CN102916922B  Adaptive search Doppler compensation method for underwater sound OFDM  Google Patents
Adaptive search Doppler compensation method for underwater sound OFDM Download PDFInfo
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 CN102916922B CN102916922B CN201210388724.9A CN201210388724A CN102916922B CN 102916922 B CN102916922 B CN 102916922B CN 201210388724 A CN201210388724 A CN 201210388724A CN 102916922 B CN102916922 B CN 102916922B
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Abstract
The invention aims to provide an adaptive search Doppler compensation method for an underwater sound OFDM (Orthogonal Frequency Division Multiplexing). The method comprises the following steps: adopting CW (Continuous Wave) signals as training sequences to roughly measure Doppler frequency deviation factors; utilizing highpower DFT to compensate the Doppler frequency deviation; taking the Doppler frequency deviation factors measured by the CW signals as original values; taking meansquare error of compensated and balanced data as a price function; and unceasingly regulating the size of the factor to perform search until the conditions are met. According to the method provided by the invention, the defect that the Doppler frequency deviation factors measured by a current block Doppler estimation or singlefrequency signal frequency measurement method have error, as well as the defect that the divergence of a data star map, namely the increasing of data meansquare error, is caused by directly utilizing the factors to compensate Doppler frequency deviation, is avoided.
Description
Technical field
What the present invention relates to is a kind of compensation method of highspeed underwater sound communication.
Background technology
Sound wave is the information carrier of carried out remote transmission unique under water, along with deepening continuously of ocean development, to the efficiency of transmission of mobile communication under water and reliability requirement also more and more higher.Orthogonal frequency division multiplexi has the advantage such as antimultipath fading, antibursty interference, and is suitable for high speed data transfer.OFDM symbol is actually multiple subcarrier signal sum through ovennodulation, utilizes orthogonality to distinguish between each subcarrier.If but the interference existed between subcarrier, this orthogonality will be destroyed, cause systematic function to decline.Cause the reason of intersubcarrier interference to have multiple, one of them is exactly that receiving terminal and transmitting terminal relative motion cause Doppler shift, destroys orthogonality between subcarrier, causes demodulation mistake.So carrying out Doppler effect correction for OFDM System for Underwater Acoustic is the key improving systematic function.
Existing OFDM System for Underwater Acoustic Doppler effect correction technology, many employing block Doppler estimate or simple signal frequency measuring method estimating Doppler frequency offset factor, are then compensated Doppler by Variable sampling method.It is respectively insert a known signal in the front and back of Frame that block Doppler estimates, at receiving terminal by carrying out with local signal the width that relevant treatment records receiving data frames, compare with transmission data frame length and record the Doppler shift factor, its certainty of measurement, by the restriction of the ambiguity figure of known signal, namely requires that the timebandwidth product of signal is enough large.Simple signal frequency measuring method is by measuring the CW signal frequency received, and compare with the CW signal frequency sent and record the Doppler shift factor, its certainty of measurement by the restriction of CW signal pulse length, and is vulnerable to noise and multipath jamming.
Summary of the invention
The object of the present invention is to provide the underwater sound OFDM adaptable search Doppler Compensation Method not being vulnerable to noise and multipath jamming.
The object of the present invention is achieved like this:
Underwater sound OFDM adaptable search Doppler Compensation Method of the present invention, is characterized in that:
(1) CW signal is adopted to carry out the bigness scale of the Doppler shift factor as training sequence: definition Doppler shift factor mu
_{d}=v/c, wherein v represents the speed of related movement of transmitting terminal and receiving terminal, and c is the speed of sound wave at water transmission, and the CW signal frequency of transmission is f
_{t}, the CW signal frequency of reception is f
_{r}, first filtering is carried out to the CW signal received, then Fourier transform is done to it, then record CW signal frequency by curveparabolafitting method and be
the then Doppler shift factor of bigness scale
(2) high power DFT is utilized to compensate Doppler shift: the OFDM symbol length of transmission is that N length N does not comprise that circulation is forward and backward sews length, because signal is subject to the impact of Doppler shift, make the time domain waveform of OFDM symbol that compression or expansion occur, namely the sampled point of a complete ofdm signal waveform becomes N ' point, does 2 after intercepting at N ' to the ofdm signal received
^{n}point DFT, recalculates the position of data subcarrier according to the Doppler shift factor recorded;
(3) the Doppler shift factor mu gone out by CW Signal estimation, as initial condition, is searched for using the MSE of the reception data after equilibrium as cost function, until the MSE calculated can find minimum:
Before intercepting OFDM timedomain symbol, the frequency offset factor μ of bigness scale is utilized to recalculate OFDM symbol length, that is:
Wherein [] represents round, high power DFT Doppler effect correction is carried out at N ' after again intercepted to OFDM symbol, OFDM symbol adopts Comb Pilot structure, Comb Pilot interpolation is utilized to carry out channel estimating, equalized data subcarriers, because the frequency offset factor of bigness scale exists error, Doppler effect correction with balanced after the mean square error of data comparatively large, so will search procedure be carried out.The mean square error now calculated is stored:
MSE(0)＝MSE
_{μ}，
Then to Doppler factor with step size mu
_{f}carry out new Doppler factor search, obtain new Doppler factor:
μ
_{k}＝μ+kμ
_{f},k＝±1,±2,...,±K
By μ
_{k}substitution formula
calculate the OFDM symbol length made new advances, and again intercept reception data, Doppler effect correction afterwards and channel equalization process repeat abovementioned steps, and the data mean square error calculated are stored, that is:
Make k=± 1, ± 2 ..., ± K, increases μ
_{k}absolute value search for, and compare the MSE value calculated, if can find the minimum of MSE, then the Doppler shift factor of its correspondence is estimated correct, jumps out circulation, exports data, completes channel disturbance and suppresses; If the minimum of MSE can not be found, then repeat said process, until find the minimum of MSE then to jump out circulation, export data, complete channel disturbance and suppress.
