A method for eliminating cross-correlation interference based on shift-and-add sequence

Abstract

A method for eliminating mutual correlation interference based on a shifted superposition sequence belongs to the field of mobile communication technology. Steps: generating a cyclic shift sequence and a shift spacing sequence, a shifted spread spectrum sequence, a superposition spread spectrum sequence, a sequence correlation operation, generating a signal sequence to be demodulated, signal sequence demodulation, residual signal demodulation, and demodulation using a traditional serial interference elimination method. The present invention generates a cyclic shift sequence on the receiver side, shifts and superimposes the local spread spectrum sequence of each user with a specific cyclic shift number, and performs a correlation operation on the shifted superposition sequence and the received CDMA superposition signal. It can uniquely determine the spread spectrum sequence of the user corresponding to each peak according to the spacing of the generated peaks, and at the same time, the peak sizes are sorted by descending powers to obtain the signal sequence to be demodulated so that multiple user signals can be demodulated at one time. When performing mutual correlation interference elimination on the receiver side, it can reduce the computational complexity of the correlation operation, increase the running speed of the demodulation, and improve the communication performance of each user in the system.

Description

Technical Field

[0001] The present invention belongs to the technical field of mobile communications, relates to a mutual correlation interference elimination technology, and particularly relates to a mutual correlation interference elimination method based on a shifted superposition sequence. Background Art

[0002] In a multi-user CDMA communication system, a receiver receives the superimposed signal composed of CDMA signals transmitted by multiple user transmitters and demodulates the data sent by each user to complete multi-user communication transmission. Because the power of each user's signal reaching the receiver after transmission varies, and the spreading sequences between users are not completely orthogonal, cross-correlation interference will occur during the receiver demodulation process, seriously affecting the system's communication performance.

[0003] To address the problem of cross-correlation interference cancellation, the traditional serial interference cancellation method first correlates the superimposed signal with each user's spread spectrum sequence at the receiver. The signal with the strongest correlation is demodulated, and this signal is remodulated to remove it from the superimposed signal to obtain a new superimposed signal. These steps are then repeated on the new superimposed signal until the signal with the weakest correlation is demodulated. This method requires repeated correlation operations during demodulation. When the number of users in the communication system is large, the computational complexity of the correlation operations increases significantly. Furthermore, demodulating only the signal with the strongest correlation at a time reduces the demodulation efficiency of this method. Summary of the Invention

[0004] The technical problem to be solved by the present invention is that the cross-correlation interference problem generated by the receiver during demodulation of superimposed signals in a multi-user CDMA communication system seriously affects the communication transmission of each user's signal. When using traditional serial interference cancellation methods to cancel cross-correlation interference, the computational complexity of the correlation operation is large, and the demodulation efficiency is low when only one signal is demodulated at a time. The purpose of the present invention is to provide a cross-correlation interference cancellation method for demodulating superimposed multi-user CDMA signals on the signal receiver side, thereby reducing the computational complexity of the demodulation correlation operation and improving the reliability of the communication system. The present invention is directed to CDMA-based multi-user communication systems, including but not limited to multi-user DS-CDMA communication systems.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A method for eliminating cross-correlation interference based on shifted superposition sequences assumes that there are K signal transmitters and one signal receiver in a CDMA communication system. The signal transmitter is used to send K CDMA signals carrying data information of each user, and the signal receiver is used to receive the CDMA superposition signals of K users. The signal receiver completes the communication transmission of K users by demodulating the superposition signal. The superposition signal received by the receiver is denoted as y sum, then y sum =y1+y2+...+y K , where y K For the received signal of the Kth user, the process of performing cross-correlation interference elimination on the CDMA superposition signals of K users at the receiver side is as follows:

[0007] Step 1: Generate cyclic shift sequence and shift spacing sequence

[0008] First, let the local spreading sequence of K users with code length L be cyclically shifted by the number of cyclic shifts a1, a2, ..., a K , where a K Denotes the cyclic shift number of the Kth user. Let a1 = 0, that is, set the local spreading sequence of the first user to be unshifted, and generate the cyclic shift sequence A = {a1, a2, ..., a K}. :

[0009]

[0010] The first formula in formula (1) represents the difference between the number of adjacent cyclic shifts and the code length and a K The difference must be greater than or equal to 2; the second formula represents the spacing between adjacent cyclic shift numbers and a K The sum of the distances from the end of the code length must be equal to the difference between the code length L and the number of system users K, that is, a i+1 -a i -1 indicates the spacing between adjacent cyclic shift numbers, La K -1 means a K The distance from the end of the code length; the third formula represents the number of cyclic shifts a i+1, i+2, ..., K i+1 ,a i+2 ,...,a K and the ith cyclic shift number a respectively i The difference must be unique and without repeated values.

