CN104753561B  Direct sequence spread spectrum modulation method for suppressing multipath interference in underwater acoustic communication  Google Patents
Direct sequence spread spectrum modulation method for suppressing multipath interference in underwater acoustic communication Download PDFInfo
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 CN104753561B CN104753561B CN201310739788.3A CN201310739788A CN104753561B CN 104753561 B CN104753561 B CN 104753561B CN 201310739788 A CN201310739788 A CN 201310739788A CN 104753561 B CN104753561 B CN 104753561B
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Abstract
Description
Technical field
The invention belongs to underwater sound spreadspectrum communication technical field, more particularly to a kind of suppress the straight of Multipath interference in underwater sound communication Connect sequence spread spectrum modulator approach.
Background technology
In underwater sound communication, the intersymbol interference that multipath effect causes（ISI）It is the main original of restricted information transfer rate Cause.In this regard, the transmission range of signal can not only be improved by using DSSS modulation, can also be sharp according to PRN code Autocorrelation performance, has very strong inhibitory action to time delay more than the multipath signalses of a spreadspectrum code chip.Therefore, modulated based on DSSS The underwater sound communication of system receives extensive research.In DSSS modulation techniques, conventional spreading code has msequence, Gold codes, mixes Ignorant code etc..But in engineering use, the spreading code of finite length can only to be used for the modulation and demodulation of signal.However, by text Offer " Welch L R.Lower bounds on the maximum crosscorrelation of signals [J] .IEEE Transactions Information Theory,1974,20:Welch circle be given in 397399. " understands, for length Limited random sequences, it is impossible to obtain preferable autocorrelation or crosscorrelation function.Therefore, nonideal autocorrelation and crosscorrelation Prevent spread spectrum system is from completely inhibiting intersymbol interference（ISI）And multiaccess inference（MAI）, the correlation spy for improving random sequences Property, become one of effective means of suppression Multipath interference.
Even eliminate intersymbol interference to reduce（ISI）And multiaccess inference（MAI）, in document " Sta ' nczak S.Boche H.Haardt M.Are LAScodes a miracle?[J].Global Telecommunications Conference,2001.GLOBECOM'01.IEEE,2001,1:Give in 589593. " for the LS of LASCDMA systems Code.The LS codes are a kind of with zero correlation window（IFW）" complementary seriess "（Complementary Series）, in zero correlation window Interior, " complementary seriess " are to preferable autocorrelation and crosscorrelation function.The proposition of this spreading code, is mainly used in mobile nothing Line communication network, to the intersymbol interference for reducing cdma system（ISI）And multiaccess inference（MAI）, improve the appearance of cellular network Amount.
The content of the invention
It is an object of the present invention to underwater sound communication can not be completely inhibited to solve existing DSSS modulator approach A kind of technical problem of middle Multipath interference, there is provided the DSSS modulator approach of Multipath interference in suppression underwater sound communication, should Complementary seriess to the pseudorandom sequence modulated as DSSS, are introduced the underwater sound and are led to by DSSS modulator approach Letter, using ideal correlation function in the zero correlation window that it is constructed, realizes than common limited long random sequences with preferably many Way rejection ability.
To achieve these goals, the present invention provides a kind of DSSS tune of Multipath interference in suppression underwater sound communication Method processed, the DSSS modulator approach includes：
Step 1）Construction two meets the sequence of " complementary seriess " to relation；
Step 2）By step 1）Two sequences of middle acquisition carry out Orthogonal Composite and generate pseudorandom sequence, with the pseudorandom sequence Row are used as spreading code；
Step 3）According to the transmission bandwidth of spreadspectrum signal, the persistent period of single spreading code chip is calculated, according to information Transfer rate, calculates the persistent period of single information chip, then calculates spreading code corresponding to single information chip Chip number；
Step 4）According to step 3）The result of middle calculating, by the information chip of transmitting according to correspondence number relation respectively with step Rapid 2）In the pseudorandom sequence that obtains do after the multiplication operation of mould 2, combination producing complex base band spreadspectrum signal；
Step 5）By step 4）In the complex base band spreadspectrum signal that obtains be multiplied with signal carrier and treating excess syndrome portion, obtain spread spectrum and send out Penetrate signal.
