Sequences with optimal autocorrelation properties play an important role in wireless communication, radar and cryptography. Interleaving is a very important method in constructing the optimal autocorrelation sequence. Tang and Gong gave three different constructions of interleaved sequences (generalized GMW sequences, twin prime sequences and Legendre sequences). Su et al. constructed a series of sequences with optimal autocorrelation magnitude via interleaving Ding-Helleseth-Lam sequences. In this paper we further study the correlation properties of interleaved Legendre sequences and Ding-Helleseth-Lam sequences.
Citation: Yixin Ren, Chenyu Hou, Huaning Liu. Correlation properties of interleaved Legendre sequences and Ding-Helleseth-Lam sequences[J]. Electronic Research Archive, 2023, 31(8): 4549-4556. doi: 10.3934/era.2023232
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Sequences with optimal autocorrelation properties play an important role in wireless communication, radar and cryptography. Interleaving is a very important method in constructing the optimal autocorrelation sequence. Tang and Gong gave three different constructions of interleaved sequences (generalized GMW sequences, twin prime sequences and Legendre sequences). Su et al. constructed a series of sequences with optimal autocorrelation magnitude via interleaving Ding-Helleseth-Lam sequences. In this paper we further study the correlation properties of interleaved Legendre sequences and Ding-Helleseth-Lam sequences.
Given a binary sequence v=(v(0),v(1),⋯,v(N−1))∈{0,1}N of period N, ZN is the integer ring of modulo N, the set Cv={t∈ZN: v(t)=1} is called the support of v. Let τ∈{1,⋯,N−1}, the autocorrelation function Rv(τ) of v is defined by
Rv(τ)=N−1∑i=0(−1)v(i)+v(i+τ), |
where i+τ is operating on modulo N. Easy to verity
Rv(τ)=N−4|(Cv+τ)∩Cv|. |
Thus Rv(τ)≡N (mod 4). In particular, Rv(τ)≡0 (mod 4) for N≡0 (mod 4).
If Rv(τ)∈{0,−4} or Rv(τ)∈{0,+4} where τ∈{1,⋯,N−1}, then v is referred to as a sequence with optimal autocorrelation value. If Rv(τ)∈{0,±4} where τ∈{1,⋯,N−1}, then v is referred to as a sequence with optimal autocorrelation magnitude. Sequences with optimal autocorrelation properties play an important role in wireless communication, radar and cryptography[1], so they have always been the focus of scholars' research (refer to [2,3,4,5,6,7,8,9,10,11]).
Gong[12] introduced powerful interleaved method to design sequences. The key idea of this method is to obtain long sequences with good correlation from shorter ones. Let
vk=(vk(0),vk(1),⋯,vk(N−1)) |
be a sequence of period N, where 0≤k≤M−1. Define the N×M matrix U=(Uij) as follows
U=[v0(0)v1(0)⋯vM−1(0)v0(1)v1(1)⋯vM−1(1)⋮⋮⋮v0(N−1)v1(N−1)⋯vM−1(N−1)]. |
An interleaved sequence u=(u(t)) of period MN is defined by
uiM+j=Uij=vj(i),0≤i<N, 0≤j<M. |
For convenience we write
u=I(v0,v1,⋯,vM−1). |
Interleaving is a very important method in constructing optimal autocorrelation sequences. The autocorrelation of the sequence u is determined by the autocorrelation and cross-correlation of the sequences v0,v1,⋯,vM−1.
Tang and Gong[10] gave three different constructions of interleaved sequences with the following structure:
u=I(a0+b(0), Ld+η(a1)+b(1), L2d(a2)+b(2), L3d+η(a3)+b(3)), | (1.1) |
where η is an integer with 0≤η<N, d is an integer with 4d≡1 (mod N), b=(b(0),b(1),b(2),b(3)) is a binary perfect sequence, ai, i=0,1,2,3, are sequences of period N taken from three different pairs of sequences (generalized GMW sequences, twin prime sequences and Legendre sequences).
