CN103905355B - A kind of virtual time reversal underwater sound OFDM channel equalization methods - Google Patents
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Abstract
The present invention relates to a kind of virtual time reversal underwater sound OFDM channel equalization methods, it is characterised in that：Step 1：The detectable signal estimated for signal is added in transmitting terminal transmission signal；Step 2：Receiving terminal completes synchronizing process, extracts the detectable signal for receiving；According to the detectable signal for receiving, channel impulse response is estimated；Step 3：According to the channel impulse response estimated, anti-OFDM channel equalizations when completing virtual；Step 4：To the signal serioparallel exchange after equilibrium, Cyclic Prefix and cyclic suffix are gone, complete demodulating process.
Description
Technical field
The present invention relates to a kind of virtual time reversal underwater sound OFDM channel equalization methods.
Background technology
The features such as there is underwater acoustic channel serious limited bandwidth, multi-path jamming, space-variant, time-varying, frequency to become, particularly with shallow sea water
Acoustic channel, is influenceed by sea-floor relief and velocity of sound distribution, and channel is extremely complex, and the time delay of way more than channel often reaches millis up to a hundred
Second, serious interference is brought to underwater sound communication.OFDM（OFDM）With band efficiency is high, resistance to frequency selective
The advantages of decline, simple equilibrium, it is widely used in Short Range High Speed underwater sound communication, but in order to overcome many way time delays of length to bring
Intersymbol interference (ISI) and inter-carrier interference (ICI), it is necessary to add during more than channel most most way before the circulation of elongatedness
Sew, and insert closeer pilot tone and addition Error Correction of Coding etc., this will cause the degradation of OFDM band efficiencies, therefore,
How to reduce interference of the multi_path channel to OFDM turns into one of key issue of high-speed underwater sound communication.
Channel equalization technique is the effective ways for overcoming the interference brought on the way more than channel, conventional OFDM channel equalization methods
It is the frequency domain channel equalization algorithm based on pilot tone, inserts known sequence by time domain or frequency domain, and estimate in receiving terminal
Channel frequency response.Existing Channel Equalization Algorithm, such as least square (LS), least mean-square error (MMSE), are required to add
Enter and overcome ISI and ICI more than the Cyclic Prefix of the most most way time delay of channel, and add closeer pilot tone to estimate channel, serious limit
The traffic rate of OFDM is made.
The content of the invention
Present invention aim at a kind of virtual time reversal underwater sound OFDM channel equalization methods are provided, can effectively shorten letter
Road length, reduces many intersymbol interferences for bringing on the way, improves OFDM band efficiencies.
Realize the object of the invention technical scheme：
A kind of virtual time reversal underwater sound OFDM channel equalization methods, add more than the most most way of channel in OFDM symbol
When elongatedness Cyclic Prefix and cyclic suffix, it is characterised in that：
Step 1：The detectable signal estimated for signal is added in transmitting terminal transmission signal；
Step 2：Receiving terminal completes synchronizing process, extracts the detectable signal for receiving；According to the detectable signal for receiving, letter is estimated
Channel shock response；
Step 3：According to the channel impulse response estimated, anti-OFDM channel equalizations when completing virtual；
Step 4：To the signal serioparallel exchange after equilibrium, Cyclic Prefix and cyclic suffix are gone, complete demodulating process.
In step 1, from pseudo-random sequence by OFDM modulation after as detectable signal.
In step 2, according to the detectable signal for receiving, amplitude, time delay and the phase of channel are estimated using matching pursuit algorithm,
Estimate channel impulse response.
In step 3, the channel impulse response time reversal of estimation, with reception signal convolution, anti-OFDM letters when completing virtual
Trace equalization.
The device have the advantages that：
The detectable signal estimated for signal is added in transmitting terminal transmission signal of the present invention, receiving terminal is according to the detection for receiving
Signal, estimates channel impulse response, anti-OFDM channel equalizations when completing virtual；Using virtual time reversal Channel Equalization Algorithm,
Channel length can effectively be shortened, received signal to noise ratio is improved, reduce ISI and ICI that multi_path channel brings, improve OFDM frequency bands
Utilization rate, realizes adaptive channel equalizer.The present invention estimates amplitude, time delay and the phase of channel using matching pursuit algorithm, estimates
Meter high precision, can accurately estimate channel impulse response, for it is virtual when inverse channel accurate channel information is provided.
