CN111865363B - High-dynamic code capture method based on conjugate frequency modulation - Google Patents

High-dynamic code capture method based on conjugate frequency modulation Download PDF

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CN111865363B
CN111865363B CN202010670247.XA CN202010670247A CN111865363B CN 111865363 B CN111865363 B CN 111865363B CN 202010670247 A CN202010670247 A CN 202010670247A CN 111865363 B CN111865363 B CN 111865363B
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conjugate
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frequency modulation
frequency
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CN111865363A (en
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韩玉兵
韩亚龙
戴佳媛
彭丽莹
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Nanjing University of Science and Technology
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
    • H04B1/7073—Synchronisation aspects
    • H04B1/7075—Synchronisation aspects with code phase acquisition

Abstract

The invention discloses a high dynamic download code capturing method based on conjugate frequency modulation.A sending end designs a combined signal which consists of two parts, wherein the front part is formed by alternately splicing a frequency modulation signal and a conjugate signal thereof, and the rear part is a spread spectrum modulation signal; the receiving end receives the combined signal, and the combined signal is sent to a non-conjugate channel and a conjugate channel for matched filtering after down-conversion processing, and the matched filtering result is subjected to modulo calculation to obtain two-channel judgment data; and sending the two-channel decision data to a decision device, determining the capturing moment according to the two-channel joint decision result, and starting to generate a local spread spectrum code. The invention eliminates the influence of frequency deviation on code capture by utilizing the reverse direction deviation of the frequency modulation signal and the autocorrelation curve of the conjugate signal thereof under the condition of the same Doppler frequency deviation and the combined technology of the frequency modulation signal and the spread spectrum modulation signal, and realizes high-precision code capture under high dynamic and large Doppler frequency deviation.

Description

High-dynamic code capture method based on conjugate frequency modulation
Technical Field
The invention belongs to the technical field of communication, and particularly relates to a high-dynamic code downloading capturing method based on conjugate frequency modulation.
Background
Spread spectrum communication has the advantages of strong anti-interference capability, convenience for code division multiple access and the like, and is widely applied to the fields of secret communication, GPS navigation and the like. The acquisition of the spread spectrum code is one of the key technologies of a spread spectrum communication system, the acquisition probability and the anti-interference capability of the spread spectrum code directly influence the system performance, and in the process of acquiring the spread spectrum code, a plurality of factors can influence the final result to cause performance reduction, wherein the low signal-to-noise ratio of an input signal and the large Doppler frequency offset are key reasons of acquisition failure, especially in short burst communication.
In the traditional spread spectrum signal capture, the sharp autocorrelation characteristic of a spread spectrum code is utilized, a received signal and a locally generated spread spectrum code are subjected to correlation operation, the result of the correlation operation is compared with a threshold, if the result is greater than the threshold, the capture is successful, and otherwise, the capture process is continued. The basic capture methods include matched filter method, sequential detection capture method and transmitted signal reference method. Under the condition of large Doppler frequency offset caused by high dynamic, the peak value of the autocorrelation curve of the spread spectrum code can be deviated and reduced, even the curve shape is distorted, and under the condition, the accuracy of code capture can be seriously influenced by using the correlation characteristic of the spread spectrum code to capture the code.
Disclosure of Invention
The invention aims to provide a high-dynamic code downloading and capturing method based on conjugate frequency modulation.
The technical scheme for realizing the aim of the invention is as follows: a high dynamic code capture method based on conjugate frequency modulation comprises the following steps:
step 1: receiving a combined signal, wherein the combined signal consists of two parts, the front part is a frequency modulation signal and a conjugate signal thereof which are spliced alternately, and the rear part is a spread spectrum modulation signal;
step 2: sending the received combined signal after digital down-conversion processing to a non-conjugate channel and a conjugate channel at the same time to obtain non-conjugate channel decision data and conjugate channel decision data;
and step 3: sending the two-channel judgment data into a joint judger, judging whether the capturing is successful or not according to a 1+1 judgment criterion, and if the capturing is successful, obtaining time points corresponding to two successful judgments;
and 4, step 4: and determining the final capturing time point according to the time points corresponding to the two successful judgments to complete the code phase capturing.
