The Terahertz (THz) band is envisioned as a promising technique to support bandwidth-hungry and secure applications. Although the significant path loss and strong directivity make THz communications secure naturally, the information security is still imperfect at near regions along the beam propagation path. In this paper, a novel distance-adaptive absorption peak modulation (DA-APM) is developed for THz covert communications, by exploring the unique spectrum features of frequency-dependent molecular absorption. Although high-attenuation molecular absorption is unfavored for communications, the main principle to enhance covertness or equivalently, minimize the eavesdroppable distance, is dynamically modulating signals under the molecular absorption peaks in the THz spectrum, where the eavesdroppable distance is defined as the threshold distance within which an eavesdropper can wiretap the transmission. Furthermore, an optimization framework is proposed to minimize the eavesdroppable distance, to which the sub-optimal solutions are derived for the multi-wideband waveform by controlling carrier frequencies, power allocation, and rate distribution on each sub-band. Extensive numerical results show that the THz-spectrum-based DA-APM approach can reduce the eavesdroppable distance by 60% compared with random spectrum selection methods, which significantly reduce the insecure area and enhance the covertness of THz wireless transmission.

中文翻译：

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太赫兹隐蔽通信的距离自适应吸收峰值调制（DA-APM）

太赫兹（THz）频段被认为是一种有前途的技术，可支持高带宽和安全的应用。尽管显着的路径损耗和强大的方向性使THz通信自然安全，但是在沿波束传播路径的附近区域，信息安全仍然不完善。在本文中，通过探索依赖频率的分子吸收的独特光谱特征，开发了一种新型的距离自适应吸收峰调制（DA-APM）用于太赫兹隐蔽通信。尽管高衰减分子吸收不利于通信，但是提高隐蔽性或等效性，最小化可窃听距离的主要原理是动态调制THz光谱中分子吸收峰下的信号，其中可窃听距离定义为窃听者可以窃听传输的阈值距离。此外，提出了一种优化框架以最小化可窃听的距离，通过控制每个子带上的载波频率，功率分配和速率分布，可以得出多宽带波形的次优解决方案。大量数值结果表明，与随机频谱选择方法相比，基于太赫兹频谱的DA-APM方法可以将可窃听距离减少60％，从而显着减少了不安全区域并提高了太赫兹无线传输的隐蔽性。通过控制每个子带上的载波频率，功率分配和速率分布，可以得出多宽带波形的次优解。大量数值结果表明，与随机频谱选择方法相比，基于太赫兹频谱的DA-APM方法可以将可窃听距离减少60％，从而显着减少了不安全区域并提高了太赫兹无线传输的隐蔽性。通过控制每个子带上的载波频率，功率分配和速率分布，可以得出多宽带波形的次优解。大量数值结果表明，与随机频谱选择方法相比，基于太赫兹频谱的DA-APM方法可以将可窃听距离减少60％，从而显着减少了不安全区域并提高了太赫兹无线传输的隐蔽性。