CN110531323A - A kind of reference signal reconstructing method suitable for MIMO/OFDM external illuminators-based radar - Google Patents
A kind of reference signal reconstructing method suitable for MIMO/OFDM external illuminators-based radar Download PDFInfo
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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
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
- G01S7/28—Details of pulse systems
- G01S7/2813—Means providing a modification of the radiation pattern for cancelling noise, clutter or interfering signals, e.g. side lobe suppression, side lobe blanking, null-steering arrays
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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
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
- G01S7/28—Details of pulse systems
- G01S7/285—Receivers
- G01S7/292—Extracting wanted echo-signals
- G01S7/2923—Extracting wanted echo-signals based on data belonging to a number of consecutive radar periods
-
- 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
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
- G01S7/35—Details of non-pulse systems
-
- 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
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
- G01S7/35—Details of non-pulse systems
- G01S7/352—Receivers
- G01S7/354—Extracting wanted echo-signals
Abstract
The invention discloses one kind to be suitable for MIMO/OFDM external illuminators-based radar reference signal reconstructing method, frame synchronization is realized first with the leading training sequence in direct-path signal, sign synchronization and Frequency Synchronization, then multipath is eliminated by channel estimation method using pilot tone, intersymbol interference caused by Doppler frequency shift etc. and inter-sub-carrier interference, initial data recovery has been carried out followed by maximum likelihood decoding algorithm, the very low bit stream of BER has been obtained after decoding error correction, re-encoding is carried out to clean bit stream again and generates transmitting end signal, finally the autoambiguity function of transmitting end signal is analyzed, and inhibited for ambiguity function submaximum and secondary lobe.Compared with directly acquiring reference signal, the present invention improves reference signal purity, is conducive to the target acquisition of radar system, and implementation method is simple.
Description
Technical field
The invention belongs to external illuminators-based radar technical field and fields of communication technology, are related to radar receiving end reference signal
Acquisition, use based on " demodulation-re-modulation-amendment " reference signal obtain and method of purification.
Background technique
In field of radar, existing a variety of counter radar technologies make the working environment of monostatic radar more and more dangerous, even
It is faced with huge test.Meanwhile gradually being opened along with China's low altitude airspace, civil aviation industry is developed rapidly,
But there is also many deficiencies with regulatory means for effective low altitude airspace monitoring.The appearance of external illuminators-based radar is to solve tradition to have
The predicament that source radar is faced at this stage provides new solution, while also preferably having complied with the requirement in epoch.External source
Third party's irradiation source that radar utilizes mainly has: ground digital television broadcast DVB-T (Digital Video
Broadcasting-Terrestrial), digital television terrestrial broadcasting DTMB (Digital Television Terrestrial
Multimedia Broadcasting), China Mobile Multimedia Broadcasting CMMB (China Mobile Multimedia
Broadcasting), FM broadcast, digital audio broadcasting DAB (Digital Audio Broadcasting), LTE (Long
Term evolution)。
Wherein, the signal based on IEEE WLAN standard (802.11n) uses MIMO/OFDM coding mode, with it
Its radiation source characteristic has biggish difference.At home and abroad main cities are universal at present for the signal, utilize it as external radiation
Source, which carries out through-wall detection, natural advantage: the covering of 802.11n signal is wide, nowadays from public domain, such as airport, coffee
, dining room, fast food restaurant, hotel, department store to private area, as family, school, government organs even spacecraft all cover
Wireless network, makes the 802.11n external source radar have many application scenarios；Meanwhile 802.11n access point is more, is conducive to carry out outer
The detection of radiation source radar network composite, promotes investigative range, and advantageously reduce cost；802.11n signal and the signal studied before
For have higher bandwidth, through-wall performance is excellent, and blind area is smaller, and have preferable distance resolution；802.11n signal tool
There is 2.4G and 5G double frequency mode, convenient for realizing dual frequency sounder in the future.
External illuminators-based radar is usually there are two channel, and one is monitoring channel, receives target scattering signal, another is ginseng
Channel is examined, the direct wave of radiation emission is received, carries out matched filtering as reference signal and monitoring signals.The matter of reference signal
Amount is very big for the influence of matched filtering result, and the pure reference signal of all acquisitions is extremely important.
There are two types of modes for the reference signal acquisition of external radiation source radar system, the first is the side for directly acquiring reference signal
Formula, i.e., receiving end directly pass through directive reference antenna be directed toward transmitting station direction, the reference channel signal that receives i.e.
For reference signal；Another reference signal acquisition modes carry out " demodulation-re-modulation " reconstruct transmitting letter to reference channel signal
Number processing to extracting more pure direct-path signal as reference signal；The present invention is in traditional " demodulation-re-modulation "
It proposes " demodulation-re-modulation-amendment " method on the basis of mode to be reconstructed, to reduce submaximum and the side of signal ambiguity function
Valve improves detection performance.
Reconstruct needs successively to synchronize reception signal, channel estimation, MIMO space-time decoding, demapping, deinterleaves, translates
Code error correction, after error correction, obtains correct bit stream, is modified according still further to transmitting signal characteristic, reduces submaximum and secondary lobe suppression
System finally generates 802.11n reference signal according to transmitting signal generation step.Wherein synchronizing mainly includes frame synchronization, symbol
Synchronous and Frequency Synchronization.Frame synchronization is the initial position in order to roughly estimate a frame signal；Sign synchronization primarily to
Find the starting point of OFDM symbol；Frequency Synchronization has different Processing Algorithms according to the size of frequency deviation, primarily to correcting frequency offset.
Synchronize be receiving end carry out subsequent processing precondition, synchronization it is not accurate, will lead to subsequent channel estimation and etc. can not
It is normally carried out, therefore synchronization is particularly significant for signal reconstruction.
