Multi-static radar distance deception jamming identification and high-speed target detection method

Abstract

The invention discloses a multi-static radar distance deception jamming recognition and high-speed target detection method, which is applied to the field of radar anti-jamming and aims at solving the problem of failure of real target detection caused by strong deception of distance deception jamming during high-speed target detection. The method comprises the following steps: firstly, establishing an echo model under an interference condition according to a multistatic radar distance historical mathematical equation, and on this basis, realizing coherent accumulation of a single-channel echo signal through Radon Fourier Transform (RFT); then, acquiring matching position information of each target in different channels by using an entropy cyclic matching method based on a topological structure; secondly, performing ellipse positioning processing on the paired position information to realize identification of distance deception interference and a target; and finally, performing constant false alarm rate detection processing on the identification result of the target and the distance deception interference to obtain a target detection result.

Description

Technical Field

[0001] The present invention belongs to the field of radar anti - interference, and particularly relates to a deception interference recognition and target detection technology. Background Art

[0002] High - speed target detection and recognition have always been hot research topics in the field of radar detection, and have broad application prospects in both civilian and military fields. However, with the continuous development of science and technology, high - speed stealth targets, due to their strong stealth ability, high speed, high mobility and other characteristics, pose a huge challenge to the accurate detection of targets. At the same time, digital radio frequency memory can generate range deception interference signals highly similar to radar signals, making target detection even more difficult.

[0003] For range deception interference, a single - station radar can use the surface scattering intensity of objects, polarization characteristic differences, and DRFM quantization errors to identify deception interference. However, the detection angle of a single - station radar is single, and it cannot obtain complete environmental information. In the face of high - fidelity active range deception interference, the anti - interference performance will be significantly reduced.

[0004] In recent years, multi - base radars can not only expand the surveillance area, but also achieve resource sharing among multiple stations at the same time, and can flexibly establish the configuration structures of single - base, dual / multi - base radars according to actual combat requirements by converting the working states of each station in real - time. They have received extensive attention in the direction of anti - deception interference and moving target accumulation detection. R. Tan et al. combined with a frequency - domain cooperative waveform strategy to complete the suppression of multi - main - lobe blanket interference, but this method requires designing different transmission signals for different base stations and does not have universality. S. Zhao et al. proposed an active false target discrimination method based on the difference in target amplitude ratios. By theoretically analyzing the random distribution of the amplitude ratios of the target in each receiving station, and according to the amplitude ratio difference, the deception interference recognition was realized by using the clustering analysis method. However, this work only considered the amplitude information of true and false targets, and the recognition rate would suffer a certain performance loss. D. Huang et al. proposed an anti - interference technology based on homologous positioning experiments for the problem of multiple range deception interferences existing in a multi - base radar system, and completed the effective recognition of deception interference, but it is not applicable to low - signal - to - noise ratio environments. S. Zhao et al. used the method of non - coherent accumulation to detect targets and designed a false target discriminator based on the Newman - Pearson criterion, but did not consider the phase information of the targets, resulting in a decrease in the accumulation detection performance. M. Wang et al. proposed a multi - channel coherent accumulation algorithm with entropy as the optimization criterion, which improved the echo signal - to - noise ratio, but it did not consider the influence of range deception interference on the echo signal.

[0005] In summary, existing studies have respectively achieved the recognition and suppression of deception jamming through transmitted waveform design, utilization of energy or spatial position differences, and non-coherent fusion accumulation. However, they all have high requirements for the signal-to-noise ratio and it is difficult to accurately recognize the range deception jamming of multi-static radar and effectively detect targets under low signal-to-noise ratio conditions. Summary of the Invention

[0006] To solve the above problems, the present invention proposes a method for recognizing range deception jamming and detecting high-speed targets of multi-static radar, which realizes the effective recognition of range deception jamming and targets under different jamming-to-signal ratio conditions.