Let N=p be an odd prime. The Legendre sequence l=(l(0),l(1),⋯,l(p−1)) of period p is defined by
l(i)={0 or 1,if i=0,1,if i is quadratic residue modulo p,0,if i is quadratic non-residue modulo p. |
In particular, l is called the first type Legendre sequence if l(0)=1 otherwise the second type Legendre sequence. Let l and l′ be the first type and the second type of Legendre sequence of period p, respectively.
Tang and Gong[10] showed that the constructions of interleaved Legendre sequence have optimal autocorrelation magnitude. Li and Tang[6] studied the linear complexity of binary sequences given in [10].
Let p=4f+1 be an odd prime, where f is a positive integer, α is a generator of Fp. Define sets
Di={αi+4j: 0≤j≤f−1},i=0,1,2,3. |
Then Di, i=0,1,2,3 are called the cyclotomic classes of order 4 with respect to Fp. It is very easy to verity that Di, i=0,1,2,3 constitute a partition of F∗p.
Ding et al.[13] constructed some binary sequences based on the cyclotomic classes of order 4 of Fp, and proved that these sequences are three-level autocorrelation.
Proposition 1.1. Let p=4f+1=x2+4y2 be an odd prime, where f,x,y are integers. Let s1,s2,s3,s4 be binary sequences of period p with supports D0∪D1, D0∪D3, D1∪D2 and D2∪D3, respectively. Then Rsi(τ)∈{1,−3} for all 1≤τ<p if and only if f is odd and y=±1.
Let a0,a1,a2,a3 be four binary sequences of period p, b=(b(0),b(1),b(2),b(3)) be a binary sequence of length 4. Construct a binary sequence u=u(t) of length 4p as follows:
u=I(a0+b(0), Ld(a1)+b(1), L2d(a2)+b(2), L3d(a3)+b(3)), | (1.2) |
where d is a integer with 4d≡1 (mod p), L is a cyclic left shift operator, when c=(c(0),c(1),⋯,c(N−1)), L(c)=(c(1),c(2),⋯,c(N−1),c(0)).
Su et al.[8] constructed a series sequences with optimal autocorrelation magnitude of period 4p via interleaving Ding-Helleseth-Lam sequences.
Proposition 1.2. Let p=4f+1=x2+4y2 be an odd prime, where x is an integer, y=±1, f is odd. Let b(0)=b(2), b(1)=b(3), and (a0,a1,a2,a3) be chosen from
(s3,s2,s1,s1),(s2,s3,s1,s1),(s1,s4,s2,s2),(s4,s1,s2,s2),(s1,s4,s3,s3),(s4,s1,s3,s3),(s3,s2,s4,s4),(s2,s3,s4,s4). | (1.3) |
Then the binary sequence u by (1.2) has Ru(τ)∈{0,±4} for all 1≤τ<4p.
Later Fan[14] showed that the above sequences have large linear complexity. Sun et al.[15] proved that the sequences have good 2-adic complexity.
This paper will further study the fourth-order correlation properties of interleaved Legendre sequences and Ding-Helleseth-Lam sequences. Our results are as follows.
