CN112436905B - Communication radar combined system - Google Patents
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- H—ELECTRICITY
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Abstract
The invention discloses a communication radar combined system, which specifically comprises the following steps: the target echo is input into a range gate rearrangement module, a rearranged two-dimensional data matrix enables a signal wave crest to be aligned to a target position through a pulse compression module, a time delay compensation module and a coherent accumulation module, then a distance estimation value is obtained through a smoothing filtering module, a distance dimension constant false alarm detection module and a distance estimation module, and then the target speed estimation value is obtained through a speed dimension constant false alarm detection module and a speed estimation module according to a distance estimation result. The invention improves the signal-to-noise ratio of the received signal through smooth filtering operation; the main-to-side ratio performance is improved by adopting a partial sequence to generate a matched filter coefficient during pulse compression processing; by optimizing the selection of the CFAR reference window, the target false alarm probability is reduced, so that the incomplete echo can still have good pulse pressure characteristics when being received, and the target detectable range is widened.
Description
Technical Field
The invention belongs to the field of communication radars. In particular to a communication radar integrated signal processing device, and a radar processing system is designed based on communication signals according to a typical preamble structure of the communication radar integrated signal processing device under the standard framework of IEEE 802.11 series communication protocols.
Background
Abbreviations and Key term definitions
| CAGO-CFAR | Cell Averaging with Greatest Of CFAR | Unit average selection maximum CFAR |
| CEF | Channel Estimation Field | Channel estimation field |
| CFAR | Constant False Alarm Rate | Constant false alarm rate |
| CPI | Coherent Processing Interval | Coherent processing interval |
| DFRC | Dual-Functional Radar-Communication | Dual function radar communication |
| FFT | Fast Fourier Transform | Fast Fourier transform |
| LRR | Long-Range Radar | Remote radar |
| PHY | Physical Layer | Physical layer |
| PRF | Pulse Repetition Frequency | Pulse repetition frequency |
| PRI | Pulse Repetition Interval | Pulse repetition interval |
| SC PHY | Single Carrier Physical Layer | Single carrier physical layer |
| STF | Short Training Field | Short training field |
| TDD | Time Division Duplex | Time division duplex |
A Dual-Functional Radar-Communication (DFRC) system can realize Communication data transmission and Radar target detection functions under the same waveform system based on unified hardware. The method can reduce the cost in hardware dimension on one hand, and can reduce the conflict between communication and the use of the radar on the frequency spectrum in frequency spectrum dimension on the other hand.
Typical design methods for DFRC include two types: 1) on the basis of the communication waveform, adding a radar function on the communication waveform; 2) based on the radar waveform, a communication function is added to the radar waveform. The invention mainly aims at the first situation, takes the physical layer waveform of an IEEE 802.11 series communication protocol as a basic frame, utilizes a typical lead code structure thereof, realizes the receiving and transmitting isolation by configuring the lead code structure into a pulse coding single-station radar, and designs a radar processing algorithm to realize the speed measurement and the distance measurement of a target.
As shown in fig. 1, the physical layer frame preamble of the IEEE 802.11 series protocol is generally composed of two parts, where the preamble sequence P1 is composed of a plurality of identical short fields a and B, the field a is used for implementing frame detection, synchronization and frequency offset estimation functions in communication data transmission, the field B is used for implementing symbol boundary detection function, and the preamble sequence P2 is composed of two repeated long fields C and cyclic prefix thereof, and is mainly used for implementing channel estimation in communication data transmission.
Due to the good autocorrelation characteristic of the preamble sequence, the target distance and speed estimation can be better realized by using the preamble sequence as a radar coding waveform. However, the design starting point of the IEEE 802.11 series protocol standard is to meet the communication transmission requirement, and the radar detection requirement is not considered at the beginning of the design.
Firstly, because a short field A in a lead code typical structure of IEEE 802.11 series protocols repeatedly appears periodically, if the lead code is simply applied to the existing radar detection method, after pulse compression processing, the main-to-side ratio performance of related peaks is very poor, and the radar distance and speed detection performance is seriously influenced;
secondly, because the single-station radar adopts a transceiving time-sharing structure, only part of target echoes can be received aiming at a near-end target, so that the near-end target echoes are incomplete. If the matched filter is designed according to the full code word during pulse compression, incomplete echoes of a near-end target can only realize partial pulse compression, and the main lobe gain after pulse compression is reduced, so that the main-to-side ratio of a related peak is further deteriorated;
thirdly, because the transmitting power of the IEEE 802.11 series equipment is often small, when the equipment is used as a radar function, the signal to noise ratio of the echo is low, and the target detection is influenced. Therefore, a radar signal processing scheme needs to be designed specifically, so as to improve the signal-to-noise ratio and reduce the false alarm probability on the basis of ensuring the detection probability.
Therefore, the present patent focuses on the above three problems, and takes the physical layer waveform of the IEEE 802.11 series communication protocol as the basic frame, and utilizes the typical preamble structure to design the radar detection scheme, so as to implement the communication transmission and radar detection functions in the IEEE 802.11 series standard.
