1.
Introduction
Throughout this paper, let p be an odd prime and q=pn for some positive integer n. Codebooks (also known as signal sets) with low coherence are typically used to distinguish signals of different users in code division multiple access (CDMA) systems. An (N,K) codebook C is a finite set {c0,c1,…,cN−1}, where the codeword ci, 0≤i≤N−1, is a unit norm 1×K complex vector over an alphabet A. The maximum inner-product correlation Imax(C) of C is defined by
where cHj denotes the conjugate transpose of cj. The maximal cross-correlation amplitude Imax(C) of C is an important index of C, as it can approximately optimize many performance metrics such as outage probability and average signal-to-noise ratio. For a fixed K, researchers are highly interested in designing a codebook C with the parameter N being as large as possible and Imax(C) being as small as possible simultaneously. Unfortunately, there exists a bound between the parameters N, K and Imax(C).
Lemma 1. ([1]) For any (N,K) codebook C with N≥K,
The bound in (1.1) is called the Welch bound of C and is denoted by Iw(C). If the codebook C achieves Iw(C), then C is said to be optimal with respect to the Welch bound. However, constructing codebooks achieving the Welch bound is extremely difficult. Hence, many researchers have focused their main energy on constructing asymptotically optimal codebooks, i.e., Imax(C) asymptotically meets the Welch bound Iw(C) for sufficiently large N [2,3,4,5,6,7].
The objective of this paper is to construct a class of complex codebooks and investigate their maximum inner-product correlation. Results show that these constructed complex codebooks are nearly optimal with respect to the Welch bound, i.e., the ratio of their maximal cross-correlation amplitude to the Welch bound approaches 1. These codebooks may have applications in strongly regular graphs [8], combinatorial designs [9,10], and compressed sensing [11,12].
This paper is organized as follows. In Section 2, we review some essential mathematical concepts regarding characters and Gauss sums over finite fields. In Section 3, we present a class of asymptotically optimal codebooks using the trace functions and multiplicative characters over finite fields. Finally, we make a conclusion in Section 4.
2.
Preliminaries
In this section, we review some essential mathematical concepts regarding characters and Gauss sums over finite fields. These concepts will play significant roles in proving the main results of this paper.
Let n be a positive integer and p an odd prime. Denote the finite field with pn elements by Fpn. The trace function Trn from Fpn to Fp is defined by
Let ζp denote a primitive p-th root of complex unity and Trn denote the trace function from Fpn to Fp. For x∈Fpn, it can be checked that χn given by χn(x)=ζTrn(x)p is an additive character of Fpn, and χn is called the canonical additive character of Fpn. Assume a∈Fpn, then every additive character of Fpn can be obtained by μa(x)=χn(ax) where x∈Fpn. The orthogonality relation of μa is given by
Let q=pn and α be a primitive element of F∗q, then all multiplicative characters of Fq are given by φj(αi)=ζijq−1, where ζq−1 denotes a primitive (q−1)-th root of unity and 0≤i,j≤q−2. The quadratic character of Fq is the character φ(q−1)/2, which will be denoted by ηn in the sequel, and ηn is extended by setting ηn(0)=0. For φj, its orthogonality relation is given by
The Gauss sum G(ηn) over Fpn is defined by
The explicit value of G(ηn) is given in the following lemma.
Lemma 2 ([13], Theorem 5.15). With symbols and notations above, we have
The following results on exponential sums will play an important role in proving the main results of this paper.
Lemma 3 ([13], p.195). With symbols and notations above, we have
where η1 denotes the quadratic character and χ1 the canonical additive character of Fp.
Lemma 4 ([13], Theorem 5.33). If f(x)=a2x2+a1x+a0∈Fpn[x] with a2≠0, then
Lemma 5 ([14], Theorem 2). Let n=2m be an even integer and z∈F∗p, then
Lemma 6 ([15]). Let n=2m be an even integer, a∈F∗pm, and b∈Fpn, then
Lemma 7 ([16], Lemma 3.12). If A and B are finite abelian groups, then there is an isomorphism
where ˆA consists of all characters of A.
By this lemma, we know that
where
for x,y∈Fpn.
