Design and processing method of CP-free OFDM integrated signal in SAR imaging

Abstract

The present invention discloses a CP-free OFDM radar communication integrated signal design and processing method in SAR imaging. When designing the signal, a comb-shaped pilot mode and a blank guard interval are used to construct an OFDM integrated signal without a cyclic prefix under a frequency division system. When processing the signal at the receiving end, the SAR echo signal of the designed transmission signal at the radar receiving end is processed according to the RD algorithm. The present invention solves the problem of low peak sidelobe ratio and integrated sidelobe ratio of SAR imaging receiving signals in traditional technologies, overcomes the deficiency of low spectrum utilization, and improves the pulse compression effect under high-speed platform motion, so that the signal has a narrower main lobe and lower sidelobe in the zero Doppler result, improves the peak sidelobe ratio and the integrated sidelobe ratio and SAR imaging performance, and reduces the communication bit error rate.

Description

Technical Field

[0001] The present invention belongs to the field of communication technology, and further relates to a design and processing method of an integrated signal without cyclic prefix CP (Cyclic Prefix) orthogonal frequency division multiplexing OFDM (Orthogonal-Frequency Division Multiplexing) in synthetic aperture radar SAR (Synthetic aperture radar) imaging in the field of radar communication technology. The present invention can be used in an air-to-ground mode, where the aircraft is an integrated device, and an integrated transmission signal is designed to perform stripe mode imaging processing on a target and to communicate with a ground communication target in real time. Background Art

[0002] Among the existing imaging methods, synthetic aperture radar imaging is not easily affected by the external environment and can ensure all-weather and all-day monitoring. Since OFDM multi-carrier signals have the advantages of large bandwidth, good orthogonality and flexible signal adjustment, OFDM-SAR imaging has attracted the attention of many researchers and carried out fruitful research. OFDM signals were first used in the field of communications. It has an important feature: by adding a sufficiently long cyclic prefix, a channel with inter-symbol interference (ISI) caused by multipath can be converted into multiple sub-channels without inter-symbol interference. Therefore, some researchers apply this feature to OFDM-SAR imaging, treating different radar range units as different sub-channels, and ISI corresponds to the crosstalk IRCI (Inter-Range-Cell Interference) between range resolution units in the imaging system. The transmitting end adopts an OFDM signal containing a sufficiently long CP, and the receiving end separates the echo signals of multiple range units into multiple independent echo signals without crosstalk between range resolution units. This method theoretically proves the absence of range sidelobe crosstalk, making the SAR imaging performance based on OFDM signals equivalent to that of ideal traditional LFM signals, greatly improving the performance of existing OFDM waveform imaging algorithms, and can further meet the needs of high-quality SAR imaging.

[0003] Xidian University has disclosed a MIMO-OFDM radar imaging signal design method based on cyclic prefix in its patent document "MIMO-OFDM radar imaging method based on cyclic prefix" (application number 201510112667.5, application publication number CN 104678395 A). The steps of implementing this method are: designing a discrete time domain waveform based on cyclic shift Zadoff-Chu sequence as the transmission of each MIMO-OFDM antenna; inserting a cyclic prefix at the beginning of each discrete time domain waveform; performing digital / analog conversion on the signal with the cyclic prefix inserted, and adding the radar carrier frequency to generate the transmission signal of each transmitting antenna; dividing the full mapping band into multiple sub-mapping bands through multiple spatial filters, obtaining the baseband discrete echo signal of each sub-mapping band, and reconstructing the distance of the signal without interference between distance units to achieve imaging. The shortcoming of this method is that a cyclic prefix is ​​introduced into the OFDM radar communication integrated signal in SAR imaging to combat the multipath effect, but the cyclic prefix will affect the orthogonality of the subcarriers and produce interference between subcarriers, thereby causing the pulse compression gain loss of the main lobe of the received signal ambiguity function, and the loss is higher than the loss of the side lobe, resulting in a decrease in the peak-to-sidelobe ratio. The main lobe of a weak target is easily submerged by the side lobe of an adjacent strong target, seriously affecting the radar SAR imaging performance.

[0004] Yu Xiang published a radar imaging signal processing method based on OFDM signals in SAR scenarios in his paper "Research on OFDM-SAR Signal Analysis and Imaging Technology" (National University of Defense Technology, Master's Degree 2016). The steps of this method are: based on the SAR system imaging scene model, the general form of the generalized fuzzy function of the OFDM signal is derived; based on the OFDM signal with cyclic prefix, the OFDM-SAR echo signal model is established; under the framework of the matched filter imaging algorithm, a set of systematic radar signal design schemes is proposed; although this method proposes an OFDM random coding radar signal form, it can process high-quality SAR imaging signals for ambiguous extended targets. However, the shortcomings of this method are that since OFDM signals are more sensitive to Doppler frequency shift, OFDM signals containing CP use CP to resist multipath effects, and introduce cyclic prefixes in SAR scenarios, which limits radar and communication capabilities, and the introduction of CP will not only reduce the communication rate, but also introduce symmetrical pseudo peaks in the radar's fuzzy function, resulting in reduced radar range and velocity resolution. Summary of the invention

[0005] The purpose of the present invention is to address the deficiencies of the above-mentioned existing technologies and propose an integrated signal design method for OFDM radar communication based on cyclic prefix-free SAR imaging, which is used to solve the problems of low resolution when imaging targets and low spectrum utilization when communicating in traditional synthetic aperture radar, and greatly improve the performance of radar systems and communication systems.

