CN112910497A - Quick code capture method for short-spreading-ratio satellite communication system - Google Patents

Quick code capture method for short-spreading-ratio satellite communication system Download PDF

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CN112910497A
CN112910497A CN202110062326.7A CN202110062326A CN112910497A CN 112910497 A CN112910497 A CN 112910497A CN 202110062326 A CN202110062326 A CN 202110062326A CN 112910497 A CN112910497 A CN 112910497A
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spreading
information
node
code
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CN112910497B (en
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倪祖耀
林志远
匡麟玲
刘秉坤
贾浩歌
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Tsinghua University
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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
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/14—Relay systems
    • H04B7/15—Active relay systems
    • H04B7/185—Space-based or airborne stations; Stations for satellite systems
    • H04B7/1851—Systems using a satellite or space-based relay
    • H04B7/18513—Transmission in a satellite or space-based system

Abstract

The invention discloses a quick code capturing method for a short spreading ratio satellite communication system, belonging to the technical field of wireless communication. In a short spreading ratio satellite communication system, the detection performance of the traditional correlator-based code acquisition algorithm is reduced due to the frequent flipping of information bits. In order to overcome the influence of information bit inversion, the method obtains an initial state value of a spread spectrum code by carrying out joint estimation on the information bit and a spread spectrum chip, and loads the initial value to a local code generator, thereby completing quick code capture. Under the condition of extremely low spread spectrum ratio, the invention can effectively realize the correct estimation of the initial phase of the spread spectrum code and avoid the search process of the chip phase, thereby saving a large amount of capture time compared with the traditional serial search method and simultaneously being lower than the parallel capture in complexity.

Description

Quick code capture method for short-spreading-ratio satellite communication system
Technical Field
The invention relates to the technical field of wireless communication, in particular to a quick code acquisition method for a short-spreading-ratio satellite communication system.
Background
At present, the aperture of the satellite terminal antenna gradually develops towards miniaturization, so that the beam width of the terminal antenna gradually increases. The increase in beam width makes the interference of the terminal to the adjacent satellites non-negligible. In order to reduce the adjacent satellite interference, the ITU (international telecommunications union) imposes a corresponding limitation on the transmission power spectral density of small-aperture satellite terminals. In order to meet ITU regulations, the terminal may reduce the transmit power spectral density in a spread spectrum manner. The satellite spectrum resource is precious, and the short spreading ratio does not need to occupy too large bandwidth, so the method is applied to vehicle-mounted, airborne and satellite Internet of things terminals.
In spread spectrum communication, in order to realize information despreading, spread spectrum code acquisition is an important link. Conventional code acquisition methods include serial search and parallel search, which are based on correlators to determine the phase of the spreading code by finding the largest correlation peak. However, in the short spreading ratio system, the sign inversion phenomenon of the information bits is not negligible. The information bits are modulated onto the spreading chips such that the symbols of the spreading chips are flipped. The phenomenon of frequent flipping causes the performance of the traditional code acquisition algorithm based on correlation to be reduced, and the detection probability is seriously reduced.
Disclosure of Invention
The invention aims to provide a quick code acquisition method for a short spreading ratio satellite communication system, which is characterized by comprising the following steps:
step 1: carrying out frequency offset and initial phase compensation on a section of receiving signal with the length of N;
step 2: acquiring system parameters including a spread spectrum code generator polynomial, a spread spectrum ratio, signal energy and a noise variance; sending the system parameters and the signals subjected to initial phase compensation in the step 1 into a spread spectrum code and information bit joint estimation module to obtain an initial state estimation value of the spread spectrum code;
and step 3: sending the initial state estimation value obtained in the step 2 into a local code generator to generate a local spread spectrum code;
and 4, step 4: performing integral and non-correlation accumulation operation on the local spread spectrum code generated in the step 3 and the signal subjected to initial phase compensation in the step 1, and comparing an operation result with a threshold value; if the operation result is larger than the threshold value, the acquisition is successful, otherwise, the acquisition is failed, another signal with the length of N is received, and the steps 1 to 3 are repeated until the acquisition is successful.
The received signal in step 1 is a discrete time signal, and has completed down-conversion, matched filtering and sampling operations, and the sampling rate is a chip rate.
