CN114338327A - Synchronous capture method of short-wave multi-carrier signal - Google Patents

Synchronous capture method of short-wave multi-carrier signal Download PDF

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CN114338327A
CN114338327A CN202210008343.7A CN202210008343A CN114338327A CN 114338327 A CN114338327 A CN 114338327A CN 202210008343 A CN202210008343 A CN 202210008343A CN 114338327 A CN114338327 A CN 114338327A
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程郁凡
董晓珂
阳珂馨
欧云瑶
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University of Electronic Science and Technology of China
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Abstract

The invention belongs to the technical field of communication, and particularly relates to a synchronous capturing method of a short-wave multi-carrier signal with a leader sequence. The method comprises the steps of performing power spectrum analysis on a signal in a sliding window mode in a time domain and a frequency domain by utilizing leading tone frequency information in the signal, wherein a time-frequency sliding window comprises a plurality of multi-carrier symbols in the time domain, frequency deviations of a plurality of positive sub-carrier intervals and negative sub-carrier intervals are covered in the frequency domain, and leading tone average signal-to-noise ratios of the symbols in the time-frequency sliding window are used as a capturing judgment standard; if the average signal-to-noise ratio of the leading sound in the length of the current sliding window is larger than the capture threshold and the number of symbols with high signal-to-noise ratio is larger than a certain value, the capture is considered to be successful; meanwhile, the time domain and the frequency domain position corresponding to the current sliding window are respectively the calculation basis of the initial time offset estimation value and the initial frequency offset estimation value; the method fully utilizes the characteristics of the front sound signal, combines the time-frequency analysis of multiple symbols, can still perform normal synchronous capture under large frequency offset, and improves the robustness of synchronous capture.

Description

Synchronous capture method of short-wave multi-carrier signal
Technical Field
The invention belongs to the technical field of communication, and particularly relates to a synchronous capturing method of a short-wave multi-carrier signal with a leader sequence.
Background
The short-wave communication is an important wireless communication mode in the field of long-distance communication by the characteristics of simple equipment, flexible communication mode, strong survivability, long communication distance and the like, is widely applied to military and civil communication, but a short-wave channel has a series of complex phenomena such as multipath time delay, fading, Doppler frequency shift, frequency shift diffusion, radio station interference and the like, and has frequency selectivity to most short-wave communication systems and short-wave sky wave channels due to the large multipath time delay, so that serious fading and symbol interference of short-wave communication signals are caused; the multi-carrier technology has outstanding advantages in the aspects of overcoming multipath propagation, channel fading, Doppler frequency shift, improving data rate and the like, so that the multi-carrier technology is widely applied to short-wave communication; in a short-wave multi-carrier communication system, synchronization is a premise of correct data transmission, and synchronization capture is taken as the most important ring in a synchronization technology and comprises two parts of time synchronization and frequency synchronization; under normal conditions, a receiving end does not know the starting position of a signal and cannot directly receive and demodulate data information, so time synchronization is needed; the carrier wave of the received signal and the local carrier wave are not completely synchronous, and frequency offset (frequency offset for short) exists, so that frequency synchronization is needed; however, for the multi-carrier system, various synchronization errors may introduce Inter-Channel Interference (ICI) and Inter-Symbol Interference (ISI), and may also destroy orthogonality of each sub-carrier in the multi-carrier system, so that a receiving end may not receive data correctly, thereby greatly affecting the performance of the multi-carrier system; therefore, how to realize the time synchronization and the frequency synchronization of the short-wave multi-carrier signal is a difficult problem of the synchronization acquisition.
Common short-wave multi-carrier signals include commercial product ARD9900 modulation signals of American worldwide radio communication company, 16-tone and 39-tone multi-carrier signals in the American short-wave communication MIL-STD-188-110 series standard and the like; these signals all have a preamble sequence, the format Of the data frame is shown in fig. 1, the beginning Of each Message has a preamble sequence, then a plurality Of frames Of data, each frame Of data is composed Of a frame synchronization sequence and a data segment, and finally an End-Of-Message (EOM) indication; usually, the preamble sequence is composed of a plurality of preamble modulation symbol sequences with different frequencies, the frequency used by the preamble sequence is also called preamble, and generally, the power of each preamble is higher than that of each subcarrier of the data segment, so that the preamble sequence can be used for synchronization.