The prior art is as follows:
an IEEE 802.11ad-based vehicle-to-vehicle communication and remote Radar (LRR) design method (the literature is P, Kumari, J, Choi, N, Gonzlez-Prelcic, and R.W,
the method is characterized in that a radar target detection, distance and speed estimation method is designed based on Single frames and multiple frames during communication data transmission in An IEEE 802.11ad-based radar, An a proproach to joint vehicle communication system, IEEE Trans. However, the scheme adopts the full duplex assumption, and the complexity of the implementation of the scheme in the millimeter wave frequency band is high on the assumption that the system can simultaneously transmit and receive.
The prior art has the following disadvantages:
the above prior art requires a system with full duplex capability to eliminate self-interference in transmission and reception. In addition, the radar detection process is based on channel estimation and time-frequency synchronization technology of communication receiving signals, and the detection performance cannot meet the requirement under low signal-to-noise ratio.
Disclosure of Invention
The invention aims to provide a communication radar combined system aiming at the problems of poor main-to-side ratio performance caused by repeated fields in a typical lead code structure of IEEE 802.11, deterioration of the main-to-side ratio caused by incomplete near-end target echoes and insufficient signal-to-noise ratio of received signals caused by serious millimeter wave signal attenuation, and aims to effectively improve the distance precision and speed precision of radar system detection.
The invention is realized by the following technical scheme: a communication radar combined system comprises a range gate rearrangement module, a pulse compression module, a time delay compensation module, a coherent accumulation module, a smooth filtering module, a distance dimension constant false alarm detection module, a distance estimation module, a speed dimension constant false alarm detection module and a speed estimation module; the radar transmits radar signals of typical communication lead code codes of IEEE 802.11 series protocols, received signals are firstly rearranged through a range gate rearrangement module, the obtained two-dimensional data matrix enables signal wave crests to be aligned to the position of a target through a pulse compression module, a time delay compensation module and a coherent accumulation module, then a distance estimation value of the target and the radar is obtained through a smooth filtering module, a distance dimension constant false alarm detection module and a distance estimation module, and finally a relative speed estimation value of the target and the radar is obtained through a speed dimension constant false alarm detection module and a speed estimation module according to the result of distance estimation;
the radar model of the system is a single base station radar operating in a time division duplex mode, with the transmission and reception of the radar being separated in time from the communication.
The range gate rearrangement module:
sequentially putting the sampled data into a two-dimensional data matrix by adopting a two-dimensional buffer memory; each cell represents an independent baseband sample; the column data is L received echo samples of a single pulse in a pulse repetition interval, and the row data is M received echo samples from the same distance; wherein, L is the sampling times in the pulse repetition interval, M is the number of the repeated pulses, and the value of M needs to ensure that the movement of the target in the coherent processing interval is smaller than the distance resolution; subsequent processing is expanded around the two-dimensional data matrix;
the pulse compression module:
adopting a matched filter to realize matched filtering one by one pulse, and obtaining the receiving time delay of a target echo from the position of the peak of an output signal; the coefficient of the matched filter is generated by a local sequence, the amplitude-frequency characteristic of the matched filter is the same as that of the echo signal, and the phase-frequency characteristic of the matched filter is conjugated with that of the echo signal;
aiming at rich side lobes caused by a preamble repeating field of a typical IEEE 802.11 series protocol, selecting a partial sequence (instead of the whole preamble sequence) as a reference sequence to generate a matched filter coefficient; the partial sequence has the following conditions:
condition 1: the number of positive values and negative values in the sequence is equal, so that peak positive and negative counteraction is realized when the positive values and the negative values are not matched;
condition 2: the sequence is long so that the main lobe peak is higher when fully matched;
condition 3: the number of the included repeated fields is small so as to reduce the number of side lobes;
condition 4: the sequence is later in the entire preamble field, and still has a good matched filter output waveform when an incomplete echo is received, i.e., a close target.
The pulse compression module: when applied to the IEEE 802.11ad protocol standard, the reference sequence for selecting the matched filter coefficients is: 4 Gray sequences with length of 128 at the end of short training field, in channel estimation field
And
。
the time delay compensation module:
the module compensates the pulse compression module to bring two parts of time delay, one is the filter time delay, and the other is the positioning time delay of the selected part of sequence which is not started from the first code word of the preamble; selecting a part of sequence with the length of K, so that the coefficient length of the filter is K, and the time delay of the filter is K-1; if the partial sequence starts from the qth code word, the positioning time delay is q-1; both delays are fixed delays, so the matched filter output is moved forward by the fixed unit and zeroed at the end, eliminating the delay.
The coherent accumulation module:
the module realizes the separation of Doppler frequency through fast Fourier transform, namely, the data matrix after time delay compensation is subjected to Fourier transform independently according to rows, and N-point Fourier transform is performed for L times in total; the output of each Doppler channel corresponds to a narrower Doppler frequency band, and the Doppler channel where the output waveform peak is located is the Doppler frequency offset value of the target.