Brief description of the drawings
Fig. 1 is the general principle figure of VTRM technologies；
Fig. 2 is transmission signal frame assumption diagram；
Fig. 3 is that VTRM realizes block diagram for the receiver of OFDM channel equalizations；
Fig. 4 is the schematic diagram that Cyclic Prefix and cyclic suffix are added in OFDM symbol.
Specific embodiment
Step 1：According to Virtual time reversal mirror（VTRM）Principle, adds what is estimated for signal in transmitting terminal transmission signal
Detectable signal；
As shown in figure 1, the general principle of VTRM technologies is, before transmitting information signal, a detectable signal, root are first sent
Channel impulse response is estimated according to detectable signal, its time-reversal signal is then done into convolution with signal is received, when obtaining virtual
Signal after anti-.
According to VTRM principles, design transmission signal frame structure is as shown in Fig. 2 frame head is entered using linear frequency modulation (LFM) signal
Row frame timing synchronization, behind add pure-tone pulse (CW) signal to be used for the estimating Doppler factor, be used to eliminate in receiving terminal
The influence of Doppler shift, is afterwards detectable signal, is finally OFDM symbol for estimating channel impulse response.Each signal
Between all leave certain protection interval, and protection interval is more than the length of the most most way time delay of channel.
Step 2：Receiving terminal completes synchronizing process, extracts the detectable signal for receiving；According to receive detectable signal, using
Amplitude, time delay and the phase of channel are estimated with tracing algorithm, channel impulse response is estimated.
Step 3：According to the channel impulse response estimated, synchronization is re-started to the signal after convolution, it is anti-when completing virtual
OFDM channel equalizations；
Step 4：To the signal serioparallel exchange after equilibrium, Cyclic Prefix and cyclic suffix are gone, complete demodulating process.
During specific implementation, as shown in figure 3, the signal for receiving is extracted after synchronizing signal finds the position of signal starting
Go out CW signals, Doppler's compressibility factor obtained by frequency measurement, Variable sampling treatment is carried out to the signal for receiving according to Doppler factor,
Eliminate influence of the Doppler shift to signal.Then detectable signal is extracted from the signal after Variable sampling, estimates that channel impulse is rung
Should, its time reversal is done into convolution with signal is received, VTRM channel equalizations are completed, the signal serioparallel exchange after equilibrium goes circulation
Prefix and cyclic suffix, carry out FFT demodulation constellation inverse mappings, complete OFDM demodulation process.
Wherein, there is a key technology to affect VTRM for the performance of OFDM channel equalizations, i.e., should in OFDM symbol
Elongatedness during most most way of inverse channel when adding cyclic suffix, and cyclic suffix and circulating prefix-length to should be greater than.Circulation
Suffix will be before OFDM symbol block several data duplications to behind symbolic blocks, as shown in Figure 4.
If actual channel impulse response is h (t), the channel impulse response of estimation isDefine virtual time reversal channel
Impulse response isCan be considered the channel that signal is eventually passed through.Assuming that direct sound wave amplitude is maximum, length is
The discrete channel of L can be expressed as h=[h (0) h (1) ... h (L-1)], when inverse channel can be expressed as h '=[h ' (1-L) ... h '
(- 1) h ' (0) h ' (1) ... h ' (L-1)], its length is 2L-1, wherein, h ' (0) is the sound ray after the superposition of each path, amplitude
It is maximum, it can be seen that when inverse channel be a non-minimum phase channel, i.e., many ways point were still suffered from before the maximum sound ray of amplitude
Amount.If with the maximum sound ray of energy as initial time when synchronous, ISI that many way components after energy maximum sound ray cause and
ICI can be overcome by Cyclic Prefix, and ISI and ICI that many way components after energy maximum sound ray cause then need
Overcome by cyclic suffix.