Currently, there are two main methods for acquiring synchronization of short-wave multi-carrier signals with preamble sequences: 1) based on the synchronous capture of the leader sequence, the method is premised on that the design rule of the leader sequence pattern of the originating signal is known at the receiving end; generally, signal correlation detection is performed based on the periodicity of a preamble sequence, so that synchronous acquisition of signals is realized; 2) detecting the power spectrum of the leader sequence by using a time domain sliding window, calculating the sum of signal-to-noise ratios of all leader tones, and setting a capture threshold for signal capture; the method 1 is suitable for cooperative communication, and cannot be used for synchronous capture in a semi-blind or full-blind state of unknown originating leader design rule of a receiving end under the non-cooperative condition; the method 2 only utilizes a time domain sliding detection window without considering the condition that the power of a preamble becomes lower when a received signal is in a larger frequency offset, so that signal missing detection is easy to occur when the frequency offset is larger, and synchronous capture cannot be realized; in addition, the method only detects the starting position of the signal roughly, and does not consider the estimation of initial time offset and frequency offset; therefore, although the two methods can achieve signal acquisition under certain conditions, the two methods have limitations under the conditions of uncooperative communication or large frequency offset, and cannot achieve correct synchronization.
Disclosure of Invention
Aiming at solving the defects of the prior art and aiming at the problem of synchronous capture of short-wave multi-carrier signals, the invention provides a synchronous capture method of short-wave multi-carrier signals with leading tones; in the method, the power spectrum analysis is carried out on the signal in a sliding window mode at the time domain and the frequency domain by utilizing the leading tone frequency information in the signal, one time-frequency sliding window comprises a plurality of multi-carrier symbols in the time domain, the frequency offset of a plurality of positive and negative sub-carrier intervals is covered on the frequency domain, and the leading tone average signal-to-noise ratio of the symbols in the time-frequency sliding window is taken as a capturing judgment standard; if the average signal-to-noise ratio of the leading sound in the length of the current sliding window is larger than the capture threshold and the number of symbols with high signal-to-noise ratio is larger than a certain value, the capture is considered to be successful; meanwhile, the time domain and the frequency domain corresponding to the current sliding window are respectively the calculation basis of the initial time offset estimation value (signal initial position) and the initial frequency offset estimation value; the method fully utilizes the characteristics of the preamble signal, combines the time-frequency analysis of multiple symbols, can still perform normal synchronous capture under large frequency offset, improves the robustness of synchronous capture, and can well complete the synchronous capture of the short-wave multi-carrier signal with the preamble sequence; meanwhile, the initial time offset and the frequency offset are estimated, so that a foundation is laid for the subsequent correct demodulation, and the method can be applied to a non-cooperative communication scene.
For convenience of describing the content of the present invention, the short-wave multi-carrier signal synchronous acquisition method is described by taking an ARD9900 short-wave Orthogonal Frequency Division Multiplexing (OFDM) signal as an example; the ARD9900 modem is a specific application of an OFDM system in short-wave communication, and has the function of transmitting digital voice, images and data; the format of the ARD9900 modem message is shown in fig. 2, and comprises a preamble segment, 1 frame data and an EOM, wherein the preamble segment is composed of a preamble sequence and a reference phase symbol, each OFDM symbol period is 20ms, the preamble sequence of the first part lasts for 84 OFDM symbol periods and is composed of 3 preamble tone system symbol sequences, the modulation mode of the preamble tone adopts a DBPSK mode, and the modulation symbols are all 1; the reference phase symbol of the second part lasts for 1 OFDM symbol period, and comprises 36 subcarriers used for initial phase reference of a subsequent frame synchronization DQPSK modulation symbol sequence, and the subcarrier interval is 62.5 Hz; the preamble sequence has longer duration and higher power than the data subcarrier, and can be used for signal acquisition, timing synchronization and frequency offset estimation.
The schematic diagram of time-frequency sliding detection is shown in FIG. 3, wherein the OFDM symbol period is NOFDMOne sample point, N in the leader sequencepA pilot of length N for received dataFFTTime domain detection windowing of individual samplesLine detection, NFFTThe number of points of FFT; to reduce the complexity of the calculation, the detection window has a step length of NstepSliding of individual sample points, Nstep＝NOFDM/NsegIn which 1 is<Nseg＜＜N OFDM2, and NsegIs NOFDMA factor of (d); when one OFDM symbol is in sliding step NstepIs an interval divided into NsegEach position corresponds to a time offset, and each sliding is NstepSampling points, taking out N from the current time offset as a starting pointFFTCarrying out FFT (fast Fourier transform) on the sampling points to carry out power spectrum analysis, wherein the same time offset positions of different OFDM (orthogonal frequency division multiplexing) symbols can be regarded as the same group; the acquisition and the time-frequency coarse synchronization can be carried out according to the Signal-to-Noise Ratio (SNR) difference of the front sound among different groups (time offsets); the preamble SNR of the time-frequency detection window can be expressed as:
Figure BDA0003456395230000031
wherein the SNRijIs time offset of (j-1) NstepSampling point and frequency offset of (i-N)sc_ofst-1) within capture window length N in case of Δ facqAverage signal-to-noise ratio of individual symbols, Nsc_ofstIs the maximum subcarrier offset, and Δ f is the subcarrier spacing; the calculation formula can be expressed as:
Figure BDA0003456395230000032
Figure BDA0003456395230000033
for N within the current acquisition windowacqN of one OFDM symbolpThe average power of the individual leading tones,
Figure BDA0003456395230000034
the frequency values of the preamble and the noise frequency point are changed according to the frequency offset setting value corresponding to the sliding window of the current frequency domain for the noise average power in the current capture window.