[0007] The technical solution adopted by the present invention is as follows: A method for recognizing range deception jamming and detecting high-speed targets of multi-static radar, including:

[0008] S1. The one-transmitter multi-receiver radar system uses an LFM signal as the transmitted signal, and the target echo signals received by each receiver are down-converted and expressed as:

[0009] S rk (t,t m )=A r S t (t-R k (t m ) / c)exp[j(-2πf c R k (t m ) / c+2πf dk t)]

[0010] Among them, S rk (t,t m ) is the target echo of each receiving node of the radar, t is the fast time, c is the propagation speed of electromagnetic waves, A r is the amplitude of the echo signal, f dk is the Doppler frequency of the target echo signal corresponding to each receiver, and R k (t m ) represents the equivalent distance of the target relative to the k-th channel of the radar system;

[0011] The down-conversion of the range deception jamming signals received by each receiver can be expressed as:

[0012]

[0013] Among them, J rk (t,t m ) is the interference echo of each receiving node of the radar, is the amplitude of the range deception jamming, N is the number of range deception jammings, is the time delay of each range deception jamming;

[0014] If the echo signals received by each channel are the superposition of the target signal and the range deception interference, the baseband echo signal can be expressed as:

[0015] Z k (t,t m )=S rk (t,t m )+J rk (t,t m )

[0016] where Z k (t,t m ) is the coupled echo waveform of each receiving node of the radar;

[0017] 2. Perform pulse compression processing on the coupled echo, and the pulse compression result of the baseband echo signal of each channel is:

[0018]

[0019] where is the pulse compression waveform of the coupled echo of each receiving node, A e is the amplitude of the pulse compression of the target echo, is the pulse compression amplitude of the range deception interference, B is the signal bandwidth, and λ is the signal wavelength;

[0020] 3. Perform RFT accumulation and threshold preprocessing on the processed echo signal to obtain the target peak estimation information of single-channel preprocessing;

[0021] 4. According to the preprocessing estimation value, construct the envelope alignment and phase compensation functions of the echoes between multiple channels, and combine the topological structure of the system to complete the cyclic matching processing of different channels;

[0022] 5. Screen each plane of the cyclic matching according to the information entropy criterion to obtain the optimal matching plane and obtain the parameter index of the system target;

[0023] 6. Combine the parameter index of the system target and the target peak estimation information of the preprocessing, and the position estimation result of the system target can be mapped;

[0024] 7. Perform elliptical positioning processing on the position estimation result of the system target to complete the identification of the target and the range deception interference;

[0025] 8. Perform constant false alarm rate detection processing according to the identification results of the target and the range deception interference in step S7 to obtain the target detection result.

[0026] Advantages of the present invention: By using the RFT algorithm to extract the target motion trajectory and correcting the range walk simultaneously, the present invention obtains the single-channel coherent integration result. Then, an entropy optimization and cyclic matching method based on topological structure is adopted to perform multi-channel signal integration enhancement and obtain the target pairing position information. Finally, the distance deception interference and the target are identified through elliptical positioning. The present invention can effectively identify the distance deception interference and the target under different signal-to-interference ratios, and at the same time realize the integration and enhancement of the target signals. Brief Description of the Drawings

[0027] Figure 1 It is a flowchart of the present invention.

[0028] Figure 2 It is the integration result of the single-channel echo RFT;

[0029] Among them, (a) is the RFT integration result of channel 1, (b) is the RFT integration result of channel 2, and (c) is the RFT integration result of channel 3.

[0030] Figure 3 It is the cyclic matching result between channel 1 and channel 2;

[0031] Among them, (a) is the correct matching result, and (b) is the incorrect matching result.

[0032] Figure 4 It is the information entropy optimization result;

[0033] Among them, (a) is the information entropy result of channels 1 and 2, and (b) is the information entropy result of channels 1 and 3.

[0034] Figure 5 It is the elliptical positioning result;

[0035] Among them, (a) is distance deception interference 1, (b) is distance deception interference 2, (c) is distance deception interference 3, and (d) is the moving target.

[0036] Figure 6 It is the detection performance curve. Detailed Embodiment

[0037] The present invention is verified by using the Matlab simulation experiment method, and the correctness and effectiveness of the present invention are verified on the scientific computing software Matlab R2019a. The technical solution of the present invention is further described below with reference to the drawings.