The smoothing filtering module:
the module is realized by averaging the same distance gate data of a plurality of repeated pulses, and reduces the input data of the distance dimension constant false alarm detection module while weakening noise; when target detection is carried out in a distance dimension, considering that the distance fluctuation between a target and a radar does not exceed a range gate in a coherent processing interval, the target distance is considered to be fixed, but channel noises are different and can be mutually offset; therefore, averaging M samples can weaken the influence of noise on the waveform, but the distance information is not changed, and the step is carried out after coherent accumulation and before distance constant false alarm probability detection.
The distance maintenance false alarm detection module:
the module processes data in the sliding window by adopting a constant false alarm rate detection technology; the sliding window sequentially covers the front reference window, the front protection window, the observation window, the rear protection window and the rear reference window; respectively aligning the front reference window and the rear reference window to the left side lobe and the right side lobe of the main lobe after echo matching filtering; with the sliding of the window, the reference window of the main lobe is aligned with the side lobe, the reference window of the side lobe is aligned with the side lobe or noise, and the reference window of the noise is aligned with the side lobe or noise; estimating unknown statistical parameters of background clutter and side lobes by using signals in the front reference window and the rear reference window, and designing a threshold value, so that only a main lobe can exceed the threshold value, and the side lobes are restricted with each other; if the threshold is lower than the power of the observation window, then 1 is marked in the distance grid corresponding to the observation window, otherwise, 0 is marked, and the output of the module is a one-dimensional array corresponding to the distance grids one by one.
The distance dimension constant false alarm detection module: when applied to the IEEE 802.11ad protocol standard, the relative positions of the main lobe and the side lobes are fixed and depend on the repetition field in the preamble
Selecting a first side lobe or a second side lobe adjacent to the observation window as a reference window object, setting the lengths of a front reference window and a rear reference window according to the major-minor ratio, and setting the lengths of the front reference window and the rear reference window according to the major-minor ratio
The length of the front and rear protection windows is set.
The distance estimation module: in order to eliminate false alarms caused by the fact that the continuous multiple lattices of the same target main lobe exceed a threshold value, the module processes an output array of the distance dimension constant false alarm detection module, and the specific processing rule is as follows: rule 1: if the single lattice is 1, no processing is carried out; rule 2: if two continuous grids are 1, only 1 at the large value is reserved, and the other grid is set to be 0; rule 3: if the three continuous grids are 1, only 1 in the median is reserved, and the other two grids are set to be 0; rule 4: if the continuous multiple lattices, namely more than or equal to 4 lattices are 1, the first three data are considered to be from the same target and are processed according to a rule 3, the remaining data in the array are the first three data which are considered to be from the same target and are processed according to the rule 3, and the like, and when the remaining data are less than three lattices, the processing is carried out according to the rule 1 or the rule 2;
after processing according to the 4 rules, the object exists at the position of the distance grid corresponding to the grid still being 1; and obtaining the receiving time delay of the target echo according to the position of the distance grid, thereby obtaining the target distance estimation value.
The speed dimension constant false alarm detection module:
selecting row data with a target from the two-dimensional matrix output by the coherent accumulation module according to the distance detection result to perform constant false alarm detection; the method for detecting the velocity dimension constant false alarm rate is consistent with the method for detecting the distance dimension constant false alarm rate, but the reference window does not need to be aligned with the side lobe and can be set according to experience; the velocity dimension constant false alarm detection is used for dealing with a scene with a plurality of targets at the same distance, and the result is the Doppler frequency offset estimation value of the targets;
and the speed estimation module converts the Doppler frequency offset estimation value of the target into a speed estimation value of the target.
Compared with the prior art, the invention has the beneficial effects that:
(1) the radar system based on the typical lead code structure of the IEEE 802.11 series protocols does not need to improve the original hardware, and can expand the radar detection function on the basis of the original data transmission function in a mode of additionally arranging a processing algorithm. Moreover, by adopting the protocol standard of the millimeter wave frequency band, the radar system can achieve high distance and speed accuracy;
(2) the problem of insufficient signal-to-noise ratio caused by low transmitting power of IEEE 802.11 equipment is solved, the signal-to-noise ratio of a received signal is improved on the premise of not losing distance information by performing smooth filtering operation before constant false alarm processing, the complexity of subsequent processing is greatly reduced, and the detection efficiency of a system is further improved;
(3) aiming at the problem of poor main-to-side ratio performance caused by repeated fields in typical lead codes of IEEE 802.11 series protocols, the invention provides that partial sequences are adopted to generate matched filter coefficients during pulse compression processing instead of directly using full code words, thereby effectively avoiding side lobe triggering false alarm and improving radar detection performance;
(4) aiming at the problem of poor main-to-side ratio performance caused by repeated fields in typical lead codes of IEEE 802.11 series protocols, the invention provides that a reference window is aligned to side lobes during constant false alarm processing, so that a main lobe can be highlighted in rich side lobes, the side lobes are mutually restricted, and the influence of the side lobes on radar detection performance is further reduced;
(5) aiming at the problem of main-to-side ratio deterioration caused by incomplete echo of a near-end target, the invention generates the coefficient of the matched filter by adopting a partial sequence, so that the near-end target still has good autocorrelation characteristic when receiving incomplete echo of the near-end target, and the detectable range of the system is widened.