Theorem 1.1. Let p>2 be a prime, b=(b(0),b(1),b(2),b(3)) be a binary perfect sequence, η be an integer with 0<η<p and let the binary sequence u be generated by (1.1). For n∈{0,1,⋯,4p−1} we have
(−1)un+un+p+un+2p+un+3p={(−1)a0(p−η)+a1(0)+a2(p−η)+a3(0)+1,n≡−4η (mod p),(−1)a0(0)+a1(η)+a2(0)+a3(η)+1,n≡0 (mod p) and n≢−4η (mod p),−1,n≢0 (mod p) and n≢−4η (mod p). |
Theorem 1.2. Let p=4f+1 be an odd prime, b(0)=b(2), b(1)=b(3) and let (a0,a1,a2,a3) be chosen from (1.3). Let the binary sequence u be defined as in (1.2). For n∈{0,1,⋯,4p−1} we have
(−1)un+un+p+un+2p+un+3p={+1,n≡0 (mod p),−1,n≢0 (mod p). |
Our results show that the fourth-order correlation for interleaved Legendre sequences and Ding-Helleseth-Lam sequences u we have (−1)un+un+p+un+2p+un+3p=−1 which the number of n is at least 4p−5 for all n∈{0,1,⋯,4p−1}. This means that there are at least four subsequences that do not satisfy lower mutual interference. Therefore, researchers should pay careful attention to these four subsequences when consider their application in communication systems.
Write
u=I(v0, v1, v2, v3)=I(a0+b(0), Ld+η(a1)+b(1), L2d(a2)+b(2), L3d+η(a3)+b(3)). |
Then
(−1)v0(n)=(−1)b(0)(−1)a0(n), | (2.1) |
(−1)v1(n)=(−1)b(1)(−1)a1(n+d+η), | (2.2) |
(−1)v2(n)=(−1)b(2)(−1)a2(n+2d), | (2.3) |
(−1)v3(n)=(−1)b(3)(−1)a3(n+3d+η). | (2.4) |
Case Ⅰ: p≡1 (mod 4). We have
p=4⋅p−14+1,2p=4⋅2(p−1)4+2,3p=4⋅3(p−1)4+3,d=3p+14. |
Write n=4i+j, where 0≤i≤p−1 and 0≤j≤3. Hence,
un+un+p+un+2p+un+3p=u4i+j+u4i+j+4⋅p−14+1+u4i+j+4⋅2(p−1)4+2+u4i+j+4⋅3(p−1)4+3. | (2.5) |
First we consider the case of j=1. By (2.1)–(2.5) we get
un+un+p+un+2p+un+3p=u4i+1+u4(i+p−14)+2+u4(i+2(p−1)4)+3+u4(i+3(p−1)4)+4=v1(i)+v2(i+p−14)+v3(i+2(p−1)4)+v0(i+3p+14)=a1(i+d+η)+b(1)+a2(i+p−14+2d)+b(2)+a3(i+2(p−1)4+3d+η)+b(3)+a0(i+3p+14)+b(0)=b(0)+b(1)+b(2)+b(3)+a0(i+d)+a1(i+d+η)+a2(i+d)+a3(i+d+η). | (2.6) |
For all j∈{0,1,2,3} we can also have
un+un+p+un+2p+un+3p=b(0)+b(1)+b(2)+b(3)+a0(i+jd)+a1(i+jd+η)+a2(i+jd)+a3(i+jd+η). | (2.7) |
Noting that 4d≡1 (mod p) and n=4i+j, thus we get
i+jd≡0 (mod p)⟺4i+j≡0 (mod p)⟺n≡0 (mod p),i+jd+η≡0 (mod p)⟺4i+j+4η≡0 (mod p)⟺n+4η≡0 (mod p). |
Therefore
(−1)un+un+p+un+2p+un+3p=(−1)a0(i+jd)+a1(i+jd+η)+a2(i+jd)+a3(i+jd+η)={(−1)a0(0)+a1(0)+a2(0)+a3(0)+1,n≡0 (mod p) and n+4η≡0 (mod p),(−1)a0(p−η)+a1(0)+a2(p−η)+a3(0)+1,n≢0 (mod p) and n+4η≡0 (mod p),(−1)a0(0)+a1(η)+a2(0)+a3(η)+1,n≡0 (mod p) and n+4η≢0 (mod p),−1,n≢0 (mod p) and n+4η≢0 (mod p),={(−1)a0(p−η)+a1(0)+a2(p−η)+a3(0)+1,n≡−4η (mod p),(−1)a0(0)+a1(η)+a2(0)+a3(η)+1,n≡0 (mod p) and n≢−4η (mod p),−1,n≢0 (mod p) and n≢−4η (mod p). |
Case Ⅱ: p≡3 (mod 4). We have
p=4⋅p−34+3,2p=4⋅2(p−1)4+2,3p=4⋅3p−14+1,d=p+14. |
Similarly we can get
(−1)un+un+p+un+2p+un+3p={(−1)a0(p−η)+a1(0)+a2(p−η)+a3(0)+1,n≡−4η (mod p),(−1)a0(0)+a1(η)+a2(0)+a3(η)+1,n≡0 (mod p) and n≢−4η (mod p),−1,n≢0 (mod p) and n≢−4η (mod p). |
This proves Theorem 1.1.