Preferably, the mathematical model of the combined signal is represented as:
Figure BDA0002582022290000021
in the formula (f)cIn order to transmit the signal at the intermediate frequency,
Figure BDA0002582022290000022
for initial phase, p (t) is the combined baseband signal.
Preferably, the combined baseband signal is specifically:
Figure BDA0002582022290000023
wherein
Figure BDA0002582022290000024
And
Figure BDA0002582022290000025
a pair of mutually conjugated FM signals, M is an even number representing the number of FM signals, T is a symbol duration, d (T) is a spread spectrum modulation symbol, c (T) is a spread spectrum code sequence, M (T) is a FM function,
Figure BDA0002582022290000026
is defined as:
Figure BDA0002582022290000027
preferably, the received signal obtained after sampling and digital down-conversion is:
Figure BDA0002582022290000028
wherein P [ k-d]Comprises the following steps:
Figure BDA0002582022290000029
where k is 0,1, 2.. N-1, N is the number of sampling points of one symbol length, fdIs the Doppler frequency, fsIs the sampling frequency and d is the transmission delay.
Preferably, the non-conjugate channel filter coefficients are:
hf[k]＝x*[tk-k]
the conjugate channel filter coefficients are:
hg[k]＝x[tk-k]
the ith original frequency modulation signal x (k-d-i2N) in the pilot signal is sent to a non-conjugate channel to obtain decision data | z1(k) I is:
Figure BDA0002582022290000031
the ith conjugate FM signal x in the pilot signal*(k-d-N-2iN) is sent into the conjugate channel to obtain the decision data | z2(k) I is:
Figure BDA0002582022290000032
preferably, the specific method for sending the two-channel decision data to the joint decision device and jointly judging whether the capturing is successful according to the 1+1 decision criterion is as follows:
step 3-1: non-conjugate channel decision data | z1(k) Comparing | with threshold value to make preliminary decision, if it is not conjugate channel decision data | z1(k) | is greater than the threshold value, and the corresponding time point is kzThen at (k)z,kz+ N/2) search for maximum value | z of decision data of non-conjugate channel1(kmax1) I, | and its corresponding time point kmax 1Wherein N is the number of sampling points of one bit data;
step 3-2: in (k)max 1+3N/4,kmax 1+5N/4) to obtain the maximum value | z of the decision data2(kmax2) I, | and its corresponding time point kmax 2And judging | z2(kmax2) If | is greater than the threshold value, if | z2(kmax2) If | is larger than the threshold value, the combined signal is successfully captured.
Preferably, the final capture time point is:
Figure BDA0002582022290000041
in the formula, kmax 2Judging the time point, k, corresponding to the maximum value of the data for the conjugated channelmax 1And judging the time corresponding to the maximum value of the data for the non-conjugated channel, wherein N is the number of sampling points with one symbol length.
Compared with the prior art, the invention has the following remarkable advantages: the invention eliminates the influence of frequency deviation on code capture by utilizing the reverse direction deviation of the frequency modulation signal and the autocorrelation curve of the conjugate signal thereof under the condition of the same Doppler frequency deviation and the combined technology of the frequency modulation signal and the spread spectrum modulation signal, and realizes high-precision code capture under high dynamic and large Doppler frequency deviation.
The present invention is described in further detail below with reference to the attached drawings.
Drawings
Fig. 1 is a diagram illustrating a design structure of a signal at a transmitting end according to the present invention.
Fig. 2 is a diagram of a baseband signal at the receiving end according to the present invention.
Fig. 3 is a block diagram of a signal processing structure of a code acquisition process at a receiving end according to the present invention.
Fig. 4 is a simulation waveform diagram of the LFM and its conjugate signal autocorrelation result Matlab under the same doppler frequency offset.
Fig. 5 is a Matlab simulation waveform diagram for implementing code acquisition by non-conjugate and conjugate channel joint decision under the 1+1 decision criterion.
Fig. 6 is a frequency-time diagram of 6 different frequency modulation schemes.