Step 2 comprises the following substeps:
step 21: generating a polynomial according to the spreading code to construct a factor graph;
step 22: initializing probability information:
Figure BDA0002902792020000021
wherein K represents a spreading ratio, and tau represents a delay variable node in a factor graph;
step 23: initializing log-likelihood ratio information:
Figure BDA0002902792020000022
Figure BDA0002902792020000023
wherein EcRepresenting the signal energy, N0Representing the variance of the noise, ykRepresents the signal after initial phase compensation in step 1, xkRepresenting the kth spreading chip variable node, g, in the factor graphiRepresenting the ith check function node, h, in the factor graphkRepresenting the kth state transfer function node in the factor graph, dkRepresents the kth information chip variable node in the factor graph;
step 24: starting iteration, setting the maximum iteration number as IterMax, if the iteration number reaches IterMax, quitting the iteration, entering the step 25, otherwise, continuing the iteration; the l iteration is as follows:
(1) updating check function node gjTo spreading chip variable node xkLog likelihood ratio information of (1):
Figure BDA0002902792020000024
wherein N (g)j) G in the representative factor graphjThe neighbor node of (2);
(2) updating spread-spectrum chip variable node xkTo channel transfer function node fkLog likelihood ratio information of (1):
Figure BDA0002902792020000025
(3) updating channel transfer function node fkTo information chip variable node dkLog likelihood ratio information of (1):
Figure BDA0002902792020000026
(4) update information chip variable node dkNode h of the state transfer functionkAnd hk-1Log likelihood ratio information of (1):
Figure BDA0002902792020000027
(5) updating state transfer function node hkProbability information to delay variable node τ:
Figure BDA0002902792020000031
wherein I (-) represents an exemplary function,
Figure BDA0002902792020000032
Figure BDA0002902792020000033
(6) confidence of update delay variable node τ:
Figure BDA0002902792020000034
(7) updating delay variable node tau to state transfer function node hkProbability information of (2):
Figure BDA0002902792020000035
(8) updating state transfer function node hkTo information chip variable node dkAnd dk+1Log likelihood ratio information of (1):
Figure BDA0002902792020000036
Figure BDA0002902792020000037
(9) update information chip variable node dkTo channel transfer function node fkLog likelihood ratio information of (1):
Figure BDA0002902792020000038
(10) updating channel transfer function node fkTo spreading chip variable node xkLog likelihood ratio information of (1):
Figure BDA0002902792020000039
(11) updating spread-spectrum chip variable node xkTo check function node giLog likelihood ratio information of (1):
Figure BDA00029027920200000310
step 25: obtaining soft information of the spreading chips:
Figure BDA00029027920200000311
step 26: to expandingAnd (3) carrying out hard decision on the frequency chips:
Figure BDA00029027920200000312
step 27: obtaining an initial state estimation value of a spreading code:
Figure BDA00029027920200000313
where u represents the estimated initial state value of the spreading code and r represents the highest degree of the generator polynomial.
The spreading code in step 2 comprises an m sequence and a Gold sequence, and the spreading ratio is a positive integer greater than 2.
The local code generator in step 3 is composed of a linear feedback shift register, the number of stages of the shift register is equal to the highest degree of the generator polynomial in step 2, and the tap coefficient of the shift register is determined by the coefficient of the generator polynomial.
The integration in the step 4 is to perform multiplication and addition operation on the local code and the received signal, and the integration length is m; the non-correlation accumulation refers to square summation of the integration results of each segment, and the summation length is n; the operation result refers to the result of the uncorrelated accumulation.
The invention has the beneficial effects that:
the invention obtains the initial state value of the spread spectrum code by carrying out joint estimation on the information bit and the spread spectrum chip, and loads the initial value to the local code generator, thereby completing the rapid code capture. Under the condition of extremely low spreading ratio, the correct estimation of the initial phase of the spreading code can be effectively realized, and the one-by-one searching process of the chip phase is avoided. Compared with the traditional serial search method, the method provided by the invention saves a large amount of capture time, and is lower than parallel capture in complexity.
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FIG. 1 is a system diagram of a fast code acquisition method for a short spreading ratio satellite communication system;
FIG. 2 is a flow chart of a method for fast code acquisition for a short spreading ratio satellite communication system;
FIG. 3 is a factor graph diagram of example 1;
FIG. 4 is a factor graph showing the results of example 2.
Detailed Description
The present invention provides a fast code acquisition method for a short spreading ratio satellite communication system, which is further described with reference to the accompanying drawings and specific embodiments.