A synchronous capture method of short wave OFDM signals with pilot tones comprises the following specific steps:
s1, initialization parameters: number N of leading tonespMaximum subcarrier offset Nsc_ofstSupported frequency offset range of [ -N [)sc_ofst△f,Nsc_ofst△f]Length of capture window NacqStep length N of slidingstepThe signal-to-noise ratio threshold of the captured signal is GtThe high SNR is scaled by a factor of λ relative to the threshold of the acquisition SNRh(if SNR is greater than λ)hGtThen high SNR) is considered, the number of symbols of high SNR is relative to the acquisition window length NacqHas a scale factor of λf；
S2, corresponding to different frequency offsets (i-N)sc_ofst-1) taking a short-wave OFDM signal pilot tone under delta f as a pilot tone set, and taking a residual sub-carrier wave after removing the pilot tone and two sub-carrier waves at the left and the right of the pilot tone in a set formed by all sub-carrier waves in a receiving and filtering bandwidth as a noise sub-carrier wave set;
s3, setting the step length as NstepThe length of the detection window is NFFTCarrying out FFT on the short wave OFDM signal data received in sequence in a sliding manner, calculating and recording NpSNR of each preamble under different sliding positions and different frequency offsets is totally N for each symbolseg(2Nsc_ofst+1) sets of time-offset frequency offsets, corresponding to Nseg(2Nsc_ofst+1) SNR values;
s4, judging whether N is processedacqA symbol, if yes, calculating the current capture window length NacqInner SNRpilot(ii) a Otherwise, go to S3;
s5, finding SNRpilotSNR of maximum value inmaxTo determine SNRmaxWhether or not greater than GtAnd SNRpilotMedium high SNRhigh(SNRhigh≥λhGt) Number N ofhighLambda greater than the length of the capture windowfIf yes, the capture is successful; otherwise, go to S3;
S6、SNRmaxat SNRpilotColumn j in (1)mThe corresponding coarse time offset estimate is (j)m-1)NstepSample point, line imThe corresponding integer frequency offset estimation value is (i)m-Nsc_ofst-1) Δ f; according to the coarse estimation value (j) of the time offsetm-1)NstepAnd current time domain sliding windowPosition Lwin(Lwin＝Nslide-NacqNseg-(mod(Nslide-1,Nseg)+1)，NslideFor the number of slips) can calculate the starting position L at which the leading data was capturedAcqI.e. LAcq＝(Lwin+jm-1)Nstep；
S7, according to NpThe leading tone l with the largest average power among the leading tonesmPerforming initial frequency offset estimation;
s8, starting position L according to the captured leading dataAcqPerforming initial time offset estimation;
further, the initial frequency offset estimation in S7 specifically includes the following steps:
s71, starting position L according to the captured leading dataAcqAnd an integer multiple frequency offset estimate (i)m-Nsc_ofst-1) Δ f vs. NpThe power average value of each leading sound in the capture window length is obtained, and the leading sound l with the maximum power average value is foundm；
S72, for NacqThe leading tone l is taken out from each symbolmAnd its left and right frequency points lm-1，lm+1Frequency domain signal V (l) of 3 subcarriers in totalm)，V(lm-1)，V(lm+1)；
S73, according to the length N of the capture windowacqThe signals of 3 subcarriers taken out from each symbol are respectively estimated by frequency offset estimation formula
Figure BDA0003456395230000041
Wherein
Figure BDA0003456395230000042
Representing the operation of the real part;
s74, for NacqCalculating maximum power pilot tone l separately by each symbolmIf the signal-to-noise ratio is less than λhGtIf so, the frequency offset estimation value of the symbol is regarded as an abnormal value; estimate absolute value if frequency offset
Figure BDA0003456395230000043
Values greater than 0.5 are also considered outliers; removing the abnormal estimation value;
s75, averaging the frequency deviation estimated by the symbols after removing the abnormal value, obtaining the decimal frequency deviation
Figure BDA0003456395230000051
Initial frequency offset estimation
Figure BDA0003456395230000052
Is fractional frequency offset
Figure BDA0003456395230000053
Plus integral frequency offset (i)m-Nsc_ofst-1) the sum of Δ f;
further, the initial time offset estimation in S8 specifically includes the following steps:
s81, retrieving the starting position L of the preamble dataAcqAnd integer frequency offset (i)m-Nsc_ofst-1) N to DeltafpIndividual leading tone average signal-to-noise ratio
Figure BDA0003456395230000054
And average signal-to-noise ratio corresponding to front and back time offsets of the signal
Figure BDA0003456395230000055
S82, calculating the decimal time error of the obtained 3 SNR values by a triangular interpolation method:
Figure BDA0003456395230000056
s83, starting position L according to captured leading dataAcqAnd initial timing error Δ nstepCalculating an initial time offset estimate
Figure BDA0003456395230000057
Namely:
Figure BDA0003456395230000058
and (5) sampling points.
