CN102118335B - Demodulation method for frequency-modulation acoustic communication system - Google Patents
Demodulation method for frequency-modulation acoustic communication system Download PDFInfo
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Abstract
A demodulation method for a frequency-modulation acoustic communication system comprises the following steps: amplifying acoustic signals acquired from an energy converter to obtain a discrete signal sequence s(n) of N sampling points; constructing Hanning window function with length N, and windowing s(n) to obtain S'(n); conducting FFT (Fast Fourier Transform) algorithm on N points to obtain a frequency domain information sequence S(n); searching peak spectral lines and left and right side spectral lines from S(n); adopting double-linear interpolation correction method to carry out frequency spectrum correction on the peak spectral lines and left and right side spectral lines, and obtaining a corrected peak position kmax; calculating a corrected frequency fmax from kmax according to the relation between frequency and sampling rate; and adopting frequency point best approximation algorithm to decode peak frequency f, and restoring original binary information. The method has the advantages of simplicity, practicality, fast calculation speed, high frequency-discrimination frequency, and stability in the demodulation and the judgment window; and the influence of Doppler shift can be maximally accommodated and the communication success rate for the acoustic communication system can be improved.
Description
Technical field
The invention belongs to the underwater sound communication field, particularly the method for a kind of windowing bilinear interpolation FFT (fast Fourier transform) mode demodulation underwater sound FSK (Frequency-shift keying, frequency shift keying) signal.
Background technology
Along with the develop rapidly of modern information technologies, water sound communication technique is the important technology part in contemporary marine resources development and Integrated Ocean Environmental Monitoring System.Extensive use in remote measuring and controlling, pollution monitoring, marine resources investigation and exploitation especially under water is one of project of the anxious exploitation to be studied of China's Marine High-technology.
Underwater acoustic channel is an extremely complexity, and when random, sky, the channel that frequently becomes, its principal character shows as: complexity, polytropy, strong many ways, and the wide and serious Doppler of limited service band disturbs, particularly the intersymbol interference of many way time domain expansion generations, be the ultimate impediment of underwater high-speed rate transfer of data.Many ways feature that ocean acoustic channel random fluctuation, time space frequency become make water sound communication technique become one of the most complicated communication technology in the present age ([1] Xu Xiaomei. shallow sea underwater sound data transmission technology research [D]. Xiamen: Xiamen University's doctorate paper, 2002; [2] Tao Yi. the anti-many way frequency-hopping communication system researchs of Shallow Water Acoustic Channels [D]. Xiamen: Xiamen University's doctorate paper, 2008; [3] Liu Baisheng etc. marine acoustics principle [M]. Harbin: publishing house of Harbin Institute of Technology, 1993).
The Design of Signal of the frequency modulation incoherent detection methods such as frequency shift keying (FSK) is considered in the shallow sea, remote many ways are serious, the position changes the unique selection in channel mutually fast usually.Especially multicarrier frequency shift keying (MFSK) is one of important method that overcomes at present Multi-path interference.It has solved carrier wave position phase tracking problem, and system rejection to disturbance, anti fading performance are good, and signal is easy to produce, demodulation is easy, so still be widely used at present in the underwater sound, in lower rate transmissions equipment.As U.S. Woods Hole institute of oceanography's use carrier frequency 20～30kHz, employing MFSK is that modulation signal (wherein is divided into 16 subbands to system, 4FSK in each subband, 64 frequencies of total use), peak transfer rate reaches 5kbit/s, be 4km in shallow sea horizontal channel operating distance, the error rate of system is 10
-2～10
-3The order of magnitude ([1] Xu Xiaomei. shallow sea underwater sound data transmission technology research [D]. Xiamen: Xiamen University's doctorate paper, 2002).