For this purpose, the orthogonality of the characteristics of the present invention is based on signal frame structures and ofdm signal, proposes one kind and is suitable for
802.11n external illuminators-based radar reference signal reconstructing method.
Summary of the invention
The not high problem of reference signal purity is directly acquired for external illuminators-based radar, the invention proposes based on " demodulation-
The reference signal method of purification of re-modulation-amendment " reconstruct.
The technical scheme adopted by the invention is that: one kind being suitable for 802.11n external illuminators-based radar reference signal reconstruct side
Method, comprising the following steps:
Step 1, roughly the reference signal in signal stream is detected by frame synchronization, finds out signal frame rough
Beginning position, specifically: using direct-path signal it is leading in 10 duplicate short training sequences, pass through the related frame synchronization that is delayed
Algorithm carries out the detection of frame starting point, is sentenced according to the variation of the superposition value of received correlation on decision metric, that is, every antenna
Disconnected, the position that decision metric peak value occurs is exactly the starting point of frame；
Step 2, coarse frequency offset is carried out to the signal in step 1 using data auxiliary law, present in correction signal compared with
Big frequency deviation；
Step 3, the initial position that OFDM symbol is found out using sign synchronization algorithm, using the synchronous related algorithm that is delayed, benefit
It is carried out with long training sequence to received sample sequence related, the starting point of OFDM symbol is estimated according to decision metric；
Step 4, correlation is carried out using the long training sequence in frame structure, fractional part of frequency offset present in system is estimated
Meter；
Step 5, using the channel estimation methods based on pilot tone, the channel value at pilot tone is estimated first, recycles interpolation
Algorithm calculates the channel value of other subcarriers；
Step 6, space-time decoding is carried out, the recovery of original data stream is carried out using maximum likelihood decoding algorithm；
Step 7, the correct bit stream according to obtained in step 6 is generated according still further to transmitting signal generation step with reference to letter
Number, and after reference signal generation, according to analysis of fuzzy functions, reference signal is modified, realizes the submaximum of ambiguity function
Inhibition and Sidelobe Suppression.
Further, rapid 1 specific implementation is as follows,
For the sample sequence r (n) of reception signal, it is calculated by the following formula to obtain the starting point of frame in step 1,
Wherein, k indicates receiving antenna quantity；L is the related accumulated value of short training sequence delay；The index of n expression subcarrier
Value；D is the length of sliding window, consistent with short training sequence length value；CkIt (n) is delay auto-correlation function, PkIt (n) is reception
The energy of the signal arrived, cor (n) are the decision metric of single antenna, and Tr is transmitting antenna number, and M is the judgement degree of more antennas
Amount, when decision metric is rapidly increased to peak value, the position that peak value occurs is exactly the starting point of frame.
Further, the specific implementation in step 2 is as follows,
If m-th of OFDM symbol n " ' sample of transmission signal on i-th transmitting terminal antenna is si(m, n), then corresponding
The symbol received is expressed as:
After mimo channel, the signal received on jth root receiving antenna is indicated are as follows:
In transmitting terminal, short training sequence generally without difference, only exists the difference in phase, then it is relevant to be delayed
Value is expressed as:
Since mimo system uses more receiving antennas, then on the basis of obtaining the value of single received antenna, also
It needs to be summed to obtain final judgment variables, then obtained frequency offseting value are as follows:
Wherein, yi(m, n) is the symbol sampler value received, fcFor the carrier frequency of transmission, TsFor sampling interval, TxFor
Number of transmission antennas, w are additive white Gaussian noise, rj(m, n) is signal on jth root receiving antenna, zjFor the relevant value that is delayed, L
For the related accumulated value of short training sequence delay, D is short training sequence length value, fΔTo receive carrier wave and sending the frequency between carrier wave
Difference, frequency offseting value areIt is indicated for the angle of phase difference, range is (- π, π)；
Therefore obtained coarse frequency offset range are as follows:
Further, the specific implementation of step 3 is as follows,
Two hairs two under 40MHz mode in 802.11n consensus standard receive system, and the length of cyclic shift is followed successively by
0ns, -400ns, Article 2 receives link advanced 8 samples compared with previous item, if the starting point for setting L-LTF corresponds to nsA sample
This, then be expressed as long training sequence is related to the sample sequence progress received at this time:
In above formula, rkWhat is indicated is the sample sequence that kth receives link receives,For long training sequence
Complex conjugate；
Decision metric at this time are as follows:
M2(n)=| Ck(n)|2+|Ck(n-8)|2
The then starting point estimation of OFDM symbol are as follows:
Further, the specific implementation of step 4 is as follows,
Smart carrier wave frequency deviation algorithm does related, use to signal is received using the long training sequence in frame structure
The HT-Mixed frame format of 802.11n WiFi signal, selection is L-LTF sequence in frame format, and L-LTF is by protection interval
It is formed with two sections long training sequence；
Wherein Λ is correlation function, r* (nL+ k) it is complete long training part, r (n) is to receive signal, nLFor long training
Sequence symbol starting point, TFFTFor the length of long training sequence, N is sub-carrier number, and k is the rope of corresponding long training symbol subcarrier
Draw value, small several times Nonlinear Transformation in Frequency Offset Estimation value is εf；
Therefore obtain smart frequency offset estimation range are as follows:
The range of smart Nonlinear Transformation in Frequency Offset Estimation is up to the half of subcarrier spacing.
