CN108549048B - Multi-frequency WiFi external radiation source radar coherent processing method - Google Patents

Multi-frequency WiFi external radiation source radar coherent processing method Download PDF

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CN108549048B
CN108549048B CN201810247032.XA CN201810247032A CN108549048B CN 108549048 B CN108549048 B CN 108549048B CN 201810247032 A CN201810247032 A CN 201810247032A CN 108549048 B CN108549048 B CN 108549048B
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饶云华
纪赛飞
万显荣
周健康
王雅莉
胡海霞
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Wuhan University WHU
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
    • G01S5/02—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using radio waves

Abstract

The invention discloses a multi-frequency WiFi external radiation source radar coherent processing method. The WiFi signal is a multi-carrier signal modulated by OFDM, so that the orthogonality of the sub-carriers can be used to perform spectrum synthesis on WiFi signals of a plurality of different frequencies to improve the detection performance. The method of the invention demodulates a plurality of paths of baseband signals by utilizing different carrier frequencies of a plurality of paths of WiFi signals, carries out spectrum shifting on one path of signals so that the frequency spectrum of the one path of signals is just spliced with the frequency spectrum of the other path of signals without overlapping, and carries out target detection by utilizing the signals after the frequency spectrum splicing. The method can improve the range resolution of the radar system of the external radiation source, simultaneously improve the output signal-to-noise ratio after matched filtering, and is beneficial to target detection.

Description

Multi-frequency WiFi external radiation source radar coherent processing method
Technical Field
The invention belongs to the technical field of external radiation source radar target detection, and particularly relates to a multi-frequency WiFi external radiation source radar coherent processing method, which is a processing method based on multi-frequency WiFi external radiation source radar signal coherent synthesis.
Background
The external radiation source radar detects a target by using electromagnetic waves emitted by a third party as an irradiation source, has the advantages of environmental protection, strong anti-striking capability, no occupation of frequency spectrum and the like, and is very favorable for passive detection because a WiFi signal based on an IEEE wireless local area network standard (802.11) covers most of the urban area and has the characteristics of wide coverage range, approximate ideal tack-shaped fuzzy function after secondary peak suppression and the like.
The WiFi signal is used as an external radiation source to carry out through-wall detection, and the target is mainly indoor active personnel, so that higher requirements are put on the range resolution of a radar system. The indoor environment is complex, a large amount of multipath clutter can be introduced into a target echo signal, so that the target echo signal is very weak, and therefore, the improvement of the signal strength is very important. The existing WiFi external radiation source radar system mostly adopts a single radiation source signal, the signal bandwidth is generally maximum 20MHz, the emission power is 50mW, the signal power is limited, and the action range, the positioning precision, the detection performance and the like are all limited. Because the working bandwidth of the WiFi signal in the frequency band of 2.4GHz is 100MHz, a plurality of WiFi signals are generally distributed in the whole 100MHz bandwidth, and therefore, the distance resolution and the signal power of the radar can be effectively improved by utilizing the detection after the synthesis processing of a plurality of radiation source signals.
The multi-frequency external radiation source radar comprehensively utilizes signals of a plurality of different frequency bands to realize integrated detection, provides a way for improving performance, and is an important development trend of the external radiation source radar. The methods for improving the detection performance by multi-frequency signal comprehensive processing are mostly as follows: and performing matched filtering on the signals of a plurality of different frequency bands to obtain multi-frequency RD (Range-Doppler) spectrums, and adding the multi-frequency RD spectrums to obtain the RD spectrums with high signal-to-noise ratios to perform target detection. The superposition of multi-frequency RD spectra is further divided into coherent accumulation (removing phase difference terms) and non-coherent accumulation (direct amplitude addition). The multi-frequency RD spectrum superposition can improve the output signal-to-noise ratio of the matched filtering, but the improvement of the range resolution is not obvious. The WiFi signal is a multi-carrier signal modulated by OFDM, frequency band synthesis can be carried out by utilizing orthogonality among sub-carriers of the multi-carrier signal, phase compensation and phase-coherent synthesis are respectively carried out on a reference signal and a monitoring signal, matched filtering is carried out by utilizing the synthesized signal, distance resolution can be improved, and meanwhile, the output signal-to-noise ratio is improved.
Disclosure of Invention
The invention aims to provide a multi-frequency WiFi external radiation source radar coherent processing method based on spectrum shifting by fully utilizing OFDM modulation characteristics of WiFi signals aiming at WiFi signals with different working frequencies in space.