Utilize FFT to carry out demodulation of frequency discriminator to underwater sound FM signal, simple due to its system configuration, computing is fast and convenient, real-time, be widely adopted always.The general process of FFT demodulation is: the discrete signal s (n) after sampling is carried out FFT obtain the spectrum distribution sequence, determine the frequency f of signal by maximum wherein, then according to certain modulating-coding, demodulation result is translated back corresponding binary message
[2]In practical application, the FFT of discrete signal S (n) as a result is also the point of series of discrete, and the frequency information of integral point position is accurately in S (n), but the frequency information of non-integral point position is unknown.In underwater acoustic channel, bandwidth is seriously limited, and designed frequency interval is often narrower.When meanwhile, Doppler frequency deviation, intersymbol interference (ISI) all can cause the signal that receives with modulation, the accurate frequency of design produces skew to a certain extent.In case the deviation of the demodulation result of the FFT of signal receiving end and transmitting terminal fsk signal original design exists, and will cause actual peak-peak not on the discrete point of S (n), therefore demodulation of frequency discriminator easily causes erroneous judgement, produces the communication error code.
The method that tradition improves FFT frequency discrimination precision is a lot, and after all, all the relation according to frequency resolution df and sample rate f s, sampling number N realizes:
In general, the way of raising frequency resolution df is logical is reduce sample rate f s or increase sampling number N, and common method has:
(1) increase the signal length of transmitting terminal, thereby improve receiving terminal sampling number N.Signal length increases, and the time that can cause sending, receive signal is elongated, and the increase of points N can make the operand of FFT increase, and reduces system effectiveness.
(2) carry out the FFT computing by the mode of zero padding.Fast fourier transform can increase N by zero padding in the process of calculating, can improve equally the amount of calculation of system.
(3) reduce sample rate f s and resample, namely owe Sampling techniques ([4] Chen Guotong. digital communication theory [M]. Harbin: publishing house of Harbin Institute of Technology, 1996).Sample rate is subject to the restriction of Nai Sikuite (Nyquist) sampling law, and Least sampling rate fs must satisfy fs＞2f
max, can obtain higher frequency resolution, but need the relatively long sampling time; In addition, the resampling of frequency-division section has increased the complexity of system, improves system cost.
Above-mentioned three class methods all can effectively improve the precision of frequency discrimination, but carry the high-precision while, the operand of system are increased, and are applied to can make in underwater acoustic system that system complexity is high, operation efficiency is low, increase the cost of underwater sound equipment.
Another kind of thinking is to use interpolation, the application interpolation is carried out certain match to the S as a result (n) of FFT, estimate reasonably that by known spectral line information ([5] Qi is clear for the position at maximum frequency place, Jia Xinle. the precision analysis of Frequency Estimation of Sinusoid Based on Interpolated FFT [J]. electronic letters, vol, 2004,32 (4): 625-629).For underwater sound FSK system, last demodulation judgement generally only needs the frequency information at peak value place, therefore, as long as use suitable interpolation formula, the frequency discrimination result is optimized, and just satisfies the needs of underwater sound FSK demodulation.Because the Interpolate estimation method is simple, do not need complicated programming, and computational complexity is low, be fit to very much the high underwater sound communication system of requirement of real-time, but choosing of interpolating function is most important, directly affects systematic function.
The demodulation of underwater sound FM signal must be considered the impact such as Doppler frequency shift under water, sound wave only has 1500m/s in water transmission speed, far away from the light velocity, than low 5 orders of magnitude of electromagnetic wave, so the relative Doppler effect in underwater sound communication is than the high several orders of magnitude of land radio communication
[2,3]The ocean channel increases along with the increase of frequency sound absorption in addition, and the frequency of sound wave that underwater sound communication uses generally only is confined to tens to hundreds of kHz
[1]Therefore, underwater sound communication more is subject to the impact of Doppler frequency shift than radio, and Doppler effect can affect carrier estimation, thereby makes whole system performance degradation.Reducing as far as possible or eliminate the impact of Doppler effect, is also one of difficulties in underwater sound communication.The demodulation frequency discrimination window stabilization energy opposing Doppler frequency shift impact of FSK system, and normal use windowing in underwater sound frequency modulation system is effectively assembled spectrum energy, suppresses spectrum leakage, improves the accuracy of demodulation, stablizes the frequency discrimination window.But some interpolation algorithms of tradition are after windowing, and error sharply changes, and causes the width of FSK frequency discrimination window unstable, causes demodulating error.If it is applied in underwater sound frequency modulation system, still remain further to study and discuss.