The invention has the beneficial effects that:
the invention can realize the reliable time frequency synchronous capture of the short wave multi-carrier signal: based on the leader sequence, capturing the multicarrier signal and simultaneously obtaining the initial position of the signal and the integral multiple frequency offset estimation value according to the SNR of the leader tone under different time offset frequency offsets as a capturing judgment standard; then, the leading tone with the maximum power is used for carrying out decimal frequency offset estimation, so that the frequency offset estimation range is improved while the frequency offset estimation precision is ensured; meanwhile, initial time offset estimation is carried out according to the captured signal initial position, and the performance of time synchronization is improved by combining multiple symbols; compared with the traditional synchronous capture method, the method can obviously improve the synchronous capture probability of the system in a short-wave channel environment with large frequency offset (for example, the frequency offset exceeds 1 time of subcarrier interval), can also carry out time-frequency offset coarse estimation, and is also suitable for a non-cooperative communication scene; the method has the advantages of low complexity, good robustness, great significance for correctly receiving signals and subsequent synchronization, demodulation and other work, and high application value.
Drawings
Fig. 1 is a schematic diagram of a short-wave multicarrier signal message format with a preamble sequence;
FIG. 2 is a message format schematic of an ARD9900 short-wave OFDM signal;
FIG. 3 is a schematic diagram of time-frequency sliding acquisition of a short-wave multicarrier signal;
fig. 4 is a flowchart of the general operation of the method for synchronously acquiring a short-wave multicarrier signal according to the present invention;
FIG. 5 is a flow chart of initial frequency offset estimation;
FIG. 6 is a flow chart of initial time offset estimation;
FIG. 7 is a graph of the simulation performance of the acquisition algorithm of the embodiment of the present invention under different channels of short wave;
FIG. 8 is a diagram of the rough frequency offset estimation simulation performance of the exemplary embodiment of the present invention in short-wave channels;
fig. 9 is a schematic diagram comparing the acquisition performance of the embodiment of the present invention with that of the conventional synchronization acquisition method under a short-wave severe channel.
Detailed Description
The technical solution of the present invention is described in detail below with reference to the embodiments and the accompanying drawings.
Taking an ARD9900 signal as an example, synchronous acquisition is carried out; the front pilot segment signal is an OFDM signal modulated by DBPSK, and comprises 3 front pilots, the positions of the front pilots are 0.5kHz, 1kHz and 1.5kHz respectively, the amplitude of the front pilots is 4 times of that of a data frame signal, and the subcarrier interval delta f is 62.5 Hz; the signal model is:
Figure BDA0003456395230000061
Tc＝1/ftis a sampling period in which ft8kHz as originating sampling frequency, Ts20ms is the OFDM symbol period, g (t) is the rectangular shaped pulse, OFDM symbol length NOFDM160 samples; the kth OFDM symbol time span t0+(k-1)(NFFT+D)Tc≤t≤t0+(NFFT+D)kTc，ck,nFor the nth sample point of the kth OFDM symbol in the time domain:
Figure BDA0003456395230000062
wherein, ak,lFor modulated data on the l sub-carrier of the k OFDM symbol in frequency domain, l is 0,1, …, NFFT-1; n (t) is additive white gaussian noise, then the received signal is: r (t) ═ s (t) + n (t), the sampled preamble data can be modeled as a sinusoidal signal in one OFDM symbol:
Figure BDA0003456395230000063
wherein, A, theta and f are respectively the amplitude, initial phase and frequency of the current leading sound signal, and frIn order to receive the sampling frequency at the end,
Figure BDA0003456395230000064
complex white gaussian noise; the synchronization capturing method adopted by the embodiment specifically comprises the following steps:
s1, initialization parameters: number N of leading tonespMaximum subcarrier offset N of 3sc_ofstIs 2, the supported frequency offset ranges are [ -2 Δ f,2 Δ f]Length of capture window NacqIs 10OFDM symbol length (typical Doppler value of short wave channel is usually 1Hz, corresponding coherent time is about 423 ms; capture window length is 200ms, which is less than half of coherent time, so that signal in this period is considered to be stable and less fluctuated by channel influence), sliding step length NstepIs 16, one OFDM symbol is divided into N seg10 positions, each position corresponds to a time offset, and the signal-to-noise ratio threshold G of the captured signaltA scaling factor lambda of high SNR relative to the threshold of the acquisition signal-to-noise ratio of 10h0.8, the number of symbols with high SNR is scaled by a factor of λ with respect to the length of the acquisition windowfIs 0.75;