[0006] The technical idea for achieving the purpose of the present invention is that the present invention uses a blank guard interval instead of a cyclic prefix to construct a CP-free OFDM signal, thereby ensuring the orthogonality between subcarriers, solving the low spectrum utilization rate caused by the multipath effect in SAR imaging, improving the communication bit error rate, completing the functions of radar detection and communication, and avoiding the problem of CP occupying resources in the OFDM communication system in the prior art. During the signal transmission process, the present invention periodically transmits known OFDM training symbols as comb pilots to perform channel estimation. Since all training signals are known signals, this comb pilot estimation method is well suitable for fast fading channels, and can maintain a good pulse compression effect under high-speed platform movement, solving the problem of low resolution of traditional CP-OFDM signals in SAR imaging. The present invention adopts Doppler frequency compensation at the receiving end, reduces the Doppler frequency deviation, makes the signal have a narrower main lobe and lower side lobe in the zero Doppler result, improves the peak sidelobe ratio and the integrated sidelobe ratio of the received signal, and the improvement of the peak sidelobe ratio reduces the mutual crosstalk between the range and azimuth resolution units, and enhances the ability of strong targets to cover weak targets; the enhancement of the integrated sidelobe ratio effectively improves the ability of the SAR imaging system to eliminate the distortion caused by adjacent distributed targets, thereby obtaining a two-dimensional high-resolution image.

[0007] The specific steps of the CP-free OFDM integrated signal design method in SAR imaging adopted by the present invention include the following:

[0008] Step 1, randomly generate a comb-shaped pilot sequence S(f) under a frequency division system;

[0009] Step 2: Use a blank guard interval to construct an OFDM integrated signal without a cyclic prefix in a frequency division system:

[0010] Step 2.1, randomly generate a communication signal S L , using symbol mapping to generate each communication symbol of the communication signal; wherein L = 0, 1, ..., N-1, N represents the total number of randomly generated communication symbols, and the value of N is equal to the total number of subcarriers of the OFDM signal transmission pulse to be generated;

[0011] Step 2.2, calculate the frequency domain signal of the OFDM signal corresponding to each communication symbol:

[0012] Step 2.3, generate an integrated transmission signal according to the following formula:

[0013]

[0014] Wherein, s(t) represents the transmitted signal after the comb pilot S(f) is inserted into the OFDM integrated frequency domain signal and the inverse fast Fourier transform IFFT is performed, N represents the total number of subcarriers of the OFDM integrated signal transmission signal, k represents the subcarrier number, S k (f) represents the kth comb pilot inserted into the OFDM frequency domain signal, exp(·) represents the exponential operation with the natural constant e as the base, j represents the imaginary unit symbol, π represents pi, and f k represents the center frequency of the kth subcarrier;

[0015] Step 3: Send an OFDM integrated signal without a cyclic prefix in a frequency division system:

[0016] Step 3.1, calculate the amplitude of the baseband signal in each OFDM transmission pulse of the transmission signal through each path at each sampling time according to the following formula:

[0017]

[0018] Among them, s qL (t) represents the amplitude of the baseband signal in the Lth OFDM transmission pulse of the transmission signal through the qth path at the tth sampling time, q = 0, 1, ..., m-1, m represents the total number of paths passed by the transmission signal, L represents the number of baseband signals in the OFDM transmission pulse, the number of baseband signals is equal to the total number of pulses of one OFDM subcarrier, L = 0, 1, ..., N L -1,N L represents the total number of pulses of the Lth OFDM subcarrier, S L [k] represents the data carried by the Lth OFDM pulse on the kth subcarrier, T r represents the repetition period of OFDM transmission pulse when blank guard interval is adopted, T represents the period of OFDM symbol when blank guard interval is adopted, rect[·] represents the rectangular window function, when 0≤t≤T, In other cases, the value is 0;

[0019] Step 3.2: All the received signals of the transmission paths are integrated within the integration window length T. w After superposition, the amplitude of the baseband signal of the OFDM transmission pulse at each sampling time is obtained.