Once how cyclic suffix overcomes ISI and ICI to lower surface analysis, if p-th current demodulated OFDM symbol is rp, it is right
The transmission signal answered is expressed as sp, a previous and latter OFDM symbol is expressed as sp-1、sp+1If not adding Cyclic Prefix and following
Ring suffix, after impulse response is for the channel of h, receiving signal can be expressed as
Wherein, rp、sp、sp-1、sp+1And npIt is N × 1 dimensional vector,WithIt is N × N-dimensional matrix, difference can
To be expressed as
Be can see from formula (1), Section 1It is desired signal, Section 2And Section 3For
The intersymbol interference that previous symbol and latter symbol bring, npIt is noise item.Wherein, distracterCan be by adding
Enter Cyclic Prefix to overcome, and distracterThen need to add cyclic suffix to overcome.If adding the circulation more than channel length L
Prefix and cyclic suffix, then receive signal remove circulation before and after sew after, can be expressed as
Wherein,It is circular matrix, can be expressed as
It can be seen that, remove after sewing before and after circulating, original sending signal is linear with channel impulse response in making time domain
Convolution is changed into circular convolution.From formula (5) as can be seen that input of the output of current sign block only with current sign block is relevant, with
Previous and latter symbolic blocks are unrelated, i.e., eliminate previous symbol and latter symbol by Cyclic Prefix and cyclic suffix
The ISI and many ICI for bringing on the way for bringing.
The key of VTRM channel equalizations is channel estimation, and the present invention is using match tracing (Matching Pursuit, MP)
Algorithm estimates channel impulse response, and compared with the method for conventional signal copy correlation estimation channel, estimated accuracy is high, and can be with
Channel phase information is estimated, the specific implementation process of MP algorithms is described below.
Consider the linear model that Sparse Problems are often used
y=Ax+v (7)
Wherein, x ∈ RMIt is sparse signal to be estimated, y ∈ RNIt is observation vector, v ∈ RNIt is Gaussian noise vector, A ∈ RN ×M, and N<M, A are represented by
A=[a1,a2,...,aM] (8)
Wherein, ai∈RN, i=1, commonly referred to as 2 ..., M, A are dictionary or atom, aiIt is the atom in dictionary.MP algorithms
Basic thought be in iterative process each time, the atom that is most matched with signal to be found from dictionary to build sparse bayesian learning,
Then signal residual error is obtained, and continues the atom for selecting and signal residual error is most matched in remaining atom, by successive ignition
Afterwards, by observation vector and the atom i.e. restructural sparse signal selected.MP algorithms do not require that the atom in dictionary is orthogonal, but
It is the norm of requirement two | | ai||2=1。
If the residual error after pth time iteration is rp, it is initialized as r0=y, the atom of the matching selected from dictionary isOften
The atom minimum with residual signals inner product in the secondary remaining atom of selection, i.e.,
Wherein, Ip-1∈{s1,s2,...,sp-1Be selected by preceding p-1 iteration matched atoms index set,
Pth time iterative estimate goes out the element of signal xCan be expressed as
Residual signals can be expressed as
Work as residual signals | | rp||2<During ε, iteration ends, ε is given residual error thresholding, is relevant with input signal-to-noise ratio
One amount.According to above-mentioned analysis, comprising the following steps that for MP algorithms is summarized：
1. initialize：Setting residual error thresholding ε, r0=y
2. the atom that selection is matched：
3. the component of signal estimated：
4. residual error：
5. pth time iteration, p>1
6. matched from remaining atom：
7. the component of signal of pth time iterative estimate：
8. the residual error of pth time iteration：
In order that estimating channel impulse response with MP algorithms, a sparse signal model should be constructed first, it is considered to detect
Signal x (n) is the channel of h (n) by channel impulse response, and receiving signal y (n) can be expressed as
Wherein,Convolution is represented, Fourier transformation is done simultaneously to formula (12) both sides, can be expressed as
Y=XH+V (13)
Wherein, Y and X are respectively the Fourier transformations of y (n) and x (n), and H is channel frequency response matrix, is channel impulse response
Fourier transformation, can be expressed as
Bring formula (14) into formula (13), can be expressed as
Wherein,It is the diagonal matrix being made up of X, h can be expressed as
h=[h(0),h(1),...,h(L)]T (16)
Wherein, []TTransposition is represented, F is Fourier transform matrix, can be expressed as
Be can see from derivation above, formula (15) meets the representation of sparse signal, Y can be expressed as observing square
Battle array,Dictionary can be expressed as, be complex matrix because X and Y are transmitting and the frequency domain representation for receiving detectable signal, thus it is logical
Cross complex gain and time delay that MP algorithms are estimated that channel.Now detectable signal should select have good auto-correlation in frequency domain
Property signal, the present invention selection one pseudo-random sequence by OFDM modulation after as detectable signal.