As the further improvement of abovementioned technical proposal, step 1）Each sequence of middle generation is represented on continuoustime domain For：
Wherein, the length of sequence be N, t express times, c_{n}The sequential value of nth pseudorandom sequence is represented, rect () is Square wave functionT_{C}Represent the persistent period of the single chip of pseudorandom sequence；
Described " complementary seriess " to correlation function be defined as：
Wherein, τ represents sequence A_{i}And A_{j}Time delay, A_{i},A_{j},B_{i},B_{j}It is the pseudorandom sequence of satisfaction " complementary seriess " relation Row, as i=j, above formula describes autocorrelation performance；I ≠ j then describes cross correlation.
As the further improvement of abovementioned technical proposal, the step 4）The information chip of middle transmitting is expressed as：
Wherein, d_{n}Represent nth information sequence value, t express times, T_{d}Represent single information sequence duration.
As the further improvement of abovementioned technical proposal, the step 5）In signal carrier be expressed as：
Wherein,Imaginary unit is represented, ω is carrier angular frequencies,For carrier wave initial phase, t express times.
As the further improvement of abovementioned technical proposal, the step 2）When middle Orthogonal Composite generation pseudorandom sequence is adopted The mode of dividing, after two sequences sequencing temporally is arranged, the value that certain length is added between two sequences is 0 sequence Row, the pseudorandom sequence for obtaining is expressed as：
Wherein, A_{n}And B_{n}Sequence is represented, W represents add between two sequences 0 number, and S represents pseudorandom sequence, Its length is 2 (N+W)；
According to the definition of correlation function, formula（5）Correlation function be expressed as：
Wherein, A_{i},A_{j},B_{i},B_{j}The pseudorandom sequence of satisfaction " complementary seriess " relation is, as i=j, above formula description is certainly Correlation properties；I ≠ j then describes cross correlation.
As the further improvement of abovementioned technical proposal, the step 5）In obtain spread spectrum transmission signal and be expressed as：
Wherein, T_{C}The persistent period of the single chip of expression pseudorandom sequence, t express times,Expression information chip,WithSequence is represented, ω is carrier angular frequenciesFor carrier wave initial phase, T_{d}Represent that single information sequence is lasting Time.
As the further improvement of abovementioned technical proposal, the step 2）Middle Orthogonal Composite generates pseudorandom sequence using load Wave phase orthogonal manner, two sequences is adjusted to respectively on the quadrature carrier that phase is 90 degree, the pseudorandom sequence for obtaining Row are expressed as：
CS=A_{n}+jB_{n}（8）
Wherein, A_{n}And B_{n}Represent sequence,Imaginary unit is represented, CS represents the pseudorandom of the plural form after combination Sequence, its length is N；
According to the definition of correlation function, formula（8）Correlation function be：
The real part of the correlation functionIt is expressed as：
The imaginary part of the correlation functionIt is expressed as：
Wherein, A_{i},A_{j},B_{i},B_{j}The pseudorandom sequence of satisfaction " complementary seriess " relation is, as i=j, above formula description is certainly Correlation properties；I ≠ j then describes cross correlation.
As the further improvement of abovementioned technical proposal, the step 5）In obtain spread spectrum transmission signal and be expressed as：
Wherein, T_{C}The persistent period of the single chip of expression pseudorandom sequence, t express times,Expression information chip,WithRepresent sequence,The pseudorandom sequence of the plural form after combination is represented, ω is carrier angular frequencies,For carrier wave initial phase, T_{d}Represent single information sequence duration.
Used as the further improvement of abovementioned technical proposal, described " complementary seriess " are to using LS codes.
The advantage of the DSSS modulator approach of Multipath interference is in a kind of suppression underwater sound communication of the present invention：
The employing " complementary seriess " of the present invention is to as the pseudorandom sequence in underwater sound spreadspectrum communication, constructing preferably Autocorrelation and crosscorrelation function, can completely inhibit in theory the Multipath interference in underwater sound spreadspectrum communication；Two kinds of " complementations are provided Sequence " to combined method, can select as the case may be when using：1）Timedivision Orthogonal Composite mode：Add between complementary seriess Plus value be 0 sequence can reduce launch dutycycle, so as to play a part of cooling power amplifier；This signal is using just Hand over insensitive to carrier phase when demodulating, it is not necessary to accurate carrier phase synchronization；When signal to noise ratio is relatively low, the demodulation of the method The bit error rate is better than mode 2）；2）Carrier phase Orthogonal Composite mode：The information emission effciency of the method with common msequence, The pseudorandom sequence such as Gold sequence band spectrum modulation is consistent, than mode 1）It is in hgher efficiency；When signal to noise ratio is higher, the demodulation of the method The bit error rate is better than mode 1）.