Write
u=I(v0, v1, v2, v3)=I(a0+b(0), Ld(a1)+b(1), L2d(a2)+b(2), L3d(a3)+b(3)). |
Then
(−1)v0(n)=(−1)b(0)(−1)a0(n), | (3.1) |
(−1)v1(n)=(−1)b(1)(−1)a1(n+d), | (3.2) |
(−1)v2(n)=(−1)b(2)(−1)a2(n+2d), | (3.3) |
(−1)v3(n)=(−1)b(3)(−1)a3(n+3d). | (3.4) |
Let b(0)=b(2) and b(1)=b(3). Noting that p≡1 (mod 4), thus we have
p=4⋅p−14+1,2p=4⋅2(p−1)4+2,3p=4⋅3(p−1)4+3,d=3p+14. |
Write n=4i+j, where 0≤i≤p−1 and 0≤j≤3. Hence,
un+un+p+un+2p+un+3p=u4i+j+u4i+j+4⋅p−14+1+u4i+j+4⋅2(p−1)4+2+u4i+j+4⋅3(p−1)4+3. | (3.5) |
For the case of j=1, by (3.1)–(3.5) we get
un+un+p+un+2p+un+3p=u4i+1+u4(i+p−14)+2+u4(i+2(p−1)4)+3+u4(i+3(p−1)4)+4=v1(i)+v2(i+p−14)+v3(i+2(p−1)4)+v0(i+3p+14)=a1(i+d)+b(1)+a2(i+p−14+2d)+b(2)+a3(i+2(p−1)4+3d)+b(3)+a0(i+3p+14)+b(0)=b(0)+b(1)+b(2)+b(3)+a0(i+d)+a1(i+d)+a2(i+d)+a3(i+d). | (3.6) |
For all j∈{0,1,2,3} we can also have
un+un+p+un+2p+un+3p=b(0)+b(1)+b(2)+b(3)+a0(i+jd)+a1(i+jd)+a2(i+jd)+a3(i+jd). | (3.7) |
Clearly,
i+jd≡0 (mod p)⟺4i+j≡0 (mod p)⟺n≡0 (mod p). |
Let (a0,a1,a2,a3) be chosen from
(s3,s2,s1,s1),(s2,s3,s1,s1),(s1,s4,s2,s2),(s4,s1,s2,s2),(s1,s4,s3,s3),(s4,s1,s3,s3),(s3,s2,s4,s4),(s2,s3,s4,s4). |
From (3.7) we get
(−1)un+un+p+un+2p+un+3p=(−1)a0(i+jd)+a1(i+jd)+a2(i+jd)+a3(i+jd)={+1,n≡0 (mod p),−1,n≢0 (mod p). |
This completes the proof of Theorem 1.2.
The authors have not used Artificial Intelligence (AI) tools in the creation of this article.
This paper is supported by National Natural Science Foundation of China under Grant No. 12071368, Shaanxi Fundamental Science Research Project for Mathematics and Physics (Grant No. 22JSY017), and Northwest University Graduate Innovation Program Funds (No. CX2023066).
The authors declare there is no conflicts of interest.
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