CN112019303A - VDES satellite receiver frame header detection method and system - Google Patents
VDES satellite receiver frame header detection method and system Download PDFInfo
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- CN112019303A CN112019303A CN202010918830.8A CN202010918830A CN112019303A CN 112019303 A CN112019303 A CN 112019303A CN 202010918830 A CN202010918830 A CN 202010918830A CN 112019303 A CN112019303 A CN 112019303A
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/0001—Systems modifying transmission characteristics according to link quality, e.g. power backoff
- H04L1/0036—Systems modifying transmission characteristics according to link quality, e.g. power backoff arrangements specific to the receiver
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- H—ELECTRICITY
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- H04B7/00—Radio transmission systems, i.e. using radiation field
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- H04B7/185—Space-based or airborne stations; Stations for satellite systems
- H04B7/18578—Satellite systems for providing broadband data service to individual earth stations
- H04B7/18582—Arrangements for data linking, i.e. for data framing, for error recovery, for multiple access
Abstract
The invention relates to the technical field of ground-end VDES signal detection, and provides a method and a system for detecting a frame header of a VDES satellite receiver, wherein the method comprises the following steps: s1: preprocessing a received signal, and constructing a periodic sequence by using the characteristics of double Barker codes in a frame structure of a ground-end VDES signal to form a preprocessed signal; s2: performing Fourier transform on the preprocessed signal, based on the periodicity of the preprocessed signal, enabling a peak value to appear in a specific position of a transformed sequence, performing constant false alarm rate detection on the ground-end VDES signal, and detecting the ground-end VDES signal; s3: and performing multiple times of constant false alarm rate detection, and when the ground end VDES signals are continuously detected for multiple times, determining that the ground end VDES signals are detected. The method has the technical characteristic of constant false alarm rate detection, and is beneficial to the satellite receiver to detect the uncoordinated communication signals.
Description
Technical Field
The invention relates to the technical field of ground VDES signal detection, in particular to a method and a system for detecting a frame header of a VDES satellite receiver.
Background
With the great success of the application of Automatic Identification Systems (AIS) for ships and the continuous development of AIS functions, the network load of AIS is getting larger and larger to impair the initial concept of AIS collision avoidance. In order to guarantee the performance of the AIS, the international telecommunication union WP5B group and the international navigation aid and navigation association E-NAV group have a conference to discuss the technical scheme and development direction of the next generation AIS, namely, the very high frequency data exchange system (VDES). On the basis of ensuring the highest priority of the AIS, the organizations such as the International maritime organization and the International telecommunication Union separate Application Specific Messages (ASM) in the AIS and allocate new channels (CH2027 and CH2028) and modulation modes (pi/4 QPSK) for the ASM. Meanwhile, a new VHF band (CH24, CH25, CH26, CH84, CH85, and CH86) and a new modulation scheme (QPSK, 8PSK, 16QAM, 16APSK, etc.) are allocated to the VDE, and a forward error correction coding technique, a continuous phase modulation spread spectrum technique, etc. are introduced into the VDE.
According to the IALA G1139 document, ground-end VDES signals (the present patent primarily considers ASM-TER and VDE-TER signals) are currently used for communication between ships and shore. In order to supervise and protect the open-sea ship, a supervision department can monitor the broadcast information of the open-sea ship by using a satellite carried related receiver. The satellite receiving the far-sea ship ground-end VDES signal faces the following technical problems:
1) the satellite has high running speed, and the ground-end VDES message received from the ship broadcast has larger Doppler frequency offset;
2) the satellite only scouts and does not participate in communication in the self-organizing time division multiplexing area, so that the time of transmitting the ground-end VDES signal by each ship cannot be known.
Disclosure of Invention
In view of the above problems, an object of the present invention is to provide a method and a system for detecting a frame header of a VDES satellite receiver, which have the technical characteristics of constant false alarm rate detection and are beneficial to a satellite receiver to perform uncoordinated communication signal detection.
