Aperiodic Total Squared Ambiguity Function: Theoretical Bounds for Binary Sequence Sets and Optimal Constructions
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摘要: 扩频序列集是直接-序列码分多址系统中的关键组成部分,其性能可通过完全平方相关进行评估。在高速移动场景中,信号在传输过程中会产生多普勒效应,需同时考虑序列的时移和多普勒移位。此时,应使用二维模糊函数替代一维相关函数。该文主要研究二元序列集的非周期完全平方模糊函数(Aperiodic Total Squared Ambiguity Function, ATSAF),推导了二元序列集的ATSAF理论下界。基于Hadamard矩阵、非周期互补集和特殊序列,设计了几类达到ATSAF理论下界的最优二元序列集。
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关键词:
- 非周期完全平方模糊函数 /
- 模糊函数 /
- 完全平方相关
Abstract:Objective In direct-sequence code division multiple access systems, the performance of spreading sequence sets is typically evaluated using the total squared correlation metric. Traditional metrics such as total squared correlation and aperiodic total squared correlation are only applicable to synchronous communication systems and asynchronous systems with time shifts only, respectively. However, in modern high-speed mobile and satellite communications, the Doppler effect becomes significant, causing both time and Doppler shifts in the received signal and consequently leading to severe signal distortion. In communication scenarios considering only time shift, the one-dimensional correlation function is typically employed to measure interference within the system. However, in high-speed mobile environments, the Doppler effect is introduced during signal transmission, necessitating the simultaneous consideration of both time shift and Doppler shift of the sequence. In such cases, the two-dimensional ambiguity function should be used in place of the one-dimensional correlation function. To mitigate Doppler effects, the research community has increasingly focused on designing Doppler-resilient sequences to address the Doppler effects present in various mobile channels. Existing studies are primarily concentrated on the theoretical bounds of the ambiguity function, namely the maximum ambiguity magnitude, with sequence sets subsequently constructed that achieve or asymptotically achieve these bounds. This research, however, focuses on the overall ambiguity function performance of binary sequence sets in asynchronous communication, namely the ATSAF. The specific objectives are as follows:1. The theoretical lower bound for the ATSAF of binary sequence sets is derived.2. Based on the derived ATSAF lower bound, several classes of optimal binary sequence sets that achieve this theoretical bound are designed. Methods The aperiodic time-phase cycling extension matrix $ {\boldsymbol{S}}_{a} $ is defined for a binary sequence set $ \boldsymbol{S} $ consisting of $ K $ sequences of length $ L $, in order to account for both time shifts and Doppler shifts. This definition transforms the problem of computing the ATSAF for the set $ \boldsymbol{S} $ into that of calculating the total squared correlation of the matrix $ {\boldsymbol{S}}_{a} $. Subsequently, the theoretical lower bounds for the ATSAF of the binary sequence set $ \boldsymbol{S} $ are derived for different combinations of the set size $ K $, sequence length $ L $, and Doppler shift $ V $. To design binary sequence sets that achieve these derived ATSAF lower bounds, it is first proven that binary aperiodic complementary sets constitute optimal binary sequence sets with respect to the ATSAF. Furthermore, based on Hadamard matrices and specific sequences, two additional classes of optimal binary sequence sets are designed, which are shown to achieve the theoretical ATSAF lower bound. Results and Discussions Existing research primarily focuses on the maximum ambiguity magnitude of sequence sets, while this study emphasizes the overall ambiguity function performance. The one-dimensional aperiodic total squared correlation analysis for asynchronous communication with delay only, as investigated by Ganapathy et al., is extended in this work to the two-dimensional aperiodic total squared ambiguity function, which incorporates both time delay and Doppler shift. This paper first defines the aperiodic time-phase cycling extension matrix $ {\boldsymbol{S}}_{a} $ for a binary sequence set $ \boldsymbol{S} $ (Definition 3). Subsequently, the theoretical lower bounds for the ATSAF of the binary sequence set $ \boldsymbol{S} $ are derived for various parameters, including the set size$ K $, sequence length $ L $, and Doppler shift $ V $ (Theorem 1). When the Doppler shift $ V=1 $, the ATSAF theoretical bound derived in this paper reduces to the aperiodic total squared correlation theoretical bound. Binary sequence sets that achieve these ATSAF lower bounds maintain the overall cross interference energy in the two-dimensional delay-Doppler domain at its theoretical minimum. To design binary sequence sets that achieve these derived ATSAF bounds, it is first proven that binary aperiodic complementary sets are ATSAF-optimal binary sequence sets (Theorem 2). Furthermore, based on Hadamard matrices and specific sequences, two additional classes of ATSAF-optimal binary sequence sets are designed (Theorems 3 and 4). Finally, an example is provided in this paper to demonstrate that the sequence set constructed in Theorem 4 is an ATSAF-optimal binary sequence set (Example 1). Conclusions In high-speed mobile communication scenarios, Doppler effects lead to distortion in the received signal. Therefore, by defining the aperiodic time-phase cycling extension matrix $ {\boldsymbol{S}}_{a} $ for a binary sequence set $ \boldsymbol{S} $, the theoretical lower bound for the ATSAF is derived, which specifies the minimum theoretical value for the total energy of the binary sequence set S in the two-dimensional delay-Doppler domain. When Doppler shifts are not considered, the derived ATSAF bound reduces to the aperiodic total squared correlation bound. Furthermore, three classes of ATSAF-optimal binary sequence sets that achieve this theoretical bound are constructed using binary aperiodic complementary sets, Hadamard matrices, and specific sequences. This study not only provides the theoretical ATSAF bound for binary sequence sets in the two-dimensional delay-Doppler domain but also designs several classes of optimal binary sequence sets that achieve this bound. These sets achieve the theoretical minimum for overall cross interference energy in the two-dimensional delay-Doppler domain. -
表 1 例1中序列集$ \boldsymbol{S} $的ATSAF值和理论下界
$ V $ 1 2 3 4 $ \text{ATSAF(}\boldsymbol{S}\text{)} $ 5516 22008 49420 87808 理论下界 5516 22008 49420 87808 -
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