MATLAB Code Implementation of EXIT Chart - Extrinsic Information Transfer Chart

EXIT Chart (Extrinsic Information Transfer Chart) is a powerful tool for analyzing the performance of iterative decoding algorithms, particularly useful in the design of channel coding schemes such as Turbo codes and LDPC codes. It predicts decoder convergence behavior by visualizing the evolution of mutual information. Implementing EXIT charts in MATLAB typically involves these key steps: First, establishing simulation models for both encoder and decoder, including modulation, channel transmission, and demodulation processes. The core implementation calculates mutual information to track information exchange between decoders. In practical implementation, one typically fixes the input extrinsic information of one decoder and computes the output extrinsic information of the other decoder through Monte Carlo simulations. A typical implementation consists of two main components: mutual information calculation for SISO (Soft Input Soft Output) decoders, and the exchange and recording of extrinsic information during iterations. Mutual information calculation commonly employs histogram estimation methods (using MATLAB's histcounts function) or parametric approaches. The curve plotting phase requires careful design of simulation parameters such as SNR ranges and iteration counts, often implemented using nested loops with proper data logging. Key interpretation of EXIT charts lies in observing the gap between two curves: if the curves intersect, it indicates that iterative decoding will stall at that SNR; if an open tunnel forms, the decoder can progressively improve performance through iterations. This analytical method predicts potential coding scheme performance faster than traditional bit error rate simulations, with MATLAB implementations typically featuring SNR sweep loops, mutual information tracking arrays, and visualization using plot functions with appropriate axis labeling.