CN109150234B - Direct sequence spread spectrum signal transmission method based on direct differential coherent accumulation - Google Patents

Direct sequence spread spectrum signal transmission method based on direct differential coherent accumulation Download PDF

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CN109150234B
CN109150234B CN201810888494.XA CN201810888494A CN109150234B CN 109150234 B CN109150234 B CN 109150234B CN 201810888494 A CN201810888494 A CN 201810888494A CN 109150234 B CN109150234 B CN 109150234B
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CN109150234A (en
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陈延涛
董彬虹
赵宇轩
李芊饶
崔亚笛
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University of Electronic Science and Technology of China
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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
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
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Abstract

The invention discloses a direct spread spectrum signal transmission method based on direct differential coherent accumulation, and belongs to the technical field of spread spectrum signal receiving and processing. A direct spread spectrum signal capturing method for carrying out forward and backward conjugate multiplication and coherent integration on the descrambled signal, wherein the operation of the forward and backward conjugate multiplication is called as differential coherence; after descrambling, the received signal should be two identical PN sequences, i.e. the transmitted pseudo-random sequence should be two identical PN sequences before scrambling. The core of the invention lies in that the coarse synchronization head of the signal is specially designed, and the receiving end eliminates the influence of frequency offset by carrying out forward and backward conjugate multiplication (differential coherence) on the coarse synchronization head of the received signal, so that the influence of data bit jump caused by Doppler frequency offset is not influenced during coherent integration. Because the required sequence length is only related to the signal-to-noise ratio, the algorithm can search the direct sequence spread signal in a very large frequency range under a certain sequence length under a certain signal-to-noise ratio.