Summary of the invention
The objective of the invention is not high for existing underwater sound frequency modulation system demodulation medium frequency resolution, be subject to the deficiencies such as Doppler's impact, provide a kind of based on windowing bilinear interpolation, not only have the advantages such as simple and practical, fast operation, the frequency discrimination precision is high, the demodulation decision window is stable, and can allow to the full extent the impact of Doppler frequency deviation, improve the demodulation method of frequency modulation underwater sound communication system of the communication success rate of underwater sound frequency modulation system.
The present invention includes following steps:
The underwater sound signal that 1) will collect from transducer end is through amplifying, after filtering, obtaining the discrete signal sequence s (n) of N sampled point;
2) structure length is the Hanning window function of N, and discrete signal sequence s (n) is carried out windowing process, obtains s ' (n);
3) s ' (n) is carried out the FFT computing that N is ordered, obtain the frequency domain information sequence S (n) of signal;
4) peak value spectral line (being designated as k), left side spectral line (being designated as k-1) and the right side spectral line (being designated as k+1) in the frequency domain information sequence S (n) of search signal, the amplitude of described peak value spectral line is designated as A
0, the amplitude of described left side spectral line is designated as A
1, the amplitude of right side spectral line is designated as A
2
5) use bilinear interpolation revised law to carry out Spectrum Correction, the peak k after being proofreaied and correct to peak value spectral line, left side spectral line and 3 spectral lines of right side spectral line
max
6) according to frequency and sample rate relation, from k
maxCalculate the frequency f after correction
max
7) approximate algorithm according to frequency, carry out decoding with crest frequency f, recovers original binary message.
In step 5) in, the peak k after described correction
maxCalculated by the bilinear interpolation formula:
Wherein, k is the peak in S (n), k
maxBe revised peak, A
0, A
1, A
2Be respectively amplitude S (k), the amplitude S (k-1) of left side spectral line, the amplitude S (k+1) of right side spectral line of peak value spectral line.
In step 6) in, the frequency f after described correction
maxFor:
f
max＝k
max·df
In step 7) in, described approximate algorithm according to frequency, the formula that crest frequency f is carried out decoding is:
f＝int(f
max×M)/M。
The demodulation method of frequency modulation underwater sound communication system of the present invention is compared with respect to existing method, has the following advantages:
(1) tradition improves all relations of sample-based formula medium frequency resolution and sample rate, sampling number of frequency resolution, and the inventive method is based on Interpolate estimation, open thinking of dealing with problems.
(2) in traditional FFT frequency, demodulation frequency modulating system, sampling number N generally need to cooperation and the design that fixes such as sample rate, frequency interval, and the present invention there is no specific (special) requirements to the design of the symbol width of transmitting terminal, has greatly improved the flexibility of system.
(3) Interpolate estimation algorithm of the present invention is simple, practical, effective, can not increase the extra operand of system, can access the result under upper frequency resolution, is very suitable for the calibration of frequency offset signal.
(4) bilinear interpolation of the present invention can be avoided the deficiencies such as the frequency discrimination directivity mistake of other interpolation method in the frequency modulation system demodulation, frequency discrimination window be stable not, reduces the Doppler frequency shift impact, more is conducive to the correct demodulation of signal.
Description of drawings
Fig. 1 is the bilinear interpolation model example in the present invention.In Fig. 1, abscissa is n, and ordinate is S (n).
Fig. 2 is that the performance simulation of not interpolation, double spectral line interpolation and bilinear interpolation of the present invention compares.In Fig. 2, abscissa is frequency shift (FS) (normalization), and ordinate is error; ... for without interpolation, * is double spectral line interpolation, ● be the bilinear interpolation, zero is double spectral line interpolation (Hanning window), and ◇ is bilinear interpolation (Hanning window).
