低信噪比隐蔽通信综合中的脉冲波形和间歇非线性滤波
Pulsed Waveforms and Intermittently Nonlinear Filtering in Synthesis of Low-SNR and Covert Communications
摘要：
在传统的扩频技术中，通过用包含相对低速率消息的窄带信号调制宽带载波来获得宽带发射信号。在接收机中，相应的解调/解扩恢复携带信息的窄带信号。在本文中，我们介绍了一种替代方法，其中低速率的信息被直接编码成一个给定带宽的宽带波形，没有物理“扩展”的载波的频率。这种方法的主要优点在于用于编码信息的扩展选项，以及保留对调制宽带波形的时间和幅度结构的可逆控制。显着的“多余的带宽”（超过需要携带的信息）使我们能够使用全通滤波器来管理这些波形的统计特性和时域外观，而不改变它们的频谱组成。例如，可以将发送波形的混合整形为低波峰因数信号（例如，以减少功率放大器上的负担），和/或使其在统计上与高斯噪声不可区分（例如，用于隐蔽传输和物理层隐写术），而可以将接收波形的所选分量变换为适合于复用和/或低SNR通信的高波峰因数脉冲串。此外，对携带低速率信息的宽带波形的时间和幅度结构的控制使得能够有效地使用非线性滤波技术。这种技术可以用于信息的鲁棒实时异步提取，以及用于具有相同频谱内容的宽带信号分量彼此的分离。这可以促进各种各样的低SNR和隐蔽通信配置的开发。
英文摘要：
In traditional spread-spectrum techniques, a wideband transmit signal is obtained by modulating a wideband carrier by a narrowband signal containing a relatively low-rate message. In the receiver, the respective demodulation/despreading restores the information-carrying narrowband signal. In this paper, we introduce an alternative approach, where the low-rate information is encoded directly into a wideband waveform of a given bandwidth, without physical "spreading" of the carrier's frequency. The main advantages of this approach lie in extended options for encoding the information, and in retaining a reversible control over the temporal and amplitude structures of the modulating wideband waveforms. Significant "excess bandwidth" (over that needed to carry the information) enables us to use allpass filters to manage statistical properties and time-domain appearances of these waveforms without changing their spectral composition. For example, a mixture of transmitted waveforms can be shaped as a low-crest-factor signal (e.g. to reduce the burden on the power amplifier), and/or made statistically indistinguishable from Gaussian noise (e.g. for covert transmissions and physical layer steganography), while the selected components of the received waveform can be transformed into high-crest-factor pulse trains suitable for multiplexing and/or low-SNR communications. Further, control over the temporal and amplitude structures of wideband waveforms carrying low-rate information enables effective use of nonlinear filtering techniques. Such techniques can be employed for robust real-time asynchronous extraction of the information, as well as for separation of wideband signal components with identical spectral content from each other. This can facilitate development of a large variety of low-SNR and covert communication configurations.
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