Drawings
Fig. 1 is a diagram of a typical preamble frame structure of the physical layer of an IEEE 802.11 series protocol.
Fig. 2 is a diagram of an IEEE 802.11ad SC PHY preamble frame structure.
Fig. 3 is a flow chart of IEEE 802.11ad based radar signal processing.
Fig. 4 is a schematic diagram of a pulsed radar signal within a CPI.
FIG. 5 is a two-dimensional data matrix of a range gate reorganization module.
Fig. 6a, 6b are pulse compression output signal power spectra for full and partial sequences, respectively.
FIG. 7 is a schematic diagram of a coherent accumulation module.
Fig. 8 is a schematic diagram of a smoothing filter module.
Fig. 9a is the 100 th pulse effect after FFT.
Fig. 9b is the output effect of the smoothing filter.
FIG. 10 is a schematic diagram of CAGO-CFAR.
Detailed Description
The first embodiment of the invention:
the first embodiment of the invention provides a scheme for realizing a radar target detection function in a dual-function communication radar system under an IEEE 802.11ad standard framework. Because the IEEE 802.11ad communication protocol works in a 60GHz frequency band, the transmission bandwidth is very large, the radar detection function is realized based on the standard, and the radar speed measurement and distance measurement precision can be obviously improved.
The radar system of the present embodiment is only in the leisure of the communication, i.e. the transmission and reception of the radar system is separated in time from the communication system. The radar signal is an IEEE 802.11ad Single Carrier Physical Layer (SC PHY) frame preamble phase coding pulse radar signal. The chip rate in this mode is 1.76GHz and the minimum synchronization unit is 128 points.
The IEEE 802.11ad SC PHY preamble has a typical IEEE 802.11 protocol frame preamble repetition structure, which includes a Short Training Field (STF) and a Channel Estimation Field (CEF), and both use golay complementary sequences. The STF field consists of 16 repeated Gray sequences Ga128And one of-Ga128Consisting of a CEF field consisting of Gu512And Gv512and-Ga128Composition, as shown in fig. 2.
The Gray complementary sequence has good autocorrelation characteristics, the autocorrelation function of the Gray complementary sequence has very sharp wave peaks, and the side lobes are mutually cancelled. Therefore, the target distance and speed can be estimated well by using the Golay complementary sequence as the radar coding waveform.
However, the performance of the radar signal main-to-sub ratio is poor due to the repeated occurrence of the gray sequence in the IEEE 802.11ad SC PHY preamble, in addition, the main-to-sub ratio is deteriorated due to incomplete near-end target echoes, and the signal-to-noise ratio of the received signal is insufficient due to serious millimeter wave signal attenuation.
Therefore, the present embodiment addresses the three problems described above, and implements radar target detection under the framework of the IEEE 802.11ad standard by designing a receiving end processing mechanism.
Fig. 3 shows the complete signal processing flow of the present embodiment. A communication radar combined system comprises a range gate rearrangement module, a pulse compression module, a time delay compensation module, a coherent accumulation module, a smooth filtering module, a distance dimension constant false alarm detection module, a distance estimation module, a speed dimension constant false alarm detection module and a speed estimation module. The radar transmits a lead code radar signal based on IEEE 802.11ad, a received signal is firstly sent to a range gate rearrangement module, a rearranged two-dimensional data matrix passes through a pulse compression module, a time delay compensation module and a coherent accumulation module, so that the peak of the signal is aligned to the position of a target, then a distance estimation value of the target and the radar is obtained through a distance dimension constant false alarm detection module and a distance estimation module, and finally a relative speed estimation value of the target is obtained through a speed dimension constant false alarm detection module and a speed estimation module by using the result of distance estimation.
The pulsed radar signal used in the present invention is represented by
Wherein the radar envelope function
Is a function of the shape of a rectangle,
j is the imaginary unit for the phase modulation function. The coding sequence IEEE 802.11ad preamble used contains only two elements, 0 and 1, so that a bi-phase coding is used, i.e. when the coding symbol is 1,
when the code symbol is 0,
。
as shown in fig. 4, the radar model of the present embodiment is a single base station radar operating in a Time Division Duplex (TDD) mode. The radar signal sub-pulse width τ = 1/1.76GHz = 0.57ns, and the preamble code length N =3328, so the radar pulse width is T = N × τ =1.89 us. The radar provides range resolution of ar = c τ/2 = 8.5cm, where c is the speed of light. The radar does not receive when sending radar signals, after the sending is finished, the receiver starts to work, the total sending-receiving duration is a Pulse Repetition Interval (PRI), and when the duty ratio is 50%, the PRI =3.78 us. And repeating the transmitting and receiving for M times and then performing signal processing. The value of M is required to ensure that the movement of the target in the Coherent Processing Interval (CPI) is less than the distance resolution, i.e. Vmax*M*PRI<Δ R, where V is the maximum speed of the target that the radar can detect. In this example, M is taken to be 120.