Description of the drawings
Fig. 1 is the flow process of the DSSS modulator approach of Multipath interference in a kind of suppression underwater sound communication of the invention Figure.
Fig. 2 be in the present invention " complementary seriess " to using time division way Orthogonal Composite generate pseudorandom sequence structural representation Figure.
Fig. 3 is to generate the autocorrelation function of pseudorandom sequence using time division way Orthogonal Composite in the present invention.
Fig. 4 is to generate the crosscorrelation function of pseudorandom sequence using time division way Orthogonal Composite in the present invention.
Fig. 5 is to generate the autocorrelation function of pseudorandom sequence using carrier phase orthogonal manner Orthogonal Composite in the present invention.
Fig. 6 is to generate the crosscorrelation function of pseudorandom sequence using carrier phase orthogonal manner Orthogonal Composite in the present invention.
Fig. 7 is the lake examination CZT timefrequency search graphics of the spread spectrum transmission signal obtained based on time division way in the present invention.
Fig. 8 is the lake examination CZT timefrequency search of the spread spectrum transmission signal obtained based on carrier phase orthogonal manner in the present invention Graphics.
Specific embodiment
With reference to the accompanying drawings and examples to the direct sequence of Multipath interference in a kind of suppression underwater sound communication of the present invention Spread spectrum modulation technique is described in detail.
As shown in figure 1, in a kind of suppression underwater sound communication of the present invention Multipath interference DSSS modulator approach, institute Stating DSSS modulator approach includes：
Step 1）Construction two meets " complementary seriess " to the sequence A code and the sequence of B codes, i.e., two of relation orthogonal sequence each other Row, and A codes aperiodic autocorrelation either crosscorrelation function and B codes autocorrelation or crosscorrelation function at zero propagation phase Deng equal in absolute value elsewhere, symbol is contrary；
Step 2）By step 1）Two sequences of middle acquisition carry out Orthogonal Composite and generate pseudorandom sequence, with the pseudorandom sequence Row are used as spreading code；
Step 3）According to the transmission bandwidth of spreadspectrum signal, the persistent period of single spreading code chip is calculated, according to information Transfer rate, calculates the persistent period of single information chip, then calculates spreading code corresponding to single information chip Chip number；
Step 4）According to step 3）The result of middle calculating, by the information chip of transmitting according to correspondence number relation respectively with step Rapid 2）In the pseudorandom sequence that obtains do after the multiplication operation of mould 2, combination producing complex base band spreadspectrum signal；
Step 5）By step 4）In the complex base band spreadspectrum signal that obtains be multiplied with signal carrier and treating excess syndrome portion, obtain spread spectrum and send out Penetrate signal.
Based on abovementioned DSSS modulator approach, it is assumed that the length of sequence is all N, then " complementary seriess " are met to closing Two sequences of system can be expressed as on continuoustime domain：
Wherein, t express times, c_{n}The sequential value of nth pseudorandom sequence is represented, rect () is square wave functionT_{C}Represent the persistent period of the single chip of pseudorandom sequence.
Described " complementary seriess " to correlation function may be defined as：
Wherein, τ represents sequence A_{i}And A_{j}Time delay, A_{i},A_{j},B_{i},B_{j}It is the pseudorandom sequence of satisfaction " complementary seriess " relation Row, as i=j, above formula describes autocorrelation performance；I ≠ j then describes cross correlation.By formula（2）As can be seen that " complementary sequence Row " are all preferable to the autocorrelation of correlation function that constructs and cross correlation, i.e., autocorrelation has individual arteries and veins at 0 time delay Punching；Outside 0 time delay, autocorrelation and crosscorrelation are all 0.