FIG. 1 is a system diagram of a fast code acquisition method for a short spreading ratio satellite communication system; fig. 2 is a flow chart of a fast code acquisition method for a short spreading ratio satellite communication system.
Example 1
Assume that the spreading code generator polynomial g (D) 1+ D15Spreading ratio K4, received signal length N1024, and signal energy E c1, noise variance is N0Setting the maximum iteration number Itermax to be 32, and setting the threshold value to be Th. Fast code acquisition can be achieved by:
1) carrying out frequency offset and initial phase compensation on a received signal with the length of 1024;
2) according to the generating polynomial g (D) ═ 1+ D15Constructing a factor graph, as shown in FIG. 3;
3) initializing probability information:
Figure BDA0002902792020000051
where τ represents the delay variable node in fig. 3;
4) initializing log-likelihood ratio information:
Figure BDA0002902792020000052
Figure BDA0002902792020000053
wherein y iskRepresents the compensated received signal, x, of 1)kRepresents the kth spreading chip variable node, g, in fig. 3iRepresents the ith check function node, d, in FIG. 3kRepresents the kth information chip variable node, h, in FIG. 3kRepresents the kth state transfer function node in fig. 3;
5) starting iteration, if the number of iterations reaches 32, exiting the iteration, and entering the step 6), otherwise, continuing the iteration. The l iteration is as follows:
a) updating check function node gjTo spreading chip variable node xkLog likelihood ratio information of (1):
Figure BDA0002902792020000054
wherein N (g)j) Is represented as g in FIG. 3jThe neighbor node of (2);
b) updating spread-spectrum chip variable node xkTo channel transfer function node fkLog likelihood ratio information of (1):
Figure BDA0002902792020000055
c) updating channel transfer function node fkTo information chip variable node dkLog likelihood ratio information of (1):
Figure BDA0002902792020000056
d) update information chip variable node dkNode h of the state transfer functionkAnd hk-1Log likelihood ratio information of (1):
Figure BDA0002902792020000057
e) updating state transfer function node hkProbability information to delay variable node τ:
Figure BDA0002902792020000058
wherein I (-) represents an exemplary function,
Figure BDA0002902792020000059
Figure BDA00029027920200000510
f) confidence of update delay variable node τ:
Figure BDA00029027920200000511
g) updating delay variable node tau to state transfer function node hkProbability information of (2):
Figure BDA0002902792020000061
h) updating state transfer function node hkTo information chip variable node dkAnd dk+1Log likelihood ratio information of (1):
Figure BDA0002902792020000062
Figure BDA0002902792020000063
i) update information chip variable node dkTo channel transfer function node fkLog likelihood ratio information of (1):
Figure BDA0002902792020000064
j) updating channel transfer function node fkTo spreading chip variable node xkLog likelihood ratio information of (1):
Figure BDA0002902792020000065
k) updating spread-spectrum chip variable node xkTo check function node giLog likelihood ratio information of (1):
Figure BDA0002902792020000066
6) obtaining soft information of the spreading chips:
Figure BDA0002902792020000067
7) and hard decision is carried out on the spreading chips:
Figure BDA0002902792020000068
8) obtaining an initial state estimation value of a spreading code:
Figure BDA0002902792020000069
9) sending the initial state estimated value to a local code generator to generate a local spread spectrum code;
performing correlation operation on the local spread spectrum code and the compensated signal in the step 1), wherein the correlation operation comprises a correlation integral and a non-correlation accumulation, the correlation integral refers to performing multiplication and addition operation on the local code and a received signal, and the integral length is 8; non-correlated accumulation refers to the square summation of the results of each segment of correlated integration, with a summation length of 256. If the correlation result is greater than the threshold value ThIndicating successful acquisition, otherwise failing to acquire, and receiving another signal, and repeating the steps 1) to 10) until successful acquisition is achieved.