s2, positive integer i (i is less than or equal to 2N)sc_ofst+1) different values of corresponding frequency offsets (i-N)sc_ofst-1) the preamble set of the signal in case of Δ f can be expressed as:
Figure BDA0003456395230000071
the preamble and the extension sets corresponding to the left and right subcarriers can be expressed as follows:
Figure BDA0003456395230000072
defining the total set of frequencies in the pass band of the receive filter as
Figure BDA0003456395230000073
Then the noise subcarrier set
Figure BDA0003456395230000074
Can be made ofRxMinus
Figure BDA0003456395230000075
Obtain (N for the number of noise subcarriers)noiseRepresents):
Figure BDA0003456395230000076
s3, step by step Nstep16, detection Window Length NFFTPerforming FFT for 128-sequence sliding received signal data to obtain frequency domain data:
Figure BDA0003456395230000077
wherein W (l) is w (n) FFT transform at frequency point l; calculate and record Np3 preamble tones
Figure BDA0003456395230000078
At different sliding positions j (j is more than or equal to 1 and less than or equal to N)seg) Different frequency offsets (i-N)sc_ofst-1) SNR at Δ f:
Figure BDA0003456395230000079
wherein the content of the first and second substances,
Figure BDA00034563952300000710
is N of the current sliding windowpThe average power of the individual leading tones,
Figure BDA00034563952300000711
the noise average power of the current window;
s4, judging whether N is processedacqA symbol, if yes, calculating the current capture window length
Figure BDA00034563952300000712
Wherein
Figure BDA00034563952300000713
Figure BDA00034563952300000714
For sliding position j, frequency offset (i-N)sc_ofst-1) the signal-to-noise ratio of the kth symbol within the window length for af; otherwise, go to S3;
s5, finding SNRpilotSNR of maximum value inmaxTo determine SNRmaxWhether greater than the capture threshold G t10 and high SNRhigh(SNRhigh≥λhGt) Number N ofhighWhether or not it is greater than lambdafNacqIf yes, the capture is successful; otherwise, go to S3;
S6、SNRmaxat SNRpilotColumn j in (1)mWhen corresponding toThe partial coarse estimate is (j)m-1)NstepLine imThe corresponding integer frequency offset estimation value is (i)m-Nsc_ofst-1) Δ f; according to the coarse estimation value (j) of the time offsetm-1)NstepAnd current time domain sliding window position Lwin(Lwin＝Nslide-NacqNseg-(mod(Nslide-1,Nseg) +1)) can calculate the starting position L at which the preamble data was capturedAcqI.e. LAcq＝(Lwin+jm-1)Nstep；
S7 maximum leading sound l according to powermPerforming initial frequency offset estimation; the method specifically comprises the following steps:
s71, starting position L according to captured leading dataAcqAnd integer frequency offset (i)m-Nsc_ofst-1) Δ f vs. NpObtaining N in the length of capture window for each leading toneacqPower mean of individual symbols
Figure BDA0003456395230000081
Acquiring a frequency domain signal of a kth symbol frequency point l in a window length; finding the mean value of the power within the length of the acquisition window
Figure BDA0003456395230000082
Leading tone corresponding to the maximum value in (1)
Figure BDA0003456395230000083
S72, extracting leading sound l with maximum powermV (l) frequency domain of 3 subcarriers left and rightm)＝Aejθx(lm)+W(lm) (order)
Figure BDA0003456395230000084
),V(lm-1)＝Aejθx(lm+1)+W(lm-1),V(lm+1)＝Aejθx(lm-1)+W(lm+1)；
S73, respectively estimating frequency offset by using a frequency offset estimation formula according to the signals of 3 subcarriers taken out by each symbol in the capture window length
Figure BDA0003456395230000085
Wherein
Figure BDA0003456395230000086
Representing the operation of the real part;
s74, calculating N respectivelyacqWithin a symbol lmSNR of (d):
Figure BDA0003456395230000087
indicating the k-th symbol frequency point lmSignal power of (A) if
Figure BDA0003456395230000088
The frequency offset estimation value of the symbol is regarded as an abnormal value; estimate absolute value if frequency offset
Figure BDA0003456395230000091
Values greater than 0.5 are also considered outliers; removing the abnormal estimation value;
s75, removing L after abnormal estimationnormal(Lnormal≤Lacq) Averaging the estimated value of the frequency deviation to obtain the decimal frequency deviation
Figure BDA0003456395230000092
Initial frequency offset estimation
Figure BDA0003456395230000093
Is the sum of the fractional frequency offset plus the integer frequency offset, i.e.