Claims (1)
1. a kind of virtual time reversal underwater sound OFDM channel equalization methods, it is characterised in that：
Step 1：According to Virtual time reversal mirror (VTRM) principle, the detection estimated for signal is added in transmitting terminal transmission signal
Signal；
Before transmitting information signal, a detectable signal is first sent, channel impulse response is estimated according to detectable signal, then will
Its time-reversal signal with receive signal do convolution, it is anti-when obtaining virtual after signal；
Transmission signal frame head is that LFM signals carry out frame timing synchronization using linear frequency modulation, behind add a pure-tone pulse be CW
Signal is used for the estimating Doppler factor, is used to eliminate the influence of Doppler shift in receiving terminal, is afterwards detectable signal, for estimating
Meter channel impulse response, is finally OFDM symbol, and certain protection interval is all left between each signal, and protection interval is more than
The length of the most most way time delay of channel；
Step 2：Receiving terminal completes synchronizing process, extracts the detectable signal for receiving；According to the detectable signal for receiving, chased after using matching
Track algorithm estimates amplitude, time delay and the phase of channel, estimates channel impulse response；
Step 3：According to the channel impulse response estimated, synchronization is re-started to the signal after convolution, anti-OFDM when completing virtual
Channel equalization；
Step 4：To the signal serioparallel exchange after equilibrium, Cyclic Prefix and cyclic suffix are gone, complete demodulating process,
The signal of reception extracts CW signals after synchronizing signal finds the position of signal starting, how general obtains by frequency measurement
Compressibility factor is strangled, Variable sampling treatment is carried out to the signal for receiving according to Doppler factor, eliminate shadow of the Doppler shift to signal
Ring, detectable signal is then extracted from the signal after Variable sampling, estimate channel impulse response, by its time reversal and reception signal
Convolution is done, VTRM channel equalizations are completed, the signal serioparallel exchange after equilibrium goes Cyclic Prefix and cyclic suffix, carries out FFT demodulation
With constellation inverse mapping, OFDM demodulation process is completed；
Wherein, there is a key technology to affect VTRM for the performance of OFDM channel equalizations, i.e., should be added in OFDM symbol
Cyclic suffix, and cyclic suffix and circulating prefix-length elongatedness, cyclic suffix during most most way of inverse channel when should be greater than
Several data duplications that will be before OFDM symbol block to behind symbolic blocks,
If actual channel impulse response is h (t), the channel impulse response of estimation isDefine virtual time reversal channel impulse
Respond and beIt is considered as the channel that signal is eventually passed through；If direct sound wave amplitude is maximum, length is discrete for L's
Channel is expressed as h=[h (0) h (1) ... h (L-1)], when inverse channel be expressed as h '=[h ' (1-L) ... h ' (- 1) h ' (0) h '
(1) ... h ' (L-1)], its length is 2L-1, wherein, h ' (0) is the sound ray after the superposition of each path, and amplitude is maximum, can see
Then inverse channel is a non-minimum phase channel, i.e., still suffered from many way components before the maximum sound ray of amplitude, if when synchronous
With the maximum sound ray of energy as initial time, before the ISI and ICI that many way components after energy maximum sound ray cause pass through circulation
Sew and overcome, and many way components ISI that causes and ICI after energy maximum sound ray then overcome by cyclic suffix,
Specifically include：
If p-th current demodulated OFDM symbol is rp, corresponding transmission signal is expressed as sp, previous and latter OFDM symbol
Number it is expressed as sp-1、sp+1If not adding Cyclic Prefix and cyclic suffix, after impulse response is for the channel of h, signal table is received
It is shown as
Wherein, rp、sp、sp-1、sp+1And npIt is N × 1 dimensional vector,WithIt is N × N-dimensional matrix, is expressed as
According to formula (1), Section 1It is desired signal, Section 2And Section 3Be previous symbol and after
The intersymbol interference that one symbol brings, npIt is noise item；Wherein, distracterOvercome by adding Cyclic Prefix, and
DistracterThen need to add cyclic suffix to overcome, if adding Cyclic Prefix and cyclic suffix more than channel length L,