In order to achieve the purpose, the invention adopts the following technical scheme:
a multi-frequency WiFi external radiation source radar coherent processing method comprises the following steps:
step 1, carrying out frequency mixing separation, sampling, clutter suppression and other processing on WiFi signals of a plurality of different frequency bands respectively to obtain multi-channel baseband WiFi signals;
step 2, estimating and compensating phase difference items of the multi-channel baseband WiFi signals respectively to obtain multi-channel coherent baseband signals;
and 3, carrying out frequency spectrum shifting on the multi-channel baseband signals after the phase compensation to synthesize the multi-channel narrow-band signals into a single-channel broadband signal, and carrying out matched filtering by using the synthesized broadband signal so as to realize target detection.
Further, the multi-path baseband reference signal in step 1 is represented as:
Figure BDA0001606820810000021
where m is the subcarrier number, Δ f is the subcarrier frequency spacing, fciIs the carrier frequency, τdFor the time delay of the signal transmitted by the router transmit antenna to the reference antenna,
Figure BDA0001606820810000022
in order to transmit the initial phase,
Figure BDA0001606820810000023
an additional phase term is introduced for the reference signal mixing process.
Further, the multipath baseband target echo signals in step 1 are represented as:
Figure BDA0001606820810000024
i＝1，2…，N
wherein τ is the two-way distance delay; f. ofdiA doppler shift of the target;
Figure BDA0001606820810000025
the additional phase introduced for the ith signal for the target,
Figure BDA0001606820810000026
an additional phase term introduced for the target echo signal mixing process.
Further, the phase difference term of the baseband reference signal in step 2 is:
Figure BDA0001606820810000027
the specific method for estimating the initial phase of signal transmission is as follows: the carrier frequency offset estimation is realized by using the correlation relationship between the preamble sequences, namely:
Figure BDA0001606820810000028
Figure BDA0001606820810000031
wherein r isnFor the received baseband signal, D is the delay of the same sample of two consecutive repeated symbols in the preamble sequence, L is the length of the repeated symbol, R is the delayed correlation sum of two consecutive repeated symbols, TsIs a symbol interval, fΔI.e. the carrier frequency error estimate. Coarse synchronization is performed using a short training sequence, and then the coarse frequency offset estimation is usedAnd correcting the long training sequence, and performing fine frequency offset estimation and correction by using the corrected long training sequence so as to realize frequency synchronization.
The phase error of the transmit-side mixer is estimated using two long training symbols, namely:
RRLTS＝(R1LTS+R2LTS)/2
Figure BDA0001606820810000032
wherein R is1LTSAnd R2LTSFor the received first and second long training symbols; l isLTSFor standard long training symbols, RRLTSIs the average of two long training symbols;
Figure BDA0001606820810000033
is the channel impulse response function in the frequency domain. To pair
Figure BDA0001606820810000034
The phase deviation between the mixers at the transmitting end can be estimated by averaging the phases of the two mixers.
Further, the phase difference term of the baseband target echo signal in step 2 is:
Figure BDA0001606820810000035
it is not only the frequency difference Δ f of each signaliThe function of (d) is also a function of the target distance delay τ, and should be compensated for by different detection units according to different channels.
Further, the step 3 of performing spectrum shifting on the multiple paths of baseband signals to synthesize multiple paths of narrowband signals into one path of wideband signal specifically includes the following steps:
1) the two baseband signals are respectively expressed as:
Figure BDA0001606820810000036
Figure BDA0001606820810000037
p, Q represents OFDM symbol number of two-path signal1Time delays caused for different signal frame starting points;
2) to s2(t) performing frame alignment, wherein the second baseband signal after frame alignment is represented as:
Figure BDA0001606820810000041
to s'2(t) moving the frequency spectrum, i.e. to s'2(t) mixing the frequency so that the mixed frequency spectrum is exactly equal to s1(t) frequency spectra are spliced without overlap, mixed signal s2"(t) is expressed as:
Figure BDA0001606820810000042
fcfor mixing frequencies, for spectral synthesis fcShould be 64 Δ f;
3) to s1(t) and s2"(t) is superimposed, and the superimposed signal s' (t) is represented as:
Figure BDA0001606820810000043
wherein c iss,0、cs,1…cs,63Is equal to ap,0、ap,1…ap,63，cs,64、cs,65…cs,127Is equal to bq,0、bq,1…bq,63。
Compared with the prior art, the invention has the beneficial effects that:
1) the invention utilizes WiFi signals with different working frequencies in space and utilizes the orthogonality among subcarriers of an OFDM system to synthesize frequency bands, thereby fully utilizing the frequency resources of the WiFi signals;
2) n WiFi signals are subjected to coherent synthesis, the peak value of a distance Doppler spectrum can be improved by N times, and the output signal-to-noise ratio can be improved by 10logNdB approximately under the condition of the same transmitting power;
4) compared with a multi-frequency RD spectrum coherent superposition method, the method has the advantages that the bandwidth is synthesized by utilizing a plurality of narrow-band signals, and the improvement on the range resolution is more obvious.