Fig. 3 be N=256 the emulation of 3 kinds of demodulation embodiments Performance Ratio.In Fig. 3, abscissa is frequency (by along step gap size normalization), and ordinate is detection probability; Correspond respectively to from top to bottom without interpolation two spectral lines, bilinear, the two spectral lines of windowing, windowing bilinear.
Fig. 4 be N=512 the emulation of 3 kinds of demodulation embodiments Performance Ratio.In Fig. 4, abscissa is frequency (by along step gap size normalization), and ordinate is detection probability; Correspond respectively to from top to bottom without interpolation two spectral lines, bilinear, the two spectral lines of windowing, windowing bilinear.
Embodiment
Utilize bilinear interpolation FFT method of the present invention to carry out demodulation emulation to underwater sound 4FSK signal.Use the 4FSK modulation in emulation, frequency range is 17～23kHz, and according to Nai Sikui sampling law, the sample rate f s of system is set to 50kHz.In order to resist the multi-path effect in underwater acoustic channel, modulation system adopts FH-MFSK (frequency shift keying of frequency hopping multicarrier), can bandwidth be divided into a plurality of subbands, adopt the 4FSK modulation in each subband, the width interval of therefore modulating between frequency is 0.25kHz, as the channel disturbance source, signal to noise ratio is 5dB with white Gaussian noise.
For example, the experiment carrier frequency is used 20.75kHz, and setting each symbol width N is 256.The contained number of data points of less symbol width is less, is convenient to system's computing.When carrier frequency used 20.75kHz to send binary message " 10 ", the frequency deviation of underwater acoustic channel was 0.1kHz, and the signal that receives of receiving terminal, be 107 through the peak k that obtains after filtering, windowing, FFT computing, respective frequencies:
Obtain near the frequency computing through searching:
f＝int(20.898kHz×4)/4＝21kHz
Therefore, the FSK demodulation result of general FFT near frequency 21kHz, will be grouped into 21kHz but not 20.75kHz due to more, search the output binary message through decoder and be " 11 ".
At this moment, if use the bilinear interpolation, search the needed A of bilinear function
0, A
1, A
2Value:
A
0＝S(107)＝57.3621，A
1＝S(106)＝34.5616，A
2＝S(108)＝21.9521
Above 3 substitution bilinear interpolation correction functions are obtained revised position k
max:
At this moment, then carry out demodulation and adjudicate:
f＝int(20.829kHz×4)/4＝20.75kHz
Search output " 10 " through decoder.As seen, correction can reduce Doppler frequency deviation to the impact of underwater sound 4FSK Demodulation Systems through interpolation.
Bilinear interpolation formula in the present invention has been constructed the interpolation model of two opposite straight lines of slope, approximately thinks that peak value spectral line the right and left is symmetric relation, and the peak-peak spectral line is by chance the angular bisector (shown in Figure 1) of two included angle of straight line.Being characterized as of bilinear model equates with two straight lines size slopes that twice maximal point consists of, opposite in sign, and its intersection point is the peak frequency loca.The derivation of this algorithm is as follows:
The real sinusoidal signal of single-frequency is expressed as:
s(t)＝acos(2πf
0t+θ
0)
By obtaining the sequence s that length is N (n) after constant duration T=1/fs (t) sampling, its corresponding discrete fast Fourier transform sequence is:
On this basis, carry out the frequency correction and mate nearby by some interpolation methods.Based on the interpolation correction of bilinear model, make maximum spectral line amplitude A
0, the spectral line amplitude on its left side is A
1, the right spectral line is A
2
Two straight lines that peak point and left and right neighbor point consist of are respectively:
y-A
1＝a[x-(k-1)]
y-A
2＝-a[x-(k+1)]
Work as A
1=A
2The time, revised peak point overlaps fully with former peak point; Work as A
1＜A
2The time, peak point drops on straight line y
1On; Work as A
1＞A
2The time, peak point drops on straight line y
2On.(as shown in Figure 1)
By can be calculated, two straight-line intersections can get interpolation correction deviator and be:
Tradition can obtain reasonable interpolation performance ([5] Y.T.Chan based on double spectral line interpolation, F.Couture.Comparison oftwo FFT-based demodulation schemes for M-ary FSK[C] .Military Communications Conference, 1992,2:603-607; [6] Qi is clear, Jia Xinle. the precision analysis of Frequency Estimation of Sinusoid Based on Interpolated FFT [J]. and electronic letters, vol, 2004,32 (4): 625-629; [7] Xu Rui. with FFT, the 8FSK signal is carried out comparison [J] communication technology of demodulation method, 2003,2:36-37.).But be applied to underwater sound communication, except considering interpolation error, must consider the performance in the windowing situation, and the directivity of interpolation correction (may directly affect demodulation result).In document [5], the method for double spectral line interpolation FFT can acquire higher precision, but because the energy information that only utilizes maximum spectral line and two spectral lines of secondary maximum spectral line carries out Interpolate estimation, as easy as rolling off a logly the orientation mistake occurs and cause error code when equating (especially the secondary maximum value of peak value the right and left).Windowing can make frequency spectrum converge, but traditional double spectral line interpolation result is vulnerable to the windowing process influence, reason is the mistake that two spectral lines easily cause the interpolation result directivity, especially the value of maximum left and right equates, double spectral line interpolation can cause interpolation correction result and real frequency location further to depart from.And the essence of bilinear interpolation is three spectral line interpolations, has considered simultaneously the information of peak value left and right spectral line, has weakened to a certain extent the some impact of " solely large " in two spectral lines.Therefore, select the bilinear interpolation method in the present invention.Experiment simulation shows, the simple and superior performance of the bilinear interpolation algorithm that the present invention proposes is adapted at using in underwater acoustic system.Especially under the situation windowing, the method performance of bilinear interpolation of the present invention obviously is better than traditional double spectral line interpolation (as shown in Figure 2).
Available bandwidth in underwater acoustic channel is usually between 10～25kHz, and is very limited.Use the frequency modulation technologies such as MFSK in this limited bandwidth, must improve as far as possible frequency resolution on the one hand, must consider on the other hand the impact of Doppler frequency shift under water.The demodulation key of underwater sound FSK system is, in the limited frequency band scope, resists many way time delays and Doppler frequency deviations.Need in reduction peaks spectrum f, it to be matched nearest modulation frequency in receiving end signal, be decoded into binary message.In water sound communication technique, use the method that resamples to carry out certain compensation to Doppler effect, but the method system complexity is higher.In general, in the situation of fixing point point to-point communication under water, Doppler frequency shift is generally between 0.05～0.2kHz, as long as the width that guarantees frequency resolution just is highly resistant to the impact of Doppler frequency shift greater than 0.25kHz.In the present invention, leakage that windowing has effectively suppressed frequency spectrum makes the model about the peak-peak symmetry closer to our design, make the stable performance of demodulation court verdict, effectively suppress the shake of frequency discrimination window, when the frequency discrimination precision improves, farthest weakened the impact of Doppler frequency deviation.
When Fig. 2 was sampling number N=256, the present invention contrasted with the error of the not 4FSK demodulation emulation of interpolation, double spectral line interpolation.Can find out from emulation, reach maximum without mean square error center position between two frequencies of the FFT demodulation frequency discrimination of interpolation; Can effectively reduce error after using interpolation correction, improve the detection performance of system.In not windowing situation, error can being controlled at below 0.5 of double spectral line interpolation, the error of bilinear interpolation is below 0.37; After windowing, the bilinear interpolation obtains better performance, further error is stablized to be controlled near 0.1, and the error of double spectral line interpolation is increasing (scope is between 0.1～0.6) on the contrary after windowing.As seen, the performance of windowing bilinear interpolation demodulation 4FSK is best.