Further, the specific implementation of step 5 is as follows,
Assuming that the sequence sent is matrix X, the signal received is matrix Y, and channel matrix H, interchannel noise Z are to meet
E { z (k) }=0,So whole system model is expressed as follows:
Y=XH+Z
The then cost function of channel matrix are as follows:
J (H)=| | Y-XH | |2=YHY-YHXH-HHXHY+HHXHXH
In above formula, Y, X are respectively to receive and dispatch end signal；
Above formula is sought into local derviation, and its value is enabled to be equal to 0, it may be assumed that
X can be obtainedHY=XHXH, further abbreviation can obtain:
H=(XHX)-1XHY=X-1Y
The Expectation-based Representation for Concepts of H is as follows:
E (H)=E (X-1Y)=E (X-1(XH+Z))=E (H+X-1Z)=E (H)
The wherein cost function of J (H) channel matrix, Y, X are respectively to receive and dispatch end signal, and H's is desired for E (H)；If Hp(k)
For the channel value of the pilot sub-carrier k estimated, L is pilot number in an OFDM symbol, first to the channel estimated at pilot tone
ValueIDFT is taken, h is obtainedp(k)；Then in time domain by by hp(k) mode of zero padding obtains time-domain signal and isWherein the number of zero padding is that sub-carrier number N subtracts pilot number；It is finally rightDo N point DFT transform
Obtain channel estimation value
The mean square error of LS channel estimation indicates as follows:
Wherein, δz, δxThe respectively variance of signal X and noise Z.
Further, the specific implementation of step 6 is as follows,
The STBC scheme that transmitting terminal is proposed using Alamouti leads to the data transmitted on two antennas of transmitting terminal
It crosses orthogonal coding to send later, encoder matrix indicates are as follows:
In above formula, s0,s1--- modulate latter two continuous symbol；
(·)*--- complex conjugate operation；
By encoder matrix it is found that in first time interval T, s0, s1It is sent simultaneously by antenna 1,2 respectively；At second
Between be spaced T ,-s* 1, s* 0, sent simultaneously by antenna 1,2 respectively；There was only 2 transmitting signals so on receiving end antenna, also
Noise exists, since Space-Time Block Coding uses orthogonal coding, so maximum likelihood decoding algorithm, which can be used, in receiving end carries out original
The recovery of beginning data, for receiving end in moment t and t+T, the signal received is as follows:
r0=r (t)=h0s0+h1s1+n0
In above formula, h0(t),h1(t) --- the channel between dual-mode antenna；
n0,n1--- the noise received；
The signal received is subjected to linear combining, obtains following expression formula:
Further abbreviation can obtain:
In above formula,--- first, the equivalent white Gaussian noise in second symbol period；
It is obtained according to the further abbreviation of the decision rule of maximum-likelihood criterion:
In above formula, s is the set of the constellation point in the modulation constellation of M system, hi,jIndicate i-th transmitting antenna and jth
Channel between root receiving antenna.
Further, reference signal is modified according to analysis of fuzzy functions in step 7, realizes the submaximum of ambiguity function
Inhibit as follows with the specific implementation of Sidelobe Suppression,
Submaximum is divided into two classes by the position and amplitude that submaximum occurs in ambiguity function, and a kind of submaximum is located at symmetrical near main peak
Place, and two width peak amplitudes and main peak amplitude are close, are caused by leading symbol；Another kind of submaximum amplitude is obviously less than normal with respect to main peak,
Distribution relatively dissipates, and is caused by cyclic prefix；
For the first kind, using the method for rejecting leading symbol, the submaximum to generate to leading symbol inhibits；Needle
To the second class, submaximum caused by cyclic prefix is removed using the method for cyclic prefix zero setting, and carry out at invalid subcarrier simultaneously
Manage suppressed sidelobes.
Compared with prior art, the obtained reference signal purity is high of the present invention, advantage is especially prominent, to promotion radar system
System detection performance has positive effect, very significant for external illuminators-based radar practical application.
Detailed description of the invention
Fig. 1: being the mimo system structure chart of the embodiment of the present invention；
Fig. 2: being the method flow diagram of the embodiment of the present invention；
Fig. 3: being the specific algorithm flow chart of frame synchronization；
Fig. 4: being channel estimation methods；
Fig. 5: being transmitting end signal product process；
Fig. 6: being emitter Signals ambiguity function figure；
Fig. 7: being long training sequence time delay auto-correlation；
Fig. 8: being short training sequence time delay auto-correlation；
Fig. 9: being the signal ambiguity functional arrangement after processing is leading；
Figure 10: being the leading ambiguity function figure with after cyclic prefix of processing；
Figure 11: being the signal ambiguity functional arrangement handled after leading and cyclic prefix and invalid subcarrier.
Specific embodiment
Understand for the ease of those of ordinary skill in the art and implement the present invention, with reference to the accompanying drawings and embodiments to this hair
It is bright to be described in further detail, it should be understood that implementation example described herein is merely to illustrate and explain the present invention, not
For limiting the present invention.
Fig. 1 and Fig. 2 is the MIMO structure chart and method flow diagram that the present invention is implemented, and signal is IEEE in the embodiment of the present invention
Signal under 802.11n consensus standard OFDM mode, modulation system 16-QAM, subcarrier spacing 312.5KHz, center frequency
Rate is 2.4GHz.The present invention provides a kind of suitable for 802.11n external illuminators-based radar reference signal reconstructing method, including following step
It is rapid:
Step 1: roughly the signal in signal stream being detected by frame synchronization, finds out the rough start bit of signal frame
It sets.Using 802.11n it is leading in 10 duplicate short training sequences, pass through the related frame synchronization algorithm that is delayed and carry out frame synchronization
The detection of starting point judges, decision metric according to the variation of the superposition value of received correlation on decision metric, that is, every antenna
The position that peak value occurs is exactly the starting point of frame, as shown in Figure 3.