The above object of the present invention is achieved by the following technical solutions:
a method for detecting a frame header of a VDES satellite receiver comprises the following steps:
s1: preprocessing a received signal, and constructing a periodic sequence by using the characteristics of double Barker codes in a frame structure of a ground-end VDES signal to form a preprocessed signal;
s2: performing Fourier transform on the preprocessed signal, based on the periodicity of the preprocessed signal, enabling a peak value to appear in a specific position of a transformed sequence, performing constant false alarm rate detection on the ground-end VDES signal, and detecting the ground-end VDES signal;
s3: and performing multiple times of constant false alarm rate detection, and when the ground end VDES signals are continuously detected for multiple times, determining that the ground end VDES signals are detected.
Further, in step S1, specifically, the method includes:
and determining that the length of the received signal is 26 symbols according to the frame header structure characteristics of the ground-end VDES signal to form an intercepted signal, and performing low-pass filtering and amplitude normalization processing, 13 symbol differential operation, 1 symbol differential operation and imaginary part taking processing on the intercepted signal to obtain a periodic preprocessing signal with the period of 2M and the total length of N being 12M.
Further, the method for detecting the frame header of the VDES satellite receiver further includes:
the frame header structure of the ground-end VDES comprises a synchronization word which consists of 1 Barker13 and anti-Barker 13 and has 27 symbols in total, and the interception length of the received signal is determined to be 26 symbols.
Further, the method for detecting the frame header of the VDES satellite receiver further includes:
the intercepted signal is processed by low-pass filtering and amplitude normalization processing, 13 symbol difference operation, 1 symbol difference operation and imaginary part taking, so as to obtain a preprocessed signal, and the specific steps are as follows:
s11: low-pass filtering and amplitude normalization processing are carried out on the intercepted signal, and r is definedn[i]Indicating that the intercepted information after normalization processing is at t ═ nTb+iTsSample point of time, TbDenotes the symbol width, TsRepresents a sampling interval;
s12: carrying out 13-symbol difference operation on the intercepted signal of 26 symbols after normalization processing to form a difference signal:
s13: performing a 1-symbol difference operation on the 13-symbol difference signal:
and taking imaginary part to obtain preprocessed signal psi with 12 symbol lengthn[i]＝Im{yn[i]}。
Further, in step S2, specifically, the method includes:
performing discrete Fourier transform on the preprocessed signal, and performing modular squaring on the transformed sequence to obtain a sequence u (k);
based on the pre-processed signal constructed using the double Barker code feature in step S1 with a period of 2M and a total length of N12M, the fourier transformed sequence will have a peak at an integer multiple of N/2M, and the energy is mainly concentrated at two points, k N/2M and k-N/2M;
and carrying out constant false alarm rate detection on the ground-end VDES signal, taking a point near k to N/2M to estimate noise, calculating a ratio R of u (N/2M) to the noise, and considering that the ground-end VDES signal is detected when R is greater than a threshold value T, wherein the threshold value T is related to false alarm detection probability.
Further, the method for detecting the frame header of the VDES satellite receiver further includes:
predefining a detection counter, wherein the detection counter accumulates 1 after the ground end VDES signal is detected, and otherwise, the detection counter is set to be 0;
the received signal is slid backward by 1 symbol length, and the execution is resumed from step S1.
Further, in step S3, the method further includes:
presetting a detection counting threshold, when the counting of the detection counter reaches the detection counting threshold, determining that the ground end VDES signal is detected, wherein the specific process of accumulating and judging by the detection counter is as follows:
judging whether the count of the detection counter reaches the detection count threshold value;
if the detection count threshold value is not reached, sliding the received signal backwards by 1 symbol length, and then re-executing step S1;
if the detection count threshold is reached, it is determined that the ground-side VDES signal is detected, the received signal is slid backward by 26 symbol lengths, the detection counter is set to 0, and step S1 is executed again.
Further, the ground-end VDES signal is modulated by pi/4 QPSK, and a mapping relationship of pi/4 QPSK modulation symbols is defined as follows:
the first symbol adopts a constellation diagram of
The constellation diagram adopted by the second symbol is {1+0j, 0+ j, -1+0j, 0-j };
the third symbol uses the same constellation diagram as the first symbol, the fourth symbol uses the same constellation diagram as the second symbol, and other symbols are alternated all the time.