Fig. 3 is sampling number N=256, and 512 o'clock, interpolation, double spectral line interpolation and bilinear interpolation of the present invention were not carried out the performance simulation of the verification and measurement ratio of 4FSK demodulation.Allow that in order to make the Doppler frequency deviation ability is maximum, the normalized frequency in frequency interval is 50% especially good in-0.5 and 0.5 position probing rate.Contrast by the three is as can be known:
FSK demodulation frequency deviation without interpolation allows ability least stable.Because frequency discrimination point needs to guarantee certain frequency resolution df, choosing of sampling number N (non-zero padding FFT) is relevant with sample rate f s, when sampling number did not mate with frequency interval and sample rate, the frequency tolerance scope was usually excessive or too small, and the frequency deviation of left and right allows ability not wait.
Can effectively improve the frequency deviation throughput of Demodulation Systems after interpolation.In addition, owing to having improved the frequency resolution df of system through Interpolate estimation, so sampling number N arrange relatively freer.Use double spectral line interpolation and bilinear interpolation all effectively to improve frequency deviation and allow ability, but still stable not, especially shake easily appears in double spectral line interpolation.
The FSK demodulation is carried out in the windowing interpolation, and two spectral line performances descend on the contrary, and it is extremely unstable that frequency deviation allows to become, and the excessive or too small fluctuating (when N=512) that even occurs occurred, and reason is that windowing causes the error increase of double spectral line interpolation to affect demodulation performance.And the bilinear interpolation can be fixed scope-0.5～0.5, and along with the edge that increases of counting is tending towards precipitous, performance more.
Claims (1)
1. the demodulation method of a frequency modulation underwater sound communication system is characterized in that comprising the following steps:
The underwater sound signal that 1) will collect from transducer end is through amplifying, after filtering, obtaining the discrete signal sequence s (n) of N sampled point;
2) structure length is the Hanning window function of N, and discrete signal sequence s (n) is carried out windowing process, obtains s ' (n);
3) s ' (n) is carried out the FFT computing that N is ordered, obtain the frequency domain information sequence S (n) of signal;
4) peak value spectral line, left side spectral line and the right side spectral line in the frequency domain information sequence S (n) of search signal, the amplitude of described peak value spectral line is designated as A
0, the amplitude of described left side spectral line is designated as A
1, the amplitude of right side spectral line is designated as A
2
5) use bilinear interpolation revised law to carry out Spectrum Correction, the peak k after being proofreaied and correct to peak value spectral line, left side spectral line and 3 spectral lines of right side spectral line
maxPeak k after described correction
maxCalculated by the bilinear interpolation formula:
Wherein, k is the peak in S (n), k
maxBe revised peak, A
0, A
1, A
2Be respectively amplitude S (k), the amplitude S (k-1) of left side spectral line, the amplitude S (k+1) of right side spectral line of peak value spectral line;
6) according to frequency resolution, from k
maxCalculate the frequency f after correction
maxFrequency f after described correction
maxFor:
f
max＝k
max·df；
Wherein, df is frequency resolution;
7) approximate algorithm according to frequency, carry out decoding with crest frequency f, recovers original binary message, described approximate algorithm according to frequency, and the formula that crest frequency f is carried out decoding is:
f=round?(f
max×4）/4。
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| CN109525529A (en) * | 2018-10-17 | 2019-03-26 | 舟山美通信息技术有限责任公司 | A kind of multiband MFSK modulator approach improving underwater sound communication rate |
| CN110095791B (en) * | 2019-05-28 | 2021-04-02 | 中国电子科技集团公司第五十四研究所 | Multi-carrier FSK modulation signal demodulation method |
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| CN101079674A (en) * | 2007-04-25 | 2007-11-28 | 北京交通大学 | A water sound communication method of shallow sea remote horizontal channel |
| CN101605001A (en) * | 2009-07-13 | 2009-12-16 | 中国船舶重工集团公司第七一五研究所 | A kind of Doppler measurement of MFSK underwater sound communication and modification method |
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| CN101605001A (en) * | 2009-07-13 | 2009-12-16 | 中国船舶重工集团公司第七一五研究所 | A kind of Doppler measurement of MFSK underwater sound communication and modification method |
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