Range gate rearrangement module
Specifically, the range gate rearrangement module uses a two-dimensional buffer memory to sequentially put the sampled data into a two-dimensional data matrix, as shown in fig. 5. Each cell represents an independent baseband sample. The column data is L received echo samples within one PRI, and the transmitter repeatedly transmits the radar pulse signal M times, so that there are M columns of data in total. Data in the same column are from the same batch of samples, sampling frequency
Is the nyquist sampling rate. L is the number of samples within PRIFrom PRI and FsAnd (4) calculating. The data of the same row have the same receiving time delay, so the target echoes from the same distance. The time required to generate an L x M two-dimensional data matrix, CPI, is M x PRI. Where the sampling frequency of the column data is fast, called fast time. And the data in the same row are separated by one PRI, and the sampling interval is large and is called slow time. Subsequent processing is spread around the two-dimensional data matrix.
Pulse compression and delay compensation module
Specifically, the pulse compression module implements matched filtering pulse by pulse, and obtains the receiving delay of the target echo from the position information of the peak of the output signal. The matched filter coefficients are generated from a known local sequence, and because the local sequence is matched to the transmitted signal, the signal can gain pulse compression. And clutter or noise does not match with the local sequence, so after pulse compression, signal echo can be more prominent in background noise, and detection is convenient. The amplitude-frequency characteristic of the matched filter coefficient is the same as the echo signal, and the phase-frequency characteristic is conjugate with the echo signal.
When performing matched filtering, the 16 segments of the repeating sequence contained in the STF field of the IEEE 802.11ad physical frame result in an output signal with a large number of side lobes with high peak values. In addition, the time width of a radar signal based on an IEEE 802.11ad physical frame is long, and a near-end target echo cannot be completely received, so that the main-to-side ratio deterioration of an output signal is aggravated. Aiming at the problem of poor performance of a main-to-side ratio caused by an IEEE 802.11ad lead code, the invention provides that partial sequences are adopted to generate the coefficients of a matched filter, rather than directly using full code words. The partial sequence has the following conditions:
condition 1: positive sequence (Ga)128、Gb128) Number of and negative sequence (-Ga)128、-Gb128) Equal in number, so that the positive and negative cancellation of the peak values can be realized when the peak values are not matched.
Condition 2: the sequences are long so that the main lobe peak is higher when there is a full match.
Condition 3: the number of the STF field is small to reduce the number of side lobes.
Condition 4: the sequence is later in the whole preamble field to have a good matched filter output waveform when an incomplete echo is received (near target).
Specifically, in this embodiment, the gray sequences with length of 128 at the end of the STF field and Gu in the CEF field are selected512And Gv512As a reference sequence for the matched filter coefficients. Wherein Gu512And Gv512Are golay complementary sequences and the other 4 golay sequences are used to neutralize the number of positive and negative sequences in the sequence. The results show that when the complete sequence is used, the ratio of the main lobe to the first side lobe is 6.03dB, compared to the highest side lobe ratio 4.85dB, as shown in fig. 6 a. And when a partial sequence is used, the ratio of the main lobe to the first side lobe is 15.89dB, and the ratio to the highest side lobe is 9.8dB, see fig. 6 b. The main-to-auxiliary ratio is improved by about 10dB by adopting a partial sequence strategy, the side lobe triggering false alarm is effectively avoided, and the radar detection performance is improved.
The pulse compression module brings two parts of time delay, namely filter time delay and positioning time delay of the selected partial sequence which is not started from the first code word of the preamble. In this embodiment, the length of the partial sequence is 1536, so that the filter coefficient length is K =1536, and the filter delay is K-1= 1535. The partial sequence starts with the q =1536 th codeword, the localization delay is q-1= 1535. Both delays are fixed delays, so moving the output of the matched filter forward by the fixed unit and zero padding at the end, the delay can be eliminated.
Coherent accumulation module
The coherent accumulation module utilizes the phase relation of the signals to obtain the superposition of the signal amplitudes before envelope detection or other processing. Coherent accumulation is the doppler processing of data across pulses, i.e., the processed data are all from the same distance.
Specifically, the doppler processing of the multiple pulses is implemented by Fast Fourier Transform (FFT), that is, the FFT is performed on the data matrix after the delay compensation independently according to rows, and there are L distance units in total, so that N-point FFT is performed for L times. Each row of data contains M samples, the number N of FFT points must be a power of 2, and must be equal to or greater than M, where N is 128 in this embodiment. After the FFT, the dimensions of the data matrix were changed from L × M to L × N, as shown in fig. 7. Where each sample in the same column of data is from a different distance, called a range gate. The FFT achieves separation of the doppler frequencies, with the output of each doppler channel in the same row of data corresponding to a narrower doppler band, called a doppler gate.