Further, in step S12, when the received signal is a double Barker13 sequence, the 13-symbol difference operation results in a sequence phase of:
wherein Δ f identifies the doppler frequency offset;
further, in step S13, in order to eliminate the effect of doppler frequency offset, 1-symbol difference operation is performed on the difference sequence of 13-symbol length, and the sequence phase of the preprocessed signal is obtained as:
in order to implement the system of the method for detecting the frame header of the VDES satellite receiver, the system includes:
the receiving signal preprocessing module is used for preprocessing a receiving signal, and constructing a periodic sequence by using the characteristics of double Barker codes in a frame structure of a ground-end VDES signal to form a preprocessed signal;
a frame header detection module of a single ground-end VDES signal, configured to perform Fourier transform on the preprocessed signal, where a peak appears in a sequence after the transform at a specific position based on periodicity of the preprocessed signal, perform constant false alarm rate detection on the ground-end VDES signal, and detect the ground-end VDES signal;
and the frame header detection module is used for carrying out continuous multiple-time constant false alarm rate detection, and when the ground end VDES signals are continuously detected for multiple times, the ground end VDES signals are considered to be detected.
Compared with the prior art, the invention has the following beneficial effects:
the invention constructs a periodic sequence by carrying out and processing on a received signal; periodically detecting the constant false alarm rate of a frame header of a VDES signal at a single ground end by utilizing the preprocessed signal; and performing multiple times of detection of the VDES signal frame header at the ground end according to the structural characteristics of the frame header. The method has the technical characteristics of constant false alarm detection, and is beneficial to the satellite receiver to detect the uncoordinated communication signals.
Drawings
FIG. 1 is a flow chart of a method for detecting a frame header of a VDES satellite receiver according to the present invention;
FIG. 2 is a diagram of ground-side VDES signal frame structures for ship-to-ship and ship-to-shore communication in accordance with an embodiment of the present invention;
FIG. 3 is a π/4QPSK modulation symbol mapping diagram according to the present invention;
FIG. 4 is a graph of discrete Fourier transform of a preprocessed signal according to an embodiment of the present invention;
FIG. 5 is a detection probability curve of single ground-side VDES signal frame header detection under different SNR in the embodiment of the present invention;
fig. 6 is a false alarm probability curve of single ground-side VDES signal frame header detection under different snr in the embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application, and it is obvious that the described embodiments are some embodiments of the present application, but not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.
As used herein, the singular forms "a", "an", "the" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
Example one
As shown in fig. 1, the present embodiment provides a method for detecting a frame header of a VDES satellite receiver, which is characterized by comprising the following steps:
s1: preprocessing a received signal, and constructing a periodic sequence by using the characteristics of double Barker codes in a frame structure of a ground-end VDES signal to form a preprocessed signal, wherein the preprocessing signal specifically comprises the following steps:
for the frame header structure characteristic of the ground-end VDES signal, determining that the length of the received signal is 26 symbols, forming an intercepted signal, and performing processing including low-pass filtering and amplitude normalization (the intercepted signal modulus value is 1 in this embodiment), 13-symbol differential operation, 1-symbol differential operation, and taking the imaginary part of the intercepted signal, to obtain a periodic preprocessed signal with a period of 2M and a total length of N-12M.
The method is characterized in that the method for intercepting the received signal with the length of 26 symbols is only an optimal scheme, received signals with other symbol lengths can be intercepted according to actual conditions, and only the finally formed preprocessing signal is ensured to be periodic.
Further, in the frame header structure of the ground-side VDES, a synchronization word consisting of 1, Barker13 and anti-Barker 13 and totaling 27 symbols is included, and the truncation length of the received signal is determined to be 26 symbols.