In addition, the step 4）The information chip of middle transmitting is represented by：
Wherein, d_{n}Represent nth information sequence value, t express times, T_{d}Represent single information sequence duration.
The step 5）In signal carrier be represented by：
Wherein,Imaginary unit is represented, ω is carrier angular frequencies,For carrier wave initial phase, t express times. In the present invention, can have much as " complementary seriess " of pseudorandom sequence pair and its building method.In the present embodiment, with LS codes As a example by, as underwater sound spreadspectrum communication " complementary seriess " to spreading code.Assume (c_{N},s_{N}) and (c '_{N},s′_{N}) it is with group complementary seriess Right, as N=2, basic complementary seriess are represented by code character：
According to following treelike iteration, iteration each time, it is possible to obtain the complementary seriess that length doubles are to code character.
Through n extension, it is possible to obtain 2^{n}It is 2 to code length^{n}" complementary seriess " of N are right.
Due to the nonperiodic autocorrelation function of C codes it is equal at zero propagation with the autocorrelation function of S codes, it is exhausted elsewhere Equal to being worth, symbol is contrary.Therefore, C codes and S codes are combined by certain mode, in the interval of orthogonality of C codes and S codes, Correlation function is superimposed, and is formed " zero correlation window ", is consequently formed LS codes.
In order to by the sequence A code and B codes of " complementary seriess " centering（The sequential value of A codes and B codes can only take+1 or 1） Construction ideal correlation function in zero correlation window, needs to ensure that related operation, i.e. sequence A code and B can not occur in sequence A code and B codes Code reaches in some sense orthogonal.In order to reach orthogonal purpose in modulation, The present invention gives two kinds of feasible methods：
1）Time division way：By the sequencing arrangement temporally of the sequence A code and B codes of " complementary seriess " centering, in sequence A The value that certain length is added between code and B codes is 0 sequence, and the Orthogonal Composite for being achieved in two sequence A codes and B codes generates puppet Random sequences.The structure of pseudorandom sequence as shown in Figure 1 is represented by：
Wherein, A_{n}And B_{n}Sequence is represented, W represents add between two sequences 0 number, and S represents pseudorandom sequence, Its length is 2 (N+W)；
According to the definition of correlation function, formula（7）Correlation function be expressed as：
Wherein, A_{i},A_{j},B_{i},B_{j}The pseudorandom sequence of satisfaction " complementary seriess " relation is, as i=j, above formula description is certainly Correlation properties；I ≠ j then describes cross correlation.
Obviously, as time delay  τ  ＜ W, using the pseudorandom sequence of " complementary seriess " of time division way Orthogonal Composite to formation Row can obtain preferable correlation function.
2）Carrier phase orthogonal manner：The sequence A code and B code divisions of " complementary seriess " centering are not adjusted in phase be On 90 degree of two quadrature carriers, the Orthogonal Composite for being achieved in two sequences generates pseudorandom sequence.The pseudorandom sequence for obtaining Row are represented by：
CS=A_{n}+jB_{n}（9）
Wherein, A_{n}And B_{n}Represent sequence,Imaginary unit is represented, CS represents the pseudorandom of the plural form after combination Sequence, its length is N；
According to the definition of correlation function, formula（9）Correlation function be expressed as：
The real part of correlation functionIt is expressed as：
The imaginary part of correlation functionIt is expressed as：
Wherein, A_{i},A_{j},B_{i},B_{j}The pseudorandom sequence of satisfaction " complementary seriess " relation is, as i=j, above formula description is certainly Correlation properties；I ≠ j then describes cross correlation.
Obviously, from formula（11）Real part correlation function can be seen that using carrier phase orthogonal manner Orthogonal Composite " complementary seriess " have preferable autocorrelation and mutually to pseudorandom sequence for being formed, the real part of its correlation function in any time delay Correlation function.
From formula（12）In as can be seen that the correlation function of the spreading code using carrier phase orthogonal manner Orthogonal Composite Imaginary part represents the result of the crosscorrelation effect of " complementary seriess " centering sequence A code and B codes, although its value is in any delay time section Inside be not constantly equal to 0, but due to sequence A code and B codes it is orthogonal, therefore the amplitude very little of imaginary part, the impact to whole correlation function Very little.In order to make full use of the preferable correlation properties of combinational code, for the correlation demodulation of this spreading code, it is only necessary to take out related As a result real part is used as effective output result.
