低信噪比下归零Turbo码码长及其帧同步识别

针对目前高斯消元法在归零Turbo码长、帧同步等参数识别过程存在容错性能低且计算复杂度高的缺点，该文提出一种低信噪比(SNR)下基于差分似然差(DLD)的识别算法。首先通过定义差分似然差的概念，利用归零Turbo码帧头两码元差分似然差为正值(“+”)的特性，构建分析矩阵实现码长的识别；其次，提出基于最小错误判决准则下的差分似然差“+”位置门限判决方法，完成帧同步；最后，从工程实际出发，遍历寄存器个数的可能值，实现码率、寄存器个数以及交织长度识别。仿真实验表明:所提算法对于归零Turbo码码长、帧同步等参数识别有效，差分似然差“+”位置分布与分析的数据结构特征一致，判决门限能够有效判断差分似然差“+”位置，同时，算法容错性能较强，在信噪比为–5 dB条件下，码长、帧同步等参数识别率能够达到90%以上，并且算法的复杂度远小于现有算法。

Blind Recognition of Code Length and Synchronization of Turbo Codes on Trellis Termination at Low SNR

In order to overcome the shortcomings of low fault-tolerance and high computational complexity in the process of parameter identification such as code length and synchronization of Turbo code, a new algorithm based on Differential Likelihood Difference (DLD) at low Signal-to-Noise Ratio (SNR) is proposed. Firstly, the concept of DLD is defined, and the analysis matrix is constructed to identify the code length by using the characteristic that the DLD between two codes in Turbo frame terminal is positive ("+"); Secondly, a method based on the minimum error decision criterion to decide DLD "+" position is proposed to complete frame synchronization. From the engineering practice, the possible values of the number of registers are traversed to realize the recognition of the code rate, the number of registers and the interleaving length. Simulation results show that the proposed algorithm is effective in identifying parameters such as code length and frame synchronization, the position distribution of DLD "+" is consistent with the data structure characteristics of the analysis, and the threshold can effectively determine the position of DLD "+". At the same time, the algorithm has strong fault-tolerant performance. Under the condition of SNR –5 dB, the identification of code length, frame synchronization and other parameters can reach more than 90%, and the complexity of the algorithm is far less than the existing algorithms.