[0020] The steps of the CP-free OFDM integrated signal processing method in SAR imaging adopted by the present invention include the following:

[0021] Step 1, processing the communication information in the echo signal of the designed transmission signal at the communication receiving end:

[0022] Step 1.1, symbol synchronization is performed on the baseband received signal, a periodic pulse sequence is used to extract the clock from the echo signal, and the position of the comb pilot training symbol information and the communication symbol information is determined;

[0023] Step 1.2, use 1 / T w The radar receiver samples the communication information carried by the subcarrier of the baseband received signal after symbol synchronization at the sampling interval of T, and calculates the communication signal after sampling; where T w Indicates the receiving window length of the OFDM baseband receiving signal, T w <T r And T w =nT, n is a positive integer;

[0024] Step 1.3, input the sampled communication signal into the carrier correction module for frequency domain compensation, and output the correction signal after frequency domain compensation;

[0025] Step 1.4, using the channel response at the time when the comb pilot is located to correct the channel at the time when the data pulse is located, to complete channel equalization;

[0026] Step 1.5, using a quadrature phase shift keying QPSK algorithm, demapping the signal obtained by passing the frequency domain compensated correction signal through the equalized channel, and then performing channel decoding on the demapped signal to obtain the original communication information;

[0027] Step 2: Process the SAR echo signal of the designed transmission signal at the radar receiving end according to the RD algorithm:

[0028] Step 2.1: Generate the amplitude of the SAR echo signal at each moment according to the following formula:

[0029]

[0030] Where s(t) represents the amplitude of the SAR echo signal at the tth moment, and σ(x,r) represents the position of the target at the tth moment in the plane polar coordinate system composed of the range x-axis and the azimuth r-axis (x i ,r i ) backscatter coefficient, W a (x i ) represents the direction x i The antenna gain at r (r i ) indicates the distance to r i The antenna gain at f c represents the center frequency of the SAR echo signal, R(x i ,r i ) means that at the tth moment, the target is located at (x i ,ri ) and the distance between the radar platform, c represents the speed of light, ω(x i ,r i ) means that the SAR target is located at (x i ,r i ) is the total noise of the echo signal at ;

[0031] Step 2.2, calculate the signal f of the SAR echo signal after range compression r (x i ,r i ) and the signal f after azimuth compression of the SAR echo signal a (x,r), the SAR echo signal at the target at time t is located at (x i ,r i ) at the amplitude s(x i ,r i ), f a (x,r) and f r (x i ,r i ) is convolved to obtain the result of CP-OFDM integrated signal processing in SAR imaging.

[0032] Compared with the prior art, the present invention has the following advantages:

[0033] First, the present invention adopts a comb-shaped pilot method to construct an OFDM integrated signal without a cyclic prefix under a frequency division system, which overcomes the shortcoming of low SAR imaging resolution of airborne platforms in the prior art, enables the present invention to maintain a high pulse compression effect under high-speed platform movement, and improves the peak sidelobe ratio, the integrated sidelobe ratio and the SAR imaging performance.

[0034] Second, the present invention adopts a blank guard interval to construct an OFDM integrated signal without a cyclic prefix under a frequency division system, which overcomes the defect of low spectrum utilization caused by the multipath effect in SAR imaging, so that the present invention can correctly receive the original signal information sent by the transmitting end at the signal receiving end, thereby reducing the communication bit error rate.

[0035] Third, the present invention uses the RD algorithm at the receiving end to process the SAR imaging echo signal, compensates for the Doppler frequency, and reduces the Doppler frequency deviation, so that the signal has a narrower main lobe and lower side lobes in the zero Doppler result, overcomes the defects of mutual crosstalk between the range and azimuth resolution units, so that the present invention can accurately detect strong targets during radar detection, and can eliminate the distortion caused by adjacent scattered targets, thereby improving the ability of the radar to obtain high-resolution imaging in SAR imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a flow chart of the present invention;

[0037] Figure 2 It is a slice diagram of the target range ambiguity function of the center point of the radar subsystem without CP in the simulation experiment 1 of the present invention;

[0038] Figure 3 It is a slice diagram of the azimuth ambiguity function of the center point target of the radar subsystem without CP in the simulation experiment 1 of the present invention;

[0039] Figure 4 This is the imaging diagram of the multi-scattering point target without CP of the radar subsystem in the simulation experiment 2 of the present invention;

[0040] Figure 5 This is a comparison diagram of the bit error rates of multipath channels with communication signal-to-noise ratios between -5 and 10 dB in the communication subsystem in simulation experiment 2 of the present invention. DETAILED DESCRIPTION

[0041] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0042] Reference Figure 1 The specific implementation steps of the present invention are further described in detail in the following embodiments.

[0043] Step 1: In a high-speed platform movement scenario, the channel has fast fading channel characteristics. According to the maximum time delay spread, a comb pilot sequence S(f) is constructed under the frequency division system.

[0044] The pilot sequence S(f) is generated by random numbers to form a comb-shaped pilot in the frequency division system, which is expressed in matrix form:

[0045]

[0046] Among them, Y c The matrix form of the comb pilot is represented by y c represents the number of rows inserted into the comb-shaped pilot. In the embodiment of the present invention, y is taken c =3.