Receive signal remove circulation before and after sew after, be expressed as
Wherein,It is circular matrix, is expressed as
Remove after sewing before and after circulating, original sending signal is changed into circumference volume with the linear convolution of channel impulse response in making time domain
Product；According to formula (5), the input exported only with current sign block of current sign block is relevant, with previous and latter symbolic blocks
It is unrelated, i.e., the ISI and many way bands that previous symbol and latter symbol bring are eliminated by Cyclic Prefix and cyclic suffix
The ICI for coming；
It is that MP algorithms estimate channel impulse response using match tracing, the MP algorithms are specifically included：
Consider the linear model that Sparse Problems are often used
Y=Ax+v (7)
Wherein, x ∈ RMIt is sparse signal to be estimated, y ∈ RNIt is observation vector, v ∈ RNIt is Gaussian noise vector, A ∈ RN×M, and
N ＜ M, A are expressed as
A=[a1,a2,...,aM] (8)
Wherein, ai∈RN, i=1,2 ..., M, A be called dictionary or atom, aiIt is the atom in dictionary；MP algorithms do not require word
Atom in allusion quotation is orthogonal, but requires two norms | | ai||2=1；
If the residual error after pth time iteration is rp, it is initialized as r0=y, the atom of the matching selected from dictionary isChoosing every time
Minimum with residual signals inner product atom in remaining atom is selected, i.e.,
Wherein, Ip-1∈{s1,s2,...,sp-1It is preceding p-1The set of the matched atoms index selected by secondary iteration, pth time iteration
Estimate the element of signal xIt is expressed as
Residual signals are expressed as
Work as residual signals | | rp||2During ＜ ε, iteration ends, ε is given residual error thresholding, is relevant with input signal-to-noise ratio
Amount；MP algorithms are comprised the following steps that：
1. initialize：Setting residual error thresholding ε, r0=y
2. the atom that selection is matched：
3. the component of signal estimated：
4. residual error：
5. pth time iteration, p ＞ 1
6. matched from remaining atom：
7. the component of signal of pth time iterative estimate：
8. the residual error of pth time iteration：
In order that estimating channel impulse response with MP algorithms, a sparse signal model should be constructed first, it is considered to detectable signal x
N () is the channel of h (n) by channel impulse response, receive signal y (n) and be expressed as
Wherein,Convolution is represented, Fourier transformation is done simultaneously to formula (12) both sides, be expressed as
Y=XH+V (13)
Wherein, Y and X are respectively the Fourier transformations of y (n) and x (n), and H is channel frequency response matrix, is Fu of channel impulse response
In leaf transformation, be expressed as
Bring formula (14) into formula (13), be expressed as
Wherein,It is the diagonal matrix being made up of X, h is expressed as
H=[h (0), h (1) ..., h (L)]T (16)
Wherein, []TTransposition is represented, F is Fourier transform matrix, is expressed as
Formula (15) meets the representation of sparse signal, and Y is expressed as observing matrix,Dictionary is expressed as, because X and Y are transmittings
It is complex matrix with the frequency domain representation for receiving detectable signal.
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| CN105356907B (en) * | 2015-09-24 | 2017-12-19 | 哈尔滨工程大学 | It is a kind of based on when anti-mirror cyclic shift energy measuring underwater acoustic communication method |
| CN106712781A (en) * | 2016-12-08 | 2017-05-24 | 浙江大学 | In-band full duplex underwater acoustic communication apparatus capable of suppressing interference from digital domain and analog domain |
| CN106936440B (en) * | 2017-02-20 | 2020-06-16 | 东南大学 | Compressed sensing observation matrix generation method and device |
| CN107454024A (en) * | 2017-07-24 | 2017-12-08 | 哈尔滨工程大学 | A kind of underwater sound OFDM MFSK channel equalization methods based on Virtual time reversal mirror |
| CN107862289A (en) * | 2017-11-10 | 2018-03-30 | 哈尔滨工程大学 | The FPGA implementation method of anti-passive Direction Finding Algorithm when virtual |
| CN108169560A (en) * | 2017-12-21 | 2018-06-15 | 哈尔滨工程大学 | A kind of segmentation Sine-Fitting decomposition method |
| CN109617662B (en) * | 2019-01-04 | 2020-05-12 | 浙江大学 | Joint resource optimization method based on underwater sound OFDM-NOMA system downlink |
| CN110602016B (en) * | 2019-09-18 | 2022-01-07 | 哈尔滨工程大学 | Underwater acoustic channel time delay estimation method based on image deconvolution |
| CN112332930B (en) * | 2020-11-06 | 2021-09-07 | 西北工业大学 | Time reversal method under moving condition |
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