For the sample sequence r (n) of reception signal, it is calculated by the following formula to obtain the starting point of frame in step 1:
Wherein, k indicates receiving antenna quantity, takes 2 herein；L is the related accumulated value of short training sequence delay, takes 5；N is indicated
The index value of subcarrier, range 0-127；D is the length of sliding window, consistent with short training sequence length value, takes 32；Ck
It (n) is delay auto-correlation function, PkIt (n) is the energy of the signal received, cor (n) is the decision metric of single antenna, and Tr is
Transmitting antenna number, generally taking 2, M is the decision metric of more antennas, when decision metric is rapidly increased to peak value, what peak value occurred
Position is exactly the starting point of frame, the specific algorithm flow chart of frame synchronization such as Fig. 3；
Step 2: coarse frequency offset is carried out to the signal in step 1 using data auxiliary law, present in correction signal compared with
Big frequency deviation.
If m-th of OFDM symbol n " ' sample of transmission signal on i-th transmitting terminal antenna is si(m, n), then corresponding
The symbol received can indicate are as follows:
After mimo channel, the signal received on jth root receiving antenna can be indicated are as follows:
In transmitting terminal, short training sequence generally without difference, only exists the difference in phase, then it is relevant to be delayed
Value can be expressed as:
Since mimo system uses more receiving antennas, then on the basis of obtaining the value of single received antenna, also
It needs to be summed to obtain final judgment variables.The frequency offseting value so obtained are as follows:
Wherein, yi(m, n) is the symbol sampler value received, fcFor the carrier frequency of transmission, TsFor sampling interval, TxFor
Number of transmission antennas, w are additive white Gaussian noise, rj(m, n) is signal on jth root receiving antenna, zjFor the relevant value that is delayed, L
For the related accumulated value of short training sequence delay, D is short training sequence length value, fΔTo receive carrier wave and sending the frequency between carrier wave
Difference, frequency offseting value areIt is indicated for the angle of phase difference, range is (- π, π).
Therefore obtained coarse frequency offset range are as follows:
Step 3: the initial position of OFDM symbol is found out using sign synchronization algorithm, using the synchronous related algorithm that is delayed, benefit
It is carried out with long training sequence to received sample sequence related, the starting point of OFDM symbol is estimated according to decision metric.
It is the same with frame synchronization, since the signal received is the sum of the signal on more transmitting antennas, and wave in order to prevent
Beam formation introduces cyclic shift and distinguishes to the signal on different transmitting antennas, so the symbol of single-input single-output SISO system
Synchronized algorithm can not be applied to mimo system, it is necessary to be correspondingly improved in conjunction with the characteristics of mimo system.The present invention is main
Two two receipts systems of hair being directed under the 40MHz mode in 802.11n consensus standard, the length of cyclic shift are followed successively by
0ns, -400ns, Article 2 receives link advanced 8 samples compared with previous item, if the starting point for setting L-LTF corresponds to nsA sample
This, then can indicate long training sequence is related to the sample sequence progress received at this time are as follows:
In above formula, rkWhat is indicated is the sample sequence that kth receives link receives,For long training sequence
Complex conjugate.
Decision metric at this time are as follows:
M2(n)=| Ck(n)|2+|Ck(n-8)|2
Then the starting point of OFDM symbol can be estimated are as follows:
Step 4: smart offset estimation.After integer-time carrier wave frequency deviation is synchronous, since algorithm for estimating precision is limited, signal is still
There are residual frequency deviations, it is also necessary to be estimated using smart carrier wave frequency deviation remaining carrier wave frequency deviation.Remaining carrier wave frequency deviation is not only
It will lead to the amplitude distortion of carrier frequency component, can also cause the distortion of phase.If do not corrected to it, will have a direct impact on
To decoded correctness.Smart Algorithm of Carrier Frequency Offset Estimation does phase with signal is received using the long training sequence in frame structure
It closes, for the present invention using the HT-Mixed frame format of 802.11n WiFi signal, selection is L-LTF sequence in frame format
Column, L-LTF are made of protection interval and two sections of long training sequences,
Wherein Λ is correlation function, r*(nL+ k) it is complete long training part, r (n) is to receive signal, nLFor long training
Sequence symbol starting point, TFFTFor the length of long training sequence, N is sub-carrier number, and k is the rope of corresponding long training symbol subcarrier
Draw value, small several times Nonlinear Transformation in Frequency Offset Estimation value is εf；
Therefore obtain smart frequency offset estimation range are as follows:
The range of smart Nonlinear Transformation in Frequency Offset Estimation is up to the half of subcarrier spacing.
Step 5: using the channel estimation methods based on pilot tone, estimate the channel value at pilot tone first, recycle interpolation
Algorithm calculates the channel value of other subcarriers.
Assuming that the sequence sent is matrix X, the signal received is matrix Y, and channel matrix H, interchannel noise Z are to meet
E { z (k) }=0,So whole system model can be expressed as follows:
Y=XH+Z
The then cost function of channel matrix are as follows:
J (H)=| | Y-XH | |2=YHY-YHXH-HHXHY+HHXHXH
In above formula, Y, X are respectively to receive and dispatch end signal.