Drawings
FIG. 1 is an IEEE802.11g protocol OFDM mode physical layer frame structure;
fig. 2 is a multi-frequency WiFi signal processing flow diagram;
fig. 3(a) - (b) are frequency spectrums obtained by mixing and superimposing one WiFi signal spectrum and two WiFi signals under the condition of 80MHz sampling rate;
FIGS. 4(a) - (b) are a single-signal ambiguity function distance spectrum and a two-path signal synthesized ambiguity function distance spectrum; FIGS. 4(c) - (d) are their 3dB distance spectra, respectively;
FIGS. 5(a) - (b) are range-Doppler and range spectra of a single signal;
FIGS. 6(a) - (b) are a range-Doppler spectrum and a range spectrum after coherent synthesis of two signals;
FIGS. 7(a) - (b) are a range-Doppler spectrum and a range spectrum after coherent synthesis of a three-way signal;
FIG. 8 is a distance spectrum comparison of a conventional RD spectrum coherence stack processing method and the coherence processing methods provided herein.
Detailed Description
The invention is described in further detail below with reference to the following figures and detailed description:
fig. 1 is a physical layer frame structure of ieee802.11g protocol OFDM mode, as can be seen from fig. 1, the signal structure mainly consists of preamble symbol, signal, and data 3, the present invention mainly uses its data part for detection, the baseband complex signal expression of the data part is:
Figure BDA0001606820810000051
Figure BDA0001606820810000052
wherein R is the serial number of the OFDM symbol of the data part, and R is the number of the OFDM symbol; k is the subcarrier number, NST64 is the number of subcarriers of one OFDM symbol; dr,kComplex modulation information of a k subcarrier for an r symbol; tp is the effective partial time length of one OFDM symbol, and the subcarrier frequency interval is delta f which is 1/Tp; t isdFor the length of time of the preamble symbol and signal part, TgIs a cyclic prefix time length, T, of one OFDM symbolsI.e. one full OFDM symbol duration.
Fig. 2 is a flowchart of multi-frequency WiFi signal processing, which includes the following specific steps:
step 1, carrying out frequency mixing separation, sampling, clutter suppression and other processing on WiFi signals of a plurality of different frequency bands respectively to obtain multi-channel baseband WiFi signals;
the multi-path baseband reference is represented as:
Figure BDA0001606820810000053
i＝1，2…，N
the multipath baseband target echo signal is represented as:
Figure BDA0001606820810000061
i＝1，2…，N
step 2, estimating and compensating phase difference items of the multi-channel baseband WiFi signals respectively to obtain multi-channel coherent baseband signals;
the phase difference term of the baseband reference signal is:
Figure BDA0001606820810000062
the specific method for estimating the initial phase of signal transmission is as follows: the carrier frequency offset estimation is realized by using the correlation relationship between the preamble sequences, namely:
Figure BDA0001606820810000063
Figure BDA0001606820810000064
wherein r isnFor the received baseband signal, D is the delay of the same sample of two consecutive repeated symbols in the preamble sequence, L is the length of the repeated symbol, R is the delayed correlation sum of two consecutive repeated symbols, TsIs a symbol interval, fΔI.e. the carrier frequency error estimate. And performing coarse synchronization by using the short training sequence, then correcting the long training sequence by using the coarse frequency offset estimation value, and performing fine frequency offset estimation and correction by using the corrected long training sequence, thereby realizing frequency synchronization.