Further, the intercepted signal is processed by low-pass filtering and amplitude normalization, 13 symbol difference operation, 1 symbol difference operation and imaginary part taking, so as to obtain a preprocessed signal, and the specific steps are as follows:
s11: low-pass filtering and amplitude normalization processing are carried out on the intercepted signal (the modulus value of the intercepted signal is 1), and r is definedn[i]Indicating that the intercepted information after normalization processing is at t ═ nTb+iTsSample point of time, TbDenotes the symbol width, TsRepresents a sampling interval;
s12: carrying out 13-symbol difference operation on the intercepted signal of 26 symbols after normalization processing to form a difference signal:
s13: performing a 1-symbol difference operation on the 13-symbol difference signal:
and taking imaginary part to obtain preprocessed signal psi with 12 symbol lengthn[i]＝Im{yn[i]}。
Further, in step S12, when the received signal is a double Barker13 sequence, the 13-symbol difference operation results in a sequence phase of:
wherein Δ f identifies the doppler frequency offset;
further, in step S13, in order to eliminate the effect of doppler frequency offset, 1-symbol difference operation is performed on the difference sequence of 13-symbol length, and the sequence phase of the preprocessed signal is obtained as:
s2: performing fourier transform on the preprocessed signal, based on the periodicity of the preprocessed signal, a peak value appears in a specific position of a transformed sequence, performing constant false alarm rate detection on the ground-end VDES signal, and detecting the ground-end VDES signal, specifically:
performing discrete Fourier transform on the preprocessed signal, and performing modular squaring on the transformed sequence to obtain a sequence u (k);
based on the preprocessed signal with a period of 2M and a total length of N12M constructed in step S1 using the double Barker code feature, the fourier transformed sequence will have peaks at integer times N/2M (6) positions, and the energy is mainly concentrated at two points, k N/2M 6 and k-N/2M-6;
and carrying out constant false alarm rate detection on the ground-end VDES signal, taking a point near k to N/2M to estimate noise (for example, taking noise near k to 2-5 and 7-10 when k to N/2M to 6), calculating a ratio R of u (N/2M) to the noise, and considering that the ground-end VDES signal is detected when R is greater than a threshold value T, wherein the threshold value T is related to false alarm detection probability.
Further, a detection counter is predefined, when the ground end VDES signal is detected, the detection counter C increments by 1, otherwise, the detection counter C is set to 0; the received signal is slid backward by 1 symbol length, and the execution is resumed from step S1.
S3: and performing multiple times of constant false alarm rate detection, and when the ground end VDES signals are continuously detected for multiple times, determining that the ground end VDES signals are detected.
Specifically, in order to suppress the false alarm probability, the ground-side VDES signal is considered to be present only if the ground-side VDES signal is detected several times in succession.
Further, in step S3, the method further includes:
performing continuous ground end VDES signal frame header detection according to the frame header structure characteristics of the ground end VDES signals;
presetting a detection counting threshold, when the counting of the detection counter reaches the detection counting threshold, determining that the ground end VDES signal is detected, wherein the specific process of accumulating and judging by the detection counter is as follows:
judging whether the count of the detection counter reaches the detection count threshold value;
if the detection count threshold value is not reached, sliding the received signal backwards by 1 symbol length, and then re-executing step S1;
if the detection count threshold is reached, it is determined that the ground-side VDES signal is detected, the received signal is slid backward by 26 symbol lengths, the detection counter is set to 0, and step S1 is executed again.
For example, setting the detection count threshold to be 3, performing continuous frame header detection of the ground-end VDES signal according to the frame header structure characteristics of the ground-end VDES signal: if the counter value C ≠ 3, the received signal is slid backward by 1 symbol length, and then the execution is restarted from step S1; if the counter value C is 3, it is determined that the ground-side VDES signal is present, and at this time, the received signal is slid backward by 26 symbol lengths, and the process is executed from step S1 again.
Further, in this embodiment, the ground-end VDES signal adopts pi/4 QPSK modulation, and a pi/4 QPSK modulation symbol mapping relationship is defined as follows:
the first symbol adopts a constellation diagram of
The constellation diagram adopted by the second symbol is {1+0j, 0+ j, -1+0j, 0-j };
the third symbol uses the same constellation diagram as the first symbol, the fourth symbol uses the same constellation diagram as the second symbol, and other symbols are alternated all the time.