[0038] As Figure 1 shown, the multi-base radar distance deception interference identification and high-speed target detection method proposed by the present invention includes the following steps:

[0039] S1. The multiple-receiver radar system uses an LFM (linear frequency modulation) signal as the transmitted signal. The down-converted target echo signals received by each receiver can be expressed as:

[0040] S rk (t,t m )=A r S t (t-R k (t m ) / c)exp[j(-2πf c R k (t m ) / c+2πf dk t)]

[0041] Among them, S rk (t,t m ) is the target echo of each receiving node of the radar, t is the fast time, c is the electromagnetic wave propagation speed, A r is the amplitude of the echo signal, f dk is the Doppler frequency of the target echo signal corresponding to each receiver, and R k (t m ) represents the equivalent distance of the target relative to the kth channel of the radar system, and its expression is:

[0042] R k (t m )=R TP0 +R PRk0 +(R TP0 ·v / R TP0 -R PRk0 ·v / R PRk0 )t m

[0043] Among them, R TP0 and R PRk0 respectively represent the initial transmission and reception distances, R TP0 and R PRk0 respectively represent the initial transmission and reception vectors, t m represents the slow time, and v represents the target movement speed. S t (t) represents the transmitted signal of the radar transmitting node, and the specific expression of S t (t) is:

[0044] S t (t)=rect(t / T P )exp[j2πf c t+jπμt 2

[0045] ​where rect(·) represents the rectangular pulse modulation function, T p represents the pulse width, μ = B / T p represents the frequency modulation slope, t is the fast time, f c is the signal carrier frequency.

[0046] The down-conversion of the range deception interference signal received by each receiver can be expressed as:

[0047]

[0048] where J rk (t, t m ) is the interference echo of each receiving node of the radar, is the amplitude of the range deception interference, N is the number of range deception interferences, is the time delay of each range deception interference;

[0049] The echo signal received by each channel is the superposition of the target signal and the range deception interference, and the baseband echo signal can be expressed as:

[0050] Z k (t, t m ) = S rk (t, t m ) + J rk (t, t m )

[0051] where Z k (t, t m ) is the coupled echo waveform of each receiving node of the radar;

[0052] The radar parameters adopted in the present invention are set as: the initial carrier frequency f c = 0.15 GHz, the signal bandwidth B = 5 MHz, the pulse repetition frequency PRF = 500 Hz, the sampling rate f s = 10 MHz, the pulse width T r = 20 μs, 1 transmitting node, 3 receiving nodes, the number of pulses is 300, the signal-to-noise ratio after pulse compression of a single channel is -12 dB, and the interference-to-signal ratio is 6 dB.

[0053] The nodes described in this step are specifically explained as: the multi-static radar includes multiple nodes, and each node is composed of a linear phased array radar.

[0054] S2. Perform pulse compression processing on the coupled echo, and the pulse compression result of the baseband echo signal of each channel can be obtained as:

[0055]

[0056] where is the pulse compression waveform of the coupled echo of each receiving node, Ae is the amplitude of the target echo pulse compression, is the pulse compression amplitude of the range deception interference, B is the signal bandwidth, and λ is the signal wavelength;

[0057] S3. Perform RFT accumulation and threshold preprocessing on the processed echo signal to obtain the target peak estimation information of single-channel preprocessing. By performing threshold preprocessing on the RFT results of each channel, the extraction of the target peak estimation information is completed, defined as where q = 1,..., N + 1. The expression of the single-channel RFT accumulation output result is:

[0058]

[0059] where RFT k represents the RFT processing result of channel k, M represents the number of pulse accumulations, and R k represents the initial bistatic range history, is the distance of each range deception interference relative to the true target, and T r is the pulse repetition time, and the search parameters r k (l k ) and v k (n k ) can be expressed as

[0060]

[0061] where l and n respectively represent the parameter indices of RFT range and velocity. △r = c(2f s ) and △v respectively represent the search intervals of range and velocity, and respectively represent the search ranges of range and velocity, and respectively represent the number of search units for range and velocity, and round(·) represents the ceiling operation.

[0062] The radar parameters adopted in the present invention are set as follows: the initial carrier frequency f c = 0.15 GHz, the signal bandwidth B = 5 MHz, the pulse repetition frequency PRF = 500 Hz, the sampling rate f s = 10 MHz, the pulse width T r = 20 us, and the number of pulses is 300. The radar is set with 1 transmitting node and 3 receiving nodes, and the parameter settings are: the transmitter position is (0,0) m, and the positions of receivers 1 to 3 are (7990.2,0) m, (30002.3,0) m, and (40026.4,0) m respectively. The initial position of the target is (45000,16000) m, and the velocity is (145,2,27,2) m·s -1, the number of range deception jammers is 3, and time delay modulations with distances of -9000m, 15000m, and 21000m are performed respectively.