Detailed Description
A high dynamic code capture method based on conjugate frequency modulation is disclosed, wherein a transmitting end signal design structure is shown in figure 1, a receiving end base signal structure is shown in figure 2, a receiving end code capture process signal processing structure is shown in figure 3, and the specific steps are as follows:
step 1: and receiving a combined signal sent by a sending end, wherein the combined signal consists of two parts, the front part is a frequency modulation signal and a conjugate signal thereof which are spliced alternately, and the rear part is a spread spectrum modulation signal. The mathematical model of the combined signal is represented as:
Figure BDA0002582022290000042
in the formula (f)cIn order to transmit the signal at the intermediate frequency,
Figure BDA0002582022290000051
for initial phase, p (t) is the combined baseband signal:
Figure BDA0002582022290000052
wherein
Figure BDA0002582022290000053
And
Figure BDA0002582022290000054
a pair of mutually conjugated FM signals, M is an even number representing the number of FM signals, T is a symbol duration, d (T) is a spread spectrum modulation symbol, c (T) is a spread spectrum code sequence, M (T) is a FM function,
Figure BDA0002582022290000055
is defined as:
Figure BDA0002582022290000056
the baseband structure of the combined signal is shown in fig. 1, when t is greater than or equal to 0 and less than or equal to MT, the transmitted signal is a pilot signal, the modulation mode is frequency modulation, and different frequency modulation modes can be adopted, such as 6 frequency modulation modes shown in fig. 6; when t > MT, the transmission signal is a spread spectrum modulation signal. And splicing the frequency modulation signal and the spread spectrum modulation signal according to the sequence. Note that the bandwidth of the pilot frequency modulated signal is equal to the intermediate bandwidth of the spread spectrum signal.
Step 2: the receiving end simultaneously sends the received combined signal after digital down-conversion processing to a non-conjugate channel and a conjugate channel for matched filtering to obtain decision data | z of the non-conjugate channel and the conjugate channel1(k) I and I z2(k)|；
Step 2-1: the received signal obtained after sampling and digital down-conversion is:
Figure BDA0002582022290000057
wherein P [ k-d]Comprises the following steps:
Figure BDA0002582022290000058
the signal structure is shown in fig. 2, where k is 0,1, 2.. N-1, N is the number of sampling points of one symbol length, and f is the number of sampling pointsdIs the Doppler frequency, fsIs the sampling frequency and d is the transmission delay.
Step 2-2: r [ k ] is]And simultaneously, the data are sent into a non-conjugate channel and a conjugate channel for matched filtering, and a matched filtering result is subjected to modulo operation to obtain judgment data. Wherein the non-conjugate channel filter coefficient is hf[k]＝x*[tk-k]Coefficient of conjugate channel filter of hg[k]＝x[tk-k]，tkN-1. When the ith original frequency modulation signal x (k-d-i2N) in the pilot signal is sent into a non-conjugate channel, the decision data | z is obtained1(k) I is:
Figure BDA0002582022290000061
when the ith conjugate FM signal x in the pilot signal*(k-d-N-2iN) into the conjugate channel to obtain decision data | z2(k) I is:
Figure BDA0002582022290000062
and step 3: two-channel decision data | z1(k) I and I z2(k) Sending the I into a joint decision device, jointly judging whether the capturing is successful according to a 1+1 decision criterion, and if the capturing is successful, obtaining a time point k corresponding to the two successful decisionsmax 1And kmax 2；
Step 3-1: non-conjugate channel decision data | z1(k) Comparing | with a threshold value to make a preliminary decision, and if | z1(k) | is greater than the threshold value, and the corresponding time point is kzThen at (k)z,kz+ N/2) search for | z1(k) Maximum value of | z1(kmax1) I, | and its corresponding time point kmax 1Where N is the number of sample points for one bit of data.
Step 3-2: in (k)max 1+3N/4,kmax 1+5N/4) to obtain the maximum value | z of the decision data2(kmax2) I, | and its corresponding time point kmax 2And judging | z2(kmax2) If | is greater than the threshold value. If z2(kmax2) If | is larger than the threshold value, the combined signal is successfully captured.