Above formula is sought into local derviation, and its value is enabled to be equal to 0, it may be assumed that
X can be obtainedHY=XHXH, further abbreviation can obtain:
H=(XHX)-1XHY=X-1Y
The expectation of H can be expressed as follows:
E (H)=E (X-1Y)=E (X-1(XH+Z))=E (H+X-1Z)=E (H)
The wherein cost function of J (H) channel matrix, Y, X are respectively to receive and dispatch end signal, and H's is desired for E (H), specific channel
Estimation method is shown in Fig. 4, H in Fig. 4pIt (k) is the channel value of the pilot sub-carrier k estimated, L is pilot tone in an OFDM symbol
Number.The algorithm is first to the channel value estimated at pilot toneIDFT is taken, h is obtainedp(k)；Then in time domain by by hp
(k) mode of zero padding obtains time-domain signal and isWherein the number of zero padding is that sub-carrier number N subtracts pilot number；Most
It is right afterwardsIt does N point DFT transform and obtains channel estimation value
The mean square error of LS channel estimation indicates as follows:
Wherein, δz, δxThe respectively variance of signal X and noise Z.
Step 6: carrying out space-time decoding after channel estimation, the extensive of original data stream is carried out using maximum likelihood decoding algorithm
It is multiple；
The STBC scheme that transmitting terminal is proposed using Alamouti leads to the data transmitted on two antennas of transmitting terminal
Orthogonal coding is crossed to send later.Its encoder matrix can indicate are as follows:
In above formula, s0,s1--- modulate latter two continuous symbol
(·)*--- complex conjugate operation
By encoder matrix it is found that in first time interval T, s0, s1It is sent simultaneously by antenna 1,2 respectively；At second
Between be spaced T ,-s* 1, s* 0, sent simultaneously by antenna 1,2 respectively.There was only 2 transmitting signals so on receiving end antenna, also
Noise exists.Since Space-Time Block Coding uses orthogonal coding, so maximum likelihood decoding algorithm, which can be used, in receiving end carries out original
The recovery of beginning data.For receiving end in moment t and t+T, the signal received is as follows:
r0=r (t)=h0s0+h1s1+n0
In above formula, h0(t),h1(t) --- the channel between dual-mode antenna
n0,n1--- the noise received
The signal received is subjected to linear combining, obtains following expression formula:
Further abbreviation can obtain:
In above formula,--- first, the equivalent white Gaussian noise in second symbol period.
It is available according to the further abbreviation of the decision rule of maximum-likelihood criterion:
In above formula, s is the set of the constellation point in the modulation constellation of M system, hi,jIndicate i-th transmitting antenna and jth
Channel between root receiving antenna.
Step 7: from the output of the space-time decoding of step 6 can be obtained the recovery of original data stream to get to correct bit
Stream.According still further to transmitting signal generation step carry out re-modulation, generate original transmitting signal, to the transmitting signal regenerated into
Reference signal after then reconstruct after row amendment, according to 802.11n standard, transmitting signal regeneration at/re-modulation process such as
It is specific as follows shown in Fig. 5:
(1) Scrambler is scrambled: data will be transmitted with transmission data progress exclusive or by scramble sequence and upset,
Long sequence 0 or 1 can thus be avoided.The formula that scramble sequence generates is as follows:
S (x)=x7+x4+1
(2) Encoder Parser encoder shunts: the data after scrambling being split processing, because of LDPC encoder
Number does not exceed 1, but the number of BBC encoder can be 1 or 2.
(3) FEC Encoder is encoded: function is to encode the data after encoder shunts.
(4) Stream Parser spatial flow shunts: the code word after coding is assigned to each space in a fixed order
Stream.The number that spatial flow shunts the inlet flow generated is not fixation, with the change of constellation point mapping mode, inlet flow
Number is also changing correspondingly.
(5) Interleaver interweaves: data meeting burst error in transmission process allows the decoder of receiving end decoding
When there are great difficulties.Being interleaved processing after coding can avoid this type of circumstances from happening.Interleaving treatment is encoded in LDPC
When do not use, only BCC coding when use.Change the sequence of bit script to prevent the adjacent long sequence for there are noise bits
Into BCC decoder, the influence to decoding module is reduced.
(6) Constellation mapper planisphere maps: the bit sequence in each spatial flow is mapped to constellation
Point (plural number) is namely modulated input data.
(7) STBC (Space-time block code Space Time Coding) module: when spatial flow is less than empty time stream,
It is encoded using STBC.STBC is by way of coding by NssSpatial flow is extended to NstsEmpty time stream.
(8) it CSD (Cyclic Shift Diversity cyclic shift diversity): prevents from forming extra interference wave beam.
(9) Spatial Mapping space reflection: function is that empty time stream is mapped as chain.There are following three kinds of mappings
Mode:
Directly mapping (direct mapping): being that the constellation point of each empty time stream is mapped directly to transmitting chain road,
Middle data flow number is equal to the quantity of transmitting antenna, and mapping matrix is unit matrix, that is, data are to map one by one to antenna.
Spatial spread (spatial expansion): when spatial flow is less than or equal to antenna amount, pass through matrix multiplication
All constellation point vectors from empty time stream are extended by operation, to generate the input of all transmitting chains.
Wave beam forming (beamforming): isospace extension is the same, when will come from every sky by multiplication of matrices operation
The constellation point vector of stream is multiplied by the matrix being made of dominant vector (steeringvectors), to generate the defeated of all transmitting chains
Enter.
(10) one group of constellation point inverse discrete fourier transform (IDFT): is transformed into time domain.
(11) Insert GI And Window is inserted into protection interval GI and adding window: GI uses last four points of OFDM symbol
One of data symbol.Adding window is to enhance logical attenuation outside a channel.
802.11n signal is produced according to above step, but the signal also needs to continue amendment to meet as external sort algorithm
The needs of radar detection signal.This is because external illuminators-based radar obtains mesh by reference to signal is to monitoring signals progress related
Mark information.Two signals carry out obtaining cross ambiguity function after cross-correlation, from the position that cross ambiguity function peak value occurs, Ke Yiti
Take out target range and speed parameter.