[0063] Figure 2 The accumulation result of the single-channel echo RFT is shown. Figure 2 (a) shows the RFT accumulation result of channel 1, with the target peak amplitude being 886.1 and the peak amplitudes of range deception jammers being 1399, 1612, and 1677 respectively; Figure 2 (b) shows the RFT accumulation result of channel 2, with the target peak amplitude being 842.3 and the peak amplitudes of range deception jammers being 1488, 1443, and 1436 respectively; Figure 2 (c) shows the RFT accumulation result of channel 3, with the target peak amplitude being 775.7 and the peak amplitudes of range deception jammers being 1756, 1602, and 1746 respectively. The signal-to-noise ratio of the single-channel echo after pulse compression is -12dB, and the echo signal is affected by noise, so there are slight differences in the peak amplitudes after single-channel RFT accumulation.

[0064] S4. According to the preprocessing estimation value, construct the envelope alignment and phase compensation functions for the echoes between multiple channels, and combine the topological structure of the system to complete the cyclic matching processing of different channels. The specific implementation method is as follows:

[0065] For the convenience of analysis, select the first target in channel 1 as the reference target, then the envelope alignment function constructed between each target in channel 2 and the reference target is:

[0066]

[0067] where, is the envelope alignment function between each target in channel 2 and the reference target, is the l-th element of is the l-th element of l = 1, 2,..., C2 represents the serial number of the targets participating in the matching in channel 2, δ(·) is the Dirac function,

[0068]

[0069] where, is the parameter estimation result of each channel target, m k = 1, 2,..., C k represents the index of each channel target, represents the relative velocity difference between each target in channel 2 and the reference target, and its expression is:

[0070]

[0071] The phase compensation function between each target and the reference target in Channel 2 can be expressed as:

[0072]

[0073] Where, is the phase compensation function between each target and the reference target in Channel 2, is the l-th element of.

[0074] The processing result of the reference target in Channel 1 cyclically matched with Channel 2 can be expressed as:

[0075]

[0076] Where, Υ l represents the matching processing result of all targets and the reference target in Channel 2.

[0077] Figure 3 Shows the cyclic matching results of Channel 1 and Channel 2. Figure 3 (a) represents the correct result of cyclic matching. Compared with the single-channel RFT accumulation result, the peaks of the target and the range deception interference both increase by about 2 times when correctly matched; Figure 3 (b) represents the wrong matching result. When wrongly matched, only the peak of the reference target pair increases by about 2 times.

[0078] S5. Screen each plane of the cyclic matching according to the information entropy criterion to obtain the optimal matching plane and obtain the parameter index of the system target. The specific implementation method is as follows:

[0079] The system target index is defined as:

[0080]

[0081] Where, I = [U1, U2,..., U q ,..., U N+1 and respectively represent the indexes of the system targets in Channel 1 and Channel 2, represents the C1 optimal matching entropy values when each target in Channel 1 is the reference target, where represents the C1 correct pairing target indexes when each target in Channel 1 is the reference target. Specifically, l1 represents the pairing target index of Channel 2 when the first target in Channel 1 is selected as the reference target, and its expression is Where represents the information entropy of the C2 cyclic matching planes when the first target in Channel 1 is the reference target. The expression of the information entropy of the cyclic matching plane is:

[0082]

[0083] Among them, E l represents the entropy value of the l-th matching plane, and g l (m,n)=Υ l represents the echo data corresponding to the l-th matching plane. represents the sum of the squares of all echo data of the l-th matching plane. Similarly, the optimal matching result between channel 1 as the reference channel and channel 3 can be obtained, and the system target index is [Q, K Q .

[0084] The information entropy results of each loop matching plane are as Figure 4 shown. Among them, Figure 4 (a) is the information entropy result of channels 1 and 2, Figure 4 (b) is the information entropy result of channels 1 and 3. In the two information entropy planes, the entropy value of the optimal matching has obvious differences compared with other matching cases. And there are 4 minimum entropy values in both entropy planes, and their indexes correspond to the position indexes of the system target in different channels.