The phase error of the transmit-side mixer is estimated using two long training symbols, namely:
RRLTS＝(R1LTS+R2LTS)/2
Figure BDA0001606820810000065
wherein R is1LTSAnd R2LTSFor the received first and second long training symbols; l isLTSFor standard long training symbols, RRLTSIs the average of two long training symbols;
Figure BDA0001606820810000066
is the channel impulse response function in the frequency domain. To pair
Figure BDA0001606820810000067
The phase deviation between the mixers at the transmitting end can be estimated by averaging the phases of the two mixers.
The phase difference term of the baseband target echo signal is as follows:
Figure BDA0001606820810000068
it is not only the frequency difference Δ f of each signaliIs a function of the target distanceThe function of the delay tau should be compensated for different detection units according to different channels.
And 3, carrying out frequency spectrum shifting on the multi-channel baseband signals after the phase compensation to synthesize the multi-channel narrow-band signals into a single-channel broadband signal, and carrying out matched filtering by using the synthesized broadband signal so as to realize target detection.
The specific process of shifting the frequency spectrum of the multiple baseband signals to synthesize the multiple narrowband signals into one broadband signal is as follows:
1) the two baseband signals are respectively expressed as:
Figure BDA0001606820810000071
Figure BDA0001606820810000072
p, Q represents OFDM symbol number of two-path signal1The time delay caused by the difference of the signal frame start points, the spectrogram of which is shown in fig. 3 (a);
2) to s2(t) performing frame alignment, wherein the second baseband signal after frame alignment is represented as:
Figure BDA0001606820810000073
3) to s'2(t) moving the frequency spectrum, i.e. to s'2(t) mixing the frequency so that the mixed frequency spectrum is exactly equal to s1(t) frequency spectra are spliced without overlap, mixed signal s2"(t) is expressed as:
Figure BDA0001606820810000074
fcfor mixing frequencies, for spectral synthesis fcShould be 64 Δ f;
4) to s1(t) and s2"(t) is superposed, and after superpositionSignal s' (t) of (a) is represented as:
Figure BDA0001606820810000075
wherein c iss,0、cs,1…cs,63Is equal to ap,0、ap,1…ap,63，cs,64、cs,65…cs,127Is equal to bq,0、bq,1…bq,63The spectrogram of the superimposed signal is shown in fig. 3 (b).
Fig. 4 shows a single-signal ambiguity function distance spectrum, a two-path signal synthesized ambiguity function distance spectrum, and a 3dB distance spectrum thereof, where 84 OFDM symbol data are selected for each path of signal, the duration of a corresponding packet is 336 μ s, and the signal sampling rate is set to 80 MHz. Therefore, the distance resolution of the synthesized two-path signals is doubled compared with that of the single-path signals.
Fig. 5, 6 and 7 show the coherent combined range-doppler spectra of a single signal, a two-way signal and a three-way signal, respectively. Setting channel center frequencies of 2412MHz, 2437MHz and 2462MHz respectively by the three routers, and setting the transmitting power of the three routers to be the same; the receiver adopts a 160MHz sampling rate to sample; distance R of target from transmitting stationT60m, distance R from the receiving stationRIs 70m, velocity vrIs 1 m/s; distance R between transmitting station and receiving stationdIs 40 m. The output signal-to-noise ratio is normalized, and the output signal-to-noise ratio can be compared by comparing the noise substrate of the range-Doppler spectrum. The average values of the noise bases of the distance elements between 300 and 600 after the single signal, the two-path signal and the three-path signal are subjected to coherent synthesis are-41.19 dB, -43.41dB and-45.53 dB respectively. It can be seen that the output signal-to-noise ratios of the two signals and the three signals after coherent synthesis are respectively improved by 2.22dB and 4.34dB, which are close to theoretical values of 10log 2-3 dB and 10log 3-4.7 dB, and the difference from the theoretical values is mainly due to target non-point source effect and energy loss in the filtering process.
Fig. 8 is a comparison of the distance spectrum of the conventional RD-spectrum coherent addition processing method and the coherent processing method provided herein, and it can be seen that the distance resolution improvement of the coherent processing method provided herein is more obvious.