3.
Proofs and main results
In this section, we always suppose that n=2m is an even integer and p is an odd prime. The set D is defined as follows:
where η1 is the quadratic character of Fp. A codebook C is constructed by
where ca,b=1√|D|(μa,b(x,y))(x,y)∈D, μa,b(x,y)=ζTrn(ax+by)p for (x,y)∈D and |D| denotes the cardinality of the set D.
Lemma 8. With symbols and notations as above, we have
Proof. Let
Note that
Together with the definition of D, we have
By definition, we have
where the last equality follows from Lemmas 2, 4, and 5. Note that ηn(z)=1 for z∈F∗p if n is even. By Lemma 3, we obtain
Using Lemmas 4 and 5, we get
The desired conclusion follows from (3.2) and (3.3). □
Example 1. Let p=5 and n=2. By the Magma program, we know that |D|=240, which is consistent with Lemma 8.
Theorem 9. Let symbols and notations be the same as before, then the codebook C defined in (3.1) has parameters [p2n,K],
and
Proof. By the definition of the set C and Lemma 8, we deduce that C is a [p2n,K] codebook. If a,b∈Fpn and (a,b)≠(0,0), then we have
This implies that
For a,b∈Fpn and (a,b)≠(0,0), we have that
where
By (2.1), we derive that
Combining Lemmas 4 and 6, we get that
where G(ηn) is given in Lemma 2. By Lemma 3, we have that
Moreover, by Lemmas 4 and 6, we obtain
It follows from Lemma 2 and (3.4) that
For any two distinct codewords cz1,z2, cz′1,z′2∈C, i.e., (z1,z2)≠(z′1,z′2), it is easy to check that
Combining (3.7) and (3.8), we get that Imax(C)=(p+1)pn−1/(2K). □
Example 2. Let f(x) be an irreducible polynomial over the field F3 and f(x)=x2+x+2 in F3[x]. Suppose that p=3, n=2, and α is a root of f(x) over F3, then m=1, q=32, and F9=F3(α). It can be verified that the set D consists of the following 30 elements:
The corresponding codebook C is given by
where ζ3=e2π√−13 and Tr2 denotes the trace function from F9 to F3.
Corollary 10. The codebook C constructed in (3.1) is asymptotically optimal with respect to the Welch bound.
Proof. The corresponding Welch bound is
We deduce that
which implies that C asymptotically meets the Welch bound. □
In Table 1, we show some parameters of some specific codebooks defined in (3.1). From this table, we conclude that Imax(C) is very close to Iw(C) for largely enough p, which ensures the correctness of Theorem 9 and Corollary 1.
4.
Concluding remarks
This paper presented a family of codebooks by the combination of additive characters and multiplicative characters over finite fields. Results show that the constructed codebooks are asymptotically optimal in the sense that the maximum cross correlation amplitude of the codebooks asymptotically achieves the Welch bound. As a comparison, parameters of some known nearly optimal codebooks and the constructed ones are listed in Table 2. From this table, we can conclude that the parameters of C are not covered by those in [2,3,4,5,6,17,18,19]. This means the presented codebooks have new parameters.
Use of AI tools declaration
The authors declare they have not used Artificial Intelligence (AI) tools in the creation of this article.
Acknowledgments
This work was supported by the Innovation Project of Engineering Research Center of Integration and Application of Digital Learning Technology (No.1221003), Humanities and Social Sciences Youth Foundation of Ministry of Education of China (No. 22YJC870018), the Science and Technology Development Fund of Tianjin Education Commission for Higher Education (No. 2020KJ112, 2022KJ075, KYQD1817), the National Natural Science Foundation of China (Grant No. 12301670), the Natural Science Foundation of Tianjin (Grant No. 23JCQNJC00050), Haihe Lab. of Information Technology Application Innovation (No. 22HHXCJC00002), Fundamental Research Funds for the Central Universities, China (Grant No. ZY2301, BH2316), the Open Project of Tianjin Key Laboratory of Autonomous Intelligence Technology and Systems (No. TJKL-AITS-20241004, No. TJKL-AITS-20241006).
Conflict of interest
The authors declare no conflicts of interest.