Advantage of the present invention is: existing piece of Doppler estimates or simple signal frequency measuring method, owing to being subject to the restriction of signal itself, and noise and multipath jamming, all there is certain error in the Doppler shift factor recorded.If directly utilize its factor recorded to carry out Doppler shift compensation, dispersing of data constellation figure can be caused, i.e. the increase of data mean square error.The present invention is using the Doppler factor of CW Signal estimation as initial value, with the mean square error of the data after compensation, equilibrium for cost function, continuous adjustment Doppler shift factor size is searched for, until the data mean square error calculated is enough little, then data have converged in each phase place of transmission planisphere, can stop search.
Accompanying drawing explanation
Fig. 1 is OFDM water sound communication signal frame structure of the present invention;
Fig. 2 is high power DFT method Doppler compensate of frequency deviation of the present invention;
Fig. 3 is adaptable search Doppler effect correction flow chart of the present invention;
Fig. 4 is the MSE of data after different frequency offset factor of the present invention compensates;
Fig. 5 is the BER of data after different frequency offset factor of the present invention compensates;
Fig. 6 is Doppler shift factor bigness scale compensation result of the present invention;
Fig. 7 is the Doppler shift factor of the present invention search compensation result.
Embodiment
Below in conjunction with accompanying drawing citing, the present invention is described in more detail:
Composition graphs 1 ~ 7, adopts CW signal to carry out the bigness scale of the Doppler shift factor as training sequence.High power DFT is utilized to compensate Doppler shift.The Doppler shift factor recorded using CW signal is as initial value, and the mean square error of the data to compensate, after equilibrium is cost function, and continuous Dynamic gene size is searched for, until satisfy condition.
Modelled signal frame structure as shown in Figure 1.Time synchronizing signal LFM is used for carrying out frame synchronization and timing synchronization, can find frequency deviation estimated signal CW by relevant treatment.Measure the CW signal frequency of reception again, compared with transmitting terminal CW signal frequency, the rough estimate of the Doppler shift factor can be obtained.
Doppler effect correction takes high power DFT method, does not comprise circulation forward and backwardly sew length if the OFDM symbol length sent is N(), the OFDM symbol length after affecting by Doppler shift is N '.N ' the point data of intercepting is done 2
^{n}point DFT, by increasing the method that DFT counts, increasing frequency resolution, correctly extracting data message after recalculating data subcarrier position.High power DFT method Doppler compensate of frequency deviation as shown in Figure 2.
The Doppler shift factor mu gone out by CW Signal estimation is as initial value, using the mean square error (MSE) of the reception data after high power DFT Doppler effect correction, channel equalization as cost function, the size of the continuous adjustment Doppler shift factor is searched for, until the MSE calculated stops search after satisfying condition.Flow chart as shown in Figure 3.
1, CW signal Doppler shift factor bigness scale
Adopt the signal frame structure shown in Fig. 1.Time synchronizing signal LFM is used for carrying out frame synchronization and timing synchronization, can find frequency deviation estimated signal CW by relevant treatment.Definition Doppler shift factor mu
_{d}=v/c, wherein v represents the speed of related movement of transmitting terminal and receiving terminal, and c is the speed of sound wave at water transmission.The CW signal frequency sent is f
_{t}, the CW signal frequency of reception is f
_{r}.First filtering is carried out to the CW signal received, then Fourier transform is done to it, then record CW signal frequency by curveparabolafitting method and be
the then Doppler shift factor of bigness scale
due to what record
with the f of reality
_{r}there is certain deviation, then also there is certain error in the Doppler shift factor of bigness scale.
