On the overshooting effect in EXIT charts of iterative source-channel decoding

Source codec parameters determined by modern source encoders are usually very sensitive to noise on the transmission link. Natural residual redundancy of these parameters can be utilized by Soft Decision Source Decoding (SDSD) [1] to increase the error resistance. Serially concatenating Forward Error Correction (FEC) and SDSD leads to a Iterative Source-Channel Decoding (ISCD) scheme [2, 3] which offers further improvements in robustness. ISCD exploits natural residual source redundancy by SDSD and artificial channel coding redundancy due to FEC in an iterative Turbo-like process [4, 5]. The convergence behavior of ISCD schemes can be analyzed by EXtrinsic Information Transfer (EXIT) charts [6, 7]. However, if an advanced ISCD system design [8-11] is applied, offering incremental quality improvements for many iterations, the EXIT curves for SDSD do not specify a tight bound for the decoding trajectory anymore. In the initial iterations, the decoding trajectory "overshoo ts" the EXIT curves of SDSD, e.g. [12, 13]. In this paper, we give reasons for this overshooting effect. In addition, we propose a novel solution for determining the EXIT curve of SDSD which allows a more precise convergence analysis.