The Doppler Frequency band covers a Frequency range of-PRF to PRF, and PRI is a Pulse Repetition Frequency (Pulse Repetition Frequency). A doppler frequency offset of the target may occur at the output of any one of the N doppler channels. If the target exists in the detection range, the Doppler channel where the output waveform peak value accumulated by the coherent accumulation is located is the Doppler frequency offset value of the target.
Smoothing filter module
Aiming at the problem of insufficient signal-to-noise ratio of a received signal caused by serious millimeter wave signal attenuation, smooth filtering operation is carried out before constant false alarm processing. When the noise is weakened, the input data of the distance dimension constant false alarm detection module is reduced, the complexity of subsequent processing is greatly reduced, and the detection efficiency of the system is further improved. In this embodiment, the smoothing filter module is implemented by averaging the same range gate data of a plurality of repetitive pulses. When the target detection is carried out in the range dimension, the change of the distance between the target and the radar within one CPI is not more than one range gate, namely, the targets are all positioned in the same range gate in M pulses repeatedly received. The received M repeated pulses are considered to be M samples, the range information of the target in each sample is unchanged, and the noise of the channel is different and can cancel each other out.
Assuming that the radar echo contains signal and additive noise, the ith pulse may be represented as
Wherein the content of the first and second substances,
is a signal echo, remains unchanged in the M repeated pulses,
is white uncorrelated additive noise with a variance of
. Correspondingly adding the parameters of the gates with the same distance of each pulse, as shown in FIG. 8, to obtain a one-dimensional vector
Constant false alarm detection module and estimation module
The distance dimension Constant False Alarm Rate detection module adopts a Constant False Alarm Rate (CFAR) detection technology, adjusts a threshold value in real time, and finds out a distance gate where a target is located from background noise. The constant false alarm detection technology estimates the unknown statistical parameters of the background clutter through signals in a fixed window, and calculates the threshold value of target detection. Specifically, in this embodiment, the fixed window means that a protection window and a reference window covering a plurality of units are respectively arranged in front of and behind the observation unit, the number of the windows is fixed, but the positions of the windows are sliding.
Conventional detection procedures require the ability to distinguish between the useful target echo and all possible clutter cases. When the radar signal based on the preamble of the IEEE 802.11ad physical frame is faced, in addition to the need to filter noise and clutter, the number of target echoes and sidelobes with high peak values need to be filtered. Aiming at the problem of poor main-to-side ratio performance caused by IEEE 802.11ad lead codes, the invention provides a strategy for adjusting a constant false alarm detection window object by utilizing the occurrence rule of an autocorrelation peak, so that a main lobe can be highlighted in rich side lobes, the side lobes are mutually restricted, and the influence of the side lobes on target detection is further reduced.
The interval between each correlation peak and each peak in the preamble autocorrelation waveform of the IEEE 802.11ad physical frame is fixed, and is 128 × 2 × 4, assuming that the transmitting end performs double sampling on the preamble sequence during phase encoding, and the receiving end performs quadruple interpolation on the received signal and then performs subsequent processing.
According to the occurrence rule of the correlation peak, the objects of the front reference window and the rear reference window are respectively designated as the left side lobe and the right side lobe of the main lobe after echo matching filtering. By configuring the reference window length and the guard window length, the present embodiment aligns an object of the reference window with a first side lobe adjacent to the main lobe. As the viewing window slides, the protection window and the reference window also slide accordingly. When the observation window is aligned with the main lobe, the reference window is aligned with the side lobe adjacent to the main lobe; when the observation window is aligned with the side lobe, the reference window is aligned with the side lobe (or the main lobe) adjacent to the side lobe; when the viewing window is aligned with the background noise, the reference window is also aligned with the background noise. The peak ratio of the main lobe to the side lobes is large and the peak ratio between the side lobes is small. The main lobe can be easily identified, and meanwhile, the side lobes are mutually restrained, and false alarms are reduced. The difference of the peak value of the side lobe is almost the same as that of the output of the smoothing filter in the graph 9b, and the difference of the main lobe and the side lobe is 11.68dB, so that the possibility of false alarm caused by the side lobe is greatly reduced, and the radar detection performance is further improved.
And estimating the power of the side lobe by using the mean power of the reference sample in the reference window, and designing a threshold value according to the estimated value, so that the threshold value can be exceeded only when the observation window is aligned with the main lobe.
The CFAR used in the system is a one-dimensional Cell Averaging with maximum Of CFAR (CAGO-CFAR), and its functional block diagram is shown in fig. 10. The input data is the modular square of one-dimensional distance dimension data, and the sliding window covers the front reference unit, the front protection unit, the observation unit, the rear protection unit and the rear reference unit. The protection unit is arranged to prevent the main lobe from being too wide, the power estimation value becomes high, and the threshold value becomes high, so that the main lobe cannot be detected. And (3) assuming that the number of the reference units is W, respectively estimating the average power of the front reference unit and the rear reference unit, then comparing the two average powers, and selecting a large value as a final power estimation value Z.