2, high power DFT Doppler shift compensates
High power DFT Doppler shift compensates flow chart as shown in Figure 2.Do not comprise circulation forward and backwardly sew length if the OFDM symbol length sent is N().Because signal is subject to the impact of Doppler shift, make the time domain waveform of OFDM symbol that compression or expansion occur, namely the sampled point of a complete ofdm signal waveform becomes N ' point.2 are done at N ' after intercepted to the ofdm signal received
^{n}point DFT, being then equivalent to carrying out zero padding after OFDM symbol, adding frequency resolution.Recalculate the position of data subcarrier according to the Doppler shift factor recorded, can data message be extracted.
3, adaptable search Doppler effect correction
As previously described, because CW signal is vulnerable to the impact of multipath jamming and noise jamming, and frequencymeasurement accuracy is limited to pulsewidth length to a certain extent, therefore can error be there is in its Doppler shift factor mu recorded, if directly utilize μ to carry out Doppler effect correction to reception data, then the planisphere of data there will be Divergent Phenomenon, is unfavorable for correct demodulation.If but correctly estimated Doppler shift factor mu, the reception data constellation figure after compensation can well be converged in send planisphere each phase place on, the error rate is corresponding reduction also.For QPSK mapping mode, then the phase place sending data is
amplitude is 1, lays respectively in the middle of four quadrants.If R
_{k}, k=0,1 ... n1 is the data point being distributed in same quadrant, and quantity is n, and lower footnote k is subcarrier ordinal number.In order to weigh the degree of divergence of data constellation figure, the mean square error MSE (dB) defining the data of each quadrant is:
Wherein
represent the mean value of the data subcarrier in a quadrant, namely
symbol * represents complex conjugate operation; The inverse exponent arithmetic that it is the end that lg represents with 10.In order to the mean square error of verification msg and the proportional relation of the error rate, under underwater sound multipath channel, ofdm system is emulated.If receiving terminal and transmitting terminal counter motion, Doppler shift factor mu
_{d}=1.7 × 10
^{4}.After different frequency offset factor compensates, as shown in Figure 4, the error rate as shown in Figure 5 for the mean square error of data.As can be seen from Fig. 4, Fig. 5, along with Doppler shift factor mu
_{d}change, MSE and the BER variation tendency of OFDM symbol data is substantially identical, all at μ
_{d}close to reaching minimum during theoretical value.In addition, the minimum point of MSE and the BER of data is not quite affected by noise, and the frequency offset factor that namely minimum is corresponding under low signaltonoise ratio is still near theoretical value.
Based on above discussion, consider that the Doppler shift factor mu gone out by CW Signal estimation is as initial condition, constantly search for as cost function, until the MSE calculated stops search after satisfying condition using the mean square error (MSE) of the reception data after equilibrium.Flow chart as shown in Figure 3.
Wherein time synchronized is identical with aforesaid method with frequency deviation bigness scale process.Before intercepting OFDM timedomain symbol, the frequency offset factor μ of bigness scale be utilized to recalculate OFDM symbol length, that is:
Wherein [] represents round.High power DFT Doppler effect correction can be carried out at N ' after again intercepted to OFDM symbol.OFDM symbol adopts Comb Pilot structure, therefore Comb Pilot interpolation can be utilized to carry out channel estimating, equalized data subcarriers.Because the frequency offset factor of bigness scale exists error, Doppler effect correction with balanced after the mean square error of data comparatively large, so will search procedure be carried out.The mean square error now calculated is stored:
MSE(0)＝MSE
_{μ} （3）
Then to Doppler factor with step size mu
_{f}carry out new Doppler factor search, obtain new Doppler factor:
μ
_{k}＝μ+kμ
_{f},k＝±1,±2,...,±K （4）
By μ
_{k}substitution formula (2) calculates the OFDM symbol length made new advances, and again intercepts reception data.Doppler effect correction afterwards and channel equalization process same as above, and the data mean square error calculated to be stored, that is:
Make k=± 1, ± 2 ..., ± K, constantly increases μ
_{k}absolute value search for, and compare the MSE value calculated, if the minimum of MSE can be found, then think that the Doppler shift factor of its correspondence is estimated correct, can circulation be jumped out, export data, complete channel disturbance and suppress; If do not met, then repeat said process, until find the minimum of MSE.
Below selfadaptive search algorithm is emulated.Still receiving terminal and transmitting terminal counter motion is supposed, Doppler shift factor mu
_{d}=1.667 × 10
^{4}.OFDM symbol subcarrier adopts QPSK modulation system, and Comb Pilot carries out channel estimating, and signal to noise ratio is 10dB.First utilize the CW signal of frame structure front portion to carry out Doppler shift bigness scale, obtain frequency offset factor and estimate μ=2.2 × 10
^{4}.Compensate with it and receive data and obtain data constellation figure as shown in Figure 6 after carrying out channel equalization.The mean value calculating four quadrant mean square errors after data point being divided by quadrant is0.692dB.
