as Then the expression μ{AF} is

[0019]

[0020] The modulation effect will deviate from the mean to a certain extent. The square of the average deviation distance is called variance, which can be represented by var{AF} in formula (6)

[0021]

[0022] For random states, it is also necessary to use the average radiated power μ{|AF| 2} to characterize the average distribution of power in the spatial domain, which is a characterization of the statistical mean square value. Its expression is

[0023]

[0024] in, That is a diagonal matrix, and define the matrix k1, k2=1, 2, …, x(m) determines the dimension of the matrix. c1(m)=∑(Q m -D m ),c2(m)=∑(x(m)D m -Q m ), which is determined by the unit excitation amplitude within each sub-array and the sub-array division structure, and ∑ is the operation symbol for adding all elements of the matrix. It represents the connection between the two sub-array units, which is determined by the unit excitation phase between the sub-arrays and the structure between the sub-arrays, k1=1,2,…,x(m1), k2=1,2,…,x(m2),

[0025] There is a constraint relationship between formulas (5)-(7): |μ{AF}| 2 +var{AF}=μ{|AF| 2}, we can see that the average radiated power is affected by both the mean and the variance. To suppress the average radiated power in the sidelobe region, both the mean and the variance must be sufficiently small. To reduce the probability of receiving signals in undesired directions, the average radiated power in the sidelobe region must be minimized.

[0026] For directional modulation, “relative variance” is used as an indicator to describe the directional modulation effect: var{AF} / |μ{AF}| 2, i.e., var{AF}(dB) - |μ{AF}|(dB). A larger relative variance indicates a greater proportion of the variance in the mean square value. This means that when the sidelobes have low average radiated power, the transmitted signal becomes more chaotic, and the probability of the signal being correctly identified decreases.

[0027] Based on the above principles, the present invention is described by taking the traditional four-dimensional antenna array transmission system as an example. The basic structure diagram is shown as follows: Figure 1 As shown. The system consists of an antenna array, an RF switch, a power divider, a signal source, a local oscillator, a mixer, a power amplifier, a switch control system, a phase shifter, and an attenuator. The signal source generates the required baseband signal and the local oscillator, which are up-converted into an intermediate frequency signal. The intermediate frequency signal is amplified by the power amplifier and transmitted to the power divider, so that the signal is evenly distributed to each transmission path. The attenuator is used to adjust the excitation amplitude distribution distributed to each array unit; the phase shifter produces different delay effects on each signal, acting as an excitation phase. The RF switch is connected between the power divider and the amplitude-phase network. The switch control system determines its conduction state according to the set timing, which determines whether the signal is radiated by the antenna.

[0028] By adjusting n(m), the average radiated power μ(|AF| in the sidelobe area 2 ) reaches the desired level, maximize the relative variance of the sidelobe area var{AF} / |μ{AF}| 2 As a design indicator, the probability of receiving a signal in the sidelobe region being detected is sufficiently low, and even if detected, the probability of identification is minimized. Using n(m) units in subarray m as the timing constraint, the switch control system generates a timing sequence that satisfies the constraint and controls the RF switches to achieve this constraint.

[0029] Based on the above principle, the present invention discloses a radio frequency stealth method based on sub-array level four-dimensional antenna array sidelobe power suppression, such as Figure 2 The process is shown in the following steps:

[0030] (1) The excitation phase required for each unit is obtained according to the desired direction; the initial tapered excitation amplitude is obtained through the traditional phased array method.

[0031] (2) Discretize the excitation amplitude, divide the array elements into sub-arrays according to the equal excitation amplitude, and determine the number of elements x(m) in each sub-array and the element number set y{m} of the sub-array.

[0032] (3) By adjusting n(m), the average radiated power μ(|AF|) in the sidelobe region is 2 ) reaches the desired level, maximize the relative variance of the sidelobe area var{AF} / |μ{AF}| 2According to n(m), the four-dimensional antenna array timing that meets the constraints can be obtained.

[0033] (4) When transmitting signals, the switch control system controls the RF switch to achieve the required constrained timing, and each timing is switched with equal probability.

[0034] Since the timing is randomly switched with equal probability, the electromagnetic characteristics of the original signal are greatly changed. Since the average radiation power in the sidelobe area is controlled, the signal is not easily received by illegal receivers in the sidelobe direction. Even if it is received, the chaotic received signal is difficult to identify due to the large relative variance, thus having a certain RF stealth effect. This scheme provides a guiding formula and design method for the dynamic modulation of a pseudo-random four-dimensional antenna array and the low average radiation power distribution in the sidelobe area, which makes up for the problems of the existing four-dimensional antenna array in achieving directional modulation, high sidelobe power, high probability of detection, and insufficient flexibility.

[0035] The basic method we provide is mainly for principle explanation. In practical application, this basic solution can be improved according to specific circumstances:

[0036] (1) Increasing the number of antenna units can achieve narrower beam pointing, that is, the sidelobe range will become wider and more random.