[0047] Step 2: Use a blank guard interval to construct an OFDM integrated signal without a cyclic prefix in a frequency division system.

[0048] Step 2.1, a communication signal is generated by random number method, and each communication symbol of the communication signal is generated by symbol mapping:

[0049] S L =[S0,S1,…,S N-1 ] T

[0050] Among them, S LIt represents the Lth communication symbol in a randomly generated communication signal, L=0, 1, ..., N-1, N represents the total number of randomly generated communication symbols, and the value of N is equal to the total number of subcarriers of the OFDM signal transmission pulse to be generated.

[0051] Step 2.2: Map each communication symbol to the frequency domain according to the following formula to form a frequency domain signal of the OFDM signal corresponding to the communication symbol:

[0052] S L (f) = exp(jπS L )

[0053] Among them, S L (f) represents the frequency domain signal of the OFDM signal corresponding to the Lth communication symbol in the communication signal, exp(·) represents an exponential operation with the natural constant e as the base, j represents the imaginary unit symbol, and π represents pi.

[0054] Step 2.3, continuously insert the comb-shaped pilot sequence S(f) into the frequency domain of the OFDM signal, and perform an inverse fast Fourier transform (IFFT) on the OFDM signal.

[0055] Generate the radar communication integrated transmission signal according to the following formula:

[0056]

[0057] Wherein, s(t) represents the transmission signal obtained by inserting the comb pilot into the OFDM radar communication integrated frequency domain signal and performing inverse fast Fourier transform (IFFT), N represents the total number of subcarriers of the OFDM radar communication integrated signal transmission signal, k represents the subcarrier number, S k (f) represents the kth comb pilot inserted into the OFDM frequency domain signal, f k Represents the center frequency of the kth subcarrier.

[0058] Step 3: Send an OFDM integrated signal without a cyclic prefix in a frequency division system.

[0059] Step 3.1, calculate the amplitude of the baseband signal in each OFDM transmission pulse of the transmission signal through each path at each sampling time according to the following formula:

[0060]

[0061] Among them, s qL (t) represents the amplitude of the baseband signal in the Lth OFDM transmission pulse of the transmission signal through the qth path at the tth sampling time, q = 0, 1, ..., m-1, m represents the total number of paths passed by the transmission signal, L represents the number of baseband signals in the OFDM transmission pulse, L = 0, 1, ..., NL -1,N L Represents the total number of pulses of the Lth OFDM subcarrier. Each OFDM transmit pulse is obtained by adding N subcarriers, and each subcarrier is composed of a different number of pulses. S L [k] represents the data carried by the Lth OFDM pulse on the kth subcarrier. When a blank guard interval is used, T r represents the OFDM transmission pulse repetition period, T represents an OFDM symbol period, rect[·] represents the rectangular window function, when 0≤t≤T, Otherwise, the value is 0

[0062] Step 3.2: Calculate the received signal of all transmission paths in the integration window length T according to the following formula: w The amplitude of the baseband signal of the OFDM transmission pulse after superposition at each sampling time:

[0063]

[0064] Among them, y L (t) indicates that all paths are superimposed on the integral window length T w The amplitude of the baseband signal of the Lth OFDM transmission pulse at the tth sampling time after superposition, τ p represents the time delay of the qth path, τ0 represents the delay of the received signal on the direct path, T w represents any integer multiple of T, is a rectangular window function, when LT r +τ0≤t≤LT r +τ0+T w When , it takes 1, otherwise, it takes 0.

[0065] Step 4, processing the communication information in the echo signal of the designed transmission signal at the communication receiving end.

[0066] Step 4.1, the receiving window length of the OFDM baseband receiving signal is T w , under the condition that T w <T r , T w =nT, n is a positive integer.

[0067] Step 4.2, symbol synchronization is performed on the baseband received signal, and a periodic pulse sequence is used to extract the clock from the echo signal to determine the location of the comb pilot training symbol information and the communication symbol information.

[0068] Step 4.3, select the sampling interval as 1 / T w At the radar receiving end, the communication information carried by the subcarrier of the baseband received signal after symbol synchronization is sampled to obtain the sampled communication signal.

[0069] The transmitted signal is reconstructed according to the following formula:

[0070]

[0071] Among them, signal Y L [k] represents the data carried by the Lth pulse corresponding to the Lth pulse of the transmitted signal on the kth subcarrier after sampling the echo signal, and the sampling interval is 1 / T L , LT r +τ0 is the start time of receiving the signal. L =nT, from the frequency domain, the sampling interval is 1 / T L =1 / nT, that is, the interval is reduced to 1 / n of the original, and the value corresponding to the sampling point when k=ni is selected as the sampling result. At this time, f i With f k Equal, S L [i] represents the amplitude after the change due to the superposition of multiple paths.

[0072] Step 4.4, the sampled communication signal passes through the carrier correction module to obtain the carrier frequency offset CFO (Carrier Frequency Offset) and perform frequency domain compensation to obtain a correction signal after frequency domain compensation.