Example two
In order to execute the method for detecting a frame header of a VDES satellite receiver in the first embodiment, this embodiment provides a system corresponding to the method in the first embodiment, and specifically includes:
the receiving signal preprocessing module 1 is used for preprocessing a receiving signal, and constructing a periodic sequence by using the characteristics of double Barker codes in a frame structure of a ground-end VDES signal to form a preprocessed signal;
a single ground-end VDES signal frame header detection module 2, configured to perform fourier transform on the preprocessed signal, where, based on periodicity of the preprocessed signal, a peak value appears in a sequence after the transform at a specific position, perform constant false alarm rate detection on the ground-end VDES signal, and detect the ground-end VDES signal;
and a frame header detection module 3 for continuously detecting the ground-end VDES signal for multiple times, wherein the frame header detection module is used for performing multiple times of constant false alarm rate detection, and when the ground-end VDES signal is continuously detected for multiple times, the ground-end VDES signal is considered to be detected.
It should be noted that the system of this embodiment includes all the details of the first embodiment, and the details of this embodiment are not repeated.
The received signal preprocessing module 1 constructs a periodic sequence by using the characteristics of the double Barker codes in the frame structure of the VDES signal at the ground end. The invention has the technical characteristic of constant false alarm rate detection, and is beneficial to the reconnaissance of uncoordinated communication signals by a satellite receiver.
Specifically, according to the IALA G1139 file, 1min is divided into 2250 slots. Each slot is about 26.67ms, and since the symbol rate of the ground-side VDES message is 9.6kbps, the data length of each slot is 256 symbols. The ground-side VDES message transmission may occupy K consecutive time slots.
The structure of the ground-side VDES signal (ASM-TER and VDE-TER signals) frame for communication between ships and shore in the VDES system is shown in fig. 2.
The sync word is in the form of 1+ Barker13+ anti-Barker 13, specifically 111111001101010000011001010. In the sync word, symbol 1 represents symbol 11 and symbol 0 represents symbol 00. According to the IALA G1139 file, the VDES message at the ground end adopts pi/4 QPSK modulation, which is consistent with the modulationThe number mapping relationship is shown in fig. 3. The pi/4 QPSK modulation symbol mapping relation is defined as follows: the first symbol adopts a constellation diagram of
(as indicated by the lighter color in fig. 3), the second symbol uses a constellation of {1+0j, 0+ j, -1+0j, 0-j } (as indicated by the darker color in fig. 3), the third symbol uses the same constellation as the first symbol, the fourth symbol uses the same constellation as the second symbol, and the other symbols are alternated.
In the received signal preprocessing module 1, a synchronization word in a frame header structure of a ground-end VDES signal consists of 1, Barker13 and anti-Barker 13, and the total number of the synchronization words is 27 symbols; determining the interception length of the received signal to be 26 symbols; firstly, low-pass filtering and amplitude normalization are carried out on the intercepted signal (the modulus value of the intercepted signal is 1), and r is definedn[i]Denotes the normalized clipped signal at t ═ nTb+iTsSample point of time, TbDenotes the symbol width, TsIndicating the sampling interval. And then carrying out 13-symbol difference operation on the 26-symbol receiving signals:
performing a 1-symbol difference operation on the difference signal of 13-symbol length:
and taking imaginary part to obtain preprocessed signal psi with 12 symbol lengthn[i]＝Im{yn[i]}. In this embodiment, M is 8.
When the received signal is a double Barker13 sequence, the 13-symbol differential operation results in a sequence phase of
Where Δ f represents the doppler shift. It can be seen that the sequence after differentiation is a periodic sequence with a period of 2 symbols.
In order to eliminate the influence of Doppler frequency offset, 1-symbol differential operation is carried out on a differential sequence with the length of 13 symbols to obtain a preprocessed signal sequence with the phase of
Discrete fourier transform of the imaginary sequence of the preprocessed signal, as shown in fig. 4.
