软判决多步法相关攻击在PN码快速捕获中的应用

针对非协作接收的扩频通信系统，低信噪比情况下长周期且生成多项式抽头数较多的扩频码的快速捕获问题一直是研究的重点和难点。本文借鉴密码学中密钥攻击基本思想，提出一种基于软判决多步法快速相关攻击的长周期扩频码快速捕获新算法。通过将PN序列看成线性分组码，将序列相位捕获看成线性分组码的解码，采用软判决方法进行分步串行译码，充分利用序列本身的软信息，实现低信噪比情况下长周期扩频序列的快速捕获。理论分析和仿真结果表明本文算法与多项式抽头个数无关，具有较好的捕获性能，能够在低信噪比下稳定工作；与硬判决相关攻击方法相比，本文算法大约有3.5dB以上的性能提升。 

Application of Multi-pass Soft Fast Correlation Attack on Rapid PN Code Acquisition

In non-cooperative spread spectrum communication system, rapid acquisition of long period spreading code has been the key problem when signal-to-noise ratio (SNR) is low. To solve the problem, we propose a new rapid PN code acquision scheme based on multi-pass soft fast correlation attack. By regarding the PN sequence as a linear block code, the acquisition problem can be viewed as a decoding problem. To make full use of the soft information of the sequence, the decoding problem is achived through a soft decision method,and thus accomplished a fast acquisition of the long period code.Theoretical analysis and simulation results show that the performance of the proposed scheme is irrelevant to the number of the feedback taps. Our methods can work steadily when SNR is low. Compared with the multi-pass hard fast correlation attack, our algorithm has more than 3.5dB performance improvement.