[0073] Step 4.5, use the channel response at the time of the comb pilot to correct the channel at the time of the data pulse to complete the channel equalization, that is, H 2η-1 (f) = H 2η (f), where H 2η-1 (f) represents the channel response at the time of the data pulse, H 2η (f) represents the channel response at the time of the training pulse, and channel equalization is completed at this point.

[0074] Step 4.6, the correction signal passing through the equalization channel is first demapped by quadrature phase shift keying (QPSK), and then the original communication information is obtained by channel decoding.

[0075] Step 5: The radar system performs SAR imaging on the echo. According to the RD algorithm, the echo signal is range compressed, range motion corrected and azimuth compressed to obtain a high-resolution point target image.

[0076] Step 5.1: Generate the amplitude of the echo signal of SAR imaging at each moment according to the following formula:

[0077]

[0078] Among them, s(t) represents the amplitude of the echo signal of SAR imaging at the tth moment, and σ(x,r) represents the position of the target at the tth moment in the plane polar coordinate system composed of the range x-axis and the azimuth r-axis (x i ,r i ) backscatter coefficient, W a (x i ) represents the direction x i The antenna gain at r (r i ) indicates the distance to r i The antenna gain at f c represents the center frequency of the SAR imaging echo signal, R(x i ,r i ) means that at the tth moment, the target is located at (x i ,r i ) and the distance between the radar platform, c represents the speed of light, ω(x i ,r i ) indicates that the SAR imaging target is located at (x i ,r i ) is the total noise of the echo signal at .

[0079] Step 5.2: Perform range compression based on the range-Doppler RD (Range-Doppler) algorithm and calculate the range reference signal of the SAR imaging echo signal according to the following formula:

[0080]

[0081] Among them, f r (t) represents the range reference signal of the SAR imaging echo signal, T p Indicates pulse width.

[0082] Step 5.3, calculate the signal f after range compression according to the following formula: r (x i ,r i )

[0083]

[0084] Among them, f r (x i ,r i ) represents the signal after the SAR echo signal is compressed in the distance direction, f r1 (x i ,r i ) is to make f in the first step r (t1) t1 takes Got it.

[0085] Step 5.4: Since the range migration of SAR imaging echo signals has an impact on azimuth compression, the echoes of the same target are located at different range gates and cannot be directly compressed. In azimuth, the SAR imaging echo signals need to be corrected for range migration, so that azimuth compression becomes a one-dimensional process.

[0086] According to the following formula, the range migration of SAR imaging echo signal is calculated:

[0087]

[0088] Where ΔR represents the range migration of the SAR imaging echo signal, t0 represents the midpoint of the synthetic aperture time, r(t) represents the instantaneous distance between the radar and the target, λ represents the wavelength of the signal, and f dc represents the Doppler center frequency, f dr Indicates the Doppler modulation frequency.

[0089] Step 5.5, calculate the azimuth reference signal of the SAR imaging echo signal according to the following formula:

[0090]

[0091] Among them, f a (t) represents the azimuth reference signal of the SAR imaging echo signal, T s Represents the synthetic aperture time.

[0092] Step 5.6, calculate the signal f after azimuth compression according to the following formula: a (x i ,r i ):

[0093]

[0094] Among them, f a (x, r) represents the signal of the SAR imaging echo signal after range compression, W a (ΔR-x i ) represents the direction x i Antenna gain corrected for range migration at f a1 (x i ,r i ) is to make f in the second step a (t3) t3 is taken Got it.

[0095] Step 5.7: Compress the azimuth signal f a (x i ,r i ), compressed distance signal f r (xi ,r i ) and the SAR echo signal at the tth moment the target is located at (x i ,r i ) at the amplitude s(x i ,r i ) and obtain the result of CP-OFDM integrated signal processing in SAR imaging.

[0096] The effects of the present invention are further described below in conjunction with the simulation experiments of the present invention.

[0097] 1. Simulation experimental conditions.

[0098] The hardware platform of the simulation experiment of the present invention includes: a CPU of Intel Core i7-7700 and a RAM of 8 GB.

[0099] The software platform of the simulation experiment of the present invention is: Windows 10 operating system and Matlab R2019a.

[0100] In order to ensure that the channel response and frequency offset estimation of the training pulse estimation can equalize and compensate the channel corresponding to the data pulse, the pulse width and pulse repetition period should not be too large. Referring to the length of an OFDM symbol in LTE, which is about 66.67us, and the traditional CP-OFDM signal contains 7 OFDM symbols in one frame, the size of the pulse repetition period ensures that a good channel equalization capability can be maintained within the length of one frame. At the same time, in order to ensure that the received signal experiences slow fading, the Doppler spread should be much smaller than the subchannel bandwidth of the baseband transmitted OFDM signal. The subchannel bandwidth is usually set to 10 times the Doppler spread. At this time, it can be guaranteed that the integrated system has good communication performance. According to the constraints between the parameters, the simulation parameters of the integrated signal and the target are given. Among them, the carrier frequency is 10GHz, the pulse repetition period is 200us, the pulse width is 8us, the signal width is 512MHz, the basic sampling rate is 512MHz, the upsampling multiple is 8, the number of subcarriers is 4096, the mapping method is QPSK, the detection compensation relative radar platform speed is 150m / s, the cooperation target relative radar platform speed is 150m / s, the detection target height is 5km, and the detection target scene center distance is The signal-to-noise ratio of the radar system is -5dB, the signal-to-noise ratio of the communication system is -5dB~10dB, and the multipath signal delay is 0 2us8us.