The threshold value S of the observation unit is then obtained
Wherein
Is a threshold product factor. The threshold factor of the CAGO-CFAR is implicitly given by
The distance estimation module estimates the relative distance of the target. In order to eliminate false alarms caused by the fact that the continuous multiple lattices of the same target main lobe exceed a threshold value, the module processes an output array of the distance dimension constant false alarm detection module, and the specific processing rule is as follows: rule 1: if the single lattice is 1, no processing is carried out; rule 2: if two continuous grids are 1, only 1 at the large value is reserved, and the other grid is set to be 0; rule 3: if the three continuous grids are 1, only 1 in the median is reserved, and the other two grids are set to be 0; rule 4: if the continuous multiple lattices, namely more than or equal to 4 lattices are 1, the first three data are considered to be from the same target and are processed according to a rule 3, the remaining data in the array are the first three data which are considered to be from the same target and are processed according to the rule 3, and the like, and when the remaining data are less than three lattices, the processing is carried out according to the rule 1 or the rule 2;
after processing according to the 4 rules, the object exists at the position of the distance grid corresponding to the grid still being 1; and obtaining the receiving time delay of the target echo according to the position of the distance grid, thereby obtaining the target distance estimation value.
And according to the distance detection result, the speed dimension constant false alarm detection module selects the line data with the target to carry out constant false alarm detection. The speed dimension constant false alarm detection method is consistent with the distance dimension constant false alarm detection method, but the reference window does not need to be aligned with the side lobe and can be set according to experience. The velocity dimension CFAR is used to cope with a scene with a plurality of targets at the same distance, and the result is an estimated doppler shift value of the target.
The speed estimation module converts the Doppler frequency offset estimation value of the target into the speed of the target.
The invention provides that when a receiving end carries out radar signal processing, part of IEEE 802.11ad preamble sequences are selected as matched filter coefficient generating sequences of pulse compression. The structure of the IEEE 802.11ad lead code is fully considered in the selection principle, and the selected sequence needs to meet 4 conditions, namely the number of positive sequences is equal to that of negative sequences; the sequence contains less STF field; the sequence is long; the selected sequence is later. The selected sequence enables the output waveform of the matched filter to have a larger major-minor ratio, so that better detection performance is shown in constant false alarm detection.
The smoothing filtering process proposed by the present invention is implemented by averaging the same range gate data across the pulse. The signal-to-noise ratio of the output is increased while the computational complexity is reduced.
The invention provides a constant false alarm processing in distance dimension, and aims a reference window at an adjacent side lobe or a second side lobe according to the peak value occurrence rule of an autocorrelation peak of an IEEE 802.11ad lead code. The main lobe has considerable main-to-side ratio relative to the side lobe, and the side lobe cannot be highlighted in the side lobe, so that the influence of the side lobe on radar detection is reduced.
The basic scheme of the invention can be applied to other scenes (for example, a radio frequency front end supports a full-duplex scene) for realizing the radar function by using the IEEE 802.11ad preamble code signal. The technical solution and the inventive concept of the present invention are considered to be within the scope of the claims of the present invention by equivalent substitutions and changes.
The second embodiment of the invention:
the protocol used in the first embodiment of the present invention is IEEE 802.11ad, but the present invention is also applicable to implementing radar function in other IEEE 802.11 series communication protocols. If in the IEEE 802.11ax agreement, if realize the radar function, can compare this patent design, when the receiving end carries out radar signal processing, should receive the signal and carry out multiple interpolation and improve the distance precision, utilize smooth filtering module increase SNR to at the constant false alarm detection module, aim at its too high side lobe with the reference window, reduce the false alarm probability, thereby realize better radar detection performance.
Claims (9)
1. A communication radar combined system is characterized by comprising a distance gate rearrangement module, a pulse compression module, a time delay compensation module, a coherent accumulation module, a smooth filtering module, a distance dimension constant false alarm detection module, a distance estimation module, a speed dimension constant false alarm detection module and a speed estimation module; the radar transmits radar signals of typical communication lead code codes of IEEE 802.11 series protocols, received signals are firstly rearranged through a range gate rearrangement module, the obtained two-dimensional data matrix enables signal wave crests to be aligned to the position of a target through a pulse compression module, a time delay compensation module and a coherent accumulation module, then a distance estimation value of the target and the radar is obtained through a smooth filtering module, a distance dimension constant false alarm detection module and a distance estimation module, and finally a relative speed estimation value of the target and the radar is obtained through a speed dimension constant false alarm detection module and a speed estimation module according to the result of distance estimation;
the radar model of the system is a single-base-station radar working in a time division duplex mode, and the transmitting, receiving and communication of the radar are separated in time;
the range gate rearrangement module:
sequentially putting the sampled data into a two-dimensional data matrix by adopting a two-dimensional buffer memory; each cell represents an independent baseband sample; the column data is L received echo samples of a single pulse in a pulse repetition interval, and the row data is M received echo samples from the same distance; wherein, L is the sampling times in the pulse repetition interval, M is the number of the repeated pulses, and the value of M needs to ensure that the movement of the target in the coherent processing interval is smaller than the distance resolution; subsequent processing is expanded around the two-dimensional data matrix;
the pulse compression module:
adopting a matched filter to realize matched filtering one by one pulse, and obtaining the receiving time delay of a target echo from the position of the peak of an output signal; the coefficient of the matched filter is generated by a local sequence, the amplitude-frequency characteristic of the matched filter is the same as that of the echo signal, and the phase-frequency characteristic of the matched filter is conjugated with that of the echo signal;
aiming at rich side lobes caused by a preamble repeated field of a typical preamble of an IEEE 802.11 series protocol, selecting a part of sequences as reference sequences to generate matched filter coefficients; the partial sequence has the following conditions:
condition 1: the number of positive and negative values in the sequence is equal;
condition 2: the sequence is long;
condition 3: the contained repeated fields are few;
condition 4: the sequence is later in the entire preamble field.