According to the DSSS modulation system of spreadspectrum signal, when dividing mode when employed, with reference to formula（1）、（3）With （7）The spread spectrum transmission signal for obtaining is expressed as：
When using carrier phase orthogonal manner, with reference to formula（1）、（3）With（9）The spread spectrum transmission signal for obtaining is represented For：
It can be seen from the basic theories of spread spectrum communication, it is assumed that have at least one complete pseudorandom to expand in 1 information chip Frequency sequence, then by formula（13）And formula（14）Correlation properties of the calculated spread spectrum transmission signal in base band, respectively by Formula（3）And formula（4）Correlation function determine.
Below in conjunction with the actual transmission signal of certain lake examination, further detailed is done to the specific embodiment of the present invention Description.
The spread spectrum transmission signal parameter based on LS codes of experiment transmitting is as follows：
Signal bandwidth：4KHz~8KHz, mid frequency：6KHz.
Step 201）：Generate " complementary seriess " to sequence A code and B codes, the step can be according to formula（5）With（6）'s Iterative manner, " complementary seriess " of length are to C codes and S codes required for generating.
Step 202）：According to step 201）In " complementary seriess " C codes and S codes are carried out Orthogonal Composite generate pseudorandom sequence Row, obtain two kinds of pseudorandom spreading sequences：1）The C codes and S codes of 512 chip lengths are generated, in C codes and S intersymbols 512 are inserted The 0 of chip, according to timedivision orthogonal form the spreading code that length is 2048 chips is combined into；2）1024 chip lengths of generation C codes and S codes, according to carrier phase orthogonal form the multiple spreading code that length is 1024 chips is combined into.
Step 203）：According to the transmission bandwidth of spreadspectrum signal, the duration T of single spreading code chip is calculated_{C}；According to The rate of information throughput, calculates the duration T of single information chip_{d}；Then the corresponding spread spectrum of single information chip is calculated The chip number of sequence.
Step 204）：According to step 203）The result of calculating, by the information chip of transmitting according to correspondence number relation difference With step 202）In two kinds of pseudorandom spreading sequences obtaining do the multiplication operation of mould 2, and according to formula（7）With（9）The group for being given Conjunction relation, combination producing complex base band spreadspectrum signal.
Step 205：According to formula（13）Or（14）DSSS modulation is carried out, by complex base band spreadspectrum signal and letter Number carrier multiplication and treating excess syndrome portion, obtain spread spectrum transmission signal.
Tested according to the examination of abovementioned lake as Fig. 3 and Fig. 4 shows, pseudorandom sequence is generated using time division way Orthogonal Composite Autocorrelation and cross correlation results, it is combined by the C codes and S codes that length is 512 chips according to timedivision orthogonal form, in C codes The 0 of 512 chip lengths is inserted and S codes between.It can be seen that the zero correlation window for being formed（IFW）Interval be（512, 512）, in zero correlation window, combinational code has preferable autocorrelation and crosscorrelation function.
Tested according to the examination of abovementioned lake as Fig. 5 and Fig. 6 shows, using carrier phase orthogonal manner Orthogonal Composite generate it is pseudo with The autocorrelation and cross correlation results of machine sequence, it is by the C codes and S codes that length is 1024 chips according to real part and imaginary part Form is combined.It can be seen that in any time delay, the real part of this combinational code correlation function has preferable autocorrelation and mutually Correlation, itself and formula（11）Described notional result is consistent.
Tested according to the examination of abovementioned lake as Fig. 7 and Fig. 8 shows, two kinds " complementary seriess " are doing to the spreadspectrum signal of combinational code The threedimensional result that the search of CZT timefrequencies is obtained.As can be seen that the region outside the main peak of Search Results is very flat, it means that Two kinds of spreadspectrum signals can have good correlation properties, can reach good multipath signalses rejection ability.
It should be noted last that, above example is only to illustrate technical scheme and unrestricted.Although ginseng The present invention has been described in detail according to embodiment, it will be understood by those within the art that, the technical side to the present invention Case is modified or equivalent, and without departure from the spirit and scope of technical solution of the present invention, it all should cover in the present invention Right in the middle of.
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