[0037] (2) Increasing the accuracy of the excitation amplitude, that is, expanding B, can achieve the effect of lower average radiation power of the side lobes and a more random distribution.

[0038] (3) This method can be applied to any distributed array form, i.e., a plane array, a circular array, etc., and the directional modulation effect will also exist at the pitch angle.

[0039] (4) This method is applicable to multiple arbitrary desired directions. Through the linear superposition property of the array, the complex excitation corresponding to each desired direction is first superimposed, and then normalized and discretized to obtain the final excitation amplitude phase.

[0040] The most significant innovation of this invention is that it achieves low average radiated power in the sidelobe region by individually controlling the conduction units of each subarray, and proposes that maximizing relative variance can improve directional modulation performance. Compared with existing four-dimensional antenna array radio frequency stealth technology, this invention has the following advantages:

[0041] (1) The proposed method is applicable to antenna arrays of arbitrary structures and is an accurate method that takes into account the differences in unit radiation patterns.

[0042] (2) Statistical formulas for the mean, variance, and RMS of pseudo-random 4D antenna arrays are presented, addressing the issue of the enormous computational resources required for large arrays and high bit amplitude accuracy. Unlike other 4D antenna array directional modulation methods, this method predetermines the distribution of statistical performance indicators, thereby deriving the desired timing, making it more purposeful.

[0043] (3) Since the design is based on the effect of low average radiation power in the side lobes, the received signal in the side lobe area is weaker, and the probability of signal detection is reduced. Due to the dynamic modulation effect, the RF stealth performance in the undesired direction is much stronger than that of static and periodic modulation effects, and the RF stealth performance is stronger. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a structural block diagram of the four-dimensional antenna array transmission system used in the present invention, which consists of an antenna array, a radio frequency switch, a power splitter, a signal source, a local oscillator, a mixer, a power amplifier, a switch control system, a phase shifter, and an attenuator.

[0045] Figure 2 It is a specific flow chart of the radio frequency stealth method based on sub-array-level four-dimensional antenna array sidelobe power suppression.

[0046] Figure 3 It is the time domain waveform and corresponding spectrum distribution of the ST-LFMCW signal, which serves as a reference for comparison with the desired direction and the undesired direction.

[0047] Figure 4 The values ​​of the amplitude and phase distribution are taken as the desired transmission direction of 10 degrees, where the square dotted line “□” is the discrete excitation amplitude of the corresponding unit, corresponding to the left vertical axis; the circular solid line “○” is the phase shift of the corresponding unit, corresponding to the right vertical axis.

[0048] Figure 5 This is the normalized distribution diagram of the array factor mean, variance, and root mean square value with 10 degrees as the desired direction, where the maximum root mean square value is used as the normalization standard.

[0049] Figure 6 This is a pseudo-randomly generated timing diagram that meets the timing requirements, where black represents the on state and white represents the off state.

[0050] Figure 7 The average radiation power is obtained by calculating the time average of the power at each angle at each moment obtained based on time sampling. It can be seen that the average radiation power of the side lobe is less than -20dB.

[0051] Figure 8 Figure 3 is the timing waveform of the ST-LFMCW signal at -15, 10, 32, and 48 degrees after passing through the four-dimensional antenna array. It can be seen that the amplitude of the waveform in the non-desired direction is very small and is very different from the original signal waveform.

[0052] Figure 9 Figure 3 shows the spectrum distribution of the ST-LFMCW signal at -15, 10, 32, and 48 degrees after passing through the four-dimensional antenna array. It can be seen that the spectrum in the non-desired direction has a relatively small amplitude and is significantly different from the spectrum structure of the original signal.

[0053] Figure 10 The figure shows the modulation effect of a unit signal (amplitude 1, phase 0) at -15, 10, 32, and 48 degrees for the given example. This shows the possible values ​​of the unit signal in several directions after passing through the four-dimensional antenna array. The circles represent the average modulation effect, while the hexagons represent the possible values ​​after modulation. Specific implementation plan

[0054] Based on the specific application of the above method in transmitting ST-LFMCW signals, the ST-LFMCW signal waveform length is 10μs, the center frequency is 1.5GHz, and the signal bandwidth is 4MHz. Its time domain waveform and spectrum distribution are as follows Figure 3 As shown, it serves as a reference for comparison with signals received in different directions.

[0055] Based on this solution, this example uses a 20-unit printed dipole array operating at 1.5 GHz with a unit spacing of 0.45 times the wavelength. The excitation amplitude is controlled by a 2-bit precision attenuator. The transmitting system is as follows: Figure 1 To achieve a high-performance RF stealth effect with an average radiated power in the sidelobe area less than -20dB, the specific steps are as follows:

[0056] Taking 10° as the desired direction as an example, the phase shift of the phase shifter is as follows: Figure 4 As shown by the solid line and the right vertical axis, the amplitude excitation after discretization is as follows Figure 4 Shown by the dotted line and the left vertical axis.