Example 2
Assume that the spreading code generator polynomial g (D) 1+ D3+D7Spreading ratio K is 8, received signal length N is 512, and signal energy E c1, noise variance is N0Setting the maximum iteration number Itermax to be 32 and setting the threshold value to be Th. Fast code acquisition can be achieved by:
1) carrying out frequency offset and initial phase compensation on a received signal with the length of 512;
2) according to the generating polynomial g (D) ═ 1+ D3+D7Constructing a factor graph, as shown in FIG. 4;
3) initializing probability information:
Figure BDA0002902792020000071
where τ represents the delay variable node in fig. 3;
4) initializing log-likelihood ratio information:
Figure BDA0002902792020000072
Figure BDA0002902792020000073
wherein y iskRepresents the compensated received signal, x, of 1)kRepresents the kth spreading chip variable node, g, in fig. 4iRepresents the ith check function node, d, in FIG. 4kRepresents the kth information chip variable node, h, in FIG. 4kRepresents the kth state transfer function node in fig. 4;
5) starting iteration, if the number of iterations reaches 32, exiting the iteration, and entering the step 6), otherwise, continuing the iteration. The l iteration is as follows:
a) updating check function node gjTo spreading chip variable node xkLog likelihood ratio information of (1):
Figure BDA0002902792020000074
wherein N (g)j) Is represented as g in FIG. 4jThe neighbor node of (2);
b) updating spread-spectrum chip variable node xkTo channel transfer function node fkLog likelihood ratio information of (1):
Figure BDA0002902792020000075
c) updating channel transfer function node fkTo information chip variable node dkLog likelihood ratio information of (1):
Figure BDA0002902792020000076
d) updating informationChip variable node dkNode h of the state transfer functionkAnd hk-1Log likelihood ratio information of (1):
Figure BDA0002902792020000077
e) updating state transfer function node hkProbability information to delay variable node τ:
Figure BDA0002902792020000078
wherein I (-) represents an exemplary function,
Figure BDA0002902792020000081
Figure BDA0002902792020000082
f) confidence of update delay variable node τ:
Figure BDA0002902792020000083
g) updating delay variable node tau to state transfer function node hkProbability information of (2):
Figure BDA0002902792020000084
h) updating state transfer function node hkTo information chip variable node dkAnd dk+1Log likelihood ratio information of (1):
Figure BDA0002902792020000085
Figure BDA0002902792020000086
i) update information chip variable node dkTo channel transfer function node fkLog likelihood ratio information of (1):
Figure BDA0002902792020000087
j) updating channel transfer function node fkTo spreading chip variable node xkLog likelihood ratio information of (1):
Figure BDA0002902792020000088
k) updating spread-spectrum chip variable node xkTo check function node giLog likelihood ratio information of (1):
Figure BDA0002902792020000089
6) obtaining soft information of the spreading chips:
Figure BDA00029027920200000810
7) and hard decision is carried out on the spreading chips:
Figure BDA00029027920200000811
8) obtaining an initial state estimation value of a spreading code:
Figure BDA00029027920200000812
9) sending the initial state estimated value to a local code generator to generate a local spread spectrum code;
performing correlation operation on the local spread spectrum code and the compensated signal in the step 1), wherein the correlation operation comprises correlation integration and non-correlation accumulation, the correlation integration refers to performing multiplication and addition operation on the local code and a received signal, and the integration length is 16; non-correlated accumulation refers to the square summation of the results of each segment of correlated integration, with a summation length of 64. If the correlation result is greater than the threshold value ThIndicating successful acquisition, otherwise failing to acquire, and receiving another signal, and repeating the steps 1) to 10) until successful acquisition is achieved.
Table 1 shows the data of the serial capture method and the present method over algorithm complexity and average capture time.
TABLE 1 data statistics table for serial capture and method
Figure BDA0002902792020000091
The meaning of the parameters in the table is as follows:
r: the stage number of the shift register is 15;
n: receiving signal length, where N is 1024 in the table;
k: spreading multiple, in this table K is 4;
IterMax, which is the number of iterations of the method, wherein IterMax in the table is 32;
tc: a chip rate;
under the specific parameters, the average capture time of the method is faster than that of serial capture by 2 orders of magnitude, and the complexity is lower by about 1 order of magnitude.
The present invention is not limited to the above embodiments, and any changes or substitutions that can be easily made by those skilled in the art within the technical scope of the present invention are also within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims (6)

1. A method for fast code acquisition for a short spreading ratio satellite communication system, comprising the steps of:
step 1: carrying out frequency offset and initial phase compensation on a section of receiving signal with the length of N;
step 2: acquiring system parameters including a spread spectrum code generator polynomial, a spread spectrum ratio, signal energy and a noise variance; sending the system parameters and the signals subjected to initial phase compensation in the step 1 into a spread spectrum code and information bit joint estimation module to obtain an initial state estimation value of the spread spectrum code;
and step 3: sending the initial state estimation value obtained in the step 2 into a local code generator to generate a local spread spectrum code;
and 4, step 4: performing integral and non-correlation accumulation operation on the local spread spectrum code generated in the step 3 and the signal subjected to initial phase compensation in the step 1, and comparing an operation result with a threshold value; if the operation result is larger than the threshold value, the acquisition is successful, otherwise, the acquisition is failed, another signal with the length of N is received, and the steps 1 to 3 are repeated until the acquisition is successful.