Description

Direct sequence spread spectrum signal transmission method based on direct differential coherent accumulation
Technical Field
The invention belongs to the technical field of spread spectrum signal receiving processing, and relates to a coarse acquisition method capable of completing direct sequence spread spectrum signal synchronization under large Doppler frequency offset. In particular to a direct sequence spread spectrum signal synchronization method based on differential conjugate multiplication and coherent integration.
Background
Direct sequence spread spectrum technology (abbreviated as "direct sequence spread spectrum") is one of core fields of digital communication, and is initially applied to military communication networks as a means for resisting interference and intercepting threats, but nowadays, with the rapid development of civil communication systems and networks represented by 4G/5G, such as mobile communication systems, cognitive radios, sensor networks, internet of things and the like, the direct sequence spread spectrum technology is widely applied to commercial systems and becomes a part of a plurality of wireless and mobile communication standards. Direct sequence spread spectrum signals are typically very covert and can be transmitted at very low signal-to-noise ratios and error-free recovery is achieved at the receiving end. However, the synchronization of the direct sequence spread spectrum signal is very sensitive to the doppler frequency offset, and the longer the pseudorandom sequence used for spreading, the more easily the synchronization fails due to data bit hopping caused by the doppler frequency offset after despreading. In the invention patent "a method for fast capturing direct sequence spread spectrum signal" with application number CN201710307117.8, a synchronization technique of direct spread spectrum signal in the presence of frequency offset is proposed. The technology firstly carries out FFT (fast Fourier transform) on zero padding of a received signal, secondly carries out cyclic shift on an FFT sequence of the received signal and conjugate multiplication on the FFT sequence of a local reference sequence, carries out IFFT and non-coherent accumulation on the conjugate multiplied FFT sequence, finally takes the maximum value as a decision value of coarse capture, and carries out FFT (fast Fourier transform) to search frequency offset frequency points when the decision value exceeds a threshold. The algorithm can capture the direct sequence spread spectrum signal under a lower signal-to-noise ratio and a certain frequency offset. However, the amount of calculation is proportional to the frequency search range, and in order to obtain a larger frequency search range, the number of points and the number of times of FFT must be increased. Therefore, a large amount of calculation is required to search for the direct spread signal under a large frequency offset. Therefore, in the wireless communication for high-speed movement, the doppler frequency offset will be larger and larger as the relative movement speed is increased, and the cost for realizing the direct sequence spread spectrum signal synchronization will be larger and larger.
Disclosure of Invention
The invention provides a direct sequence spread spectrum signal transmission method based on direct differential coherent accumulation, which aims at the problem that data bit jumping occurs after despreading a direct sequence spread spectrum signal under large frequency deviation, and is used for completing synchronous coarse acquisition of the direct sequence spread spectrum signal under the large frequency deviation.
The invention relates to a direct sequence spread spectrum signal capturing method which carries out conjugate multiplication and coherent integration on descrambled signals, wherein the operation of conjugate multiplication is called as differential coherence; after descrambling, the received signal should be two identical PN sequences, i.e. the transmitted pseudo-random sequence should be two identical PN sequences before scrambling. The core of the invention lies in that the coarse synchronization head of the signal is specially designed, and the receiving end eliminates the influence of frequency offset by carrying out forward and backward conjugate multiplication (differential coherence) on the coarse synchronization head of the received signal, so that the influence of data bit jump caused by Doppler frequency offset is not influenced during coherent integration. Because the required sequence length is only related to the signal-to-noise ratio, the algorithm can search the direct sequence spread signal in a very large frequency range under a certain sequence length under a certain signal-to-noise ratio.
The technical scheme of the invention is a direct spread spectrum signal transmission method based on direct differential coherent accumulation, which comprises the following steps:
step 1, setting a sampling rate of a transmitting end, and ensuring that at least one spread spectrum chip is sampled for more than two times; setting parameters including the length of the spread spectrum code, setting the PN code PNcode used by spread spectrum, and the spread spectrum pseudo-random sequence s [ n ] of the transmitting end]For two sections of same PN sequences, the former section is PN1The latter section is PN2Then s [ n ]]The following format should be satisfied:
s[n]＝[PN1PN2]＝[PNCode PNCode]
step 2, scrambling s [ n ], wherein the pseudo code used for scrambling is ScrPNcode, the cycle of the ScrPNcode is equal to the length of s [ n ], so that the effect of despreading cannot be achieved by directly correlating the previous pseudo code and the next pseudo code after s [ n ] is scrambled;
step 3, the receiving end samples the received signal according to the fixed time interval, still guarantee at least one spread spectrum chip to sample more than twice; pressing the currently sampled data into an RAM in a storage queue; the RAM satisfies:
RAM＝[RAM1RAM2]
step 4, descrambling the data in the RAM by using a pseudo code ScrPNcode;
step 5, RAM after descrambling1Sequence and RAM in2The sequence conjugate multiplication in (1) to obtain a related sequence:
R[n]＝RAM1[n]*RAM2[n]*；
step 6, performing coherent integration on the R [ n ], and obtaining a final correlation value COR (sum of Σ R [ n ]) by taking a modulus value;
step 7, comparing COR with a set Threshold value Threshold, if COR is greater than Threshold, indicating that the signal is captured, and obtaining the initial position of the received signal to finish the coarse capture of the signal; and carrying out a subsequent frequency offset estimation process.
Therefore, the method has three characteristics: firstly, the coarse synchronization head is specially designed, so that the receiving end plays a role in despreading when the step 5 is carried out; the second, front-back conjugate multiplication has the effect of demodulation, and can remove frequency offset, so that the method only needs to be performed from the anti-noise perspective when the use of the sequence length is considered. Thirdly, since the above operations are repeated every half spreading chip, the real-time performance of the system is high. From the analysis of the algorithm complexity, the conjugate multiplication operation of step 5 needs more multipliers to be completed, which is a main source of the required calculation amount of the method.
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FIG. 1 is a flow chart of the initialization of necessary parameters at the originating end of the present invention;
FIG. 2 is a flow chart of the present invention processing at the receiving end.
Detailed Description
The technical scheme of the invention is detailed below by combining the accompanying drawings and the embodiment. It should be understood that the scope of the present invention is not limited to the following examples, and any techniques implemented based on the present disclosure are within the scope of the present invention.
Fig. 1 is a flowchart illustrating initialization of necessary parameters at a transmitting end according to an embodiment of the present invention.
The method comprises the following steps:
step 1, setting 2 sampling points in a spread spectrum chip period, namely, the sampling rate is twice of the data rate and is exactly equal to the Nyquist rate.
And 2, selecting an m sequence with the length of 1024 as the PNcode.
Step 3, copying PNcode into two sections of same PN sequence to form coarse synchronization head spread spectrum sequence sn, where the length of sn should be 2048
Step 4, selecting a section of m sequence with length of 2048 as a pseudo code ScrPNcode, and making the first 1024 bits of the pseudo code ScrPNcode be Scr1The last 1024-bit data is Sc2r, i.e. ScrPNcode ═ Scr1Scr2]. For s [ n ]]Scrambling to obtain a scrambled transmission signal sScr＝s*ScrPNCode＝[PNCode*Scr1PNCode*Scr2]
Fig. 2 is a block diagram of a receiving end processing system according to the embodiment of fig. 1.
The method comprises the following steps:
step 1, not considering noise, and setting that a sending signal is completely received, so that after sampling, a received signal r [ n ] under frequency offset is:
1.
Figure BDA0001756268720000031
wherein
Figure BDA0001756268720000032
And delta theta represents the phase difference of the corresponding position between the frequency offset of the rear 1024-bit data band and the frequency offset of the front 1024-bit data band. R [ n ]]Is pushed into the storage queue RAM, then
Figure BDA0001756268720000033
Figure BDA0001756268720000034
Step 2, descrambling the data in the RAM to obtain a descrambled information sequence m [ n ]:
2.
Figure BDA0001756268720000035
it can be seen from the above operation that scrambling has no effect on the reception quality of the signal.
Step 3, conjugate multiplication of the front part and the rear part of the RAM sequence m [ n ] after descrambling to obtain a related sequence R [ n ]:
Figure BDA0001756268720000041
from the above operation, it can be seen that the frequency offset is removed after the forward and backward conjugate phases match.
Step 4, setting N as 1024; to R < n >]Coherent integration is performed to obtain sigma Rn]＝Ne-jΔθThus COR | ∑ R [ n |)]N. And after threshold judgment, the capturing situation is obtained. Since the transmitted signal is completely received, the result of the Threshold decision is necessarily COR ═ N > Threshold, indicating that the acquisition was successful. So that the descrambled m n stored in the queue at this time is stored]May be used to perform frequency offset estimation. About frequencyThe methods of partial estimation are numerous and are not enumerated here.
It is noted that when considering the influence of noise, the front and back conjugate multiplication amplifies the noise due to the multiplication. The equivalent spreading gain at this time is actually:
Figure BDA0001756268720000042
if conventional direct sequence spreading is used, the spreading gain is now:
Figure BDA0001756268720000043
the comparison shows that the algorithm is more sensitive to noise, so that a longer sequence length is required to resist noise with the same power than the conventional direct sequence spread spectrum. But since the frequency offset is removed, the amount of calculation does not increase as the frequency search range is enlarged.

