CN109870712B - Method for eliminating Doppler effect of spread spectrum code - Google Patents

Method for eliminating Doppler effect of spread spectrum code Download PDF

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CN109870712B
CN109870712B CN201811630401.XA CN201811630401A CN109870712B CN 109870712 B CN109870712 B CN 109870712B CN 201811630401 A CN201811630401 A CN 201811630401A CN 109870712 B CN109870712 B CN 109870712B
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doppler
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fpga circuit
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殷君
李志强
孙健俊
曹寿琦
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Space E Star Communication Technology Co ltd
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Abstract

The invention discloses a method for eliminating Doppler effect of a spread spectrum code. The method comprises the steps of stripping carrier waves of received direct sequence spread spectrum signals through frequency mixing, utilizing a single code NCO to generate a local spread spectrum code for despreading, then sequentially carrying out discrete Fourier transform, wedge transform and inverse Fourier transform, and carrying out spectrum analysis to obtain correlation peak values under different carrier Doppler and code phase conditions. By the method, especially the wedge-shaped conversion method is adopted, the calculation amount can be effectively reduced, the consumption of ROM and multiplication resources in an FPGA hardware circuit is saved, and the Doppler effect of the spread spectrum code is effectively eliminated.

Description

Method for eliminating Doppler effect of spread spectrum code
Technical Field
The invention relates to the field of satellite communication, in particular to a method for eliminating Doppler effect of a spread spectrum code.
Background
In a satellite measurement and control system, a direct sequence spread spectrum signal is often adopted to realize precise distance measurement and interference resistance, and the precise synchronization of the spread spectrum signal is based on the premise of precise distance measurement. The satellite is in a high-speed relative motion state relative to the ground measurement and control station, so that a large Doppler frequency shift exists in a spread spectrum measurement and control signal between the satellite-borne transponder and the ground measurement and control station, and the large Doppler frequency shift is specifically represented by carrier Doppler frequency shift and spread spectrum code Doppler effect of the signal.
The problem of acquisition of spread spectrum signals is resolved by finding the maximum correlation peak of the received-local signal through two-dimensional search of the phase difference of the spread spectrum code of the received-local signal and Doppler frequency offset. Therefore, the receiver circuit must eliminate the spreading code doppler value in the received signal, otherwise the code phase difference of the received-local signal increases with the accumulation time due to the spreading code doppler value, and when the receiving-transmitting spreading code phase difference exceeds 1 chip, the signal capture probability is sharply reduced.
In the current direct sequence spread spectrum signal capturing system based on the FPGA platform, a parallel multipath code correlator with different spread spectrum code Doppler values is often used for eliminating code Doppler of a direct spread spectrum signal. The method is simple in concept and easy to implement, but needs to consume a large amount of ROM and multiplication resources, and the larger the Doppler value of the spread spectrum code is, the more FPGA operation resources are consumed. Since the signal processing resources of the satellite-borne FPGA chip are very precious, an effective and resource-saving scheme for eliminating the doppler value of the spreading code is urgently needed to be found.
Disclosure of Invention
The invention mainly solves the technical problem of eliminating the Doppler effect of a spread spectrum code, and solves the problems of computational complexity and hardware resource consumption caused by the acquisition of the spread spectrum code by the Doppler effect in the prior art of satellite communication.
