  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.