Figure BDA0003456395230000094
S8, starting position L according to the captured signalAcqPerforming an initial time offset estimation, comprising:
s81, retrieving the starting position L of the preamble dataAcqAnd integer frequency offset (i)m-Nsc_ofst-1) N to DeltafpIndividual leading tone average signal-to-noise ratio
Figure BDA0003456395230000095
And its average signal-to-noise ratio corresponding to left and right time offsets
Figure BDA0003456395230000096
S82, calculating the decimal time error of the obtained 3 SNR values by a triangular interpolation method:
Figure BDA0003456395230000097
s83, starting position L according to captured leading dataAcqAnd initial timing error Δ nstepCalculating an initial time offset estimate
Figure BDA0003456395230000098
Namely:
Figure BDA0003456395230000099
FIG. 7 is a graph of the acquisition algorithm simulation performance for different channels according to the embodiment of the present invention; the SNR of the abscissa is the in-band signal-to-noise ratio of the complete frame of the ARD9900 multi-carrier signal in the bandwidth of 2.1875kHz, and the ordinate is the capturing probability; the simulation parameter settings of the capture algorithm are shown in table 1, and the simulation channel parameters are shown in table 2.
TABLE 1 Capture Algorithm simulation parameters
Figure BDA00034563952300000910
Figure BDA0003456395230000101
Table 2 simulation channel parameter table
Figure BDA0003456395230000102
For the three channels simulated, the false alarm probability is less than 1%. The algorithm provided by the invention is based on the fact that the signal-to-noise ratio of the pilot tone is captured in the time-frequency domain at the same time, the performance is efficient and stable, and the false alarm probability of the algorithm is very small. The acquisition probabilities for different channels differ significantly at low signal-to-noise ratios.
Under an Additive White Gaussian Noise (AWGN) channel, since a signal is affected by uniform White Gaussian Noise in a full frequency band, characteristics of each preamble channel are consistent, and thus a capture probability increases with an increase in a signal-to-Noise ratio; when the in-band SNR is greater than-1.5 dB, the capture probability can reach more than 95%.
Under the short wave fading channel, because the frequency interval of the leading tone frequency point is large and exceeds the coherent bandwidth of the short wave channel, the fading characteristics of each leading tone are inconsistent, at the moment, the leading tone power mean value in the combined multi-symbol time-frequency sliding window has equal-gain combination gain of frequency diversity, and the capture performance under the short wave fading channel under the low signal-to-noise ratio (in-band SNR < -1.8dB) is superior to that of a Gaussian channel.
The size of the channel coherent bandwidth enables the capturing performance under different fading channels to generate difference, the multipath coherent coefficient is set to be 0.5, the coherent bandwidth of the short wave medium channel and the short wave severe channel is respectively 400Hz and 200Hz, the coherent bandwidth of the short wave medium channel is close to the interval of the preamble tone by 500Hz, the equal gain combining gain in the average power band of the preamble tone is lower than that of the short wave severe channel, and the possibility of fading of a plurality of preamble tones simultaneously exists, so the synchronous capturing performance under the short wave severe channel is better than that of the short wave medium channel.
Under a short wave fading channel, when the in-band SNR < -1.8dB, the equal gain combination gain of power is larger than the influence caused by fading, and sudden small-probability successful capture exists, at the moment, the capture performance under the short wave fading channel is superior to that of a Gaussian channel, the capture probability of a short wave medium channel is about 0.7, and the capture probability of a short wave bad channel is about 0.9; when the SNR is less than 2dB in the band of-1.8, the burst small probability brought by the combining gain is successfully captured, and the power loss brought by deep fading cannot be eliminated, at the moment, the capture performance under a short wave fading channel is worse than that of a Gaussian channel, and the estimation performance gradually approaches to 1 along with the improvement of the SNR in the band; under the influence of burst deep fading, when the in-band 2< SNR <5.2dB, a small probability of acquisition failure still occurs; when the in-band SNR is greater than 5.2dB, the ARD9900 multi-carrier signal acquisition algorithm can achieve more than 95% error-free acquisition in a short-wave fading channel.