In order to solve the above technical problem, one technical solution adopted by the present invention is to provide a method for eliminating a doppler effect of a spreading code, comprising the following steps: firstly, a receiving end utilizes a locally generated radio frequency local oscillator signal to mix frequency of an input direct sequence spread spectrum signal x (n) and strip a carrier wave, and the following results are obtained:
Figure BDA0001928860240000021
Figure BDA0001928860240000022
wherein A is the signal amplitude, c (nT)s) For spreading code sample values, n denotes the sample number, TsRepresenting the sampling period, τ the propagation delay, fdDenotes carrier doppler frequency offset, γ denotes normalized spreading code doppler value, γ ═ fd/f0，f0Is the center frequency of the radio frequency carrier wave,
Figure BDA0001928860240000023
is the phase of the RF carrier, which follows the RF carrier center frequency f0And time variation; second, a local spreading code is generated using a single code NCO
Figure BDA0001928860240000024
For x1(n) despreading to obtain:
Figure BDA0001928860240000025
wherein the content of the first and second substances,
Figure BDA0001928860240000026
l represents the code phase difference, k represents the code doppler value,
Figure BDA0001928860240000027
Tcindicating a chip period, TPMFThe method comprises the steps of representing the segment duration of a received spread spectrum code, wherein L represents the code phase difference, K represents the code Doppler value, L represents the value upper limit of the code phase difference L, and K represents the value upper limit of the code Doppler value K; and thirdly, performing discrete Fourier transform on s (m, l) to obtain: s1(m,l')＝FFT[s(m,l)](ii) a The fourth step, for S1(m, l') is subjected to wedge transformation to obtain S3(m, l'); the fifth step, for S3(m, l ') inverse Fourier transform in the l' domain, yielding: s4(m,l)＝IFFT(S3(m, l')); sixth, to S4And (m, l) performing spectrum analysis in m dimensions to obtain correlation peak values under different carrier Doppler and code phase conditions.
In another embodiment of the method for eliminating Doppler effect of spread spectrum code of the present invention, the Doppler effect is eliminated by S1(m, l') is subjected to wedge transformation to obtain S3The (m, l') process comprises: the first stage of transformation:
Figure RE-GDA0002033771050000031
wherein
Figure RE-GDA0002033771050000032
B is the bandwidth of the spread spectrum signal, 2 times the spreading code rate is taken, and M represents S1(m, l') number of spots; and (3) second-stage transformation: s3(m,l')＝FFT[S2(m',l')]I.e. to S2(m ', l ') performing an m ' IFFT transform to obtain:
Figure RE-GDA0002033771050000033
in another embodiment of the method for canceling doppler effect of spreading codes according to the present invention, the first stage transform further includes: 1) selecting a minimum integer N which satisfies N ≧ 2M-1 and N ═ 2iFor all l', let
Figure RE-GDA0001970175290000034
Figure RE-GDA0001970175290000035
2) Calculate the N-point FFT of g (m), the result is:
Figure RE-GDA0001970175290000036
3) order sequence
Figure RE-GDA0001970175290000037
And calculating N-point FFT of h (m), and the result is:
Figure RE-GDA0001970175290000038
r-0, 1,2,. N-1; 4) Multiplying g (r) and h (r) to obtain q (r) ═ g (r) h (r); 5) obtaining the N-point IFFT transform of Q (r), namely: q (m) ═ IFFT [ Q (r)](ii) a 6) Taking the first M term of q (M) as q (M'), obtaining
Figure RE-GDA0001970175290000039
In another embodiment of the method for eliminating the doppler effect of the spreading code, the method for implementing the first-stage transform in the FPGA circuit includes: subjecting the mixture obtained in the step 1)
Figure BDA00019288602400000310
Calculated in advance, then stored in a first ROM of the FPGA circuit, and then utilized a first multiplier and S in the FPGA circuit1(m, l') are multiplied; performing N-point FFT operation on g (m) in the step 2) by utilizing a first FFT operation core in an FPGA circuit; performing N-point FFT operation on h (m) in the step 3) by using a second FFT operation core in the FPGA circuit to obtain H (r), and storing the H (r) into a second ROM; the second multiplier in the FPGA circuit is utilized to finish the step 4)G (r) and H (r) are multiplied to obtain Q (r); performing N-point IFFT on Q (r) in the step 5) by using a third FFT operation core in the FPGA circuit to obtain q (m'); using third ROM stored in FPGA circuit
Figure BDA0001928860240000041
And using a third multiplier in the FPGA circuit to finish the step 6)
Figure BDA0001928860240000042
To obtain S2(m',l')。
In another embodiment of the method for eliminating the doppler effect of the spreading code according to the present invention, the first FFT operation core, the second FFT operation core, and the third FFT operation core are the same FFT operation core multiplexed in the FPGA circuit.
