Easily and flexibly deployable ad-hoc communication networks emerging in tactical military or even civilian contexts, frequently suffer from poor synchronization due to lack of coordinating infrastructure. In addition to synchronization issues, and especially in military settings, security from the aspect of detectability is also of crucial importance. Imperfect synchronization can be dealt with by making use of Quasi-Synchronous Code Division Multiple Access (QS-CDMA), relying on Loosely Synchronous Codes to maintain orthogonality in the presence of limited time delays. Security, in terms of low probability of detection (LPD) from the standpoint of a malicious adversary, can be improved (reduced detection) by employing randomization techniques that disrupt the inherent structure of the transmitted QS-CDMA signals. This is based on the fact that QS-CDMA signals are Cyclostationary, having (almost) periodic Auto-Correlation functions (ACF) due to eminent signal periodicities (such as spreading code repetition). In this paper, we propose techniques to disturb the ACF and equivalently the Spectral Correlation function, and reduce the Degree of Cyclostationarity (DCS), our LPD measure. Specifically, we investigate randomization via 1) random per symbol time dithering and 2) random selection of spreading sequences, as well as a hybrid approach combining time dithering and code randomization. In all proposed techniques knowledge of the randomization pattern is not required at the legitimate receiver. We derive the Spectral Correlation function of the QS-CDMA signal under the proposed randomization schemes and compare it to simulations. We show through analysis and extensive numerical simulations that the proposed technique can reduce the DCS by almost two orders of magnitude. We also show that enhanced LPD can be achieved using the proposed techniques while sacrificing a part of the reduced time synchronization requirement. We further analyze the implications of the friendly receiver not knowing the randomization pattern and present results on the resulting communication performance.

中文翻译：

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采用随机化技术改进 QS-DS-CDMA 的 LPD 特性

在战术军事甚至民用环境中出现的可轻松灵活部署的临时通信网络，由于缺乏协调基础设施，经常会出现同步不佳的情况。除了同步问题，尤其是在军事环境中，可检测性方面的安全性也至关重要。不完全同步可以通过使用准同步码分多址 (QS-CDMA) 来处理，依靠松散同步码在有限时间延迟的情况下保持正交性。从恶意对手的角度来看，就低检测概率 (LPD) 而言，安全性可以通过采用破坏所传输 QS-CDMA 信号固有结构的随机化技术来提高（减少检测）。这是基于 QS-CDMA 信号是循环平稳的，由于显着的信号周期性（例如扩频码重复）而具有（几乎）周期性的自相关函数 (ACF)。在本文中，我们提出了干扰 ACF 和等效频谱相关函数的技术，并降低循环平稳度 (DCS)，我们的 LPD 测量。具体来说，我们通过 1) 每符号时间随机抖动和 2) 随机选择扩展序列以及结合时间抖动和代码随机化的混合方法来研究随机化。在所有提议的技术中，合法接收器不需要随机化模式的知识。我们在建议的随机化方案下推导出 QS-CDMA 信号的频谱相关函数，并将其与模拟进行比较。我们通过分析和广泛的数值模拟表明，所提出的技术可以将 DCS 减少近两个数量级。我们还表明，使用所提出的技术可以实现增强的 LPD，同时牺牲一部分减少的时间同步要求。我们进一步分析了友好接收者不知道随机化模式的影响，并展示了由此产生的通信性能的结果。