Claims (2)

1. A direct spread spectrum signal transmission method based on direct differential coherent accumulation comprises the following steps:
step 1, setting a sampling rate of a sending end, and ensuring that at least one spread spectrum chip is sampled for more than two times; setting parameters including the length of the spread spectrum code, setting the PN code PNcode used by spread spectrum, and the spread spectrum pseudo-random sequence s [ n ] of the transmitting end]For two sections of same PN sequences, the former section is PN1The latter section is PN2Then s [ n ]]The following format should be satisfied:
s[n]＝[PN1 PN2]＝[PNCode PNCode]
step 2, for s [ n ]]Scrambling is carried out, and the pseudo code used for scrambling is ScrPNcode, the cycle of the ScrPNcode and s [ n ]]Are equal in length so that the pairs s n]After scrambling, directly correlating the previous section of pseudo code with the next section of pseudo code without despreading; selecting a section of m sequence as a pseudo code ScrPNcode, and making the first half section of the pseudo code ScrPNcode be Scr1The second half data is Scr2；
Step 3, the receiving end samples the received signal according to the fixed time interval, still guarantee at least one spread spectrum chip to sample more than twice; pressing the currently sampled data into an RAM in a storage queue; the RAM satisfies:
RAM＝[RAM1 RAM2]
wherein the content of the first and second substances,
Figure FDA0002899462270000011
Figure FDA0002899462270000012
for frequency deviation, [ PNcode x Scr1 PNCode*Scr2]Is the sending signal after scrambling; delta theta represents the phase difference of the corresponding position between the frequency offset of the second half-segment data band and the frequency offset of the first half-segment data band;
step 4, descrambling the data in the RAM by using a pseudo code ScrPNcode;
step 5, RAM after descrambling1Sequence and RAM in2The sequence conjugate multiplication in (1) to obtain a related sequence:
R[n]＝RAM1[n]*RAM2[n]*；
step 6, performing coherent integration on the R [ n ], and obtaining a final correlation value COR (sum of Σ R [ n ]) by taking a modulus value;
step 7, comparing COR with a set Threshold value Threshold, if COR is greater than Threshold, indicating that the signal is captured, and obtaining the initial position of the received signal to finish the coarse capture of the signal; and carrying out a subsequent frequency offset estimation process.
2. The direct sequence spread spectrum signal transmission method based on direct differential coherent accumulation according to claim 1, wherein in step 1, the PNCode is an m-sequence with a length of 1024; delta theta in the step 3 is the phase difference of the corresponding position between the frequency offset of the rear 1024-bit data band and the frequency offset of the front 1024-bit data band; selecting a length of 2048 m sequence as a pseudo code ScrPNcode, and making the first 1024 bits as Scr1The last 1024 bits data is Scr2。
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