The invention has the beneficial effects that: the invention discloses a method for eliminating Doppler effect of a spread spectrum code. The method comprises the steps of stripping carrier waves of received direct sequence spread spectrum signals through frequency mixing, utilizing a single code NCO to generate local spread spectrum codes for de-spreading, then sequentially carrying out discrete Fourier transform, wedge transform and Fourier inverse transform, and carrying out spectrum analysis to obtain correlation peak values under different carrier Doppler and code phase conditions. By the method, especially the wedge-shaped conversion method is adopted, the calculation amount can be effectively reduced, the consumption of ROM and multiplication resources in an FPGA hardware circuit is saved, and the Doppler effect of the spread spectrum code is effectively eliminated.
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FIG. 1 is a block diagram of an embodiment of a method for canceling the Doppler effect of a spreading code according to the prior art;
FIG. 2 is a block diagram of an embodiment of a method for canceling the Doppler effect of a spreading code according to the present invention;
FIG. 3 is a block diagram of a wedge transform in an embodiment of a method for canceling the Doppler effect of a spreading code according to the present invention;
fig. 4 is a schematic diagram of an FPGA circuit in an embodiment of the method for eliminating the doppler effect of the spreading code according to the present invention.
Detailed Description
In order to facilitate an understanding of the invention, the invention is described in more detail below with reference to the accompanying drawings and specific examples. Preferred embodiments of the present invention are shown in the drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.
It is to be noted that, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.
Fig. 1 shows a block diagram of prior art acquisition of spreading codes. In a direct sequence spread spectrum signal receiving and transmitting system, because the arrival time of a signal is unknown and the relative motion speed of the receiving and transmitting parties is unknown, a receiving end needs to realize receiving and transmitting synchronization through time domain search and frequency domain search. The received spread spectrum is discretely sampled and expressed as:
Figure BDA0001928860240000051
wherein A is the signal amplitude, c (nT)s) For spreading code sample values, fdDenotes carrier doppler frequency offset, γ denotes normalized code doppler value, γ ═ fd/f0，f0Is the center frequency of the radio frequency carrier,
Figure BDA0001928860240000052
is the carrier phase, which follows the radio frequency carrier center frequency f0And time variation.
The general steps of the prior art are as follows:
1. and (5) stripping the carrier wave. The receiver carries out the loading of the received signal x (n)Wave stripping, which is to mix frequency mainly by local carrier to obtain received signal x with carrier Doppler frequency offset and code Doppler value1(n) is:
Figure BDA0001928860240000053
2. the time domain and code doppler values are searched in parallel. In the time domain, N paths of spread spectrum code branches with half chip code phase difference are locally generated to realize parallel search; meanwhile, code Doppler adopts a parallel or time-series mode, so that the code Doppler value of the local spread spectrum code is changed and has different code phase differences
Figure BDA0001928860240000054
A plurality of code NCO are used to generate spreading codes with different code rates gamma on the basis of the N code branches. The local spreading code is represented as:
Figure BDA0001928860240000055
when searching for code phase and code Doppler values, the order
Figure BDA0001928860240000061
Figure BDA0001928860240000062
TcDenotes the chip period, TPMFThe method comprises the steps of representing the segment duration of a received spread spectrum code, representing the code phase difference by L, representing the code Doppler value by K, representing the value upper limit of the code phase difference L by L, and representing the value upper limit of the code Doppler value by K.