FIG. 8 shows the performance of frequency offset estimation of the present invention tested in different channels; the abscissa is an in-band signal-to-noise ratio, the ordinate is frequency offset estimation NRMSE, and the normalization standard is that the subcarrier interval delta f is 62.5 Hz; the simulation parameters are the same as the simulation parameter settings of fig. 7; the frequency offset estimation performance of the short wave fading channel is superior to that of the AWGN channel under the condition of low signal-to-noise ratio, and the reason is analyzed as above; when the SNR is-3 dB, the frequency offset estimation NRMSE of the short wave medium channel is 5.2 multiplied by 10-2Namely, the frequency offset error is about 3.2Hz, the frequency offset error of the short wave severe channel is about 3.6Hz, and the frequency offset error under the AWGN channel is about 4.9Hz, namely the frequency offset error does not exceed 5Hz under the condition of testing the lowest signal to noise ratio; along with the increase of the signal-to-noise ratio, the frequency offset estimation performance under each channel gradually becomes better, but the fluctuation under a short-wave severe channel is larger, because the fluctuation of the signal-to-noise ratio is larger under the channel, the probability of deep fading is higher, and the frequency offset estimation value is not stable; the frequency offset estimation NRMSE under the conditions of AWGN channel and short-wave medium channel can reach 1.6 multiplied by 10dB when the signal-to-noise ratio SNR is 10dB-2The horizontal frequency deviation error is about 1Hz, and the NRMSE is 2.8 multiplied by 10-2The horizontal frequency deviation error is about 1.75 Hz; therefore, the method has the advantages of small error of the frequency deviation rough estimation result during synchronous acquisition, excellent performance and good foundation for the fine synchronization later.
FIG. 9 is a graph showing a comparison of the probability of acquisition in a short-wave severe channel according to an embodiment of the present invention and a conventional acquisition method based on a time-domain sliding window correlation; wherein, the abscissa is the in-band signal-to-noise ratio, the ordinate is the capture probability, the simulation parameters are set the same as the simulation parameters of fig. 7, the frequency deviation of each simulation is a random number within a set range, and the frequency deviation is divided into three conditions: 0.6 delta f is less than or equal to xi0≤0.8△f，1.2△f≤ξ0Delta f is less than or equal to 1.4 and delta f is less than or equal to 1.60Less than or equal to 1.8 delta f; simulation results show that delta f is less than or equal to xi at the initial frequency offset of 0.60When SNR is less than or equal to 0.8 delta f<4dB time of the inventionCompared with the traditional algorithm, the acquisition probability performance of the extraction algorithm has the signal-to-noise ratio gain of about 2dB, when the SNR is more than or equal to 4dB, the acquisition performance of the extraction algorithm is similar to that of the traditional method, and the acquisition probability approaches to 1; as the frequency deviation increases, delta f is less than or equal to xi at the initial frequency deviation of 1.20When the capture probability is less than or equal to 1.4 delta f, the algorithm is obviously superior to the traditional algorithm, and when the capture probability is 90%, the signal-to-noise ratio gain is about 10dB compared with the traditional algorithm; when the frequency deviation is close to the interval of two times of subcarriers, namely 1.6 delta f is less than or equal to xi0When the capturing probability is 75%, the signal-to-noise ratio gain of about 12.5dB is obtained compared with the traditional algorithm; the figure 9 and the above analysis show that the method has the outstanding advantages under the condition of large frequency deviation, namely the synchronous capturing performance is very robust even under the condition of large frequency deviation and is consistent with the performance under the condition of small frequency deviation, and the capturing capability of the traditional algorithm is increasingly poor along with the increase of the frequency deviation.