In a single ground end VDES signal frame header detection module, discrete Fourier transform is carried out on the preprocessed signal, and a sequence u (k) is obtained by performing modular squaring on the transformed sequence. Since the preprocessed signal period constructed by the double Barker code characteristic in step S1 is 2 symbols and the total length is 12 symbols, the fourier transformed sequence will have a peak at an integer multiple of 6, and the energy is mainly concentrated at two points, k-6 and k-6. And (3) taking points near k 6 to estimate the noise, such as k 2-5 and 7-10. The ratio R of u (6) to noise is calculated. When R is larger than a threshold T (the threshold T is related to false alarm detection probability), considering that a ground end VDES signal is detected, and accumulating the counter value C by 1; if R is less than or equal to the threshold value T, the ground-end VDES signal is not detected, at this time, the counter value C is set to 0, the received signal slides backwards by 1 symbol length, and the execution is started again from the received signal preprocessing module. Under the condition that the threshold is set to be 11 and the bit signal-to-noise ratio is [ -5dB 15dB ], 10000 times of simulation are run for each signal-to-noise ratio, a detection probability curve of single ground-end VDES signal frame header detection is shown in figure 5, and a false alarm probability curve is shown in figure 6.
In a continuous multiple ground-end VDES signal frame header detection module, if the counter value C is not equal to 3, sliding the received signal backwards for 1 symbol length to restart the preprocessing and single ground-end VDES signal frame header detection operation; if the counter value C is equal to 3, it is determined that there is a terrestrial-side VDES signal, and at this time, the received signal is slid backward by 26 symbol lengths, and the preprocessing and the single-time header operation of the terrestrial-side VDES signal are restarted.
The above description is only a preferred embodiment of the present invention, and the protection scope of the present invention is not limited to the above embodiments, and all technical solutions belonging to the idea of the present invention belong to the protection scope of the present invention. It should be noted that modifications and embellishments within the scope of the invention may occur to those skilled in the art without departing from the principle of the invention, and are considered to be within the scope of the invention.
The technical features of the embodiments described above may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments described above are not described, but should be considered as being within the scope of the present specification as long as there is no contradiction between the combinations of the technical features.
It should be noted that the above embodiments can be freely combined as necessary. The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Those of skill would further appreciate that the various illustrative elements and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both, and that the various illustrative components and steps have been described above generally in terms of their functionality in order to clearly illustrate this interchangeability of hardware and software. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the implementation. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention.
Claims (10)
1. A method for detecting a frame header of a VDES satellite receiver is characterized by comprising the following steps:
s1: preprocessing a received signal, and constructing a periodic sequence by using the characteristics of double Barker codes in a frame structure of a ground-end VDES signal to form a preprocessed signal;
s2: performing Fourier transform on the preprocessed signal, based on the periodicity of the preprocessed signal, enabling a peak value to appear in a specific position of a transformed sequence, performing constant false alarm rate detection on the ground-end VDES signal, and detecting the ground-end VDES signal;
s3: and performing multiple times of constant false alarm rate detection, and when the ground end VDES signals are continuously detected for multiple times, determining that the ground end VDES signals are detected.
2. The method for detecting the frame header of the VDES satellite receiver according to claim 1, wherein step S1 specifically comprises:
and determining that the length of the received signal is 26 symbols according to the frame header structure characteristics of the ground-end VDES signal to form an intercepted signal, and performing low-pass filtering and amplitude normalization processing, 13 symbol differential operation, 1 symbol differential operation and imaginary part taking processing on the intercepted signal to obtain a periodic preprocessing signal with the period of 2M and the total length of N being 12M.
3. The VDES satellite receiver frame header detection method of claim 2, further comprising:
the frame header structure of the ground-end VDES comprises a synchronization word which consists of 1 Barker13 and anti-Barker 13 and has 27 symbols in total, and the interception length of the received signal is determined to be 26 symbols.
