利用EXIT图分析信源码率对双LDPC码系统的影响

双LDPC( DLDPC)码系统是一种高性能的信源信道联合编码系统，但是其性能在很大程度上受到信源码率的影响。为了从理论上分析信源码率对DLDPC码系统的影响，推导了外信息转移( EX-IT)函数，并利用EXIT图分析了信源码率对DLDPC码系统的影响，理论分析结果与实际仿真结果一致。进而利用EXIT图估算出DLDPC码系统在不同信源熵情况下的信源码率门限值，该门限值可以用来指导实际系统中信源码率的选取。所提出的分析方法比传统的蒙特卡罗仿真法更加简捷、直观。

Using EXIT chart to analyze impact of source code rate on double LDPC code systems

Double low-density parity-check(DLDPC) code system is a class of joint source and channel coding system with good performance.However,the performance of DLDPC code system is significantly influenced by the source code rates.In order to analyze the impacts of source code rates on DLDPC code system theoretically,extrinsic information transfer(EXIT) functions are derived and the performance of DLDPC code system with different source code rates is analyzed based on EXIT charts.The theoretical analysis is in good agreement with simulation result.Furthermore,the thresholds of source code rates in DLDPC code system are estimated by EXIT charts.The thresholds can be used to optimize the source code rates of real DLDPC code system.The proposed analysis method is easier and more straightforward than the traditional Monte Carlo simulation method.