And 4, step 4: according to the conjugate channel and the conjugate channel peak value corresponding to the time point kmax 1And kmax 2Determining a final acquisition time kt。
Let k1And k2For a certain sampling time point of two channels, as can be seen from equations (4) and (5), when k is1And k2Satisfy k1-tk-d-2iN＝-(k2-tk-d-N-2iN) i.e. k1+k2-2tk-2d-4iN-N ═ 0,
Figure BDA0002582022290000071
i.e. the two channels output decision data are equal at this time. Considering that in one symbol period, | z1(k) I and I z2(k) I has a unique peak at which time the two values are equal, i.e., z1(kmax1)|＝|z2(kmax2) L, so kmax 1And kmax 2Satisfies the following conditions:
kmax1+kmax2-2tk-2d-4iN-N＝0 (7)
and because if the peak value of the conjugated channel corresponds to the time point kmax 2As the acquisition time, the timing error due to doppler frequency offset is:
Δk2＝kmax2-tk-d-N-2iN (8)
in conjunction with equations (7) and (8), one can deduce:
Figure BDA0002582022290000072
timing errors caused by Doppler frequency offset are eliminated.
So the final acquisition instant can be determined as:
Figure BDA0002582022290000073
furthermore, since the receiving combined signal is formed by splicing the frequency modulation signal and the spread spectrum modulation signal according to the sequence, capturing the frequency modulation signal in the combined signal is equivalent to capturing the spread spectrum signal connected later.
Example 1
To further illustrate the advantages of code acquisition using a conjugate frequency modulated signal, an example is provided.
Presetting 8 bits of pilot data as 01010101, 0 corresponding to an upper frequency sweep LFM signal, 1 corresponding to a lower frequency sweep LFM conjugate signal, and then adopting DSSS-BPSK modulation on the spread spectrum signal, wherein the symbol rate Rb is 0.25Mbps, the sampling rate Fs is 200MHz, the spread spectrum code of the DSSS-BPSK signal is a 32-bit truncation gold code, the code rate Rc is 8Mcps, and the modulation bandwidth F of the LFM signal is 16MHz, namely, the intermediate frequency bandwidths of the two modulation modes are the same. Presetting 10 information bits, adopting DSSS-BPSK signals, presetting Doppler frequency offset f with parameters consistent with pilot data d40 KHz. The simulation was performed using Matlab software.
The autocorrelation curves of the LFM signal and its conjugate signal are shown in fig. 4. It can be seen from the figure that the autocorrelation curves of the LFM signal and its conjugate signal coincide without doppler frequency offset. Compared with the condition without Doppler frequency offset, under the condition of 40KHz frequency offset, the autocorrelation peak value of the upper frequency sweep LFM signal is shifted to the left by 2 sampling points, and the correlation peak value of the lower frequency sweep LFM signal is shifted to the right by 2 sampling points, namely, the correlation peak is in bilateral symmetry relative to the condition without frequency offset, the influence of Doppler frequency offset can be eliminated by utilizing the symmetry of the correlation peak, and accurate positioning is realized.
The matching filtering and decision results of the non-conjugate channel and the conjugate channel at the receiving end are shown in fig. 5, the maximum value of the non-conjugate channel decision data at the 802 th sampling point is greater than the adaptive constant false alarm threshold, the maximum value of the conjugate channel decision data at the 1598 th sampling point is greater than the threshold value, at this moment, the combined signal is successfully captured, the final capture moment is determined to be the 1600 th sampling point according to the serial numbers of the two decision sampling points, and at this moment, the local spread spectrum code starts to be generated.
In summary, compared with the conventional code capture method, the method has the advantages that by using the reverse-direction offset of the autocorrelation curve of the LFM and the conjugate signal thereof under the same frequency offset condition and the splicing technology of the LFM signal and the spread spectrum signal, the influence of the doppler frequency offset on the code capture can be eliminated, the accurate code capture under the condition of high dynamic large frequency offset is realized, and the method has stronger anti-doppler capability.