4. The method for detecting a frame header of a VDES satellite receiver according to claim 2, wherein the step of performing the processes including low pass filtering and amplitude normalization, 13 symbol difference operation, 1 symbol difference operation and taking the imaginary part thereof on the clipped signal to obtain a preprocessed signal comprises the steps of:
s11: low-pass filtering and amplitude normalization processing are carried out on the intercepted signal, and r is definedn[i]Indicating that the intercepted information after normalization processing is at t ═ nTb+iTsSample point of time, TbDenotes the symbol width, TsRepresents a sampling interval;
s12: carrying out 13-symbol differential operation on the intercepted signal of the 26 symbols after the normalization processing to form a differential signal：
S13: performing a 1-symbol difference operation on the 13-symbol difference signal:
and taking imaginary part to obtain preprocessed signal psi with 12 symbol lengthn[i]＝Im{yn[i]}。
5. The method for detecting the frame header of the VDES satellite receiver according to claim 1, wherein step S2 specifically comprises:
performing discrete Fourier transform on the preprocessed signal, and performing modular squaring on the transformed sequence to obtain a sequence u (k);
based on the pre-processed signal constructed using the double Barker code feature in step S1 with a period of 2M and a total length of N12M, the fourier transformed sequence will have a peak at an integer multiple of N/2M, and the energy is mainly concentrated at two points, k N/2M and k-N/2M;
and carrying out constant false alarm rate detection on the ground-end VDES signal, taking a point near k to N/2M to estimate noise, calculating a ratio R of u (N/2M) to the noise, and considering that the ground-end VDES signal is detected when R is greater than a threshold value T, wherein the threshold value T is related to false alarm detection probability.
6. The VDES satellite receiver frame header detection method of claim 5, further comprising:
predefining a detection counter, wherein the detection counter accumulates 1 after the ground end VDES signal is detected, and otherwise, the detection counter is set to be 0;
the received signal is slid backward by 1 symbol length, and the execution is resumed from step S1.
7. The method for detecting the header of a VDES satellite receiver frame according to claim 6, wherein in step S3, the method further comprises:
presetting a detection counting threshold, when the counting of the detection counter reaches the detection counting threshold, determining that the ground end VDES signal is detected, wherein the specific process of accumulating and judging by the detection counter is as follows:
judging whether the count of the detection counter reaches the detection count threshold value;
if the detection count threshold value is not reached, sliding the received signal backwards by 1 symbol length, and then re-executing step S1;
if the detection count threshold is reached, it is determined that the ground-side VDES signal is detected, the received signal is slid backward by 26 symbol lengths, the detection counter is set to 0, and step S1 is executed again.
8. The VDES satellite receiver frame header detection method of claim 1, further comprising:
the ground-end VDES signal is modulated by pi/4 QPSK, and the mapping relation of the pi/4 QPSK modulation symbols is defined as follows:
the first symbol adopts a constellation diagram of
The constellation diagram adopted by the second symbol is {1+0j, 0+ j, -1+0j, 0-j };
the third symbol uses the same constellation diagram as the first symbol, the fourth symbol uses the same constellation diagram as the second symbol, and other symbols are alternated all the time.
9. The VDES satellite receiver frame header detection method of claim 4, further comprising:
in step S12, when the received signal is a double Barker13 sequence, the 13-symbol difference operation results in a sequence phase of:
wherein Δ f identifies the doppler frequency offset;
in step S13, in order to eliminate the effect of doppler frequency offset, 1-symbol difference operation is performed on the difference sequence of 13-symbol length, and the sequence phase of the preprocessed signal is obtained as follows:
10. a system for performing the method for detecting frame headers of VDES satellite receiver according to claims 1-9, comprising:
the receiving signal preprocessing module is used for preprocessing a receiving signal, and constructing a periodic sequence by using the characteristics of double Barker codes in a frame structure of a ground-end VDES signal to form a preprocessed signal;
a frame header detection module of a single ground-end VDES signal, configured to perform Fourier transform on the preprocessed signal, where a peak appears in a sequence after the transform at a specific position based on periodicity of the preprocessed signal, perform constant false alarm rate detection on the ground-end VDES signal, and detect the ground-end VDES signal;
and the frame header detection module is used for carrying out continuous multiple-time constant false alarm rate detection, and when the ground end VDES signals are continuously detected for multiple times, the ground end VDES signals are considered to be detected.
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