[0101] 2. Analysis of simulation content and results.

[0102] The simulation experiment of the present invention uses the method proposed by the present invention and an existing technology (CP-OFDM method) to generate two integrated waveform signals, and then performs communication performance analysis and radar performance analysis on the generated two integrated waveform signals. The slice diagram of the normalized fuzzy function diagram of the center point target distance is drawn by the simulation software Matlab R2019a. Figure 2 The slice diagram of the normalized fuzzy function diagram of the center point target azimuth is shown as follows: Figure 3 The target imaging diagram at the center point is shown in Figure 4 The simulation software Matlab R2019a is used to draw the OFDM signal with CP and no relative speed, the OFDM signal without CP and a relative speed of 75m / s, the OFDM signal without CP and Doppler compensation, and the theoretical bit error rate of the set Rayleigh channel for comparative analysis, corresponding to Figure 5 The inverted triangle curve, fork curve, hexagonal star curve and pentagonal star curve shown.

[0103] Figure 2 It is the center point target azimuth slice diagram, where the x-axis represents the direction and the y-axis represents the normalized amplitude. Figure 2 The simulation results show that the sidelobes of the range ambiguity function of the OFDM signal decay very quickly, and the peak sidelobes of the OFDM signal without CP are all above -13dB. Compared with the traditional signal with CP, it has a narrower mainlobe and lower sidelobes in the zero Doppler result, and the peak-to-sidelobe ratio is increased, which reduces the mutual crosstalk between the range and azimuth resolution units, enhances the ability of strong targets to cover weak targets, and enables the system to detect targets more accurately.

[0104] Figure 3 It is the center point target azimuth slice diagram, where the x-axis represents the direction and the y-axis represents the normalized amplitude. Figure 3 The zero-delay results show that the OFDM signal without CP does not show a decrease in velocity resolution, which is comparable to the signal with CP. The enhancement of the integrated sidelobe ratio effectively improves the ability of the SAR system to eliminate the distortion caused by adjacent distributed targets. These extremely low sidelobes prove that the OFDM waveform without CP is one of the best candidate waveforms for high-resolution radar imaging.

[0105] Figure 4It is a channel position target imaging diagram, where the x-axis represents the azimuth dimension sampling points, and the y-axis represents the distance dimension sampling points. For the distance direction, there is a one-to-one correspondence between the target position and the imaging position in the distance direction. In the azimuth dimension, the single-station movement speed is 50m / s, and 1000 pulses are used to image the area. The pulse repetition period of the signal is 100us, and the corresponding 1000 pulse time is 100ms. Combined with the platform movement speed, the 1000-pulse platform moves 5 meters. The target is 10 kilometers away from the center of the platform, which corresponds to a range of about 25 kilometers presented in the scene. The corresponding distance of each point in the azimuth direction is 25 meters. The horizontal positions of the points from left to right in the figure are 483, 501, and 517, respectively. After calculation, the interval where the imaging target is located is approximately consistent with the set scene, which meets the expected effect. From Figure 4 It was found that the imaging points were not strictly aligned. Through calculation, it was found that the point position difference was 1 slow time sampling point, and the corresponding distance was 10 meters. The azimuth resolution was 24 meters, which met the performance requirements. The high-speed movement of the platform under the frequency division system will cause pulse compression loss. Compared with the time division system, it still has a better compression effect. It can be applied to high-speed scenes and can avoid the mutual influence between various communication symbols. It can be used in high-speed scenes to build an integrated radar communication system.

[0106] Figure 5 This is a curve comparison chart of the bit error rate changing with the signal-to-noise ratio in four cases, where the x-axis represents the signal-to-noise ratio in decibels and the y-axis represents the bit error rate. Figure 5 The curve marked with a cross symbol in the figure represents the curve with the highest bit error rate and remains unchanged when the signal-to-noise ratio increases when the transmitted signal is an OFDM signal with no CP and a relative speed of 75m / s. The curve marked with an inverted triangle symbol represents the curve with a bit error rate that decreases significantly with the increase of the signal-to-noise ratio when the transmitted signal is an OFDM signal with CP and no relative speed. The curve marked with a hexagonal star symbol represents the curve with a bit error rate that decreases with the increase of the signal-to-noise ratio when the transmitted signal is an OFDM signal with no CP and Doppler compensation. The curve marked with a five-pointed star symbol represents the theoretical bit error rate curve of the Rayleigh channel.