Claims (3)

1. A synchronous capture method of short wave multi-carrier signals is characterized in that the capture method is to utilize the frequency information of the leading tone in the signals to perform power spectrum analysis on the signals in a sliding window mode at the time domain and the frequency domain, a time-frequency sliding window comprises a plurality of multi-carrier symbols in the time domain, frequency deviation of a plurality of positive and negative sub-carrier intervals is covered on the frequency domain, and the average signal-to-noise ratio of the leading tone of a plurality of symbols in the time-frequency sliding window is taken as a capture judgment standard, and specifically comprises the following steps:
s1, initialization parameters: number N of leading tonespMaximum subcarrier offset Nsc_ofstSupported frequency offset range of [ -N [)sc_ofst△f,Nsc_ofst△f]Δ f is the subcarrier spacing, capture window length NacqStep length N of slidingstepThe signal-to-noise ratio threshold of the captured signal is GtThe high SNR is scaled by a factor of λ relative to the threshold of the acquisition SNRhThe number of symbols with high SNR relative to the acquisition window length NacqHas a scale factor of λfThe high SNR means that the SNR is greater than lambdahGt；
S2、Will correspond to different frequency offsets (i-N)sc_ofst-1) as set of lead tones at Δ f, i ═ 1,2sc_ofst+1, removing the pilot tone and the left and right subcarriers from the set formed by all subcarriers in the receiving filter bandwidth, and using the remaining subcarriers as a noise subcarrier set;
s3, setting the step length as NstepThe length of the detection window is NFFTCarrying out FFT on the short wave OFDM signal data received in sequence in a sliding manner, calculating and recording NpSNR of each preamble under different sliding positions and different frequency offsets is totally N for each symbolseg(2Nsc_ofst+1) sets of time-offset frequency offsets, corresponding to Nseg(2Nsc_ofst+1) SNR values;
s4, judging whether N is processedacqA symbol, if yes, calculating the current capture window length NacqInner SNRpilot：
Figure FDA0003456395220000011
SNRijIs capturing the window length within NacqAverage signal-to-noise ratio of individual symbols; otherwise, go to S3;
s5, finding SNRpilotSNR of maximum value inmaxTo determine SNRmaxWhether or not greater than GtAnd SNRpilotHigh SNR in Medium number NhighLambda greater than the length of the capture windowfIf yes, the capture is successful; otherwise, go to S3;
S6、SNRmaxat SNRpilotColumn j in (1)mThe corresponding coarse time offset estimate is (j)m-1)NstepSample point, line imThe corresponding integer frequency offset estimation value is (i)m-Nsc_ofst-1) Δ f; according to the coarse estimation value (j) of the time offsetm-1)NstepAnd current time domain sliding window position Lwin，Lwin＝Nslide-NacqNseg-(mod(Nslide-1,Nseg)+1)，NslideCalculating the starting position L of captured leading data for the sliding timesAcqI.e. LAcq＝(Lwin+jm-1)Nstep；
S7, according to NpThe leading tone l with the largest average power among the leading tonesmPerforming initial frequency offset estimation;
s8, starting position L according to the captured leading dataAcqAn initial time offset estimation is performed.
2. The method for synchronously acquiring short-wave multicarrier signal according to claim 1, wherein the initial frequency offset estimation in step S7 specifically comprises the following steps:
s71, starting position L according to the captured leading dataAcqAnd an integer multiple frequency offset estimate (i)m-Nsc_ofst-1) Δ f vs. NpThe power average value of each leading sound in the capture window length is obtained, and the leading sound l with the maximum power average value is foundm；
S72, for NacqThe leading tone l is taken out from each symbolmAnd its left and right frequency points lm-1，lm+1Frequency domain signal V (l) of 3 subcarriers in totalm)，V(lm-1)，V(lm+1)；
S73, according to the length N of the capture windowacqThe signals of 3 subcarriers taken out from each symbol are respectively estimated by frequency offset estimation formula
Figure FDA0003456395220000021
Wherein
Figure FDA0003456395220000022
Representing the operation of the real part;
s74, for NacqCalculating maximum power pilot tone l separately by each symbolmIf the signal-to-noise ratio is less than λhGtIf so, the frequency offset estimation value of the symbol is regarded as an abnormal value; estimate absolute value if frequency offset
Figure FDA0003456395220000023
Values greater than 0.5 are also considered outliers; removing the abnormal estimation value;
s75, averaging the frequency deviation estimated by the symbols after removing the abnormal value, obtaining the decimal frequency deviation
Figure FDA0003456395220000024
Initial frequency offset estimation
Figure FDA0003456395220000025
Is fractional frequency offset
Figure FDA0003456395220000026
Plus integral frequency offset (i)m-Nsc_ofst-1) sum of Δ f.
3. The method for synchronously acquiring short-wave multicarrier signal according to claim 1, wherein the initial time offset estimation of step S8 specifically comprises the following steps:
s81, retrieving the starting position L of the preamble dataAcqAnd integer frequency offset (i)m-Nsc_ofst-1) N to DeltafpIndividual leading tone average signal-to-noise ratio
Figure FDA0003456395220000031
And average signal-to-noise ratio corresponding to front and back time offsets of the signal
Figure FDA0003456395220000032
S82, calculating the decimal time error of the obtained 3 SNR values by a triangular interpolation method:
Figure FDA0003456395220000033
s83, starting position L according to captured leading dataAcqAnd initial timing error Δ nstepCalculating an initial time offset estimate
Figure FDA0003456395220000034
Namely:
Figure FDA0003456395220000035
and (5) sampling points.
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