Cross ambiguity function figure by the generated signal of Fig. 5 method is Fig. 6, can be found by Fig. 6, ambiguity function in addition to main peak with
Outside, there are some submaximums according further to certain rule, the presence of submaximum will affect the judgement to target position, when submaximum peak value
Crossing conference allows main peak to be submerged, therefore to be accurately detected target, it is necessary to pre-process to direct wave, that is, by mould
The submaximum occurred in paste function is inhibited, to reduce submaximum to influence caused by target acquisition.
Submaximum can be divided into two classes by the position and amplitude occurred according to submaximum in Fig. 6, and a kind of submaximum is located at right near main peak
At title, and two width peak amplitudes and main peak amplitude com parison are close, the distance spectrum as shown in Fig. 6 (b)；Another kind of submaximum amplitude is relatively main
Peak is obviously less than normal, and distribution relatively dissipates, the Doppler domain in the range Doppler spectrogram as shown in Fig. 6 (a).
First kind submaximum: the analysis of submaximum caused by leading symbol and inhibition
Some in 802.11n signal frame format is leading symbol, leading by taking HT-Mixed frame format therein as an example
Symbol is made of L-STF, L-LTF, HT-STF, HT-LTF and immobilizes in the appearance position of every frame, then identical symbol
Carrying out auto-correlation just will appear peak value.
For long training sequence, it is divided into two parts L-LTF and HT-LTF.For L-LTF, it is identical by two
Long symbol composition, duration of each symbol is 3.2us, is done after auto-correlation as shown in Fig. 7 (a) to L-LTF, L-LTF phase
Occur two peak values after closing, the position that two peak values occur is respectively the 1st point and the 128th point, and two peak intervals are
128 sampled points.For HT-LTF, the empty time stream number of this paper studies is 2, then HT-LTF is by the long training sequence of 2 data fields
Column composition, and the duration of each symbol is 3.2us, such as Fig. 7 (b) is shown after doing auto-correlation to HT-LTF, and HT-LTF is related
Also occur two peak values later, the position that two peak values occur is divided into and the 128th point at other 1st point, and two peak intervals are also
128 sampled points.
Similarly, for containing L-STF and HT-STF for short training sequence.Wherein L-STF is by 10 short set of symbols
At each symbol is identical and the duration is 0.8us, and such as Fig. 8 (a) is shown after doing auto-correlation to L-STF, goes out after related
Ten peak values are showed, the position that peak value occurs is respectively the 1st point, the 33rd point, the 65th point, the 97th point, the 129th
Point, the 161st point, the 193rd point, the 225th point, the 257th point, the 289th point, and 32 are spaced between two neighboring peak value
A sampled point.For HT-STF, protection interval is removed there are also 4 short training sequences, each sequence time duration is
0.8us, such as Fig. 8 (b) is shown after doing auto-correlation to HT-STF, occurs four peak values after related, and the position that peak value occurs is divided
Not Wei the 1st point, the 33rd point, the 65th point, the 97th point, and between two neighboring peak value between be divided into 32 sampled points.
Based on the above analysis, it is leading in can cause submaximum after each sequence auto-correlation.In order to eliminate caused by leading symbol
Submaximum seeks to remove the correlation between leading symbol, and the present invention uses the method for rejecting leading symbol, pair
The submaximum that leading symbol generates is inhibited.802.11n ambiguity function such as Fig. 9 is obtained after rejecting to leading symbol.Fig. 9 (a)
Doppler domain in range-Doppler spectrum has been suppressed, Fig. 9 (b) is in spectrogram without the submaximum of lesser regularity
Main peak both sides also without lesser submaximum, have also been suppressed.Comparison diagram 6 and Fig. 9 can see, by leading after leading rejecting
Caused submaximum has obtained good inhibition, i.e. amplitude is not present with respect to the submaximum of main peak very little in Fig. 7.But at this time
Submaximum is still had in 802.11n signal ambiguity function, and remaining submaximum ratio is stronger by leading caused submaximum, this mainly by
Cyclic prefix causes.
Second class submaximum: the analysis of submaximum caused by cyclic prefix and inhibition
OFDM symbol live part is after the processing such as encode, interweave, and data have randomness, the correlation between the part
It is not have submaximum to occur.But cyclic prefix is made of the tail portion Forward in valid data domain, therefore the part number
It can cause corresponding submaximum according to correlation is carried out.As seen from Figure 9, in main peak both sides, there are apparent submaximums, and appear in
It is at 32 from main peak distance unit.Because it is submaximum at this at 32 that the position that cyclic prefix occurs, which is also in distance unit,
It is largely caused by the cyclic prefix of insertion.
It is also required to remove this its correlation to eliminate submaximum caused by cyclic prefix, the present invention is used and is identical to
The method for inhibiting the submaximum as caused by leading symbol, is herein handled cyclic prefix.It will be leading in 802.11n signal
The results are shown in Figure 10 after symbol, circulation prefix processing, and submaximum has been suppressed in Figure 10 (a) distance-Doppler spectrogram, Figure 10
(b) submaximum in spectrogram has also been suppressed.
The submaximum that comparison diagram 8 and Figure 10 can be seen that at distance unit is 32 has disappeared, that is to say, that by that will recycle
The method of prefix zero setting removes submaximum caused by cyclic prefix.Simultaneously it can also be seen that by by leading rejecting, cyclic prefix
Drawing pin shape is presented in zero setting treated 802.11n ambiguity function, only in τ=0, fdThere are a main peak, remaining submaximum at=0
It is suppressed, but secondary lobe at this time is still relatively high.