[0085] S6. Combining the parameter index of the system target and the preprocessing estimation value, the position estimation result of the system target can be mapped. The specific implementation method is as follows:

[0086] The mapping relationship between the position index of the system target and the preprocessing estimation result is shown in Table 1:

[0087] Table 1 Mapping relationship table between the position index of the system target and the preprocessing estimation result

[0088]

[0089] Among them, represents the index of the system target in channel 3. Thus, the position estimation information of the system target in different channels can be redefined, as shown in Table 2 specifically:

[0090] Table 2 Position estimation information table of the system target in different channels

[0091]

[0092] S7. Perform elliptical positioning processing on the position estimation result of the system target to complete the identification of the target and the range deception interference. The specific implementation method is as follows:

[0093] The elliptical equation corresponding to any target q can be expressed as:

[0094]

[0095] Among them, the semi-major axis of the ellipse is half of the estimated radial distance of target q in channel k, and the focal length 2c of the ellipse k = x Rk is the distance between the transmitting station and the receiving station in channel k.

[0096] By using mathematical methods to solve equations 1 and 2 of the above formula simultaneously, the coordinate of the intersection position of the corresponding ellipse can be expressed as:

[0097] x 12 = -a1 2 b2 2 (x R1 - x R2 ) / (2a1 2 b2 2 - 2a2 2 b1 2 ) + x R1 / 2

[0098]

[0099] Similarly, solving the remaining combined forms of the above formula can obtain x 13 , y 13 , x 23 and y 23 . Substituting the above system parameters, if target q is a real target, then x 12 = x 13 = x 23 , y 12 = y 13 = y 23 ; if target q is a range deception jammer, then x 12 ≠ x 13 ≠ x 23 , y 12 ≠ y 13 ≠ y 23 . That is, the real target must be located on the circumferences of each ellipse, while the same range deception jammer in each channel does not satisfy the topological structure of the multiple-receiver radar system and will not satisfy the above results. In summary, the ellipse positioning solution in the first quadrant realizes the accurate identification of range deception jammers and targets.

[0100] The ellipse positioning results of range deception jammers and targets are as Figure 5 shown. Among them, Figure 5 (a), (b) and (c) respectively represent the ellipse positioning results of range deception jammers 1, 2 and 3, and they all produce three pairwise intersections in the first quadrant, Figure 5 (d) represents the ellipse positioning result of the moving target, and it only produces a unique intersection in the first quadrant.

[0101] S8. Perform a constant false alarm rate detection process based on the recognition result of the target and range deception interference in step S7 to obtain the target detection result. Figure 6 shows the detection performance curves of the target and range deception interference of the method proposed in the present invention under different interference-to-signal ratios. The false alarm probability is P f = 10 -3 , and the input signal-to-noise ratio (SNR, Signal to Noise Ratio) after pulse compression ranges from -22 dB to -6 dB. From Figure 6 it can be seen that at the same interference-to-signal ratio, the detection probability of the target and range deception interference increases with the increase of the signal-to-noise ratio; at the same signal-to-noise ratio, with the increase of the interference-to-signal ratio, the detection probability of the target and range deception interference gradually increases, but when the interference-to-signal ratio exceeds 2 dB, the detection probability approaches the saturation state and there is no obvious increase. It can be seen that the method proposed in the present invention can still effectively complete the recognition of range deception interference and the accumulation and enhancement processing of target signals under the condition of variable interference amplitude.

[0102] Those of ordinary skill in the art will realize that the embodiments described herein are for helping the reader understand the principles of the present invention, and it should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. For those skilled in the art, various changes and modifications can be made to the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the scope of the claims of the present invention.