[0011] Secondly, the distance x between the i-th cyclic shift number and the i+1, i+2, ..., K-th cyclic shift numbers ij for:

[0012] x ij =a j -a i -1,i=1,2,...,K-1,j=i+1,i+2,...,K, (2)

[0013] Finally, the shift interval sequence X={x 12 ,x 13 ,...,x1K ,x 23 ,x 24 ,...,x 2K ,...,x (K-1)K}, where X is a triangular sequence.

[0014] Step 2: Shift the spreading sequence

[0015] According to the cyclic shift sequence A generated in step 1, the local spreading sequences s1, s2, ..., s of K users are K Circular shift a1, a2, ..., a respectively K bits, and the shift spread spectrum sequence s1',s2',...,s K ', where s K represents the local spreading sequence of the Kth user, s K ' represents the local spreading sequence of the Kth user after cyclic shift, that is, the shifted spreading sequence.

[0016] Step 3: Superimpose the spreading sequence

[0017] The shifted spreading sequences s1',s2',...,s of K users are K 'Perform the superposition operation of the sequence to obtain the shift superposition sequence s with the same code length L sum , that is, s sum =s1'+s2'+...+s K '.

[0018] Step 4: Serial Correlation Operation

[0019] The shifted superposition sequence s obtained in step 3 sum With the superimposed signal y sum Perform correlation operation and perform peak detection on the correlation operation result. If there is a peak in the detection result, record the peak value e1, e2, ..., e n and peak positions A1, A2, ..., A n , execute step 5, where n represents the total number of detected peaks; if the detection result shows no peak or only a single peak is generated, execute step 8.

[0020] Step 5: Generate the signal sequence to be demodulated

[0021] According to the obtained peak positions A1, A2, ..., A n , calculate the adjacent peak spacing x according to formula (3) m ', we get the sequence of adjacent peak distances {x1',x2',...,x n-1 '}, traverse the shift interval sequence X and find the peak interval x m 'Equal shift spacing xij , then the spreading sequence of the signal corresponding to the two peaks is s i and s j , thereby determining the spreading sequence s1”, s2”, ..., s corresponding to each peak signal n ", and record each signal as a combination of the peak value and its corresponding spreading sequence (e n ,s n ”).

[0022] x m '=A m+1 -A m -1,m=1,2,...,n-1 (3)

[0023] According to the peak value, all combinations are sorted in descending order to obtain the signal sequence D to be demodulated, D={(E1,S1),(E2,S2),...,(E n ,S n )}. Among them, (E n ,S n ) represents the signal combination of the nth position after sorting, E n is the peak size, S n is the spreading sequence corresponding to the signal.

[0024] Step 6: Signal Sequence Demodulation

[0025] The signal sequence D to be demodulated obtained in step 5 is used to demodulate the signal that generates the peak.

[0026] (6.1) Despreading: Despread the signal using the spread spectrum sequence S1 combined with the first bit of the sequence D to obtain the amplitude estimate G1 and the original transmitted data b1.

[0027] (6.2) Reconstruction: The signal is remodulated using the amplitude estimate G1, the original data b1, and the spreading sequence S1 to obtain the reconstructed signal y', ie, y' = G1·b1·S1.

[0028] (6.3) Remove: The superimposed signal y sum Subtract the reconstructed signal y' to get the new superimposed signal y sum ', that is, y sum '=y sum -y', complete the demodulation of the first bit signal.

[0029] For the new superimposed signal y sumRepeat steps (6.1), (6.2) and (6.3) until all signals in sequence D are demodulated. If the number of detected peaks n is equal to the number of users K, the cross-correlation interference of K user signals is eliminated and demodulation is completed; if the number of peaks n is less than the number of users K, there are remaining undemodulated signals, and the final superimposed signal is recorded as y sum_new , proceed to step 7.

[0030] Step 7: Demodulate the remaining signal

[0031] Superimposed signal y sum_new Contains the remaining undemodulated CDMA signal, for y sum_new Execute step 4. If a peak value is detected in the correlation operation result, continue to execute steps 5 and 6. If no peak value or only a single peak value is generated, execute step 8.