3. And (6) performing correlation operation. The local code and the received spread spectrum code containing code Doppler value are segmented for a time length of TPMFObtaining M segment correlation operation results under different code Doppler value conditions:
Figure BDA0001928860240000063
where m denotes the sequence number of the segment,
Figure BDA0001928860240000064
general formula
Figure BDA0001928860240000065
The rewrite is:
Figure BDA0001928860240000066
where l represents the code phase difference and k represents the code doppler value.
When there is a code doppler difference between the received signal and the local signal, the received and local code phase difference can be expressed as: τ ═ τ0+γmTPMF；
4. And (4) carrier Doppler searching. Discrete Fourier transform is carried out on the result of the sectional correlation to obtain a carrier Doppler frequency offset value fdAnd the capturing is completed.
The problem with the above acquisition process is that the receiver needs to traverse the possible l and k search gammaands
Figure BDA0001928860240000067
Step 4 can be performed after all s (m, l, k) are stored, and a circuit consumes a large amount of computing resources.
The invention mainly eliminates code phase sliding caused by code Doppler effect by adopting a wedge-shaped conversion method. The method mainly adopts a wedge-shaped transformation algorithm to eliminate code Doppler and code correlation peak position sliding caused by the code Doppler, and uses Chirp-Z transformation to reduce the complexity of the wedge-shaped transformation algorithm in the algorithm realization. The processing flow is as shown in FIG. 2:
firstly, in the embodiment of the scheme, when despreading, despreading is completed by using a single code NCO to obtain:
Figure BDA0001928860240000068
wherein the content of the first and second substances,
Figure BDA0001928860240000071
l represents the code phase difference, k represents the code doppler value,
Figure BDA0001928860240000072
Tcindicating a chip period, TPMFThe time length of the received spread spectrum code is shown, L represents the code phase difference, and L represents the upper limit of the code phase difference L.
Then eliminating code phase slip caused by code Doppler value by using wedge transform method, which includes:
1. fourier transform is carried out on s (m, l) in a distance domain, namely an l domain, and a result of a corresponding frequency domain, namely an l' domain, is obtained:
S1(m,l')＝FFT[s(m,l)]
2. to S1(m, l') performing a wedge transformation, the wedge transformation comprising two stages of transformations:
the first stage of transformation:
Figure BDA0001928860240000073
wherein
Figure BDA0001928860240000074
B is the bandwidth of the spread spectrum signal, 2 times the spreading code rate is taken, and M represents S1(m, l') number of spots;
and (3) second-stage transformation:
S3(m,l')＝FFT[S2(m',l')]
preferably, in order to reduce the amount of computation, the present invention further preferably refines the first-stage transformation into 6 steps:
1) selecting a minimum integer N which satisfies N ≧ 2M-1 and N ═ 2iFor all l', let
Figure BDA0001928860240000075
Wherein
Figure BDA0001928860240000076
B is the bandwidth of the spread spectrum signal, 2 times the spreading code rate is taken, and M represents S1(m, l') number of spots;
2) calculate the N-point FFT of g (m), the result is:
Figure BDA0001928860240000077
3) order sequence
Figure BDA0001928860240000081
And calculating N-point FFT of h (m) to obtain the result:
Figure BDA0001928860240000082
4) multiplying g (r) and h (r) to obtain q (r) ═ g (r) h (r);
5) obtaining an N-point IFFT of q (r), that is:
q(m)＝IFFT[Q(r)]；
6) taking the first M term of q (M) as q (M'), obtaining
Figure BDA0001928860240000083
In the second substep, pair S2(m ', l ') further performing an IFFT transform of m ' domain to obtain S3(m,l')：
Figure BDA0001928860240000084
3. To S3(m, l ') inverse Fourier transform is carried out in a distance frequency domain, namely an l' domain, and a result of an l domain, namely a spreading code phase domain is obtained:
S4(m,l)＝IFFT(S3(m,l'))；
4. to S4And (m, l) performing spectrum analysis on m dimensions to obtain correlation peak values under different carrier Doppler and code phase conditions.