To find out its cause, secondary lobe occur due to invalid subcarrier presence.802.11n using HT-Mixed frame lattice
Formula, 40MHz bandwidth, effective data subcarrier number are 108, obtain ambiguity function after invalid subcarrier is handled
Such as Figure 11, it can find that submaximum and secondary lobe have all been suppressed in Figure 11 (a) distance-Doppler spectrogram, Figure 11 (b) is in spectrogram
Submaximum and secondary lobe can find also all to be suppressed.It is found that drawing pin type is presented in ambiguity function at this time, satisfaction is made by comparison diagram 10 and Figure 11
For the requirement of external illuminators-based radar signal source.
Ideal thumbtack ambiguity function has been obtained after side peaks suppression and secondary lobe processing, it is general to greatly reduce false-alarm
Rate, so that the performance of target acquisition has very big improvement.
The above-mentioned description for preferred embodiment is more detailed, therefore can not be considered to protect model to the invention patent
The limitation enclosed, those skilled in the art under the inspiration of the present invention, are not departing from what the claims in the present invention were protected
Under ambit, replacement or deformation can also be made, is fallen within the scope of protection of the present invention, of the invention is claimed model
Enclosing should be determined by the appended claims.
Claims (8)
1. a kind of reference signal reconstructing method suitable for MIMO/OFDM external illuminators-based radar, which is characterized in that including following step
It is rapid:
Step 1, roughly the reference signal in signal stream is detected by frame synchronization, finds out the rough start bit of signal frame
Set, specifically: using direct-path signal it is leading in 10 duplicate short training sequences, pass through the related frame synchronization algorithm that is delayed
The detection for carrying out frame starting point, judges according to the variation of the superposition value of received correlation on decision metric, that is, every antenna, sentences
The position that certainly measurement peak value occurs is exactly the starting point of frame；
Step 2, coarse frequency offset is carried out to the signal in step 1 using data auxiliary law, it is biggish present in correction signal
Frequency deviation；
Step 3, the initial position that OFDM symbol is found out using sign synchronization algorithm utilizes length using the synchronous related algorithm that is delayed
Training sequence is related to the progress of received sample sequence, and the starting point of OFDM symbol is estimated according to decision metric；
Step 4, correlation is carried out using the long training sequence in frame structure, fractional part of frequency offset present in system is estimated；
Step 5, using the channel estimation methods based on pilot tone, the channel value at pilot tone is estimated first, recycles interpolation algorithm
Calculate the channel value of other subcarriers；
Step 6, space-time decoding is carried out, the recovery of original data stream is carried out using maximum likelihood decoding algorithm；
Step 7, the correct bit stream according to obtained in step 6 generates reference signal according still further to transmitting signal generation step, and
After reference signal generation, according to analysis of fuzzy functions, reference signal is modified, realize the side peaks suppression of ambiguity function with
Sidelobe Suppression.
2. a kind of reference signal reconstructing method suitable for MIMO/OFDM external illuminators-based radar according to claim 1,
Be characterized in that: the specific implementation of step 1 is as follows,
For the sample sequence r (n) of reception signal, it is calculated by the following formula to obtain the starting point of frame in step 1,
Wherein, k indicates receiving antenna quantity；L is the related accumulated value of short training sequence delay；The index value of n expression subcarrier；D
It is consistent with short training sequence length value for the length of sliding window；CkIt (n) is delay auto-correlation function, Pk(n) it receives
The energy of signal, cor (n) they are the decision metric of single antenna, and Tr is transmitting antenna number, and M is the decision metric of more antennas, when
Decision metric is rapidly increased to peak value, and the position that peak value occurs is exactly the starting point of frame.
3. a kind of reference signal reconstructing method suitable for MIMO/OFDM external illuminators-based radar according to claim 2,
Be characterized in that: the specific implementation in step 2 is as follows,
If m-th of OFDM symbol n " ' sample of transmission signal on i-th transmitting terminal antenna is si(m, n), then corresponding receive
To symbol be expressed as:
After mimo channel, the signal received on jth root receiving antenna is indicated are as follows:
In transmitting terminal, short training sequence generally without difference, only exists the difference in phase, then the relevant value table that is delayed
It is shown as follows:
Since mimo system uses more receiving antennas, then on the basis of obtaining the value of single received antenna, it is also necessary to
It is summed to obtain final judgment variables, then obtained frequency offseting value are as follows:
Wherein, yi(m, n) is the symbol sampler value received, fcFor the carrier frequency of transmission, TsFor sampling interval, TxTo emit day
Line number amount, w are additive white Gaussian noise, rj(m, n) is signal on jth root receiving antenna, zjFor the relevant value that is delayed, L is short training
Practice the related accumulated value of sequence delay, D is short training sequence length value, fΔTo receive carrier wave and sending the frequency difference between carrier wave, frequency
Rate deviant isAngle (*) is that the angle of phase difference indicates, range is (- π, π)；
Therefore obtained coarse frequency offset range are as follows:
4. a kind of reference signal reconstructing method suitable for MIMO/OFDM external illuminators-based radar according to claim 3,
Be characterized in that: the specific implementation of step 3 is as follows,
Two hairs two under 40MHz mode in 802.11n consensus standard receive system, and the length of cyclic shift is followed successively by 0ns ,-
400ns, Article 2 receives link advanced 8 samples compared with previous item, if the starting point for setting L-LTF corresponds to nsA sample, then
It is expressed as at this time by long training sequence is related to the sample sequence progress received:
In above formula, rkWhat is indicated is the sample sequence that kth receives link receives,For the multiple total of long training sequence
Yoke；
Decision metric at this time are as follows:
M2(n)=| Ck(n)|2+|Ck(n-8)|2
The then starting point estimation of OFDM symbol are as follows:
5. a kind of reference signal reconstructing method suitable for MIMO/OFDM external illuminators-based radar according to claim 4,
Be characterized in that: the specific implementation of step 4 is as follows,
Smart Algorithm of Carrier Frequency Offset Estimation does related, use to signal is received using the long training sequence in frame structure
The HT-Mixed frame format of 802.11n WiFi signal, selection is L-LTF sequence in frame format, and L-LTF is by protection interval
It is formed with two sections long training sequence；
Wherein Λ is correlation function, r*(nL+ k) it is complete long training part, r (n) is to receive signal, nLFor long training sequence
Symbol initial point, TFFTFor the length of long training sequence, N is sub-carrier number, and k is the index value of corresponding long training symbol subcarrier,
Small several times Nonlinear Transformation in Frequency Offset Estimation value is εf；
Therefore obtain smart frequency offset estimation range are as follows:
The range of smart Nonlinear Transformation in Frequency Offset Estimation is up to the half of subcarrier spacing.