[0032] Step 8: Demodulation using traditional serial interference cancellation methods

[0033] For the CDMA superposition signal with no peak or single peak after the correlation operation, the traditional serial interference cancellation method is used to demodulate the signal, and finally the cross-correlation interference of K user signals is eliminated, completing the communication transmission of multiple user CDMA signals.

[0034] The effects and benefits of the present invention are as follows:

[0035] Compared with the traditional method, the present invention generates a cyclic shift sequence on the receiver side, shifts and superimposes the local spread spectrum sequence of each user with a specific cyclic shift number, and performs correlation operation on the shifted and superimposed sequence and the received CDMA superimposed signal. It can uniquely determine the spread spectrum sequence of the user corresponding to each peak according to the spacing of the generated peaks, and at the same time, the peak sizes are sorted by descending power to obtain the signal sequence to be demodulated, so that multiple user signals can be demodulated at one time. When performing cross-correlation interference elimination on the receiver side, the computational complexity of the correlation operation can be reduced, the demodulation running speed can be increased, and the communication performance of each user in the system can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a specific flow chart of the present invention for performing cross-correlation interference elimination on multi-user CDMA superimposed signals.

[0037] In the figure: 1 is the shift spread spectrum sequence; 2 is the superposition shift spread spectrum sequence; 3 is the received superposition signal y sum ; 4 is the correlation operation; 5 is the detection and recording of the peak value; 6 is the traversal with the shift interval sequence X; 7 is the generation of the signal sequence to be demodulated; 8 is the signal despreading; 9 is the signal reconstruction; 10 is the signal subtraction; 11 is the repeated signal demodulation steps; 12 is the traditional method of demodulating the signal. DETAILED DESCRIPTION

[0038] The specific implementation of the present invention is described in detail below in conjunction with the technical solution (and drawings).

[0039] A method for eliminating cross-correlation interference based on shifted superposition sequences is proposed. Assume that there are four user signal transmitters in a synchronous DS-CDMA communication system, each transmitting a CDMA signal carrying the data information of each user. The spreading sequence of the four users is a Gold sequence with a code length of L=31. A signal receiver is used to receive and demodulate the superposition signal composed of the four user signals to complete the communication transmission of the four users. The signal received by the receiver is denoted as y sum , that is, y sum =y1+y2+y3+y4. To reduce the cross-correlation interference generated when the receiver demodulates the four user signals, the receiver performs cross-correlation interference elimination on the superimposed signals of the four users as follows:

[0040] Step 1: First, let the local spreading sequence of the four users at the receiver side, that is, the Gold sequence with a code length of 31, be cyclically shifted by the number of cyclic shifts a1, a2, a3, a4. Let the first Gold sequence not be shifted. Then the cyclic shift sequence A generated by equation (1) is A = {0, 4, 14, 20}. Secondly, the shift interval sequence X = {3(x 12 ),13(x 13 ),19(x 14 ),9(x 23 ),15(x 24 ),5(x 34 )}.

[0041] Step 2: Using the cyclic shift sequence A generated in step 1, cyclically shift the spread spectrum sequences s1, s2, s3, and s4 of the four users by 0, 4, 14, and 20 bits respectively to obtain the shifted spread spectrum sequences s1', s2', s3', and s4', as shown in the attached figure. Figure 1 As shown in 1.

[0042] Step 3: Superimpose the four shifted spread spectrum sequences s1', s2', s3', and s4' to obtain the shifted superposition sequence s with the same code length of 31. sum , that is, s sum =s1'+s2'+s3'+s4', as shown in the attached Figure 1 As shown in 2 places.

[0043] Step 4: Transform the signal received by the receiver into sum , as attached Figure 1 As shown in 3, the shift superposition sequence s generated in step 3 sum Perform related operations, such as Figure 1As shown in 4. The presence of a peak is detected by the gross error detection method. The gross error detection method is divided into three steps: first, the absolute value of the correlation operation result is taken; second, the absolute value result is subjected to the Laida criterion outlier detection, and the size and position of the outlier are recorded; finally, the recorded outlier is subjected to the Globus criterion outlier detection. If two outliers appear in the final test result, it proves that there are two peaks and the peak sizes e1, e2 and peak positions A1, A2 are recorded in turn, as shown in the attached figure. Figure 1 As shown in 5, go to step 5.