Claims (7)

1. A high dynamic code capture method based on conjugate frequency modulation is characterized by comprising the following steps:
step 1: receiving a combined signal, wherein the combined signal consists of two parts, the front part is a frequency modulation signal and a conjugate signal thereof which are spliced alternately, and the rear part is a spread spectrum modulation signal;
step 2: sending the received combined signal after digital down-conversion processing to a non-conjugate channel and a conjugate channel at the same time to obtain non-conjugate channel decision data and conjugate channel decision data;
and step 3: sending the two-channel judgment data into a joint judger, judging whether the capturing is successful or not according to a 1+1 judgment criterion, and if the capturing is successful, obtaining time points corresponding to two successful judgments;
and 4, step 4: and determining the final capturing time point according to the time points corresponding to the two successful judgments to complete the code phase capturing.
2. The conjugate frequency modulation-based high dynamic code acquisition method as claimed in claim 1, wherein the mathematical model of the combined signal is represented as:
Figure FDA0003518980910000011
in the formula (f)cIn order to transmit the signal at the intermediate frequency,
Figure FDA0003518980910000012
for initial phase, p (t) is the combined baseband signal.
3. The method as claimed in claim 2, wherein the combined baseband signal is specifically:
Figure FDA0003518980910000013
wherein
Figure FDA0003518980910000014
And
Figure FDA0003518980910000015
a pair of mutually conjugated FM signals, M is an even number representing the number of FM signals, T is a symbol duration, d (T) is a spread spectrum modulation symbol, c (T) is a spread spectrum code sequence, M (T) is a FM function,
Figure FDA0003518980910000016
is defined as:
Figure FDA0003518980910000017
4. the method of claim 1, wherein the received signal after sampling and digital down-conversion is:
Figure FDA0003518980910000018
wherein P [ k-d]Comprises the following steps:
Figure FDA0003518980910000021
where k is 0,1, 2.. N-1, M is an even number indicating the number of frequency modulation signals, N is the number of sampling points of one symbol length, and f is the number of sampling points of one symbol lengthdIs the Doppler frequency, fsIs the sampling frequency and d is the transmission delay.
5. The method of claim 1, wherein the non-conjugate channel filter coefficients are:
hf[k]＝x*[tk-k]
the conjugate channel filter coefficients are:
hg[k]＝x[tk-k]
the ith original frequency modulation signal x (k-d-i2N) in the pilot signal is sent to a non-conjugate channel to obtain decision data | z1(k) I is:
Figure FDA0003518980910000022
the ith conjugate FM signal x in the pilot signal*(k-d-N-2iN) is sent into the conjugate channel to obtain the decision data | z2(k) I is:
Figure FDA0003518980910000031
where k is 0,1, 2.. N-1, N is the number of sampling points of one symbol length, tk＝N-1，fdIs the Doppler frequency, fsIs the sampling frequency and d is the transmission delay.
6. The conjugate frequency modulation-based high-dynamic code download method as claimed in claim 1, wherein the two channel decision data are sent to the joint decision device, and the specific method for jointly judging whether the capture is successful according to the 1+1 decision criterion is as follows:
step 3-1: non-conjugate channel decision data | z1(k) Comparing | with threshold value to make preliminary decision, if it is not conjugate channel decision data | z1(k) | is greater than the threshold value, and the corresponding time point is kzThen at (k)z,kz+ N/2) search for maximum value | z of decision data of non-conjugate channel1(kmax1) I, | and its corresponding time point kmax1Wherein N is the number of sampling points of one bit data;
step 3-2: in (k)max1+3N/4,kmax1+5N/4) to obtain the maximum value | z of the decision data2(kmax2) I, | and its corresponding time point kmax2And judging | z2(kmax2) If | is greater than the threshold value, if | z2(kmax2) If | is larger than the threshold value, the combined signal is successfully captured.
7. The method according to claim 1, wherein the final acquisition time point is:
Figure FDA0003518980910000032
in the formula, kmax2Judging the time point, k, corresponding to the maximum value of the data for the conjugated channelmax1And judging the time corresponding to the maximum value of the data for the non-conjugated channel, wherein N is the number of sampling points with one symbol length.
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