[0107] Depend on Figure 5It can be seen that in a multipath channel, the bit error rate of the signal directly demodulated by compensating for the frequency offset caused by speed is very high. After frequency offset compensation, the bit error rate performance has been effectively improved. For OFDM signals with CP, when the length of CP is less than the maximum multipath delay, compensating for the frequency offset caused by speed or increasing the signal-to-noise ratio has limited improvement on the bit error rate. This is because in this case, the CP length is not enough to counteract the inter-carrier interference caused by multipath, and the subcarriers in the integration interval are no longer orthogonal, so the corresponding weights cannot be correctly extracted. Under the same circumstances, for OFDM symbols without CP, due to the increase in the integration interval, all carrier cycles are received back, and the orthogonality between subcarriers can be maintained by extracting sampling points, thereby eliminating the impact of multipath. From the above simulation results, it can be seen that under the condition of high signal-to-noise ratio, the designed CP-free OFDM signal has a very low bit error rate, and the communication integration system maintains a high accuracy.

Claims

1. A method for designing a CP-free OFDM integrated signal in SAR imaging, characterized in that: A comb-shaped pilot sequence under a frequency division system is randomly generated, and a blank guard interval is used to construct an OFDM integrated signal without a cyclic prefix under the frequency division system; the steps of the design method include the following: Step 1, randomly generate a comb-shaped pilot sequence S(f) under a frequency division system; Step 2: Use a blank guard interval to construct an OFDM integrated signal without a cyclic prefix in a frequency division system: Step 2.1, randomly generate a communication signal S L , using symbol mapping to generate each communication symbol of the communication signal; wherein L = 0, 1, ..., N-1, N represents the total number of randomly generated communication symbols, and the value of N is equal to the total number of subcarriers of the OFDM signal transmission pulse to be generated; Step 2.2, calculate the frequency domain signal of the OFDM signal corresponding to each communication symbol: Step 2.3, generate an integrated transmission signal according to the following formula: Wherein, s(t) represents the transmitted signal after the comb pilot S(f) is inserted into the OFDM integrated frequency domain signal and the inverse fast Fourier transform IFFT is performed, N represents the total number of subcarriers of the OFDM integrated signal transmission signal, k represents the subcarrier number, S k (f) represents the kth comb pilot inserted into the OFDM frequency domain signal, exp(·) represents the exponential operation with the natural constant e as the base, j represents the imaginary unit symbol, π represents pi, and f k represents the center frequency of the kth subcarrier; Step 3: Send an OFDM integrated signal without a cyclic prefix in a frequency division system: Step 3.1, calculate the amplitude of the baseband signal in each OFDM transmission pulse of the transmission signal through each path at each sampling time according to the following formula: Among them, s qL (t) represents the amplitude of the baseband signal in the Lth OFDM transmission pulse of the transmission signal through the qth path at the tth sampling time, q = 0, 1, ..., m-1, m represents the total number of paths passed by the transmission signal, L represents the number of baseband signals in the OFDM transmission pulse, the number of baseband signals is equal to the total number of pulses of one OFDM subcarrier, L = 0, 1, ..., N L -1,N L represents the total number of pulses of the Lth OFDM subcarrier, S L [k] represents the data carried by the Lth OFDM pulse on the kth subcarrier, T r represents the repetition period of OFDM transmission pulse when blank guard interval is adopted, T represents the period of OFDM symbol when blank guard interval is adopted, rect[·] represents the rectangular window function, when 0≤t≤T, In other cases, the value is 0; Step 3.2: All the received signals of the transmission paths are integrated within the integration window length T. w After superposition, the amplitude of the baseband signal of the OFDM transmission pulse at each sampling time is obtained.

2. The signal design method according to claim 1, characterized in that: The frequency domain signal of the OFDM signal calculated in step 2.2 is obtained by the following formula: S L (f)=exp(jπS L ) Among them, S L (f) represents the frequency domain signal of the OFDM signal corresponding to the Lth communication symbol in the communication signal.

3. The signal design method according to claim 1, characterized in that: The calculation of the received signals of all transmission paths in the integration window length T as described in step 3.2 w The amplitude of the baseband signal of the superimposed OFDM transmission pulse at each sampling time is obtained by the following formula: Among them, y L (t) indicates that all paths are superimposed on the integral window length T w The amplitude of the baseband signal of the Lth OFDM transmission pulse at the tth sampling time after superposition, τ p represents the time delay of the qth path, τ0 represents the delay of the received signal on the direct path, T w represents any integer multiple of T, is a rectangular window function, when LT r +τ0≤t≤LT r +τ0+T w When , it takes 1, otherwise, it takes 0.