The following circuit structure suitable for FPGA implementation is designed according to the above algorithm, and with reference to steps 1) to 6) in FIG. 3, it can be seen that H (r) in step 3), and steps 1) and 6) in step 3)
Figure BDA0001928860240000085
May be pre-calculated and stored in ROM. The 2-time FFT and the one-time IFFT are both N-point Fourier transform, the FFT and the IFFT are realized by multiplexing an FFT operation core in the FPGA, and the FPGA circuit module with wedge-shaped operation is designed as shown in figure 4. The design of the invention uses a plurality of ROM and FFT operation cores to replace a huge code correlator branch, and the larger the Doppler effect of the spread spectrum code is, the more hardware resources can be saved.
With reference to fig. 4, for circuit implementation, in combination with steps 1) to 6) of the first-stage transformation, the method for implementing the first-stage transformation in the FPGA circuit includes: subjecting the mixture obtained in the step 1)
Figure BDA0001928860240000091
Pre-calculated, stored in the first ROM of the FPGA circuit, and then used in the first multiplier and S of the FPGA circuit1(m, l') are multiplied; performing N-point FFT operation on g (m) in the step 2) by utilizing a first FFT operation core in an FPGA circuit; performing N-point FFT operation on h (m) in the step 3) by using a second FFT operation core in the FPGA circuit to obtain H (r), and storing the H (r) into a second ROM; utilizing a second multiplier in the FPGA circuit to finish the multiplication of G (r) and H (r) in the step 4) to obtain Q (r); performing N-point IFFT on Q (r) in the step 5) by using a third FFT operation core in the FPGA circuit to obtain q (m'); using third ROM stored in FPGA circuit
Figure BDA0001928860240000092
And using a third multiplier in the FPGA circuit to finish q (m') in the step 6)
Figure BDA0001928860240000093
To obtain S2(m',l')。
Preferably, the first FFT operation core, the second FFT operation core and the third FFT operation core are the same FFT operation core multiplexed in the FPGA circuit.
Therefore, the invention discloses a method for eliminating the Doppler effect of the spread spectrum code. The method comprises the steps of stripping carrier waves of received direct sequence spread spectrum signals through frequency mixing, utilizing a single code NCO to generate a local spread spectrum code for despreading, then sequentially carrying out discrete Fourier transform, wedge transform and inverse Fourier transform, and carrying out spectrum analysis to obtain correlation peak values under different carrier Doppler and code phase conditions. By the method, code phase sliding caused by code Doppler is eliminated, code Doppler and code correlation peak position sliding caused by the code Doppler are eliminated mainly by adopting a wedge-shaped transformation algorithm, Chirp-Z transformation is used in algorithm implementation to reduce complexity of the wedge-shaped transformation algorithm, calculated amount can be effectively reduced, consumption of ROM and multiplication resources in an FPGA hardware circuit is saved, and spread spectrum code Doppler effect is effectively eliminated.
The above description is only an embodiment of the present invention, and not intended to limit the scope of the present invention, and all equivalent structural changes made by using the contents of the present specification and the drawings, or applied directly or indirectly to other related technical fields, are included in the scope of the present invention.