Claims (3)

1. A multi-frequency WiFi external radiation source radar coherent processing method is characterized by comprising the following steps: carrying out bandwidth synthesis in a frequency domain by utilizing WiFi signals with different working frequencies in space to change the synthesized signals into large-bandwidth signals, thereby improving the distance resolution; the received signals of different frequency bands have phase difference items, phase compensation and coherent synthesis are respectively carried out on the multi-path signals, and matched filtering is carried out by utilizing the synthesized signals, so that the detection performance can be obviously improved;
the method comprises the following steps:
step 1, performing frequency mixing separation, sampling and clutter suppression processing on WiFi signals of a plurality of different frequency bands respectively to obtain a plurality of paths of baseband WiFi signals;
step 2, estimating and compensating phase difference items of the multi-channel baseband WiFi signals respectively to obtain multi-channel coherent baseband signals;
step 3, carrying out frequency spectrum shifting on the multi-path baseband signals after phase compensation to synthesize multi-path narrow-band signals into a path of broadband signals, and carrying out matched filtering by using the synthesized broadband signals so as to realize target detection;
the multi-channel baseband reference signal in step 1 is represented as:
Figure FDA0003240638140000011
i＝1，2…，N；
where m is the subcarrier number, Δ f is the subcarrier frequency spacing, fciIs the carrier frequency, τdFor the time delay of the signal transmitted by the router transmit antenna to the reference antenna,
Figure FDA0003240638140000012
in order to transmit the initial phase,
Figure FDA0003240638140000013
an additional phase term introduced for reference signal mixing processing;
the multi-path baseband target echo signals in the step 1 are expressed as follows:
Figure FDA0003240638140000014
i＝1，2…，N；
wherein τ is the two-way distance delay; f. ofdiA doppler shift of the target;
Figure FDA0003240638140000015
the additional phase introduced for the ith signal for the target,
Figure FDA0003240638140000016
additional phase terms introduced for target echo signal mixing processing;
the specific process of performing spectrum shift on the multiple paths of baseband signals to synthesize multiple paths of narrow-band signals into one path of broadband signal in the step 3 is as follows:
1) the two baseband signals are respectively expressed as:
Figure FDA0003240638140000021
Figure FDA0003240638140000022
p, Q represents the number of OFDM symbols of two signals, t1Time delays caused for different signal frame starting points;
2) to s2(t) performing frame alignment, wherein the second baseband signal after frame alignment is represented as:
Figure FDA0003240638140000023
to s'2(t) moving the frequency spectrum, i.e. to s'2(t) mixing the frequency so that the mixed frequency spectrum is exactly equal to s1(t) frequency spectra are spliced without overlap, mixed signal s2"(t) is expressed as:
Figure FDA0003240638140000024
fcfor mixing frequencies, for spectral synthesis fcShould be 64 Δ f;
to s1(t) and s2"(t) is superimposed, and the superimposed signal s' (t) is represented as:
Figure FDA0003240638140000025
wherein c iss,0、cs,1…cs,63Is equal to ap,0、ap,1…ap,63，cs,64、cs,65…cs,127Is equal to bq,0、bq,1…bq,63。
2. The method according to claim 1, wherein the phase difference term of the baseband reference signal in step 2 is:
Figure FDA0003240638140000026
the specific method for estimating the initial phase of signal transmission is as follows:
the carrier frequency offset estimation is realized by using the correlation relationship between the preamble sequences, namely:
Figure FDA0003240638140000031
Figure FDA0003240638140000032
wherein r isnFor the received baseband signal, D is the delay of the same sample of two consecutive repeated symbols in the preamble sequence, L is the length of the repeated symbol, R is the delayed correlation sum of two consecutive repeated symbols, TsIs a symbol interval, f△Namely the carrier frequency error estimated value; coarse synchronization is carried out by adopting a short training sequence to obtain a coarse frequency offset estimation value f△Then, the coarse frequency offset estimation value is adopted to correct the long training sequence, and the corrected long training sequence is utilized to carry out fine frequency offset estimation and correction, so that frequency synchronization is realized;
the phase error of the transmit-side mixer is estimated using two long training symbols, namely:
RRLTS＝(R1LTS+R2LTS)/2
Figure FDA0003240638140000033
wherein R is1LTSAnd R2LTSFor the received first and second long training symbols; l isLTSFor standard long training symbols, RRLTSIs the average of two long training symbols;
Figure FDA0003240638140000034
a channel impulse response function which is a frequency domain; to pair
Figure FDA0003240638140000035
The phase deviation between the mixers at the transmitting end can be estimated by averaging the phases of the two mixers.
3. The method according to claim 2, wherein the phase difference term of the baseband target echo signal in step 2 is:
Figure FDA0003240638140000036
it is not only the frequency difference Deltaf of each signaliThe function of (d) is also a function of the target distance delay t, and should be compensated for by different detection units according to different channels.
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