6. a kind of reference signal reconstructing method suitable for MIMO/OFDM external illuminators-based radar according to claim 5,
Be characterized in that: the specific implementation of step 5 is as follows,
Assuming that the sequence sent is matrix X, the signal received is matrix Y, and channel matrix H, interchannel noise Z are to meet E { z
(k) }=0,So whole system model is expressed as follows:
Y=XH+Z
The then cost function of channel matrix are as follows:
J (H)=| | Y-XH | |2=YHY-YHXH-HHXHY+HHXHXH
In above formula, Y, X are respectively to receive and dispatch end signal；
Above formula is sought into local derviation, and its value is enabled to be equal to 0, it may be assumed that
X can be obtainedHY=XHXH, further abbreviation can obtain:
H=(XHX)-1XHY=X-1Y
The Expectation-based Representation for Concepts of H is as follows:
E (H)=E (X-1Y)=E (X-1(XH+Z))=E (H+X-1Z)=E (H)
The wherein cost function of J (H) channel matrix, Y, X are respectively to receive and dispatch end signal, and H's is desired for E (H)；If HpIt (k) is estimation
The channel value of pilot sub-carrier k out, L is pilot number in an OFDM symbol, first to the channel value estimated at pilot toneIDFT is taken, h is obtainedp(k)；Then in time domain by by hp(k) mode of zero padding obtains time-domain signal and isWherein the number of zero padding is that sub-carrier number N subtracts pilot number；It is finally rightDo N point DFT transform
Obtain channel estimation value
The mean square error of LS channel estimation indicates as follows:
Wherein, δz, δxThe respectively variance of signal X and noise Z.
7. a kind of reference signal reconstructing method suitable for MIMO/OFDM external illuminators-based radar according to claim 6,
Be characterized in that: the specific implementation of step 6 is as follows,
The STBC scheme that transmitting terminal is proposed using Alamouti, by the data transmitted on two antennas of transmitting terminal by just
Coding is handed over to send later, encoder matrix indicates are as follows:
In above formula, s0,s1--- modulate latter two continuous symbol；
(·)*--- complex conjugate operation；
By encoder matrix it is found that in first time interval T, s0, s1It is sent simultaneously by antenna 1,2 respectively；Between second time
Every T ,-s* 1, s* 0, sent simultaneously by antenna 1,2 respectively；There was only 2 transmitting signals so on receiving end antenna, there are also noises
In the presence of since Space-Time Block Coding uses orthogonal coding, so maximum likelihood decoding algorithm, which can be used, in receiving end carries out original number
According to recovery, for receiving end in moment t and t+T, the signal received is as follows:
r0=r (t)=h0s0+h1s1+n0
In above formula, h0(t),h1(t) --- the channel between dual-mode antenna；
n0,n1--- the noise received；
The signal received is subjected to linear combining, obtains following expression formula:
Further abbreviation can obtain:
In above formula,--- first, the equivalent white Gaussian noise in second symbol period；
It is obtained according to the further abbreviation of the decision rule of maximum-likelihood criterion:
In above formula, s is the set of the constellation point in the modulation constellation of M system, hi,jIndicate i-th transmitting antenna and jth piece-root grafting
Receive the channel between antenna.
8. a kind of reference signal reconstructing method suitable for MIMO/OFDM external illuminators-based radar according to claim 1,
Be characterized in that: according to analysis of fuzzy functions in step 7, reference signal being modified, realize the side peaks suppression of ambiguity function with
The specific implementation of Sidelobe Suppression is as follows,
Submaximum is divided into two classes by submaximum occurs in ambiguity function position and amplitude, and a kind of submaximum is located at main peak and nearby symmetrically locates,
And two width peak amplitude and main peak amplitude it is close, caused by leading symbol；Another kind of submaximum amplitude is obviously less than normal with respect to main peak, distribution
It relatively dissipates, is caused by cyclic prefix；
For the first kind, using the method for rejecting leading symbol, the submaximum to generate to leading symbol inhibits；For
Two classes remove submaximum caused by cyclic prefix using the method for cyclic prefix zero setting, and carry out invalid subcarrier processing suppression simultaneously
Secondary lobe processed.
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| CN113281732A (en) * | 2021-05-27 | 2021-08-20 | 华中科技大学 | MIMO radar target positioning method and system based on space-time coding |
| CN114205199A (en) * | 2021-11-30 | 2022-03-18 | 成都中科合迅科技有限公司 | Method for identifying WIFI signal in complex electromagnetic environment |
| CN114205199B (en) * | 2021-11-30 | 2023-10-20 | 成都中科合迅科技有限公司 | WIFI signal identification method in complex electromagnetic environment |
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