Claims (4)

1. A method for eliminating Doppler effect of spread spectrum codes is characterized by comprising the following steps:
firstly, the receiving end utilizes the local radio frequency local oscillation signal generated locally to mix the frequency of the input direct sequence spread spectrum signal x (n) and strip the carrier wave to obtain x1(n) wherein,
Figure FDA0002701978080000011
Figure FDA0002701978080000012
a is the signal amplitude, c: (nTs) For spreading code sample values, n denotes the sample number, TsRepresenting the sampling period, τ the propagation delay, fdDenotes carrier doppler frequency offset, γ denotes normalized spreading code doppler value, γ ═ fd/f0，f0Is the center frequency of the radio frequency carrier wave,
Figure FDA0002701978080000013
is the phase of the RF carrier, which follows the RF carrier center frequency f0And time variation;
second, a local spreading code is generated using a single code NCO
Figure FDA0002701978080000014
For x1(n) despreading to obtain:
Figure FDA0002701978080000015
wherein the content of the first and second substances,
Figure FDA0002701978080000016
l represents the phase difference of the code phase,
Figure FDA0002701978080000017
Tcdenotes the chip period, TPMFThe method comprises the steps of representing the segment duration of a received spread spectrum code, wherein L represents a code phase difference, and L represents the upper limit of the value of the code phase difference L;
and thirdly, performing discrete Fourier transform on s (m, l) to obtain:
S1(m,l')＝FFT[s(m,l)]；
the fourth step, for S1(m, l') is subjected to wedge transformation to obtain S3(m,l')；
The fifth step, for S3(m, l ') inverse Fourier transform in the l' domain, yielding:
S4(m,l)＝IFFT(S3(m,l'))；
sixth, to S4Dimension m of (m, l)Analyzing a line frequency spectrum to obtain correlation peak values under different carrier Doppler and code phase conditions;
from S1(m, l') is subjected to wedge transformation to obtain S3The (m, l') process comprises:
the first stage of transformation:
Figure FDA0002701978080000021
wherein
Figure FDA0002701978080000022
B is the bandwidth of the spread spectrum signal, 2 times the spreading code rate is taken, and M represents S1(m, l') number of spots;
and (3) second-stage transformation:
S3(m,l')＝FFT[S2(m',l')]i.e. to S2(m ', l ') performing an IFFT transformation of m ' domain to obtain:
Figure FDA0002701978080000023
2. the method of claim 1, wherein the first stage transform further comprises:
1) selecting a minimum integer N, wherein N is more than or equal to 2M-1 and N is 2iFor all l', let
Figure FDA0002701978080000024
2) Calculate the N-point FFT of g (m), the result is:
Figure FDA0002701978080000025
3) order sequence
Figure FDA0002701978080000026
And calculating N-point FFT of h (m), the result is H (r):
Figure FDA0002701978080000027
4) multiplying g (r) and h (r) to obtain q (r) ═ g (r) h (r);
5) obtaining an N-point IFFT of q (r), that is:
q(m)＝IFFT[Q(r)]；
6) taking the first M term of q (M) as q (M'), obtaining
Figure FDA0002701978080000031
3. The method for canceling doppler effect of spreading codes according to claim 2, wherein the implementation method of the first stage transform in the FPGA circuit comprises:
subjecting the mixture obtained in the step 1)
Figure FDA0002701978080000032
Pre-calculated, stored in the first ROM of the FPGA circuit, and then used in the first multiplier and S of the FPGA circuit1(m, l') are multiplied;
performing N-point FFT operation on g (m) in the step 2) by utilizing a first FFT operation core in an FPGA circuit;
performing N-point FFT operation on h (m) in the step 3) by using a second FFT operation core in the FPGA circuit to obtain H (r), and storing the H (r) into a second ROM;
utilizing a second multiplier in the FPGA circuit to complete the multiplication of G (r) and H (r) in the step 4) to obtain Q (r);
performing N-point IFFT on Q (r) in the step 5) by using a third FFT operation core in the FPGA circuit to obtain q (m');
using third ROM stored in FPGA circuit
Figure FDA0002701978080000033
And using a third multiplier in the FPGA circuit to finish the step 6)
Figure FDA0002701978080000034
To obtain S2(m',l')。
4. The method for canceling doppler effect of spreading codes according to claim 3, wherein said first FFT computation core, said second FFT computation core and said third FFT computation core are the same FFT computation core multiplexed in FPGA circuit.
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CN102868659A (en) * 2012-08-31 2013-01-09 中国船舶重工集团公司第七一五研究所 Symbol synchronization and Doppler compensation method for mobile orthogonal frequency division multiplexing (OFDM) underwater sound communication signal

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