2. The method of claim 1, wherein the received signal in step 1 is a discrete time signal, and the down-conversion, matched filtering and sampling operations are completed, and the sampling rate is a chip rate.
3. The method of claim 1, wherein the step 2 comprises the sub-steps of:
step 21: generating a polynomial according to the spreading code to construct a factor graph;
step 22: initializing probability information:
Figure FDA0002902792010000011
wherein K represents a spreading ratio, and tau represents a delay variable node in a factor graph;
step 23: initializing log-likelihood ratio information:
Figure FDA0002902792010000012
Figure FDA0002902792010000013
wherein EcRepresenting the signal energy, N0Representing the variance of the noise, ykRepresents the signal after initial phase compensation in step 1, xkRepresenting the kth spreading chip variable node, g, in the factor graphiRepresenting the ith check function node, h, in the factor graphkRepresenting the kth state transfer function node in the factor graph, dkRepresents the kth information chip variable node in the factor graph;
step 24: starting iteration, setting the maximum iteration number as IterMax, if the iteration number reaches IterMax, quitting the iteration, entering the step 25, otherwise, continuing the iteration; the l iteration is as follows:
(1) updating check function node gjTo spreading chip variable node xkLog likelihood ratio information of (1):
Figure FDA0002902792010000021
wherein N (g)j) G in the representative factor graphjThe neighbor node of (2);
(2) updating spread-spectrum chip variable node xkTo channel transfer function node fkLog likelihood ratio information of (1):
Figure FDA0002902792010000022
(3) updating channel transfer function node fkTo information chip variable node dkLog likelihood ratio information of (1):
Figure FDA0002902792010000023
(4) update information chip variable node dkNode h of the state transfer functionkAnd hk-1Log likelihood ratio information of (1):
Figure FDA0002902792010000024
(5) updating state transfer function node hkTo delay variable nodeProbability information of point τ:
Figure FDA0002902792010000025
wherein I (-) represents an exemplary function,
Figure FDA0002902792010000026
Figure FDA0002902792010000027
(6) confidence of update delay variable node τ:
Figure FDA0002902792010000028
(7) updating delay variable node tau to state transfer function node hkProbability information of (2):
Figure FDA0002902792010000029
(8) updating state transfer function node hkTo information chip variable node dkAnd dk+1Log likelihood ratio information of (1):
Figure FDA00029027920100000210
Figure FDA0002902792010000031
(9) update information chip variable node dkTo channel transfer function node fkLog likelihood ratio information of (1):
Figure FDA0002902792010000032
(10) updating channel transfer function node fkTo spreading chip variable node xkLog likelihood ratio information of (1):
Figure FDA0002902792010000033
(11) updating spread-spectrum chip variable node xkTo check function node giLog likelihood ratio information of (1):
Figure FDA0002902792010000034
step 25: obtaining soft information of the spreading chips:
Figure FDA0002902792010000035
step 26: and hard decision is carried out on the spreading chips:
Figure FDA0002902792010000036
step 27: obtaining an initial state estimation value of a spreading code:
Figure FDA0002902792010000037
where u represents the estimated initial state value of the spreading code and r represents the highest degree of the generator polynomial.
4. The method of claim 1, wherein the spreading codes in step 2 comprise m-sequences and Gold sequences, and the spreading ratio is a positive integer greater than 2.
5. The method of claim 1, wherein the local code generator in step 3 is comprised of a linear feedback shift register, the number of stages of the shift register is equal to the highest degree of the generator polynomial in step 2, and tap coefficients of the shift register are determined by coefficients of the generator polynomial.
6. The method as claimed in claim 1, wherein the integration in step 4 is performed by performing a multiply-add operation on the local code and the received signal, and the integration length is m; the non-correlation accumulation refers to square summation of the integration results of each segment, and the summation length is n; the operation result refers to the result of the uncorrelated accumulation.
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