[0057] The array is divided into subarrays based on discrete excitation amplitudes, and units with the same excitation amplitude are placed in the same group. Here, four subarrays are selected. Under the premise that the average sidelobe radiation power is less than -20dB, the relative variance is maximized. The number of fixed conduction units in each subarray group is 4, 3, 3, and 6 respectively. The normalized distribution of their mean, variance, and RMS values ​​is as follows: Figure 5 As shown in the figure, normalization is performed based on the maximum mean square value. It can be seen that the sidelobe mean square value (average radiated power) is less than -20dB, indicating that the signal is difficult to detect. At this point, the relative variance has reached its maximum, indicating the best directional modulation effect. The variance is -50dB in the desired direction, indicating that the signal has almost no directional modulation effect in the desired direction.

[0058] The above constrained timing is applied to the four-dimensional antenna array with equal probability. The timing diagram is shown as follows: Figure 6 By averaging the radiation power at all times, the average radiation power distribution is obtained as shown in Figure 7 shown. Figure 7 and Figure 5 The normalized mean square value distribution of is basically the same, which verifies the correctness of the proposed statistical formula. Both are less than -20dB in the sidelobe area, indicating that the signal is not easy to detect in this area. The time domain signals received in different directions are as follows: Figure 8 As shown, the spectrum distribution is Figure 9 As shown, and with Figure 3 The original signal is compared with the original signal. It can be seen that the received signal in the desired direction is consistent with the original signal in both the time and frequency domains, and has a higher intensity. However, the time domain signal and spectrum intensity received in the undesired direction are both very weak and difficult to detect. Furthermore, the waveform and spectrum structure of the signal have undergone significant random changes, making it impossible to restore key information, resulting in excellent RF stealth performance. In summary, based on the statistical expressions of the mean, variance, and root square value of the random distribution, the proposed method has the effect of lower average radiated power in the sidelobes compared to the traditional four-dimensional array. Furthermore, by maximizing the relative variance, the directional modulation performance is optimized, thus achieving high-performance RF stealth.

[0059] In order to more clearly present the radio frequency stealth performance, the possible values ​​of the unit signal with an amplitude of 1 and a phase of 0 after being modulated by the four-dimensional antenna array are placed on Figure 10 , where the circle represents the average modulation effect, and the hexagon represents the possible values ​​after modulation. It can be seen that the modulation effect of the proposed method is a change in amplitude and phase. Since the level in the undesired direction is not high, it is difficult to detect. The large relative variance leads to a chaotic distribution, which greatly increases the difficulty of identification, while there is only a slight signal change in the desired direction. For digital and analog signals with more complex amplitude and phase distributions, the signal received in the undesired direction will be a more chaotic random signal.

[0060] The above description of the present invention and its embodiments is provided for the benefit of those skilled in the art. Such description is to be construed as illustrative rather than restrictive. Relevant engineers and technicians may implement the present invention in accordance with the concepts set forth in the claims and may make various modifications in form and detail without departing from the spirit and scope of the present invention. All of the foregoing should be considered within the scope of the present invention.

Claims

1. A radio frequency stealth method based on subarray-level four-dimensional antenna array sidelobe power suppression, characterized by The steps include: S1. Obtain the required excitation phase according to the desired direction; obtain the initial tapered excitation amplitude of the four-dimensional antenna array according to phased array theory; S2. Discretize the initial excitation amplitude of the four-dimensional antenna array, divide the antenna array elements into sub-arrays according to equal excitation amplitudes, and determine the number of elements in each sub-array and the set of element numbers of the sub-arrays; S3. RF switches exist in each channel after the power divider. By controlling the conduction of the RF switches in each channel, the number of conduction sub-arrays in each group can be adjusted separately, so that the relative variance of the time array factor AF of the antenna array in the sidelobe area is maximized under the premise that the average radiation power in the sidelobe area reaches the desired level. It is defined as var{AF} / |μ{AF}| 2 , where μ{AF} represents the mean of AF and var{AF} represents the variance of AF. The four-dimensional antenna array timing that satisfies the constraints can be obtained based on the number of sub-arrays turned on in each group. S4. When transmitting signals, the switch control system controls the RF switch to achieve the required constrained timing, and each timing is switched with equal probability.

2. The radio frequency stealth method based on subarray-level four-dimensional antenna array sidelobe power suppression according to claim 1 is characterized in that: Antenna array elements with the same excitation amplitude are classified into the same sub-array. By fixing the number of conducting elements in each group of equally excited sub-arrays, equal probability switching is performed within the sub-array, and the sub-arrays work independently.

3. The radio frequency stealth method based on subarray-level four-dimensional antenna array sidelobe power suppression according to claim 1 is characterized in that: The mean radiated power is controlled by adjusting the mean square value of the antenna array time factor (AF), and the relative variance of AF is adjusted to control the level of clutter in signals received in undesired directions.

Images