Claims

1. A method for identifying range deception jamming and detecting high-speed targets in a multi-static radar, characterized in that, Including: S1. The multi-receiver radar system uses an LFM signal as the transmitted signal. The down-converted target echo signals received by each receiver are expressed as: S rk (t, t m ) = A r S t (t - R k (t m ) / c) exp[j(-2πf c R k (t m ) / c + 2πf dk t)] Among them, S rk (t, t m ) is the target echo of each receiving node of the radar, t is the fast time, c is the electromagnetic wave propagation speed, A r is the amplitude of the echo signal, f dk is the Doppler frequency of the target echo signal corresponding to each receiver, R k (t m ) represents the equivalent distance of the target relative to the radar system channel k; The down-converted distance deception interference signals received by each receiver can be expressed as: Among them, J rk (t, t m ) is the interference echo of each receiving node of the radar, is the amplitude of the range deception interference, N is the number of range deception interferences, is the time delay of each range deception interference; The echo signals received by each channel are the superposition of the target echo signal and the distance deception interference signal. Then the baseband echo signal can be expressed as: Z k (t, t m ) = S rk (t, t m ) + J rk (t, t m ) Among them, Z k (t, t m ) is the coupled echo waveform of each receiving node of the radar; S2. Perform pulse compression processing on the coupled echo to obtain the pulse compression results of the baseband echo signals of each channel as: Among them, is the pulse compression waveform of the coupled echo of each receiving node, and A e is the amplitude of the pulse compression of the target echo, is the pulse compression amplitude of the range deception jamming, B is the signal bandwidth, and λ is the signal wavelength; S3. Perform RFT accumulation on the processed echo signals. Use threshold preprocessing to preprocess the RFT accumulation results of each channel to obtain the target peak estimation information of single-channel preprocessing; S4. According to the preprocessing estimation value, construct the envelope alignment and phase compensation functions of the echoes between multiple channels, and combine the topological structure of the system to complete the cyclic matching processing of different channels; S5. Screen each plane of the cyclic matching according to the information entropy criterion to obtain the optimal matching plane and obtain the parameter index of the system target; S6. Combine the parameter index of the system target and the target peak estimation information of the preprocessing, and the position estimation result of the system target can be obtained by mapping; S7. Perform elliptical positioning processing on the position estimation result of the system target to complete the identification of the target and the distance deception interference; S8. Perform constant false alarm rate detection processing according to the identification result of the target and the distance deception interference in step S7 to obtain the target detection result.

2. The method for identifying range deception jamming and detecting high-speed targets of a multi-static radar according to claim 1, characterized in that The expression of the output result of single-channel RFT accumulation is: Among them, RFT k represents the RFT processing result of channel k, M represents the number of pulse integrations, A e is the amplitude of the pulse compression of the target echo, is the pulse compression amplitude of the range deception interference, B is the signal bandwidth, λ is the signal wavelength, R k represents the initial bistatic range history, c is the propagation speed of electromagnetic waves, is the range of each range deception interference relative to the true target, T r is the pulse repetition time, r k (l k ) and v k (n k ) represent the search parameters of range and velocity respectively.

3. A method for multi-base radar range deception interference recognition and high-speed target detection according to claim 2, characterized in that Step S4 specifically uses a certain target in one of the channels as the reference target, and constructs the corresponding envelope alignment function and phase compensation function between the targets of other channels and the reference target according to the peak estimation information of the reference target.

4. A method for identifying multi - base radar range deception jamming and detecting high - speed targets according to claim 3, characterized in that The expression of the information entropy of the cyclic matching plane is: Among them, E l represents the entropy value of the l-th matching plane, where l is the target serial number participating in the matching in other channels, and g l (m,n) = Υ l represents the echo data corresponding to the l-th matching plane, represents the sum of squares of all echo data of the l-th matching plane.

5. A method for multi-base radar range deception interference recognition and high-speed target detection according to claim 4, characterized in that, Υ l The calculation formula is as follows: Among them, Υ l represents the echo data corresponding to the l-th matching plane, represents the phase compensation function corresponding to the l-th target and the reference target in other channels, represents the envelope alignment function corresponding to the l-th target and the reference target in other channels.

6. A method for multi-base radar range deception interference recognition and high-speed target detection according to claim 5, characterized in that The calculation formula is as follows: wherein, is the relative distance difference between the l-th target and the reference target in other channels, is the relative velocity difference between the l-th target and the reference target in other channels, and δ(·) is the Dirac function.

7. A method for multi - base radar range deception interference recognition and high - speed target detection according to claim 6, characterized in that, The phase compensation function is expressed as: Among them, is the l-th element of.

8. A method for multi - base radar range deception interference recognition and high - speed target detection according to claim 7, characterized in that, Step S7 is specifically: If the coordinates obtained by solving according to several elliptical equations corresponding to the target are on the circumferences of each ellipse, it is identified as a real target, otherwise it is a distance deception interference signal.

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