[0044] Step 5: If the distance between the two peaks is calculated as x1'=A2-A1-1=9 by formula (3), the shift interval sequence X is traversed, as shown in the following figure. Figure 1 As shown in 6, the shift spacing corresponding to the spacing x1' is x 23 , then it is determined that the peak position A1 corresponds to the shifted spread spectrum sequence s2', that is, the spread spectrum sequence s2 of user 2, and its signal combination is (e1, s2); similarly, the peak position A2 corresponds to the shifted spread spectrum sequence s3', which corresponds to the spread spectrum sequence s3 of user 3, and its signal combination is (e2, s3). If e2>e1, the two signal combinations are sorted in descending order according to the peak value, and the demodulated signal sequence D = {(e2, s3), (e1, s2)} is obtained. Figure 1 As shown in 7.

[0045] Step 6: Demodulate the signal in sequence D

[0046] (6.1) Despread the first bit signal by the spreading sequence s3 to obtain the amplitude estimate G3 and the original data b3, as shown in the attached figure. Figure 1 As shown in 8.

[0047] (6.2) The signal is remodulated by the amplitude estimate G3, the original data b3 and the spread spectrum sequence s3 to obtain the reconstructed signal y3', that is, y3' = G3·b3·s3, as shown in the attached figure. Figure 1 As shown in 9.

[0048] (6.3) The superimposed signal y sum Subtract the reconstructed signal y3' to get the new superimposed signal y sum ', that is, y sum '=y sum -y3', as attached Figure 1 As shown in 10.

[0049] For the new superimposed signal y sum 'Execute steps (6.1), (6.2) and (6.3) again to complete the demodulation of the second signal, as shown in the attached figure. Figure 1 As shown in 11, the final superposition signal is recorded as y sum_newSince there are still two signals that have not been demodulated, proceed to step 7.

[0050] Step 7: Superimpose signal y sum_new Execute step 4, after the peak detection is performed by the gross error detection method, no peak appears, and the superimposed signal y sum_new Go to step 8.

[0051] Step 8: Superimpose signal y sum_new The traditional serial interference cancellation method is used to demodulate the remaining two signals to obtain the original data b1 and b4 of user 1 and user 4, as shown in the attached figure. Figure 1 As shown in 12, the cross-correlation interference elimination between the four user signals is completed at the receiver side, and the communication transmission of the four user signals in the system is realized.

[0052] The above-described embodiments merely express the implementation methods of the present invention, but should not be understood as limiting the scope of the patent of the present invention. It should be pointed out that for those skilled in the art, several variations and improvements can be made without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention.

Claims

1. A method for eliminating cross-correlation interference based on shift-and-add sequence, characterized in that: Assume that there are K signal transmitters and one signal receiver in a CDMA communication system. The signal transmitter is used to send K CDMA signals carrying the data information of each user. The signal receiver is used to receive the CDMA superposition signal of K users. The signal receiver completes the communication transmission of K users by demodulating the superposition signal. The superposition signal received by the receiver is denoted as y sum , then y sum =y1+y2+...+y K , where y K For the received signal of the Kth user, the steps of performing cross-correlation interference elimination on the CDMA superposition signals of K users at the receiver side are as follows: Step 1: Generate a cyclic shift sequence and a shift spacing sequence; Step 2: Shift the spreading sequence; Step 3: superimpose the spreading sequence; Step 4: Sequence correlation operation; Step 5: Generate a signal sequence to be demodulated; Step 6: Signal sequence demodulation; Step 7: Demodulate the remaining signal; Step 8: Demodulation using traditional serial interference cancellation methods; The step 1 is specifically as follows: First, let the local spreading sequence of K users with code length L be cyclically shifted by the number of cyclic shifts a1, a2, ..., a K , where a K represents the cyclic shift number of the Kth user; let a1 = 0, that is, set the local spreading sequence of the first user to be unshifted, and generate the cyclic shift sequence A = {a1, a2, ..., a K };: The first formula in formula (1) represents the difference between the number of adjacent cyclic shifts and the code length and a K The difference must be greater than or equal to 2; the second formula represents the spacing between adjacent cyclic shift numbers and a K The sum of the distances from the end of the code length must be equal to the difference between the code length L and the number of system users K, that is, a i+1 -a i -1 indicates the spacing between adjacent cyclic shift numbers, La K -1 means a K The distance from the end of the code length; the third formula represents the number of cyclic shifts a i+1, i+2, ..., K i+1 ,a i+2 ,...,a K and the ith cyclic shift number a respectively i The difference must be unique and have no repeated values; Secondly, the distance x between the i-th cyclic shift number and the i+1, i+2, ..., K-th cyclic shift numbers ij for: x ij =a j -a i -1, i=1,2,...,K-1,j=i+1,i+2,...,K, (2) Finally, the shift interval sequence X={x 12 ,x 13 ,...,x 1K ,x 23 ,x 24 ,...,x 2K ,...,x (K-1)K }, where X is a triangular sequence.