Data constellation figure disperses comparatively serious as can be seen from Figure 6, illustrates that the frequency offset factor of bigness scale is inaccurate.Below by step size mu
_{f}=0.5 × 10
^{4}search for, result as shown in Figure 7.As seen from Figure 7, MSE corresponding during k=+1 is minimum, i.e. μ
_{k}=1.7 × 10
^{4}for the final Doppler shift factor is estimated.
Claims (1)
1. underwater sound OFDM adaptable search Doppler Compensation Method, is characterized in that:
(1) CW signal is adopted to carry out the bigness scale of the Doppler shift factor as training sequence: definition Doppler shift factor mu
_{d}=v/c, wherein v represents the speed of related movement of transmitting terminal and receiving terminal, and c is the speed of sound wave at water transmission, and the CW signal frequency of transmission is f
_{t}, the CW signal frequency of reception is f
_{r}, first filtering is carried out to the CW signal received, then Fourier transform is done to it, then record CW signal frequency by curveparabolafitting method and be
the then Doppler shift factor of bigness scale
(2) high power DFT is utilized to compensate Doppler shift: the OFDM symbol length of transmission is that N length N does not comprise that circulation is forward and backward sews length, because signal is subject to the impact of Doppler shift, make the time domain waveform of OFDM symbol that compression or expansion occur, namely the sampled point of a complete ofdm signal waveform becomes N ' point, does 2 after intercepting at N ' to the ofdm signal received
^{n}point DFT, recalculates the position of data subcarrier according to the Doppler shift factor recorded;
(3) the Doppler shift factor mu gone out by CW Signal estimation, as initial condition, is searched for using the MSE of the reception data after equilibrium as cost function, until the MSE calculated can find minimum:
Before intercepting OFDM timedomain symbol, the frequency offset factor μ of bigness scale is utilized to recalculate OFDM symbol length, that is:
Wherein [] represents round, high power DFT Doppler effect correction is carried out at N ' after again intercepted to OFDM symbol, OFDM symbol adopts Comb Pilot structure, Comb Pilot interpolation is utilized to carry out channel estimating, equalized data subcarriers, because the frequency offset factor of bigness scale exists error, Doppler effect correction with balanced after the mean square error of data comparatively large, so will search procedure be carried out; The mean square error now calculated is stored:
MSE(0)＝MSE
_{μ}，
Then to Doppler factor with step size mu
_{f}carry out new Doppler factor search, obtain new Doppler factor:
μ
_{k}＝μ+kμ
_{f},k＝±1,±2,...,±K
By μ
_{k}substitution formula
calculate the OFDM symbol length made new advances, and reception data are intercepted again, Doppler effect correction afterwards and channel equalization process repeats step (2), and the data mean square error calculated is stored, that is:
Make k=± 1, ± 2 ..., ± K, increases μ
_{k}absolute value search for, and compare the MSE value calculated, if can find the minimum of MSE, then the Doppler shift factor of its correspondence is estimated correct, jumps out circulation, exports data, completes channel disturbance and suppresses; If the minimum of MSE can not be found, then repeat said process, until find the minimum of MSE then to jump out circulation, export data, complete channel disturbance and suppress.
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CN107896204A (en) *  20171011  20180410  中国船舶重工集团公司第七〇五研究所  A kind of OFDM underwater sound Modem timefrequency twodimensionals search Doppler compensator and compensation method based on FPGA 
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CN103354538B (en) *  20130720  20161005  华南理工大学  A kind of method that Doppler effect correction is carried out to the receipt signal in underwater sound communication 
CN106027116B (en) *  20160707  20180831  哈尔滨工程大学  A kind of mobile underwater sound communication Doppler coefficient method of estimation based on chirp signals 
CN106100692A (en) *  20160829  20161109  东南大学  MIMO OFDM underwater sound communication system doppler spread method of estimation 
CN106330251B (en) *  20160829  20190813  东南大学  Underwater sound communication system doppler spread estimation method based on zero correlation band sequence 
CN108566354A (en) *  20180403  20180921  哈尔滨工程大学  DPFFT timevarying broadband Doppler Compensation Method in underwater sound OFDM 
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CN102664840A (en) *  20120426  20120912  哈尔滨工程大学  Underwater sound OFDM (orthogonal frequency division multiplexing) Doppler estimation method based on cyclic prefixes 
CN102724147A (en) *  20120627  20121010  哈尔滨工程大学  Channel estimation method for underwater sound orthogonal frequency division multiplexing 

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