2. The system of claim 1, wherein the pulse compression module: when applied to the IEEE 802.11ad protocol standard, the reference sequence for selecting the matched filter coefficients is: 4 Gray sequences with length of 128 at the end of short training field, in channel estimation field
And
。
3. the system of claim 1, wherein the delay compensation module:
selecting a part of sequence with the length of K, so that the coefficient length of the filter is K, and the time delay of the filter is K-1; if the partial sequence starts from the qth code word, the positioning time delay is q-1; both delays are fixed delays, so the matched filter output is moved forward by the fixed unit and zeroed at the end, eliminating the delay.
4. The system of claim 1, wherein the coherent accumulation module:
the module realizes the separation of Doppler frequency through Fourier transform, namely, the data matrix after time delay compensation is subjected to Fourier transform independently according to rows, and N-point Fourier transform is performed for L times in total; the output of each Doppler channel corresponds to a narrower Doppler frequency band, and the Doppler channel where the output waveform peak is located is the Doppler frequency offset value of the target.
5. The system of claim 1, wherein the smoothing filter module:
the module is implemented by averaging the same range gate data of a plurality of repetitive pulses, which is performed after coherent accumulation and before range constant false alarm probability detection.
6. The system of claim 1, wherein the distance-preserving false alarm detection module:
the module processes data in the sliding window by adopting a constant false alarm rate detection technology; the sliding window sequentially covers the front reference window, the front protection window, the observation window, the rear protection window and the rear reference window; respectively aligning the front reference window and the rear reference window to the left side lobe and the right side lobe of the main lobe after echo matching filtering; with the sliding of the window, the reference window of the main lobe is aligned with the side lobe, the reference window of the side lobe is aligned with the side lobe or noise, and the reference window of the noise is aligned with the side lobe or noise; and estimating unknown statistical parameters of background clutter and side lobes by using signals in the front reference window and the rear reference window, designing a threshold, if the threshold is lower than the power of an observation window, recording 1 in a distance grid corresponding to the observation window, otherwise, recording 0, and outputting a module as a one-dimensional array corresponding to the distance grids one by one.
7. The system of claim 6, wherein the distance-preserving false alarm detection module: when applied to the IEEE 802.11ad protocol standard, the relative positions of the main lobe and the side lobes are fixed and depend on the repetition field in the preamble
Selecting a first side lobe or a second side lobe adjacent to the observation window as a reference window object, setting the lengths of a front reference window and a rear reference window according to the major-minor ratio, and setting the lengths of the front reference window and the rear reference window according to the major-minor ratio
The length of the front and rear protection windows is set.
8. The system of claim 1, wherein the distance estimation module:
the module processes the output array of the distance dimension constant false alarm detection module, and the specific processing rule is as follows: rule 1: if the single lattice is 1, no processing is carried out; rule 2: if two continuous grids are 1, only 1 at the large value is reserved, and the other grid is set to be 0; rule 3: if the three continuous grids are 1, only 1 in the median is reserved, and the other two grids are set to be 0; rule 4: if the continuous multiple lattices, namely more than or equal to 4 lattices are 1, the first three data are considered to be from the same target and are processed according to a rule 3, the remaining data in the array are the first three data which are considered to be from the same target and are processed according to the rule 3, and the like, and when the remaining data are less than three lattices, the processing is carried out according to the rule 1 or the rule 2;
after processing according to the 4 rules, the object exists at the position of the distance grid corresponding to the grid still being 1; and obtaining the receiving time delay of the target echo according to the position of the distance grid, thereby obtaining the target distance estimation value.
9. The system of claim 1, wherein the velocity-preserving false alarm detection module:
selecting row data with a target from the two-dimensional matrix output by the coherent accumulation module according to the distance detection result to perform constant false alarm detection; the method for detecting the velocity dimension constant false alarm rate is consistent with the method for detecting the distance dimension constant false alarm rate, but the reference window does not need to be aligned with the side lobe and can be set according to experience; the result of the velocity dimension constant false alarm detection module is a Doppler frequency offset estimation value of the target;
and the speed estimation module converts the Doppler frequency offset estimation value of the target into a speed estimation value of the target.
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