2. The method for eliminating cross-correlation interference based on shift-and-add sequence according to claim 1, characterized in that: The specific steps are as follows: Step 2: Shift the spreading sequence According to the cyclic shift sequence A generated in step 1, the local spreading sequences s1, s2, ..., s of K users are K Circular shift a1, a2, ..., a respectively K bits, and the shift spread spectrum sequence s1',s2',...,s K ', where s K represents the local spreading sequence of the Kth user, s K ' represents the local spreading sequence of the Kth user after cyclic shift, i.e., the shifted spreading sequence; Step 3: Superimpose the spreading sequence The shifted spreading sequences s1',s2',...,s of K users are K 'Perform the superposition operation of the sequence to obtain the shift superposition sequence s with the same code length L sum , that is, s sum =s1'+s2'+...+s K '; Step 4: Serial Correlation Operation The shifted superposition sequence s obtained in step 3 sum With the superimposed signal y sum Perform correlation operation and perform peak detection on the correlation operation result; if there is a peak in the detection result, record the peak value e1, e2, ..., e n and peak positions A1, A2, ..., A n , execute step 5, where n represents the total number of detected peaks; if the detection result shows no peak or only a single peak is generated, execute step 8; Step 5: Generate the signal sequence to be demodulated According to the obtained peak positions A1, A2, ..., A n , calculate the adjacent peak spacing x according to formula (3) m ', we get the sequence of adjacent peak distances {x1',x2',...,x n-1 '}, traverse the shift interval sequence X and find the peak interval x m 'Equal shift spacing x ij , then the spreading sequence of the signal corresponding to the two peaks is s i and s j , thereby determining the spreading sequence s1”, s2”, ..., s corresponding to each peak signal n ", and record each signal as a combination of the peak value and its corresponding spreading sequence (e n ,s n ”); x m '=A m+1 -A m -1, m=1,2,...,n-1 (3) According to the peak value, all combinations are sorted in descending order to obtain the signal sequence D to be demodulated, D={(E1,S1),(E2,S2),...,(E n ,S n )}; Among them, (E n ,S n ) represents the signal combination of the nth position after sorting, E n is the peak size, S n is the spreading sequence corresponding to the signal; Step 6: Signal Sequence Demodulation The signal sequence D to be demodulated obtained in step 5 is used to demodulate the signal that generates the peak value; (6.1) Despreading: Despread the signal using the spreading sequence S1 combined with the first bit of sequence D to obtain the amplitude estimate G1 and the original transmitted data b1. (6.2) Reconstruction: Remodulate the signal using the amplitude estimate G1, the original data b1, and the spreading sequence S1 to obtain the reconstructed signal y', i.e., y' = G1·b1·S1; (6.3) Remove: The superimposed signal y sum Subtract the reconstructed signal y' to get the new superimposed signal y sum ', that is, y sum '=y sum -y', complete the demodulation of the first bit signal; For the new superimposed signal y sum Repeat steps (6.1), (6.2) and (6.3) until all signals in sequence D are demodulated. If the number of detected peaks n is equal to the number of users K, the cross-correlation interference of K user signals is eliminated and demodulation is complete. If the number of peaks n is less than the number of users K, there are remaining undemodulated signals. The final superimposed signal is recorded as y sum_new , proceed to step 7; Step 7: Remaining Signal Demodulation Superimposed signal y sum_new Contains the remaining undemodulated CDMA signal, for y sum_new Execute step 4. If the correlation operation result has a peak after detection, continue to step 5 and step 6. If there is no peak or only a single peak is generated, proceed to step 8. Step 8: Demodulation using traditional serial interference cancellation methods For the CDMA superposition signal with no peak or single peak after the correlation operation, the traditional serial interference cancellation method is used to demodulate the signal, and finally the cross-correlation interference of K user signals is eliminated, completing the communication transmission of multiple user CDMA signals.

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