An MSE Based Ttransfer Chart to Analyze Iterative Decoding Schemes

An alternative to extrinsic information transfer (EXIT) charts called mean squared error (MSE) charts that use a measure related to the MSE instead of mutual information is proposed. Using the relationship between mutual information and minimum mean squared error (MMSE), a relationship between the rate of any code and the area under a plot of MSE versus signal to noise ratio (SNR) is obtained, when the log likelihood ratios (LLR) can be assumed to be from a Gaussian channel. Using this result, a theoretical justification is provided for designing concatenated codes by matching the EXIT charts of the inner and outer decoders, when the LLRs are Gaussian which is typically assumed for code design using EXIT charts. Finally, for the special case of AWGN channel it is shown that any capacity achieving code has an EXIT curve that is flat. This extends Ashikhmin et als results for erasure channels to the Gaussian channel.