4. A CP-free OFDM integrated signal processing method based on the signal design method of claim 1, characterized in that: The SAR echo signal of the designed transmission signal at the radar receiving end is processed according to the RD algorithm; the steps of the processing method include the following: Step 1, processing the communication information in the echo signal of the designed transmission signal at the communication receiving end: Step 1.1, symbol synchronization is performed on the baseband received signal, a periodic pulse sequence is used to extract the clock from the echo signal, and the position of the comb pilot training symbol information and the communication symbol information is determined; Step 1.2, use 1 / T w The radar receiver samples the communication information carried by the subcarrier of the baseband received signal after symbol synchronization at the sampling interval of T, and calculates the communication signal after sampling; where T w Indicates the receiving window length of the OFDM baseband receiving signal, T w <T r And T w =nT, n is a positive integer; Step 1.3, input the sampled communication signal into the carrier correction module for frequency domain compensation, and output the correction signal after frequency domain compensation; Step 1.4, using the channel response at the time when the comb pilot is located to correct the channel at the time when the data pulse is located, to complete channel equalization; Step 1.5, using a quadrature phase shift keying QPSK algorithm, demapping the signal obtained by passing the frequency domain compensated correction signal through the equalized channel, and then performing channel decoding on the demapped signal to obtain the original communication information; Step 2: Process the SAR echo signal of the designed transmission signal at the radar receiving end according to the RD algorithm: Step 2.1: Generate the amplitude of the SAR echo signal at each moment according to the following formula: Where s(t) represents the amplitude of the SAR echo signal at the tth moment, and σ(x,r) represents the position of the target at the tth moment in the plane polar coordinate system composed of the range x-axis and the azimuth r-axis (x i ,r i ) backscatter coefficient, W a (x i ) represents the direction x i Antenna gain at , W r (r i ) indicates the distance to r i The antenna gain at f c represents the center frequency of the SAR echo signal, R(x i ,r i ) means that at the tth moment, the target is located at (x i ,r i ) and the distance between the radar platform, c represents the speed of light, ω(x i ,r i ) means that the SAR target is located at (x i ,r i ) is the total noise of the echo signal at ; Step 2.2, calculate the signal f of the SAR echo signal after range compression r (x i ,r i ) and the signal f after azimuth compression of the SAR echo signal a (x,r), the SAR echo signal at the target at time t is located at (x i ,r i ) at the amplitude s(x i ,r i ), f a (x,r) and f r (x i ,r i ) is convolved to obtain the result of CP-OFDM integrated signal processing in SAR imaging.

5. The signal processing method according to claim 4, characterized in that: The communication signal after calculation and sampling described in step 1.2 is obtained by the following formula: Among them, signal Y L [k] represents the data carried by the Lth pulse corresponding to the Lth pulse of the transmitted signal on the kth subcarrier after sampling the echo signal, and the sampling interval is 1 / T L , L T r +τ0 is the start time of receiving the signal. L =nT, from the frequency domain, the sampling interval is 1 / T L =1 / nT, that is, the interval is reduced to 1 / n of the original, and the value corresponding to the sampling point when k=ni is selected as the sampling result. At this time, f i With f k Equal, S L [i] represents the amplitude after the change due to the superposition of multiple paths.

6. The signal processing method according to claim 4, characterized in that: s(x) described in step 2.2 i ,r i ) is to let t of s(t) in step 2.1 be Got it.

7. The signal processing method according to claim 4, characterized in that: The signal f after the range compression of the SAR echo signal is calculated as described in step 2.2 r (x i ,r i ) are as follows: The first step is to calculate the SAR echo signal range reference signal according to the following formula: Among them, f r (t) represents the SAR echo signal range reference signal, T p Indicates pulse width; The second step is to calculate the signal after range compression according to the following formula: Among them, f r (x i ,r i ) represents the signal after the SAR echo signal is compressed in the range direction, f r1 (x i ,r i ) is to make f in the first step r (t1) t1 takes Got it.

8. The signal processing method according to claim 4, characterized in that: The signal f after azimuth compression of the SAR echo signal is calculated as described in step 2.2 a The specific steps of (x,r) are as follows: The first step is to calculate the range migration of the SAR echo signal according to the following formula: Where ΔR represents the range migration of the SAR imaging echo signal, t0 represents the midpoint of the synthetic aperture time, r(t) represents the instantaneous distance between the radar and the target, λ represents the wavelength of the signal, and f dc represents the Doppler center frequency, f dr Indicates the Doppler modulation frequency; The second step is to calculate the SAR imaging echo signal azimuth reference signal according to the following formula: Among them, f a (t) represents the azimuth reference signal of the SAR imaging echo signal, T s represents the synthetic aperture time; The third step is to calculate the signal f after azimuth compression according to the following formula: a (x i ,r i ): Among them, f a (x, r) represents the signal of the SAR imaging echo signal after range compression, W a (ΔR-x i ) represents the direction x i Antenna gain corrected for range migration at f a1 (x i ,r i ) is to make f in the second step a (t3) t3 is taken Got it.

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