基于调频广播及非相干扩频水印的隐蔽通信系统关键技术研究

Information hiding and digital watermarking techniques can embed imperceptible data into public media to realize cover communication which is important for information security and anti-intercept communications. Analog FM broadcasting provides a good carrier for covert communication, and could be a touchstone to test the performance of information hiding methods. Most of current schemes adopt the traditional technique of coherent spread spectrum (SS) watermarking. However, the frequency-selective fading property of FM channel will cause disperse and inversion of the correlation peaks of SS sequences, and then will lead to high decoding error rate when using coherent demodulation that depends on the polarity of correlation peaks. This project proposes a strategy of applying non-coherent SS watermarking instead of coherent methods, to solve the problem of sensitivity to frequency-selective interference. Besides, focusing on the host interference to SS watermark, this project proposes the theory of informed non-coherent SS watermarking, in which the host is suppressed and utilized at the embedder to improve the correlation property of SS watermark and then to obtain better detection and extraction performance. Based on the above techniques, a covert communication system via FM broadcasting is designed. The proposed watermarking method robust to frequency-selective interference can also be applied to other information hiding systems, and the strategy of utilizing the host to improve the watermark robustness is aggressive for SS watermarking. Hiding in severe channels such as FM broadcasting will promote the progress of information hiding techniques.

信息隐藏和数字水印技术可在公开的媒介中嵌入不被察觉的微弱信息，进而实现隐蔽通信，这对保障信息安全、提高通信的抗截获能力具有重要意义。模拟制式的调频广播是隐蔽通信的优良载体，也是检验信息隐藏性能的试金石。现有方案多采用相干扩频水印技术，但调频广播信道的频率选择性衰落将导致扩频序列相关峰的扩散和极性翻转，从而使依赖相关峰极性的相干解调产生高误码。本项目提出以非相干扩频代替相干扩频的策略，解决了现有方案对频率选择性干扰敏感的问题；同时，针对载体对水印的干扰问题，提出利用边信息的非相干扩频水印理论，在嵌入端抑制载体干扰并利用载体改善扩频水印的相关特性，使其更易被检测和提取。基于上述技术构建适合调频广播的隐蔽通信系统。本项目所提出的抗频率选择性干扰的水印技术对信息隐藏的其它应用具有借鉴意义，所倡导的利用载体改善水印鲁棒性的策略具有先进性，模拟广播这一恶劣信道的攻克也有利于推动信息隐藏技术的进步。

针对传统相干扩频水印对频率选择性干扰敏感的问题，本项目提出以非相干扩频代替相干扩频的策略，研究了利用边信息的非相干扩频水印理论，设计了基于非相干扩频的音频水印方案，并构建了基于调频广播的隐蔽通信系统。主要研究成果包括：.1）利用边信息的非相干扩频水印理论.建立了利用边信息的二进制及多进制非相干扩频水印的数学模型，在嵌入端利用载体与扩频序列的相关性来补偿和抵消载体在接收端对水印的干扰，同时，利用载体增强扩频序列相关峰以增强水印检测和解调的鲁棒性。在此基础上提出了利用边信息的循环码移键控等非相干扩频水印实现方法。.2）基于非相干扩频的音频水印方案.将非相干扩频水印理论应用于音频水印，提出了基于心理声学模型的序列整形技术，使嵌入水印前后的音频信号具有相近的特征，保证了嵌入信息后的声音质量；提出一种零阈值同步捕获方法，克服了传统的基于阈值的同步方法受信号幅度波动影响大、虚警概率不易控制等缺点，改善了水印检测的灵敏度和可靠性；提出一种基于局部统计特征的信道补偿及相关峰修复技术，补偿了频率选择性信道导致的相关峰能量的发散，提高了水印检测和解调的可靠性。.3）基于调频广播及非相干扩频水印的隐蔽通信系统.在上述技术的基础上，构建了基于调频广播的隐蔽通信系统并进行了测试，在40bps的速率下水印解调误比特率不超过0.1%，优于现有模拟广播中的水印方案，为实现安全、高效、可靠的隐蔽通信提供了解决方案。.本项目研究的非相干扩频水印技术弥补了传统的相干扩频水印在频率选择性信道中的缺陷，所贯彻的利用载体改善扩频水印鲁棒性的思想具有创新性和先进性，模拟广播这一恶劣信道的攻克也有利于推动信息隐藏理论和技术的完善。